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Institute of Neurobiology and Department of Anatomy, University of Puerto Rico, San Juan, Puerto Rico
Submitted 2 January 2004; accepted in final form 7 November 2004
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ABSTRACT |
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INTRODUCTION |
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; Chan-Palay and Palay 1984; Kupfermann 1991; Lundberg and Hökfelt 1986). Our present understanding of neurotransmitter colocalization and cotransmission has been broadened by studies using the large identified neurons that are found in invertebrate model systems (Adams and O’Shea 1983; Katz and Harris-Warrick 1989, 1990; Weiss et al. 1992). While the majority of these studies have focused on neurons with well-defined and accessible peripheral projections (Bishop et al. 1987; Brezina et al. 1994a, b; Vilim et al. 1996a, b; Whim et al. 1993), recent investigations have addressed the significance of cotransmission within the central pattern generator (CPG) circuits that regulate motor behavior (Koh et al. 2003; Marder 1999; Nusbaum et al. 2001; Thirumalai and Marder 2002).
As in many species, the feeding behavior of Aplysia californica consists of a highly variable appetitive phase that is followed by a more stereotyped consummatory phase (Kupfermann 1974a). Consummatory behaviors include multiple ingestive (such as biting and swallowing) and egestive (such as rejection) actions that are executed by intersecting sets of specialized organs and muscles. This motor system is governed by a multifunctional or polymorphic CPG network that is located primarily within the buccal and cerebral ganglia (Cropper et al. 2004; Kupfermann 1974b; Kupfermann and Weiss 2001; Morton and Chiel 1994). By regulating the phase relations of motor neuron firing, this CPG can determine the qualitative (ingestive vs. egestive) and quantitative features of the behavior that is produced by the system (Church and Lloyd 1994; Hurwitz et al. 1996; Jing and Weiss 2001, 2002; Morgan et al. 2002; Morton and Chiel 1993a, b).
B20 and B65 are two buccal interneurons that are capable of initiating coordinated buccal motor patterns (Kabotyanski et al. 1998; Teyke et al. 1993). Moreover, each is thought to play a critical role in determining the functional output of the feeding CPG. Recently, it was found that the motor programs elicited by B20 tend to be egestive and that this neuron contributes in a significant fashion to the production of egestive motor programs elicited by higher-order cerebral-buccal interneurons (CBIs) (Jing and Weiss 2001; Proekt et al. 2004). While B65 also evokes egestive BMPs (Due et al. 2004; Kabotyanski et al. 1998), its repeated firing can also achieve a transition of the buccal CPG toward ingestive motor programs (Kabotyanski et al. 1998).
B20 and B65 both exhibit catecholaminergic histofluorescence (Díaz-Rios et al. 2002; Kabotyanski et al. 1998; Teyke et al. 1993), and both contain tyrosine hydroxylase-like immunoreactivity (Díaz-Ríos et al. 2002). In addition to containing markers for catecholamines, B20 and B65 exhibit GABA-like immunoreactivity (Díaz-Ríos et al. 1999, 2002; Jing et al. 2003). As several neurons that receive direct excitatory synaptic input from these two influential interneurons are identified (Due et al. 2004; Jing and Weiss 2001; Kabotyanski et al. 1998; Teyke et al. 1993), the present study was undertaken to explore the possible roles of dopamine and GABA in their rapid synaptic signaling. Some of these observations have been reported in abstract form (Díaz-Ríos and Miller 2002; Díaz-Ríos et al. 2003).
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METHODS |
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Experiments were conducted on specimens of Aplysia californica (150–250 g) that were purchased from the Aplysia Resource Facility and Experimental Hatchery (University of Miami, Coral Gables, FL) or from Marinus (Long Beach, CA). Animals were maintained in refrigerated aquaria (14–16°C) and fed dried seaweed twice per week.
Electrophysiology and pharmacology
Neurons were identified in preparations consisting of the paired buccal and cerebral ganglia. Extracellular signals were recorded with polyethylene suction electrodes and AC-coupled amplifiers (Model 1700, AM Systems). The typical configuration consisted of two en passant recordings from the cerebral-buccal connectives and one cut-end recording from a major branch of the radula nerve. Intracellular microelectrodes filled with 2 M KCl (10–20 M
) were used to record from known synaptic follower cells of neurons B20 (Teyke et al. 1993) and B65 (Kabotyanski et al. 1998). A second microelectrode was used to locate the presynaptic neuron. After presynaptic cell identification, another intracellular electrode (5–10 M) was introduced for passing current into the postsynaptic neuron. Experiments were conducted at room temperature (19–21°C) in artificial seawater (ASW) containing elevated concentrations of divalent cations (2.2 x [Ca2+] and 2 x [Mg2+]) (Liao and Walters 2002) to attenuate polysynaptic activity.
Solutions of drugs were prepared from powder in high-divalent ASW immediately before their application. Methylergonovine maleate salt, ergonovine (ergometrine), chlorpromazine hydrochloride, fluphenazine dihydrochloride, clozapine, (±)-butaclamol, S-(–)-raclopride, (±)-baclofen, saclofen, picrotoxin, and bicucculine were obtained from Sigma Chemical Co. (St. Louis, MO). Piperidine-4-sulfonic acid (piperidine-4S) and (±)-nipecotic acid were purchased from Research Biochemicals International (Natick, MA). All experiments shown in this study were performed with application of agonists and antagonists at a concentration of 1 mM. This concentration was chosen due to its reliable production of effects that were reversible. Moreover, as there is evidence for receptors with higher sensitivity in this system (see DISCUSSION), elevated concentrations were chosen to increase the likelihood that observations reflected actions at receptors within the synapse, where high levels of DA are likely to mediate rapid signaling. Preparations were superfused with the ASW solution at a rate of 0.5 ml/min using a gravity-fed multi-channel system (ALA Scientific Instruments, Model BM4). Responses were assessed 2, 5, and 10 min after perfusion switches. Baclofen, GABA, and dopamine were also applied through a puffer electrode (0.5- to 1-s pulses) at a concentration of 1 mM using a Picospritzer II (General Valve, Fairfield, NJ) pressure ejection system. For these tests, the superfusion was briefly interrupted and the neurotransmitter or agonist was mixed with a small amount of dextran fluorescein (Molecular Probes, Eugene, OR) dye to aid visualization of the puff. The pipette tip was typically placed close to the soma and initial segment of the target neuron. The ASW superfusion was reinitiated immediately after application to ensure rapid removal of the ejected solution. Application of dextran fluorescein alone did not produce any detectable effects.
All results reported in this study were observed in a minimum of three specimens. Experiments conducted on the left and right buccal hemiganglia were pooled. Measurements are reported as the means ± SE. Due to the presence of long-lasting dopaminergic (see DISCUSSION) and GABAergic (Díaz-Ríos and Miller 2002, Díaz-Ríos et al. 2003) actions in this system, statistical tests (Student’s t-test; 2-tailed) were performed by comparing measurements obtained prior to drug application to those attained at the peak of the response. A value of P < 0.05 was established as the criterion for significance.
Neurobiotin injections
After identification of specific neurons based on location, size, pigmentation, electrophysiological features, and synaptic connectivity, one KCl microelectrode was withdrawn and replaced with one containing Neurobiotin. Injections were modified from the methods described by Delgado et al. (2000). The microelectrode tips were filled with 4% Neurobiotin (Vector Laboratories, Burlingame, CA) dissolved in 0.5 M KCl and 50 mM Tris (pH 7.6). The electrode shafts were filled with 2 M KCl, resulting in resistances ranging from 15 to 30 M. Depolarizing current pulses (1–2 nA; 0.5 s; 1 Hz; 10–30 min) were used to eject the Neurobiotin. This procedure did not appear to affect the resting potential or spontaneous electrical activity of the injected neuron. The preparations were usually left at room temperature for 2–3 h to allow material to diffuse from the injection site (cell body) into small and distant processes. They were then repinned if necessary, and fixed in 4% paraformaldehyde (1–4 h). The fixed ganglia were transferred to microcentrifuge tubes, and washed five times (20 min each) with a phosphate buffer containing 1% Triton X-100 and 0.1 mM sodium azide (PTA solution). They were then incubated in Rhodamine600 Avidin D (Vector Labs) diluted (1:200–1:3,000) in PTA (24–48 h, room temperature). Tissues were washed five times with PTA and viewed on a Nikon Eclipse TE200 fluorescence microscope. Images were captured using the ACT-1 software package (Nikon) and processed using Photoshop (Version 6.0) and Corel Draw (Version 9.0).
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RESULTS |
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B20 is a small elongated bipolar neuron located in the medial region of the ventral surface of each buccal hemiganglion (Fig. 1A1) (see also Díaz-Ríos et al. 2002; Teyke et al. 1993). Previous investigations have demonstrated direct excitatory synaptic signaling from B20 to B16 and B8 (Fig. 1, A2 and B), two buccal motor neurons that participate in closure of the food-grasping radula (Jing and Weiss 2001; Teyke et al. 1993). B16 is a large motor neuron located in the medial region of the ventral motor neuron cluster (Fig. 1A1) (see Cohen et al. 1978; Kreiner et al. 1987). Its axon projects across the hemiganglion in the lateral direction to exit via buccal nerve 3 and innervate the ipsilateral I5 (also known as the accessory radula closer or ARC) muscle (Cohen et al. 1978) and the I4 muscle (Jordan et al. 1993). The cell body of B8 (actually 2 neurons designated B8a and B8b that are indistinguishable by presently known physiological criteria) is located in the most lateral region of the ventral motor neuron cluster (Fig. 1A1) (see Church and Lloyd 1991, 1994; Morton and Chiel 1993b; Rosen et al. 2000). Its axon traverses the hemiganglion, exits via the radula nerve, and innervates the ipsilateral I4 and I6 muscles.
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30 ms; Fig. 1B). The amplitude of the EPSP recorded in the ipsilateral B8 cell body (3.9 ± 0.23 mV, n = 5) was larger than that recorded in the ipsilateral B16 (1.7 ± 0.26 mV, n = 5). The relation between postsynaptic membrane potential and the amplitude of the B20-evoked fast EPSP was examined in B16 and B8 by setting the two postsynaptic cells to hyperpolarized levels with current injected through a second microelectrode (Fig. 1C, 1 and 2). The peak amplitudes of the EPSPs evoked in both motor neurons were increased at hyperpolarized membrane potentials and they exhibited similar extrapolated reversal potentials (between –5 and –15 mV; n = 3 for each cell, Fig. 1C3).
RAPID SYNAPTIC SIGNALING OF B20: TESTING THE ROLE OF GABA.
Excitatory GABAergic signaling contributes to the generation of feeding motor programs in pulmonate and pteropod mollusks (Arshavsky et al. 1993; Bravarenko et al. 2001; Norekian 1999; Norekian and Satterlie 1993; Richmond et al. 1993). The known ability of GABA to produce excitatory responses on Aplysia neurons (Yarowsky and Carpenter 1977, 1978), coupled with the presence of GABA-like immunoreactivity in B20 (Díaz-Ríos et al. 2002), therefore prompted us to examine the possible contribution of GABA to the signaling of this cell to the radula closer motor neurons B16 and B8.
Application of GABA produced differential effects on the resting membrane potentials of the two motor neurons (Fig. 2). No changes in the membrane potential of B16 were detected with application of GABA to its cell body and initial segment (Fig. 2A; n = 4). To ensure that the absence of a response was not due to the membrane potential (Vm) of B16 being close to the GABA reversal potential, additional tests were made with the Vm adjusted to various levels (–40 to –80 mV), but no responses were observed (not shown). Puffed application of GABA (1 mM) to the soma of B8 produced a biphasic response consisting of an initial hyperpolarization followed by a long-lasting depolarization (Fig. 2B1; n = 3). A picrotoxin-sensitive hyperpolarizing response to GABA was recently demonstrated in B8, and inhibitory GABAergic PSPs were shown to originate from an identified interneuron, B40 (Jing et al. 2003; see DISCUSSION). The long-lasting depolarization was further explored using baclofen, an agonist of GABAB type receptors in vertebrates. Bath application of baclofen produced a monophasic depolarization of the B8 membrane potential (Fig. 2B2; n = 3). This depolarization was often irregular and gradual in onset, but it was sustained for the duration of baclofen exposure. It was reversed when washed with normal ASW (Fig. 2B2, 
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In contrast to the stable voltage deflections produced in B16 with hyperpolarizing current pulses, those elicited in B8 exhibited a prominent time-dependent decline or "sag" (compare Fig. 2C, 2 and 1). The GABA-induced increase in the B8 input resistance was nearly equivalent throughout the pulse. Conspicuous effects on the magnitude or kinetics of the sag itself were not noted.
All GABAergic antagonists that were tested, including agents that block GABA-mediated inhibitory postsynaptic potentials (IPSPs) in Aplysia (picrotoxin and bicucculine) (Jing et al. 2003), a GABAB antagonist (saclofen), and compounds that block GABAergic EPSPs in the feeding system of Clione limacina (piperidine-4-sulfonic acid and 5-aminovaleric acid) (Norekian 1999) did not produce detectable effects on the EPSPs produced by B20 in B16 or B8 (all tested at concentrations 
1 mM). Moreover, nipecotic acid (1 mM), an inhibitor of GABA reuptake that augments signaling at GABAergic synapses in mollusks (see Jing et al. 2003; Norekian 1999) did not affect the EPSPs from B20 to either of its followers.
Recent studies have shown that application of exogenous GABA can occlude GABAergic PSPs in this circuit (Jing et al. 2003; Wu et al. 2003). The actions of GABAergic agonists were therefore evaluated on the EPSPs produced by B20 in B16 and B8. Bath application of GABA produced a partial (38.7 ± 5.1%, n = 5) reduction of the amplitude of the EPSP evoked by B20 in B16 (t = 9.93, P < 0.05, Fig. 3A1). Exogenous baclofen produced a comparable reduction (47.2 ± 7.1% decrease, n = 4) of the B20-to-B16 EPSP (t = 8.66, P < 0.05, Fig. 3A2). In contrast, application of GABA enhanced (102.5 ± 8.2% increase; n = 5) the amplitude of the EPSP evoked by B20 in B8 (t = 12.70, P < 0.05, Fig. 3B1). Baclofen (1 mM) also augmented (56.4 ± 7.5% increase; n = 4) the B20-to-B8 EPSP (t = 14.66, P < 0.05, Fig. 3B2).
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, 2003).
RAPID SYNAPTIC SIGNALING OF B20: TESTING THE ROLE OF DOPAMINE.
Dopamine promotes motor programs in the feeding systems of several mollusks (Kyriakides and McCrohan 1989; Quinlan et al. 1997; Trimble and Barker 1984; Wieland and Gelperin 1983), including Aplysia (Kabotyanski et al. 2000; Teyke et al. 1993). Previous experiments suggested participation of dopamine in the generation of buccal motor programs elicited by interneuron B20 but left unresolved its involvement in its fast synaptic signaling (Teyke et al. 1993). As our findings did not support the role of GABA in this capacity (see preceding text), experiments were conducted to test the participation of dopamine in the mediation of the rapid EPSPs from B20 to B16 and B8.
Exposure to exogenous DA produced depolarizations of both motor neurons. A brief (0.5 s) pulsed application to the soma region of B16 elicited complex responses, consisting of a rapid depolarization (time to peak, 100 ms), an irregular secondary phase, and a smaller sustained late depolarization (Fig. 4A; n = 4). When the membrane potential of B16 was set to hyperpolarized levels prior to DA application, the three components of its DA response were all increased in amplitude (Fig. 4, A and C).
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4 s) and long-lasting (Fig. 4B; n = 5) (see also Due et al. 2004). When B8 was at its resting potential, the response to DA could exceed threshold, producing repetitive impulses (Fig. 4B, top). In contrast to the responses evoked in B16, the DA response in B8 was decreased when the membrane potential was set to hyperpolarized levels (Fig. 4, B and C; n = 4).
The possible role of dopamine in mediating synaptic signaling from B20 was further tested by examining its effect on the input resistance and excitability of B16 and B8 (Fig. 5). As these protocols required bath application of DA (see preceding text), observed effects are likely to reflect the summed and weighted contributions of all dopamine receptors with electrical influence on the soma. Moreover, the slow kinetics of this mode of agonist application would be expected to diminish the contribution of those DA receptors that display desensitization (see Ascher 1972). Finally, bath application of DA could produce release of other neuroactive substances from sources within the ganglion. These considerations notwithstanding, DA produced decreases in the membrane input resistance of B16 (Fig. 5A, 1 and 2; n = 4) and B8 (Fig. 5B, 1 and 2; n = 4).
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). This observation is consistent with attributing the principal action of exogenous DA to its activation of ligand-gated synaptic receptors. If the depolarizations of B16 and B8 produced by dopamine reflect its activation of the postsynaptic receptors that respond to the excitatory neurotransmitter of B20, then these depolarizations could be expected to have an excitatory influence on these cells. This was tested by adjusting depolarizing current pulses to evoke a moderate number of impulses in each motor neuron (Fig. 5, A3 and B3, top traces) prior to DA application. In both cases, identical current pulses produced a greater number of impulses in the presence of dopamine, indicative of an excitatory dopaminergic influence on B16 and B8 (Fig. 5, A3 and B3, bottom traces). In B16, the number of evoked impulses was increased from 12.2 ± 2.4 to 31.6 ± 6.3 (n = 7) by DA (t = 14.60; P < 0.05; Fig. 5A3). In B8, an increase from 3.6 ± 1.5 to 6.8 ± 1.2 (n = 6) impulses was observed (t = 14.26, P < 0.05; Fig. 5B3). As these tests were made using the bath mode of DA application, they were subject to the same assumptions and limitations that applied to tests on input resistance (preceding text). Comparable increases in excitatory responsiveness were observed, however, when brief puffed applications of DA were applied to the somata of B16 and B8 and followed immediately by test current pulses (not shown).
If the effects of DA on B16 and B8 reflect its activation of the synaptic receptors that mediate fast signaling from B20, then application of a high concentration of exogenous DA could be expected to attenuate or occlude these signals. When B20 was fired in the presence of DA (1 mM), its EPSP was essentially eliminated in B16 (98.9 ± 3.2% reduction; n = 3) and B8 (97.6 ± 2.8% reduction; n = 3; Fig. 6, A and B).
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; Wieland and Gelperin 1983) and methylergonovine (Nargeot et al. 1999), were ineffective in blocking B20-evoked EPSPs (see also Teyke et al. 1993). Also without effect were the D1 receptor antagonist, butaclamol, the D2 antagonist, raclopride, and the typical and atypical neuroleptics fluphenazine, chlorpromazine, haloperidol, and clozapine (all tested 1 mM concentrations). Effects were observed with the D2 receptor antagonist sulpiride, which has been shown to block dopaminergic synapses and responses to dopamine in mollusks, including Aplysia (Due et al. 2004; Magoski et al. 1995; Quinlan et al. 1997). Application of sulpiride (1 mM, 10 min) produced a blockade of the EPSPs recorded in both B16 (reduction of 98.1 ± 2.2%, n = 3) and B8 (reduction of 98.8 ± 1.8%, n = 3; Fig. 6C). Block of the EPSPs in the two cells followed identical time courses. They were fully accomplished within 10 min of exposure and both required prolonged wash with normal ASW to achieve reversal (Fig. 6C, right panels). Together, the observed dopaminergic actions on the membrane potential, input resistance, and synaptic responses in B16 and B8, coupled with the block of these PSPs by sulpiride, support the role of DA as the principal neurotransmitter mediating rapid EPSP signaling from B20 to these two buccal motor neurons.
Rapid excitatory signaling from B65 to B8 and B4/5
B65 is a moderately sized buccal interneuron that is located between the origins of the esophageal nerve and buccal nerve 1 near the caudal surface of each hemiganglion (Fig. 7A) (Díaz-Ríos et al. 2002; Kabotyanski et al. 1998). As B65 shares the GABA-immunoreactive/catecholamine phenotype with B20 (Díaz-Ríos et al. 2002; Due et al. 2004; Jing et al. 2003), its direct EPSPs to identified follower neurons provided an opportunity to explore the generality of the preceding observations implicating dopamine as the mediator of B20’s fast synaptic signaling.
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). The somata of two direct synaptic followers, B8 (see also Fig. 1) and B4/5, were readily accessible from the caudal surface of the contralateral hemiganglion, due to their large size and marginal positions (Fig. 7A). B4/5 (actually 1 of a pair of cells that are not distinguishable by currently known physiological criteria) is a multifunctional neuron that is located in the most ventromedial edge of each hemiganglion (Fig. 7A). It projects an axon to the ipsilateral I3 muscle and is thought to have sensory, motor, and interneuronal functions (Church and Lloyd 1994; Gardner 1971; Jahan-Parwar et al. 1983; Rosen et al. 1982; Sossin et al. 1987). Simultaneous recording from B8 and B4/5 while firing B65 in high-divalent ASW (Fig. 7B) confirmed that each follower neuron received a fast PSP with a brief latency (4–5 ms) and rapid time to peak (35 ms). In agreement with previous observations (Kabotyanski et al. 1998), the amplitude of the PSP recorded in the contralateral B4/5 cell body (5.25 ± 0.28 mV, n = 5) was larger than that recorded in the contralateral B8 (0.55 ± 0.31 mV, n = 5). Both EPSPs were increased in amplitude when the membrane potentials of the postsynaptic cells were adjusted to more hyperpolarized levels (Fig. 7C, 1 and 2). Reversal of the B4/5 EPSP was in the range of –10 to –5 mV (n = 3), while that in B8 was estimated to occur at +5 to +10 mV (n = 3; Fig. 7C3).
RAPID SYNAPTIC SIGNALING OF B65: TESTING THE ROLE OF DOPAMINE.
In view of the observations indicating the role of DA in mediation of the B20-evoked EPSPs (preceding text), the dopaminergic pharmacology of the EPSPs originating from B65 was examined initially. The EPSP produced in B8 was virtually occluded in the presence of 1 mM DA (98.7 ± 1.3% reduction, n = 3; Fig. 8A) and blocked by sulpiride (95.4 ± 3.5% reduction, n = 3; Fig. 8C) (see also Due et al. 2004). Surprisingly, however, the amplitude of the EPSP produced by B65 in the contralateral B4/5 was only partially reduced by 1 mM DA (37.2 ± 7.3% decrease, n = 3; Fig. 8B). Moreover, the antagonism of the B65-to-B4/5 synapse by sulpiride differed from that in B8 (Fig. 8C). After 10 min of sulpiride (1 mM) exposure, the B65-to-B8 EPSP was completely blocked (see also Due et al. 2004), but the EPSP in B4/5 was only partially reduced (48.4 ± 8.5% decrease, n = 3; Fig. 8C, compare top and bottom).
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) that was accompanied by a small decrease in input resistance (Fig. 9B; n = 4). When assessed with depolarizing intrasomatic current pulses, the DA-induced hyperpolarization was found to have an inhibitory influence on B4/5, reducing the number of evoked impulses from a control level of 14.7 ± 3.2 to 1.6 ± 1.2, n = 5; t = 29.49, P < 0.05, Fig. 9C) (see also Kabotyanski et al. 2000). With each of these tests, therefore the responses of B4/5 to application of DA differed qualitatively from those observed on B16 and B8 (see Figs. 4 and 5).
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). However, the lower efficacy of DA and sulpiride in reducing the B65-to-B4/5 EPSPs (Fig. 8), coupled with the predominantly inhibitory actions of exogenous dopamine on B4/5 (Fig. 9), leave unresolved the identity of the mediator and receptor type responsible for transmission of these signals. RAPID SIGNALING FROM B65 TO B4/5: TESTING THE ROLE OF GABA. The inconclusive results of experiments testing the role of DA in the mediation of fast EPSPs from B65 to B4/5 (preceding text) led us to examine the actions of GABA on these signals. Focal application of GABA to the cell body region of B4/5, as well as bath application to the entire cell, did not produce detectable changes in the somatic membrane potential (Fig. 10A). In agreement with this observation, no consistent effects were produced by bath applied GABA on the input resistance of B4/5 (Fig. 10B).
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), a GABAB antagonist (saclofen), or compounds that block GABAergic EPSPs in the feeding system of Clione limacina (piperidine-4S and 5-aminovaleric acid) (Norekian 1999) (all tested 1 mM). Moreover, the GABA uptake inhibitor nipecotic acid did not influence B65-to-B4/5 signaling. Finally, application of GABA itself did not produce desensitization/occlusion of this EPSP. GABA did effect a partial (36.7 ± 5.4%, n = 3) decrease of the EPSP produced by B65 in B4/5 (t = 8.66, P < 0.05; Fig. 10C1). As observed with GABAergic actions on signaling originating from B20 (Fig. 4, preceding text), the effects of GABA on the B65-to-B4/5 EPSP were mimicked by the GABAB agonist baclofen (41.2 ± 6.8% reduction, n = 4; Fig. 10C2). In sum, the absence of GABAergic actions on the B4/5 membrane potential and input resistance, coupled with the inefficacy of GABAergic pharmacological manipulations to influence synaptic signals originating from B65, do not support the participation of GABA in the mediation of the fast EPSP between these cells. As was observed with the signaling of B20 to its followers, GABA was found to be capable of modifying these signals via the activation of GABAB-like receptors.
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DISCUSSION |
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). Our results indicate that dopamine acts as the primary neurotransmitter mediating the rapid EPSPs from B20 to two of its direct followers, the radula closer motor neurons B16 and B8 (Fig. 11). Recent findings have shown that fast excitatory signaling from B65 to neurons that participate in radula protraction (interneurons B31/32 and motor neuron B61) is also mediated by dopamine (Due et al. 2004). In agreement with observations reported in that study, we observed that convergent fast EPSPs produced in B8 by B65 are mediated by dopamine acting on receptors that are similar, or identical, to those activated by B20 (Fig. 11) (see Due et al. 2004).
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Dopaminergic signals and receptors in molluscan feeding
The pharmacological observations reported in this study support the role of dopamine as the principal neurotransmitter mediating rapid EPSP signaling from two buccal interneurons to their direct follower motor neurons. While dopaminergic signaling in the vertebrate nervous system is commonly mediated by G-protein-coupled receptors that act via second messengers (Civelli et al. 1993; Greengard 2001), considerable evidence supports the presence of ligand-gated dopaminergic synaptic signaling in mollusks (Due et al. 2004; Green et al. 1996; Magoski et al. 1995; Quinlan et al. 1997).
The occlusion of synaptic signals in the presence of high concentrations of DA observed in this study could be produced by one, or a combination, of several mechanisms including: activation of nonsynaptic receptors that depolarize the postsynaptic neuron to the reversal potential of the B20-evoked PSPs, shunting of synaptic currents, desensitization of the synaptic receptors, or complete occupancy of synaptic receptors preventing their further activation by the synaptic transmitter. Nonspecific depolarization is unlikely to be responsible because elimination of B20-evoked PSPs by DA was also observed when the follower motor neurons were set at membrane potentials 40 mV more hyperpolarized than rest (not shown). Some shunting of synaptic current is likely to occur, but the moderate decreases in motor neuron input resistance produced by DA (Fig. 5, A and B) would not be expected to produce such complete elimination of PSPs. A contribution from desensitization is suggested by the observation that the depolarizations produced by sustained application of DA were modest in comparison to the apparent reversal potential of its predominant responses (Fig. 4) (see also Ascher 1972) and the reversal potential of the B20-evoked EPSPs (Fig. 1C). However, once the DA-induced depolarizations were achieved, no decay was observed in their amplitude during prolonged (30 min) bath application of the agonist. Thus although the relative contributions of desensitization and occlusion remain unresolved, these results support the proposal that exogenous DA attenuates B20- and B65-evoked PSPs in B8 and B16 by acting as an agonist at the receptors that mediate these signals.
Exposure to comparatively low concentrations (1 x 10–6 M to 1 x 10–5 M) of dopamine is known to promote coordinated BMPs in several molluscan feeding systems (Kabotyanski et al. 2000; Kyriakides and McCrohan 1989; Quinlan et al. 1997; Trimble and Barker 1984; Wieland and Gelperin 1983). Thus in addition to the receptors mediating fast synaptic signaling originating from B20 and B65, additional and more sensitive dopaminergic receptor types are likely to be present in the buccal system. The presence of such receptors, and their probable role in configuring the buccal circuit, is supported by the demonstrated ability of the DA antagonist ergonovine (10–8–10–7 M) to block BMPs produced by firing B20 without affecting the B20-to-B8 EPSP (Teyke et al. 1993; our observations). Also, low concentrations methylergonovine block the enhancement of motor patterns produced by stimulation of the esophageal nerve in an analogue of operant conditioning (Nargeot et al. 1999). Interestingly, there is evidence that additional DA receptor types are present on B8 and B4/5 themselves. In the case of B4/5, an EPSP that is thought to be produced by dopaminergic fibers in the esophageal nerve is blocked by ergonovine (Nargeot et al. 1999), an antagonist that did not affect EPSPs originating from B65. Also, the presence of presynaptic DA receptors is indicated by the observation that cholinergic inhibitory PSPs from B4/5 to B8 are down-regulated by low concentrations (5 x 10–6) of dopamine (Kabotyanski et al. 2000) (see Fig. 11 of this article).
The apparent presence of multiple dopamine receptor types in direct follower neurons (preceding text) raises the possibility that DA originating from B20 and B65 could be acting both as the fast excitatory neurotransmitter and as a modulator at these synapses. In a previous study, Kabotyanski et al. (1998) demonstrated that, in addition to producing EPSPs in B8 and B4/5, repetitive trains of B65 impulses elicited slow and persistent actions in both follower neurons. It was proposed that such actions resulted from progressively increased levels of released dopamine. In this respect, B65 was likened to an intrinsic modulatory element (the dorsal swim interneuron: DSI) in the escape swimming CPG of Tritonia diomedea (Katz and Frost 1995, 1996; Katz et al. 1994). We have observed similar long-lasting effects in B8 with repeated stimulation of B20 (Díaz-Ríos and Miller 2002; unpublished data). If dopamine is responsible for both rapid and persistent signaling from B65 and B20, then these interneurons would share another property with the DSIs, i.e., rapid and prolonged signaling by a single biogenic amine neurotransmitter (Getting 1983; Katz and Frost 1995; McClellan et al. 1994). The ability to block the fast conventional EPSPs originating from B20 and B65 with sulpiride (see also Due et al. 2004) should facilitate characterization of additional synaptic actions and clarify the possible contribution of intrinsic dopaminergic modulation to regulation of the buccal CPG of Aplysia.
GABAergic signals and receptors in molluscan feeding
Although participation of GABA in the rapid synaptic signaling of B20 and B65 was not substantiated in this study, considerable evidence supports the involvement of GABA in the regulation of molluscan feeding (Arshavsky et al. 1991, 1993; Cooke et al. 1985; Norekian 1999; Norekian and Satterlie 1993; Richmond et al. 1991). Within the feeding-related circuits of Aplysia, two GABA-immunoreactive cerebral-buccal interneurons (CBIs; CBI-11 and -3) and two buccal-cerebral interneurons (BCIs: B34 and B40) have been shown to produce fast picrotoxin-sensitive IPSPs in identified follower neurons (Jing et al. 2003; Wu et al. 2003). These demonstrations of GABAergic signaling by GABA-immunoreactive neurons seem to support the validity of this method for the localization of authentic GABA. However, it remains plausible that the dopaminergic neurons examined in this study contain a specific epitope that is speciously detected with this antiserum. Although GABAli is colocalized with additional transmitters in some of the inhibitory interneurons [CBI-3: APGWamide (Jing and Weiss 2001); B34: ACh (Hurwitz et al. 2003)], in no case is it colocalized with dopamine (see also Díaz-Ríos et al. 2002).
Although there is presently little evidence for the presence of rapid excitatory GABAergic signals in the feeding system of Aplysia, this possibility warrants further scrutiny in view of the their established role in the consummatory behaviors of other mollusks (Arshavsky et al. 1991, 1993; Norekian 1999; Norekian and Satterlie 1993). Within the Aplysia CNS, GABA is known to be capable of producing excitatory responses on specific neurons (Yarowsky and Carpenter 1977). Our observations do suggest the presence of GABA receptors that share pharmacological properties with the GABAB classification of vertebrates. The ability of these receptors to modify the excitatory signaling of key interneurons like B20 and B65 is consistent with the observations of Richmond et al. (1993), who showed that activation of GABAB-like receptors could regulate the feeding CPG of the snail Helisoma. In that study, and in ours, baclofen mimicked several long-lasting GABAergic responses (see also Díaz-Ríos et al. 2003) but did not appear to activate GABAA-like receptors. Interestingly, the GABAergic interneuron B40 produces a long-lasting depolarization of B8 that follows its inhibitory actions (Jing and Weiss 2002; Jing et al. 2003), but the possible participation of GABA in this long-lasting signal has not been explored. Baclofen should serve as a selective pharmacological tool for further disclosing the properties and functions of pre- and postsynaptic GABAB-like receptors in the Aplysia CNS (see Philippe et al. 1981).
Functional considerations
Recent studies on the neural circuit controlling Aplysia consummatory behavior have led to an increased appreciation for its multifunctionality and its diverse sources of activation (Horn and Kupfermann 2002; Hurwitz et al. 1996; Kupfermann and Weiss 2001; Morton and Chiel 1994; Proekt and Weiss 2003). Multifunctionality of the Aplysia CPG is achieved, in part, by a type of multiplexing of behavioral components. Although ingestive and egestive behaviors share common and fairly invariant features of radula protraction and retraction, it is the phasing of radula closure with respect to its protraction and retraction that determines whether food will be ingested or egested (Church and Lloyd 1994; Hurwitz et al. 1996; Jing and Weiss 2001, 2002; Morgan et al. 2002; Morton and Chiel 1993a, b). If the radula is closing during its retraction phase, it will push food toward the esophagus (ingestion), and if it is closing during the protraction phase, it will tend to push food out of the mouth (egestion). B20 and B65 both fire primarily during the protraction phase of biting, and both are thought to dictate egestive motor patterns, largely due to their direct activation of the radula closer motor neurons B8 and B16 (Jing and Weiss 2001; Kabotyanski et al. 1998; but see following text). In both instances, the fast excitatory signaling from B20 and B65 that achieves radula closure during protraction and thus promotes egestive behavior, appears to be mediated by dopamine (Fig. 11). In the case of B65, the significance of this signaling is underscored by observations of Due et al. (2004), who showed that sulpiride and bilateral hyperpolarization of B65 produced comparable decreases in the egestiveness of evoked buccal motor programs.
B4/5 fires during the retraction phase of feeding and has widespread interneuronal actions that influence the phasing of many neurons involved in radula protraction-retraction and closure (Church and Lloyd 1994; Gardner 1971; Gardner and Kandel 1977; Kabotyanski et al. 1998; Nagahama and Takata 1990; Sossin et al. 1987). Kabotyanski and coworkers (1998) showed that repeated bursts of B65 firing could exert a prolonged inhibition of B4/5. These investigators also found that application of dopamine (5 x 10–5 M) produced a downregulation of fast IPSP signaling from B4/5 to B8 during retraction (Kabotyanski et al. 2000) (see Fig. 11 of this article). They proposed that a build-up of DA with repetitive B65 bursts could achieve a transition from an egestive (radula closure during protraction) to an ingestive (radula closure during retraction) buccal motor pattern (Kabotyanski et al. 1998, 2000). The inhibitory actions of DA on B4/5 observed in this study are consistent with this proposal. By using the same neurotransmitter, dopamine, to both excite B8 directly and to downmodulate the interposed inhibitory neuron B4/5, B65 may achieve such a transition by imposing a graded downregulation of B4/5 that is proportional to its cumulative excitation of B8 over a period of time (Fig. 11) (see also Kabotyanski et al. 1998).
In common with most complex behaviors, Aplysia feeding can be activated by multiple stimuli (Horn and Kupfermann 2002; Kupfermann and Weiss 2001; Kupfermann et al. 1991). B20 and B65 both have the capacity to evoke coordinated motor programs in buccal ganglia that are isolated from the remainder of the CNS (Jing and Weiss 2001; Kabotyanski et al. 1998; Teyke et al. 1993). Recently, B20-induced BMPs were shown to be egestive (Jing and Weiss 2001). Moreover, B20 was shown to be preferentially active during egestive motor programs that were elicited by specific higher order cerebral-buccal interneurons (Jing and Weiss 2001) or stimulation of the esophageal nerve (Proekt et al., 2004). B65, which is not recruited into CBI-evoked motor programs (Jing et al. 2003), has been studied primarily in isolated buccal ganglia, where it appears to produce egestive patterns that gradually become ingestive during prolonged firing of the interneuron (Kabotyanski et al. 1998) (see preceding text). Interestingly, a catecholaminergic buccal neuron in the snail Helisoma (designated N1a) that is thought to correspond to B65 also promotes ingestive BMPs in isolated buccal ganglia. Although the natural mode of stimulation of B65 remains uncertain, N1a was shown to be activated by natural feeding stimulants (watermelon) within the oral cavity (Murphy et al. 2001). It appears, therefore that although B20 and B65 are likely to differ in their implementation and consequences, they share the ability to influence the polymorphic buccal CPG in a qualitative fashion. Their unusual GABA-immunoreactive/dopaminergic phenotype, which is only known to occur in one additional neuron in the Aplysia CNS (Díaz-Ríos et al. 2002), is likely to contribute to this common capability. The use of dopaminergic signaling by these cells is consistent with observations in mammals, where DA is thought to be critically involved in coordinating information flow and behavioral responses to changing environmental and internal conditions (Grace 2002).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for reprint requests and othercorrespondence: M. W. Miller Institute of Neurobiology, University of Puerto Rico 201 Blvd del Valle, San Juan, Puerto Rico 00901 (E-mail: mmiller{at}neurobio.upr.clu.edu)
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REFERENCES |
|---|
|
Arshavsky YI, Deliagina TG, Gamkrelidze GN, Orlovsky GN, Panchin YV, Popova LB, and Shupliakov OV. Pharmacologically induced elements of the hunting and feeding behavior in the pteropod mollusk Clione limacina. I. Effects of GABA. J Neurophysiol 69: 512–521, 1993.
Arshavsky YL, Gamkrelidze GN, Orlovsky GN, Panchin YV, and Popova LB. Gamma-aminobutyric acid induces feeding behavior in the marine mollusk, Clione limacina. Neuroreport 2: 169–172, 1991.[Web of Science][Medline]
Ascher P. Inhibitory and excitatory effects of dopamine on Aplysia neurons. J Physiol 225: 173–209, 1972.
Bishop CA, Wine JJ, Nagy F, and O’Shea MR. Physiological consequences of a peptide cotransmitter in a crayfish nerve-muscle preparation. J Neurosci 7: 1769–1779, 1987.[Abstract]
Bravarenko NI, Ierusalimsky VN, Korshunova TA, Malyshev AY, Zakharov IS, and Balaban PM. Participation of GABA in establishing behavioral hierarchies in the terrestrial snail. Exp Brain Res 141: 340–348, 2001.[CrossRef][Web of Science][Medline]
Brembs B, Lorenzetti FD, Reyes FD, Baxter DA, and Byrne JH. Operant reward learning in Aplysia: neuronal correlates and mechanisms. Science 296: 1706–1709, 2002.
Brezina V, Evans CG, and Weiss KR. Enhancement of Ca current in the accessory radula closer muscle of Aplysia californica by neuromodulators that potentiate its contractions. J Neurosci 14:4393–4411, 1994a.[Abstract]
Brezina V, Evans CG, and Weiss KR. Activation of K current in the accessory radula closer muscle of Aplysia californica by neuromodulators that depress its contractions. J Neurosci 14:4412–4432, 1994b.[Abstract]
Brezina V, Orekhova IV, and Weiss KR. Functional uncoupling of linked neurotransmitter effects by combinatorial convergence. Science 273: 806–810, 1996.[Abstract]
Burnstock G. Purines and cotransmitters in adrenergic and cholinergic neurones. Prog Brain Res 68: 193–203, 1986.[Web of Science][Medline]
Chan-Palay V and Palay S, eds. Coexistence of Neuroactive Substances in Neurons. New York: Wiley, 1984.
Church PJ and Lloyd PE. Expression of diverse neuropeptide cotransmitters by identified motor neurons in Aplysia. J Neurosci 11: 618–625, 1991.[Abstract]
Church PJ and Lloyd PE. Activity of multiple identified motor neurons recorded intracellularly during evoked feeding-like motor programs in Aplysia. J Neurophysiol 72: 1794–1809, 1994.
Civelli O, Bunzow JR, and Grandy DK. Molecular diversity of the dopamine receptors. Annu Rev Pharmacol Toxicol 33: 281–307, 1993.[CrossRef][Web of Science][Medline]
Cohen JL, Weiss KR, and Kupfermann I. Motor control of buccal muscles in Aplysia. J Neurophysiol 41: 157–180, 1978.
Cooke IRC, Delaney K, and Gelperin A. Complex computation in a small neural network. In: Memory Systems of the Brain, edited by Weinberger NM, McGaugh JL, and Lynch G. New York: Guilford Press, 1985, p. 173–191.
Cropper EC, Evans CG, Hurwitz I, Jing J, Proekt A, Romero A, and Rosen SC. Feeding neural networks in the mollusc Aplysia. Neurosignals 13: 70–86, 2004.[CrossRef][Web of Science][Medline]
Delgado JY, Oyola E, and Miller MW. Localization of GABA- and glutamate-like immunoreactivity in the cardiac ganglion of the lobster Panulirus argus. J Neurocytol 29: 605–619, 2000.[CrossRef][Web of Science][Medline]
Díaz-Ríos M and Miller MW. GABA and dopamine colocalization in interneurons of the Aplysia feeding circuit: presynaptic actions of GABA. Soc Neurosci Abstr 367.12, 2002.
Díaz-Ríos M, Oyola E, and Miller MW. Colocalization of 
-aminobutyric acid-like immunoreactivity and catecholamines in the feeding network of Aplysia californica. J Comp Neurol 445: 29–46, 2002.[CrossRef][Web of Science][Medline]
Díaz-Ríos ME, Sosa MA, and Miller MW. Divergent and convergent synaptic signaling by interneurons that contain GABA and dopamine in the feeding circuitry of Aplysia. Soc Neurosci Abstr 44.7, 2003.
Díaz-Ríos M, Suess E, and Miller MW. Localization of GABA-like immunoreactivity in the central nervous system of Aplysia californica. J Comp Neurol 413: 255–270, 1999.[CrossRef][Web of Science][Medline]
Due MR, Jing J, and Weiss KR. Dopaminergic contributions to modulatory functions of a dual-transmitter interneuron in Aplysia. Neurosci Lett 358: 53–57, 2004.[CrossRef][Web of Science][Medline]
Gardner D. Bilateral symmetry and interneuronal organization in the buccal ganglia of Aplysia. Science 173: 550–553, 1971.
Gardner D and Kandel ER. Physiological and kinetic properties of cholinergic receptors activated by multiaction interneurons in buccal ganglia of Aplysia. J Neurophysiol 40: 333–348, 1977.
Getting PA. Emerging principles governing the operation of neural networks. Annu Rev Neurosci 12: 185–204, 1983.[CrossRef]
Grace AA. Dopamine. In: Neuropsychopharmacology: The Fifth Generation of Progress, edited by: Davis KL, Charney D, Coyle JT, and Nemeroff C. Lippincott Williams and Wilkins: Philadelphia, 2002, p. 120–132.
Green KA, Harris SJ, and Cottrell GA. Dopamine directly activates a ligand-gated channel in snail neurones. Pfluegers 431: 639–644, 1996.[CrossRef]
Greengard P. The neurobiology of dopamine signaling. Biosci Rep 21: 247–269, 2001.[CrossRef][Web of Science][Medline]
Harris-Warrick RM, Coniglio LM, Levini RM, Gueron S, and Guckenheimer J. Dopamine modulation of two subthreshold currents produces phase shifts in activity of an identified motoneuron. J Neurophysiol 74: 1404–1420, 1995.
Horn CC and Kupfermann I. Egestive feeding responses in Aplysia persist after sectioning of the cerebral-buccal connectives: evidence for multiple sites of control of motor programs. Neurosci Lett 323: 175–178, 2002.[CrossRef][Web of Science][Medline]
Hurwitz I, Kupfermann I, and Weiss KR. Fast synaptic connections from CBIs to pattern-generating neurons in Aplysia: initiation and modification of motor programs. J Neurophysiol 89: 2120–2136, 2003.
Hurwitz I, Neustadter D, Morton DW, Chiel HJ, and Susswein AJ. Activity patterns of the B31/B32 pattern initiators innervating the I2 muscle of the buccal mass during normal feeding movements in Aplysia californica. J Neurophysiol 75: 1309–1326, 1996.
Jahan-Parwar B, Wilson AH Jr, and Fredman SM. Role of proprioceptive reflexes in control of feeding muscles of Aplysia. J Neurophysiol 49: 1469–1480, 1983.
Jing J and Weiss KR. Neural mechanisms of motor program switching in Aplysia. J Neurosci 21: 7349–7362, 2001.
Jing J and Weiss KR. Interneuronal basis of the generation of related but distinct motor programs in Aplysia: implications for current neuronal models of vertebrate intralimb coordination. J Neurosci 22: 6228–6238, 2002.
Jing J, Vilim FS, Wu J-S, Park J-H, and Weiss KR. Concerted GABAergic actions of Aplysia feeding interneurons in motor program specification. J Neurosci 23: 5283–5294, 2003.
Jordan R, Cohen KP, and Kirk MD. Control of intrinsic buccal muscles by motoneurons B11, B15, and B16 in Aplysia californica. J Exp Zool 265: 496–506, 1993.[CrossRef][Web of Science][Medline]
Kabotyanski EA, Baxter DA, and Byrne JH. Identification and characterization of catecholaminergic neuron B65 that initiates and modifies patterned activity in the buccal ganglion of Aplysia. J Neurophysiol 79: 605–621, 1998.
Kabotyanski EA, Baxter DA, Cushman SJ, and Byrne JH. Modulation of fictive feeding by dopamine and serotonin in Aplysia. J Neurophysiol 83: 374–392, 2000.
Katz PS. Intrinsic and extrinsic neuromodulation of motor circuits. Curr Opin Neurobiol 5: 799–808, 1995.[CrossRef][Web of Science][Medline]
Katz PS. Comparison of extrinsic and intrinsic neuromodulation in two central pattern generator circuits in invertebrates. Exp Physiol 83: 281–292, 1998.[Abstract]
Katz PS, ed. Beyond Neurotransmission: Neuromodulation and Its Importance for Information Processing. Oxford, UK: Oxford Univ. Press, 1999.
Katz PS and Frost WN. Intrinsic neuromodulation in the Tritonia swim CPG: serotonin mediates both neuromodulation and neurotransmission by the dorsal swim interneurons. J Neurophysiol 74: 2281–2294, 1995.
Katz PS and Frost WN. Intrinsic neuromodulation: altering neuronal circuits from within. Trends Neurosci 19: 54–61, 1996.[CrossRef][Web of Science][Medline]
Katz PS and Harris-Warrick RM. Serotonergic/cholinergic muscle receptor cells in the crab stomatogastric nervous system. II. Rapid nicotinic and prolonged modulatory effects on neurons in the stomatogastric ganglion. J Neurophysiol 62: 571–581, 1989.
Katz PS, Getting PA, and Frost WN. Dynamic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit. Nature 367: 729–731, 1994.[CrossRef][Medline]
Katz PS and Harris-Warrick RM. Neuromodulation of the crab pyloric central pattern generator by serotonergic/cholinergic proprioceptive afferents. J Neurosci 10: 1495–1512, 1990.[Abstract]
Klein AN, Weiss KR, and Cropper EC. Glutamate is the fast excitatory neurotransmitter of small cardioactive peptide-containing Aplysia radula mechanoafferent neuron B21. Neurosci Lett 289: 37–40, 2000.[CrossRef][Web of Science][Medline]
Koh HY, Vilim FS, Jing J, and Weiss KR. Two neuropeptides colocalized in a command-like neuron use distinct mechanisms to enhance its fast synaptic connection. J Neurophysiol 90: 2074–2079, 2003.
Kreiner T, Kirk MD, and Scheller RH. Cellular and synaptic morphology of a feeding motor circuit in Aplysia californica. J Comp Neurol 264: 311–325, 1987.[CrossRef][Web of Science][Medline]
Kupfermann I. Modulatory actions of neurotransmitters. Annu Rev Neurosci. 2: 447–465, 1979.[CrossRef][Web of Science]
Kupfermann I. Feeding behavior in Aplysia: a simple system for the study of motivation. Behav Biol 10: 1–26, 1974a.[CrossRef][Web of Science][Medline]
Kupfermann I. Dissociation of the appetitive and consummatory phases of feeding behavior in Aplysia: a lesion study. Behav Biol 10: 89–97, 1974b.[CrossRef][Web of Science][Medline]
Kupfermann I. Functional studies of cotransmission. Physiol Rev 71: 683–732, 1991.
Kupfermann I and Weiss KR. Motor program selection in simple model systems. Curr Opin Neurobiol 11: 673–677, 2001.[CrossRef][Web of Science][Medline]
Kyriakides MA and McCrohan CR. Effects of putative neuromodulators on rhythmic motor output in Lymnaea stagnalis. J Neurobiol 20: 635–650, 1989.[CrossRef][Web of Science][Medline]
Lechner HA, Baxter DA, and Byrne JH. Classical conditioning of feeding in Aplysia. I. Behavioral analysis. J Neurosci 20: 3369–3376, 2000a.
Lechner HA, Baxter DA, and Byrne JH. Classical conditioning of feeding in Aplysia. II. Neurophysiological correlates. J Neurosci 20: 3377–3386, 2000b.
Liao X and Walters ET. The use of elevated divalent cation solutions to isolate monosynaptic components of sensorimotor connections in Aplysia. J Neurosci Methods 120: 45–54, 2002.[CrossRef][Web of Science][Medline]
Lundberg JM and Hökfelt T. Multiple co-existence of peptides and classical transmitters in peripheral autonomic and sensory neurons—functional and pharmacological implications. Prog Brain Res 68: 241–262, 1986.[Web of Science][Medline]
Magoski NS, Bauce LG, Syed NI, and Bulloch AG. Dopaminergic transmission between identified neurons from the mollusk, Lymnaea stagnalis. J Neurophysiol 74: 1287–1300, 1995.
Marder E. Neural signaling: Does colocalization imply cotransmission? Curr Biol 9: R809–811, 1999.[CrossRef][Web of Science][Medline]
McClellan AD, Brown GD, and Getting PA. Modulation of swimming in Tritonia: excitatory and inhibitory effects of serotonin. J Comp Physiol [A] 174: 257–266, 1994.[Medline]
Miller MW, Rosen SC, Schissel SL, Cropper EC, Kupfermann I, and Weiss KR. A population of SCP-containing neurons in the buccal ganglion of Aplysia are radula mechanoafferents and receive excitation of central origin. J Neurosci 14: 7008–7023, 1994.[Abstract]
Morgan PT, Jing J, Vilim FS, and Weiss KR. Interneuronal and peptidergic control of motor pattern switching in Aplysia. J Neurophysiol 87: 49–61, 2002.
Morton DW and Chiel HJ. In vivo buccal nerve activity that distinguishes ingestion from rejection can be used to predict behavioral transitions in Aplysia. J Comp Physiol [A] 172: 17–32, 1993a.[CrossRef]
Morton DW and Chiel HJ. The timing of activity in motor neurons that produce radula movements distinguishes ingestion from rejection in Aplysia. J Comp Physiol [A] 173: 519–536, 1993b.[Medline]
Morton DW and Chiel HJ. Neural architectures for adaptive behavior. Trends Neurosci 17: 413–420, 1994.[CrossRef][Web of Science][Medline]
Murphy AD. The neuronal basis of feeding in the snail, Helisoma, with comparisons to selected gastropods. Prog Neurobiol 63: 383–408, 2001.[CrossRef][Web of Science][Medline]
Nagahama T and Takata M. Neural mechanism generating firing patterns in jaw motoneurons during the food-induced response in Aplysia kurodai. II. Functional role of premotor neurons on generation of firing patterns in motoneurons. J Comp Physiol [A] 166: 277–286, 1990.
Nargeot R, Baxter DA, and Byrne JH. Contingent-dependent enhancement of rhythmic motor patterns: an in vitro analog of operant conditioning. J Neurosci 17: 8093–8105, 1997.
Nargeot R, Baxter DA, Patterson GW, and Byrne JH. Dopaminergic synapses mediate neuronal changes in an analogue of operant conditioning. J Neurophysiol 81: 1983–1987, 1999.
Norekian TP. GABAergic excitatory synapses and electrical coupling sustain prolonged discharges in the prey capture neural network of Clione limacina. J Neurosci 19: 1863–1875, 1999.
Norekian TP and Satterlie RA. FMRFamide and GABA produce functionally opposite effects on prey-capture reactions in the pteropod mollusk Clione limacina. Biol Bull 185: 248–262, 1993.[Abstract]
Nusbaum MP, Blitz DM, Swensen AM, Wood D, and Marder E. The roles of co-transmission in neural network modulation. Trends Neurosci 24: 146–154, 2001.[CrossRef][Web of Science][Medline]
Philippe E, Grenon G, and Tremblay JP. Baclofen modifies via the release of monoamines the synaptic depression, frequency facilitation, and posttetanic potentiation observed at an identified cholinergic synapse of Aplysia californica. Can J Physiol Pharmacol 59: 244–252, 1981.[Web of Science][Medline]
Proekt A and Weiss KR. Convergent mechanisms mediate preparatory states and repetition priming in the feeding network of Aplysia. J Neurosci 23: 4029–4033, 2003.
Proekt A, Brezina V, and Weiss KR. Dynamic basis of intentions and expectations in a simple neuronal network. Proc Natl Acad Sci USA 101: 9447–9452, 2004.
Quinlan EM, Arnett BC, and Murphy AD. Feeding stimulants activate an identified dopaminergic interneuron that induces the feeding motor program in Helisoma. J Neurophysiol 78: 812–824, 1997.
Richmond JE, Bulloch AGM, Bauce L, and Lukowiak K. Evidence for the presence, synthesis, immunoreactivity, and uptake of GABA in the nervous system of the snail Helisoma trivolvis. J Comp Neurol 307: 131–143, 1991.[CrossRef][Web of Science][Medline]
Richmond JE, Murphy AD, Lukowiak K, and Bulloch AGM. GABA regulates the buccal motor output of Helisoma by two pharmacologically distinct mechanisms. J Comp Physiol [A] 174: 593–600, 1993.
Rosen SC, Miller MW, Evans CG, Cropper EC, and Kupfermann I. Diverse synaptic connections between peptidergic radula mechanoafferent neurons and neurons in the feeding system of Aplysia. J Neurophysiol 83: 1605–1620, 2000.
Rosen SC, Teyke T, Miller MW, Weiss KR, and Kupfermann I. Identification and characterization of cerebral-to-buccal interneurons implicated in the control of motor programs associated with feeding in Aplysia. J Neurosci 11: 3630–3655, 1991.[Abstract]
Rosen SC, Weiss KR, Cohen JL, and Kupfermann I. Interganglionic cerebral-buccal mechanoafferents of Aplysia: receptive fields and synaptic connections to different classes of neurons involved in feeding behavior. J Neurophysiol 48: 271–288, 1982.
Sossin WS, Kirk MD, and Scheller RH. Peptidergic modulation of neuronal circuitry controlling feeding in Aplysia. J Neurosci 7: 671–681, 1987.[Abstract]
Susswein AJ and Byrne JH. Identification and characterization of neurons initiating patterned neural activity in the buccal ganglia of Aplysia. J Neurosci 6: 1513–1527, 1988.
Teyke T, Rosen SC, Weiss KR, and Kupfermann I. Dopaminergic neuron B20 generates rhythmic neuronal activity in the feeding motor circuitry of Aplysia. Brain Res 630: 226–237, 1993.[CrossRef][Web of Science][Medline]
Thirumalai V and Marder E. Colocalized neuropeptides activate a central pattern generator by acting on different circuit targets. J Neurosci 22: 1874–1882, 2002.
Trimble DL and Barker DL. Activation by dopamine of patterned motor output from the buccal ganglia of Helisoma trivolvis. J Neurobiol 15: 37–48, 1984.[CrossRef][Web of Science][Medline]
Vilim FS, Cropper EC, Price DA, Kupfermann I, and Weiss KR. Release of peptide cotransmitters in Aplysia: regulation and functional implications. J Neurosci 16: 8105–8114, 1996a.
Vilim FS, Price DA, Lesser W, Kupfermann I, and Weiss KR. Costorage and corelease of modulatory peptide cotransmitters with partially antagonistic actions on the accessory radula closer muscle of Aplysia californica. J Neurosci 16: 8092–8104, 1996b.
Weiss KR, Brezina V, Cropper EC, Hooper SL, Miller MW, Probst WC, Vilim FS, and Kupfermann I. Peptidergic co-transmission in Aplysia: functional implications for rhythmic behaviors. Experientia 48: 456–463, 1992.[CrossRef][Web of Science][Medline]
Whim MD, Church PJ, and Lloyd PE. Functional roles of peptide cotransmitters at neuromuscular synapses in Aplysia. Mol Neurobiol 7: 335–347, 1993.[Web of Science][Medline]
Wieland SJ and Gelperin A. Dopamine elicits feeding motor program in Limax maximus. J Neurosci 3: 1735–1745, 1983.[Abstract]
Wu JS, Jing J, Diaz-Rios M, Miller MW, Kupfermann I, and Weiss KR. Identification of a GABA-containing cerebral-buccal interneuron-11 in Aplysia californica. Neurosci Lett 341: 5–8, 2003.[CrossRef][Web of Science][Medline]
Yarowsky PJ and Carpenter DO. GABA mediated excitatory responses on Aplysia neurons. Life Sci 20: 1441–1448, 1977.[CrossRef][Web of Science][Medline]
Yarowsky PJ and Carpenter DO. Receptors for gamma-aminobutyric acid (GABA) on Aplysia neurons. Brain Res 144: 75–94, 1978.[CrossRef][Web of Science][Medline]
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F. D. Reyes, R. Mozzachiodi, D. A. Baxter, and J. H. Byrne Reinforcement in an in vitro analog of appetitive classical conditioning of feeding behavior in Aplysia: Blockade by a dopamine antagonist Learn. Mem., May 1, 2005; 12(3): 216 - 220. [Abstract] [Full Text] [PDF]
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ABSTRACT
INTRODUCTION
) and B8 (
). When amplitudes were normalized by setting the maximal EPSP = 1.0, the fast EPSPs in the 2 motor neurons were found to be similarly influenced by membrane potential. The extrapolated reversal potential (x intercept) was –8 mV for B8 and –11 mV for B16.
). B2: bath application of baclofen (1 mM) produced a depolarization of B8 that was slow in onset (resting potential = –46 mV denoted by · · · ). The baclofen response persisted until it was reversed by wash with normal artificial seawater (ASW; wash). Note different time calibrations in B, 1 and 2. C: effects of GABA on the input resistance of 2 follower cells of interneuron B20. C1: B16, under control conditions a hyperpolarizing current pulse (8 nA, 2 s, bottom record) produced a voltage deflection of 
), was diminished at hyperpolarized postsynaptic membrane potentials in a nonlinear fashion.
), or immediately prior to its termination (con, 
), dopamine produced a decrease in the input resistance (DA, 
and 
). B3: dopamine increased the excitatory response to depolarizing current pulses in B8. Inset: under control conditions, the amplitude of a 2-s depolarizing current pulse was adjusted (1.5 nA, bottom record) to evoke 2–4 impulses in B8 (con, top trace; Vm = –46 mV). In the presence of dopamine (1 mM), an identical pulse (DA) evoked a greater number of impulses (8 in the record shown, middle trace; Vm = –41 mV). Graph: data from four experiments such as B1, revealed a significant (t = 14.26, P < 0.05) increase from 3.6 ± 1.5 to 6.8 ± 1.2 (n = 6) in the number of impulses evoked in B8 in the presence of dopamine.
) from B20 and B65, 2 interneurons in which dopamine and GABA are colocalized. The configuration of this circuit includes divergent and convergent signaling to radula closer motor neurons B16 and B8. In addition, the multifunctional interneuron B4/5 receives divergent synaptic excitation from B65 and, in turn, inhibits B8 (
). At each terminal, the neurotransmitter proposed to be mediating fast synaptic transmission appears between the synaptic terminal and the postsynaptic membrane. Neurotransmitters that can modify these EPSPs are shown in italics outside each terminal, with the direction of their modulatory actions indicated by superscripts. The depiction of the modulators is intended to underscore the present uncertainty regarding their cellular origin and their locus of action. 1, Due et al. (2004)