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1Hotchkiss Brain Institute, 2Department of Physiology and Biophysics, and 3Department of Clinical Neurosciences, University of Calgary, Calgary, Alberta, Canada
Submitted 9 June 2006; accepted in final form 5 September 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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1 and 2 agonists and found that the suppressive effects of NA on the rhythm could be reproduced by adding the 2 agonist, clonidine, to the bath. In contrast, bath applying the 1 agonist, phenylephrine, increased the amplitude and duration of the cycle period. Finally, the suppressive effects of DA were not replicated by the administration of D1, D2, or D3 agonists although application of NA 2 antagonists reversed the effects of DA. Application of D1 agonists, increased the amplitude of the bursts but did not affect the cycle period. Our results indicate that monoamines can control the expression, pattern, and timing of cauda-equina-evoked locomotor patterns in developing mice. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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). As early as birth, tail-pinch-evoked stepping behavior appears to be tightly controlled by descending projections in neonatal rats. After a complete transection of the thoracic spinal cord, a tail pinch can evoke longer bouts of locomotor activity (Norreel et al. 2003). This suggests that descending bulbospinal or propriospinal projections can functionally regulate afferent transmission onto spinal pattern-generating circuits. The monoamines serotonin (5-HT), noradrenaline (NA), and dopamine (DA) are likely candidates for modulating the expression of this air-stepping behavior. Monoaminergic projections are among the first supraspinal inputs to reach the spinal cord where they modulate and influence the development of spinal reflexes and spinal locomotor circuits (Allain et al. 2005; Ballion et al. 2002; Branchereau et al. 2002; Jordan and Schmidt 2002; Kiehn et al. 1999; Liu and Jordan 2005; Schmidt and Jordan 2000; Sillar et al. 1998; Sqalli-Houssaini and Cazalets 2000).
A great deal of insight regarding the rhythmogenic and modulatory capability of monoamines has been generated by bath application of drugs in experiments using neonatal preparations. However, the interpretation of the modulatory capabilities of these drugs is complicated by the fact that monoamines are often used to elicit a baseline rhythm. This rhythm tends to be slow, and it is unclear how physiologically relevant it may be. Recent studies demonstrate that activation of sacral afferents in vitro can readily evoke rhythmic locomotor-like activity in neonatal mouse or rat spinal cord preparations (Delvolve et al. 2001; Gabbay et al. 2002; Lev-Tov and Delvolve 2000; Lev-Tov et al. 2000; Strauss and Lev-Tov 2003; Whelan et al. 2000). It is likely that this system is the in vitro analog (Lev-Tov et al. 2000; Whelan et al. 2000) of escape-like locomotor behavior that can be elicited from neonatal pups in vivo by pinching the tail (Norreel et al. 2003). Electrical stimulation of cauda equina afferents is advantageous for producing locomotor activity because the basic circuitry necessary for producing this behavior is contained within the spinal cord (Strauss and Lev-Tov 2003). Another advantage is that because descending projections are cut, there is likely little to no monoaminergic modulation of this self-contained circuit. Finally, the cycle period of the rhythm is 
1 s, which is close to the observed behavior in vivo (Cheng and P. J. Whelan, unpublished observations). Thus cauda-equina-evoked rhythms offer an excellent base rhythm to examine the effects of monoaminergic modulation.
Here we test the hypothesis that monoamines can modulate rhythms evoked by cauda equina stimulation in the neonatal mouse. Our data suggest that even at an early stage of development, monoamines can effectively modulate escape-like responses in a physiologically appropriate frequency range. A portion of this work has been published in abstract form (Gordon and Whelan 2004).
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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30 min before being transferred to the recording chamber and superfused with oxygenated (95% O2-5% CO2) ACSF. The bath solution was heated gradually from room temperature to 27°C. Then the preparation was allowed to equilibrate for 30 min. Electrophysiological recordings and activation of locomotor networks
Neurograms were recorded with suction electrodes into which segmental ventral roots were drawn. Generally, neurograms were recorded from the following ventral roots: the left and right lumbar 2 (L2) ventral roots, an L5 ventral root, and in select experiments, a sacral 4 (S4) root as well. The neurograms were amplified (100–20,000 times), filtered (100 Hz to 1 kHz or DC–1 kHz), and digitized (Axon Digidata 1322A) for future analysis. Alternating segmental and ipsilateral ventral root bursting patterns were taken to be indicative of fictive locomotion (Whelan et al. 2000).
Constant current stimulus trains (A360 World Precision Instruments, AMPI Master 8 pulse generator) were delivered to coccygeal roots via a suction electrode. To determine the stimulus threshold (T), single pulses were delivered at increasing intensities until a polysynaptic reflex response was elicited (2–350 µA). Pulses were delivered every three minutes (4 Hz, 40 pulses, T-3T range) at a constant intensity throughout an experiment (Whelan et al. 2000).
After recording 15 min of control rhythms, monoaminergic agonists or antagonists were added to the recirculating ACSF, and the rhythm was recorded for another 45 min. Finally, between 500 and 1,000 ml of preheated and oxygenated ACSF was washed through the system, and 30 min of washout sweeps were recorded. Temperature data from the bath were stored in a separate channel during sweep acquisition.
Pharmacology
These experiments utilized a variety of monoaminergic agonists and antagonists. Agonist concentrations were established using previous reports and by testing in preliminary experiments whether the effects were reversible using appropriate antagonists. These drugs included: NA (4 µM, Sigma-Aldrich), DA (10–20 µM, Sigma-Aldrich), 5-HT (10 µM, Sigma-Aldrich), phenylephrine, an 1 adrenergic receptor agonist (25 µM, Sigma-Aldrich), clonidine, an 2 adrenergic receptor agonist (0.5 µM, Sigma-Aldrich), SKF-81297, a D1 dopaminergic receptor agonist (10 µM, Sigma-Aldrich), LE300, a D1 dopaminergic receptor antagonist (1 µM, Tocris), quinpirole, a D2 dopaminergic receptor agonist (10–40 µM, Sigma-Aldrich), PD-128907, a D3 dopaminergic receptor agonist (10 µM, Tocris), 5-CT, a 5-HT1/7 serotonergic receptor agonist (1–10 µM, Sigma-Aldrich), WAY-100635, a 5-HT1A serotonergic receptor antagonist (1–5 µM, Sigma-Aldrich), GR 127935 hydrochloride, a 5-HT1B serotonergic receptor antagonist (0.1 –1 µM, Sigma-Aldrich), and -methyl-5-HT, a 5-HT2 serotonergic receptor agonist (2 µM, Sigma-Aldrich). We checked for selectivity of the agonists in preliminary experiments using ketanserin, a 5-HT2 antagonist (4 µM, Sigma-Aldrich), SB-269970, a 5-HT7 serotonergic receptor antagonist (5 µM, Tocris), prazosin, an 1 adrenergic receptor antagonist (2 µM, Sigma-Aldrich), and yohimbine, an 2 adrenergic receptor antagonist (2 µM, United States Biochemical). Fusaric acid (100 µM, Sigma-Aldrich) was also used to block dopamine-
-hydroxylase activity in select experiments.
Data analyses
Data were analyzed using custom written programs (MatLab, MathWorks, Natick, MA) as well as commercially available programs (Statistica, StatSoft, Tulsa, OK). The section of data analyzed for rhythmicity was defined as the duration of the stimulus train for cauda equina experiments. The data were digitally high pass filtered at 100 Hz, rectified, and then low-pass filtered before being reduced by a factor of 10. A cospectral density analysis comparing the segmental L2 roots was performed using Statistica. The cospectral density and primary frequency were determined as the maximum value of the cospectral density of the compared neurograms. Phase was calculated by averaging the phase shift values for the frequencies that represented the primary 75% of the component frequencies (Strauss and Lev-Tov 2003). Phase and period values were not compared when the cospectral density was reduced <10% of control. To measure the average long-latency polysynaptic potential (LLPP) elicited from cauda equina stimulation, we calculated the mean amplitude for a 10- to 200-ms window after the stimulus initiation and subtracted an average baseline value from 200 ms of data just prior to stimulation. When modulatory effects on the rhythm were recorded, we averaged bursts together to examine mean changes on the burst amplitude and duration. To improve the clarity of the traces, we removed the stimulus artifacts.
Statistics
For reporting purposes, cospectral density, and LLPP data were normalized to control values set to the arbitrary value of 100%. Subsequent trial conditions were compared with control using a one-tailed t-test and significance was set at P < 0.05 (Statistica). Phase values and cycle periods were not normalized. For multiple comparisons, we used a repeated-measures one-way ANOVA followed by a Tukey post hoc test to detect significant differences where P < 0.05 (SigmaStat). For some data, we examined the correlation between the LLPP and the change in cycle period. Goodness of fit was calculated to determine if linear regression was suitable. To determine whether the correlation was significant we tested whether the slope of the linear regression line was greater than zero (GraphPad Prism).
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RESULTS |
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; Whelan et al. 2000). Alternate bursting of segmental L2 roots and ipsilateral L2–L5 roots were taken to indicate locomotor-like activity (Bonnot et al. 2002; Whelan et al. 2000). To determine the stability of the evoked rhythm, we ran control experiments in which C8-cauda equina spinal cords were stimulated every 3 min for a minimum of 90 min. The rhythm was assessed for the stability of the primary cospectral density, phase and frequency, and the amplitude of the long latency polysynaptic potential (LLPP) was measured (Fig. 1). All of these values remained stable >90 min.
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To determine the effect of monoaminergic neurotransmitters on the neural circuits that mediate afferent-evoked locomotion, we bath applied each of the monoamines at concentrations lower than those previously found to induce pharmacological locomotor patterns (Kiehn et al. 1999; Madriaga et al. 2004; Whelan et al. 2000). When DA (10–20 µM) was bath applied, the rhythm was completely blocked as revealed by the significant reduction in the cospectral density (n = 5, P < 0.05). Similarly, the addition of NA (4 µM) also blocked the afferent-evoked pattern (n = 5, P < 0.05). Phase and cycle period values are not reported for DA or NA as the cospectral density was reduced <10% of control (Fig. 2). Although NA or DA blocked the rhythm, baseline neurogram activity from all roots increased and sporadic bouts of rhythmic bursting were found to persist in between stimulus trains. The application of 5-HT (10 µM) did not reduce the cospectral density (n = 5, P > 0.1). However, the burst duration did increase (data not shown), and the cycle period was also significantly increased (n = 5, P < 0.01, Fig. 2), whereas the phase was unaffected (n = 5, P > 0.1, data not shown). Like NA and DA, 5-HT increased the baseline activity for all recorded neurograms (data not shown). The average initial polysynaptic potentials in the right and left L2 as well as the L5 neurograms were significantly decreased with the addition of 5-HT (10 µM, n = 5, P < 0.001), DA (10–20 µM, n = 5, P < 0.05), or NA (4 µM, n = 5, P < 0.001, Fig. 3). This inhibition was reversed for all roots recorded following washout (n = 5, P > 0.1).
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). However, in our study, the effects of dopamine were not long-lasting, and generally we saw a return to control conditions within the first half hour of wash. Activation of 5-HT2 receptors mimics the modulatory effect of 5-HT
Next we tested whether activation of specific classes of serotonergic receptors with bath-applied agonists could replicate the effects of 5-HT (Fig. 4). Addition of the 5-HT2 receptor agonist, -methyl-5-HT (2 µM) did not significantly affect the cospectral density (n = 5, P > 0.1) or the phase (n = 5, P > 0.1). However, both the cycle period (n = 5, P < 0.01, Fig. 4) and burst duration were increased (supplementary Fig. 1B1 ). Next we activated the 5-HT7 receptors in isolation by applying WAY-100635, a 5-HT1A receptor antagonist (1 µM), and GR 127935 hydrochloride, a 5-HT1B receptor antagonist (1 µM), in combination with 5-CT (1 µM). This resulted in a significant destabilization of rhythmicity as measured by the cospectral density (n = 5, P < 0.05). Neither the phase nor the period (n = 5, P > 0.1) was significantly affected (Fig. 4). Finally, the LLPP was significantly reduced following activation of either the 5-HT2 (n = 5, P < 0.05) or the 5-HT7 receptors (n = 5, P < 0.01, Fig. 4). We did not observe a correlation between the reduction in the LLPP by 5-HT2 and the modulation of the cycle period (P > 0.1; see supplementary Fig. 2A).
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2 receptors mimics the inhibitory effect of NA
To investigate which receptors may be underlying the inhibitory actions of NA, we bath applied adrenergic 2 and 1 receptor agonists (Fig. 5). First, we utilized the 1 agonist phenylephrine (25 µM) and found that although it did not decrease the cospectral density (n = 5, P > 0.1) or the phase of the rhythm (n = 5, P > 0.1), it did affect the cycle period, significantly slowing the rhythm (n = 5, P < 0.01, Fig. 5) accompanied by an increase in the burst duration (see supplementary Fig. 1C). Alternatively, clonidine (500 nM), an 2 agonist, blocked rhythmicity, significantly lowering the cospectral density (n = 5, P < 0.01). Finally, we found that the LLPP was significantly reduced by application of either phenylephrine or clonidine (n = 5, P < 0.01, Fig. 5D). We did not observe a correlation between the reduction in the LLPP by phenylephrine and the modulation of the cycle period (P > 0.1; see supplementary Fig. 2).
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In the next series of experiments, we examined the ability of DA receptor agonists to mimic the inhibitory action of DA. When applied at 20 µM, the D1 agonist, SKF-81297, did not result in a decreased cospectral density between the segmental L2 neurograms. In fact, SKF-81297 stabilized the alternating rhythm, an effect similar to that found on drug-induced rhythms (Madriaga et al. 2004) as shown by the significant increase in cospectral density (n = 7, P < 0.05, Fig. 6B). Quinpirole, the D2 agonist (10–40 µM), also did not mimic DA's action and produced no effect on the cospectral density (at 40 µM, n = 5, P > 0.01, Fig. 6B). Conversely, both the D1 and D2 agonists decreased the LLPP significantly (n = 7 and n = 5, respectively, P < 0.05, Fig. 6D). Finally, the D3 agonist, PD-128907 (10 µM), failed to decrease the cospectral density (n = 5, P > 0.1, Fig. 6A) or the LLPP (n = 5, P > 0.1, Fig. 6D). The cycle period was unaffected by the addition of D1 or D3 agonists (n = 5–7, P > 0.1, Fig. 6C); however, the addition of quinpirole, the D2 agonist did significantly increase the cycle period (n = 5, P < 0.05). Phase values also remained constant following D1, D2, or D3 agonist addition (n = 5–7, P > 0.1). SKF-81297 did increase the amplitude of averaged bursts (see supplementary Fig. 1A), likely contributing to part of the increase in the cospectral density.
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The D2 or D3 agonists did not suppress rhythmicity, and in fact, D1 agonists may have increased the stability of the rhythm and amplitude of the bursts. However, when bath applied at 10–20 µM, DA completely inhibits the afferent-evoked rhythm (Fig. 2Aiii). Therefore to determine if DA was being converted to NA, which was then activating 2 receptors to suppress the rhythm, we conducted a series of experiments in which the dopamine--hydroxylase inhibitor fusaric acid (100 µM) was added to the bath. After recording five control sweeps, we added 20 µM DA as previously described and found that DA was still capable of inhibiting the rhythm as measured by the cospectral density (n = 4, P < 0.01, data not shown). Consequently, the dopaminergic effect was not primarily mediated by a conversion of DA to NA. Finally, previous results have shown that DA is capable of binding to 2 receptors directly in the preoptic area of the rat (Cornil et al. 2002). Thus we tested whether this cross-reaction was also occurring in our preparation. Addition of the 2 adrenergic antagonist yohimbine (2 µM) after DA (10–20 µM) reversed the dopaminergic effect, allowing the locomotor rhythm to recover to control levels (n = 5, P > 0.1, supplementary Fig. 3). The addition of yohimbine without previous administration of DA did not affect the rhythm (cospectral density, n = 5, P > 0.1). Initially, we hypothesized that the addition of yohimbine after DA would lead to an increase in the cospectral density above control levels due to an unmasking of DA's effect on D1 receptors. Therefore our protocol included the application of the D1 antagonist LE300 (1 µM) to reverse the predicted effect. However, although we observed a trend for an increase, this was not significant, and so it is not surprising that bath application of LE300 did not reduce the cospectral density when compared with the yohimbine trials (n = 5, P > 0.1).
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DISCUSSION |
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Monoaminergic depression of rhythmic activity
Activation of lumbar networks by cauda equina stimulation depends on relay propriospinal interneurons within the sacral spinal cord (Strauss and Lev-Tov 2003). Presumably, one site of action of NA, DA, and 5-HT would be on the first-order interneurons within the dorsal horn of the sacral cord (Millan 2002). The suppression of the afferent-evoked rhythm and polysynaptic reflexes by clonidine, which mimicked the effects of NA, are consistent with the powerful inhibitory effects of 2 agonists on afferent transmission (Millan 2002) and with the high 2 receptor expression in the superficial dorsal horn, lamina X, and dorsal laminae (Shi et al. 1999; Stone et al. 1998).
Because DA potently blocked cauda equina evoked locomotor activity, it was surprising that none of D1, D2, or D3 receptor agonists could block rhythmicity. This was in spite of the fact that D1 and D2 receptor agonists reduced evoked polysynaptic potentials as predicted by previous reports (Clemens and Hochman 2004). This suggests that transmission from propriospinal relay interneurons in the sacral cord to the lumbar CPG was not affected. Although surprising, these data are consistent with reports demonstrating differential control of nociceptive and group II reflexes by monoamines in adult animals (Jankowska 2001; Jankowska and Hammar 2002; Millan 2002). Our data suggest that DA inhibition of the evoked rhythm is mediated through activation of 2 receptors. These effects were not due to conversion to NA because blocking dopamine--hydroxylase did not block DA-mediated inhibition. DA has been observed to cross react with 2 receptors in other parts of the brain (Cornil et al. 2002), and paracrine release of DA within the dorsal horn could potentially activate neighboring synapses (Mundorf et al. 2001; Ridet et al. 1993). One issue is that our DA bath concentrations were high, and concentration-dependent effects on evoked reflexes have been reported in the 10- to 20-µM range using a mouse spinal cord preparation (Clemens and Hochman 2004). Nevertheless, it does not detract from our observations that direct activation of DA receptors does not suppress cauda equina evoked locomotor-like activity.
5-HT is known to decrease polysynaptic reflexes, and our data confirm these findings in the mouse (Wallis et al. 1993a,b; Yomono et al. 1992). Although 5-HT suppressed polysynaptic potentials, we did not observe a disruption of rhythmic activity. 5-HT is known to affect afferent transmission, and the lack of an large inhibitory effect is most likely because 5-HT activates 15 receptor subtypes, many of which contribute to network excitation as described in the following text.
Monoaminergic modulation of rhythmic activity
Although bath application of NA and DA blocked the cauda-equina-evoked rhythm, both NA and DA modulate bath-evoked rhythms in neonates (Barriere et al. 2004; Kiehn et al. 1999; Madriaga et al. 2004; Sqalli-Houssaini and Cazalets 2000; Whelan et al. 2000), and it is well known that the precursor L-DOPA facilitates locomotor networks (Jankowska et al. 1967a,b; Tucker and Stehouwer 2000). When we used agonists for monoaminergic receptors that are preferentially expressed in the ventral horn, the rhythm slowed down, suggesting a site of action within the CPG. Activation of either 1 or 5-HT2 receptors significantly decreased the frequency of the evoked rhythm and increased the duration of the bursts. Even though the rhythm slowed compared with control conditions, the frequency was within a physiologically normal range.
Despite having similar effects on the frequency, 5-HT2 agonists increased the quality of the L5 pattern to a greater extent compared with 1 agonists. This agrees with previous electrophysiological and immunocytochemical data showing uniform rostrocaudal expression of 5-HT2 receptors in the lumbosacral segments (Liu and Jordan 2005; Schmidt and Jordan 2000) and early expression within the embryonic spinal cord of mice (Lauder et al. 2000). The modulatory actions of 5-HT2 receptors on CPG frequency and burst duration are consistent with their high expression in the ventral horn (Basura et al. 2001; Fonseca et al. 2001; Schmidt and Jordan 2000), and 5-HT can alter the excitability of commissural interneurons located in the ventral horn (Zhong et al. 2006a,b). On the basis of previous reports, we were expecting 5-HT7 agonists to potentiate and modulate the rhythm (Liu and Jordan 2005; Madriaga et al. 2004; Pearlstein et al. 2005). This was not the case, and instead the rhythm was reversibly weakened after addition of 5-HT7 agonists. Possible explanations include actions at the dorsal horn where 5-HT7 receptors are known to be clustered in addition to the ventral horn, which could explain the concomitant reduction in polysynaptic potentials (Cina and Hochman 1998; Doly et al. 2005; Schmidt and Jordan 2000).
Our data that show 1 agonists slowing the cauda-equina-evoked locomotor rhythm and increasing burst duration also suggests a site of action within the CPG. The data are comparable to immunocytochemical and electrophysiological data alluding to a locus of action within the ventral horn (Jones et al. 1985; Sqalli-Houssaini and Cazalets 2000). One notable difference is that our work demonstrates that 1 agonists slow the cauda-equina-evoked rhythm down, whereas the opposite result was obtained using pharmacologically evoked rhythms in the neonatal rat (Sqalli-Houssaini and Cazalets 2000). Modeling studies illustrate that increases in inhibitory commissural drive slow the rhythm down, whereas decreases in inhibitory drive can increase the strength of bursts as well as the frequency of the rhythm (Dale 1995; Hellgren et al. 1992). However, this may not be the only possibility because recent data show that inhibition of inhibitory ipsilateral V1 interneurons slow down the rhythm (Gosgnach et al. 2006).
Interestingly, both 1 and 5-HT2 agonists inhibited the amplitude of polysynaptic reflexes recorded from both L2 and L5 neurograms. Other reports have showed inhibition of spinal reflexes by 5-HT2 in neonatal rats (Yomono et al. 1992), and high expression of 5-HT2 receptors have been reported in lamina II of rats (Doly et al. 2004). The inhibition of afferent transmission may be due to potentiation of glycine conductances in first-order dorsal horn neurons by activation of inhibitory interneurons (Li et al. 2002). For both 5-HT2 and 1 agonists, we observed no correlation between the depth of the polysynaptic reflex modulation and the modulation of network frequency and burst duration. This suggests a complex control of polysynaptic reflexes and networks at birth similar to the proposed role for monoamines in adults (Bannatyne et al. 2003; Hammar and Jankowska 2003; Hammar et al. 2004; Jacobs and Fornal 1993; Jankowska and Hammar 2002; Jankowska et al. 2005).
In contrast to the effects of 5-HT2 or 1 agonists, D1 agonists increased the amplitude of the burst but did not affect the timing of the rhythm. Qualitatively similar effects were observed when DA was applied in the presence of 2 antagonists. These results are consistent with reports of high levels of D1 immunoreactive receptors in ventral horn neurons (Dubois et al. 1986; Holstege et al. 1996; Weil-Fugazza and Godefroy 1993) and with electrophysiological data that demonstrate that DA increases ventral neuronal excitability (Barasi and Roberts 1977; Smith et al. 1995). However, our data suggest that D1 actions are mainly on motoneurons and not on the network, which is contrary to reports that demonstrate that D1 agonists either alone or in combination with 5-HT can evoke and modulate rhythms in the rat and mouse (Barriere et al. 2004; Madriaga et al. 2004; Whelan et al. 2000). One explanation could be the differences in the drug concentrations used or the different methods used to evoke locomotor activity.
Functional considerations
Several lines of evidence suggest monoamines could produce similar effects in the intact neonate. 5-HT and NA fibers exist at all spinal segments in the perinatal rodent (Ballion et al. 2002; Rajaofetra et al. 1992). At least a portion of these fibers are likely functional because stimulation of the parapyramidal area in the brain stem elicits 5-HT-dependent locomotor-like activity (Liu and Jordan 2005) and separate studies confirm the release of 5-HT and NA metabolites in the ventral horn of adult and neonatal rats during brain stem stimulation or exercise (Gerin et al. 1994, 1995; Jordan and Schmidt 2002). Less data are available for the dopaminergic system; however, DA fibers and receptors are present in the rat spinal cord (Holstege et al. 1996; Ridet et al. 1992; Yoshida and Tanaka 1988) and are functional (Fleetwood-Walker et al. 1988). The extent to which DA, 5-HT, or NA contributes to reflex and network modulation in the intact neonate is unknown. However, our data allude to monoamines exerting a complex set of actions on the expression of escape-like behavior as early as birth. Taken together, this suggests that the descending control of reflex pathways onto motor circuits may be well developed at early stages of development. It will be interesting to test this hypothesis using new strains of mice where classes of interneurons forming reflex and locomotor spinal circuits can be identified (Gordon and Whelan 2006).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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1 The online version of this article contains supplemental data. 
Address for reprint requests and other correspondence: P. J. Whelan, HSC 2119, Hotchkiss Brain Institute, University of Calgary, Calgary, AB T2N4N1, Canada (E-mail: whelan{at}ucalgary.ca)
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REFERENCES |
|---|
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
Ballion B, Branchereau P, Chapron J, and Viala D. Ontogeny of descending serotonergic innervation and evidence for intraspinal 5-HT neurons in the mouse spinal cord. Brain Res Dev Brain Res 137: 81–88, 2002.[CrossRef][Medline]
Bannatyne BA, Edgley SA, Hammar I, Jankowska E, and Maxwell DJ. Networks of inhibitory and excitatory commissural interneurons mediating crossed reticulospinal actions. Eur J Neurosci 18: 2273–2284, 2003.[CrossRef][ISI][Medline]
Barasi S and Roberts MT. Responses of motoneurones to electrophoretically applied dopamine. Br J Pharmacol 60: 29–34, 1977.[ISI]
Barriere G, Mellen N, and Cazalets JR. Neuromodulation of the locomotor network by dopamine in the isolated spinal cord of newborn rat. Eur J Neurosci 19: 1325–1335, 2004.[CrossRef][ISI][Medline]
Basura GJ, Zhou SY, Walker PD, and Goshgarian HG. Distribution of serotonin 2A and 2C receptor mRNA expression in the cervical ventral horn and phrenic motoneurons following spinal cord hemisection. Exp Neurol 169: 255–263, 2001.[CrossRef][ISI][Medline]
Bonnot A, Whelan PJ, Mentis GZ, and O'Donovan MJ. Locomotor-like activity generated by the neonatal mouse spinal cord. Brain Res Brain Res Rev 40: 141–151, 2002.[CrossRef][Medline]
Branchereau P, Chapron J, and Meyrand P. Descending 5-hydroxytryptamine raphe inputs repress the expression of serotonergic neurons and slow the maturation of inhibitory systems in mouse embryonic spinal cord. J Neurosci 22: 2598–2606, 2002.
Cina C and Hochman S. Sertonin receptor pharmacology of the mammalian locomotor CPG: activation by a 5-HT receptor agonist in the isolated rat spinal cord. Soc Neurosci Abstr 24: 654.624, 1998.
Clemens S and Hochman S. Conversion of the modulatory actions of dopamine on spinal reflexes from depression to facilitation in D3 receptor knock-out mice. J Neurosci 24: 11337–11345, 2004.
Cornil CA, Balthazart J, Motte P, Massotte L, and Seutin V. Dopamine activates noradrenergic receptors in the preoptic area. J Neurosci 22: 9320–9330, 2002.
Dale N. Experimentally derived model for the locomotor pattern generator in the Xenopus embryo. J Physiol 489: 489–510, 1995.
Delvolve I, Gabbay H, and Lev-Tov A. The motor output and behavior produced by rhythmogenic sacrocaudal networks in spinal cords of neonatal rats. J Neurophysiol 85: 2100–2110, 2001.
Doly S, Fischer J, Brisorgueil MJ, Verge D, and Conrath M. Pre- and postsynaptic localization of the 5-HT7 receptor in rat dorsal spinal cord: immunocytochemical evidence. J Comp Neurol 490: 256–269, 2005.[CrossRef][ISI][Medline]
Doly S, Madeira A, Fischer J, Brisorgueil MJ, Daval G, Bernard R, Verge D, and Conrath M. The 5-HT2A receptor is widely distributed in the rat spinal cord and mainly localized at the plasma membrane of postsynaptic neurons. J Comp Neurol 472: 496–511, 2004.[CrossRef][ISI][Medline]
Dubois A, Savasta M, Curet O, and Scatton B. Autoradiographic distribution of the D1 agonist [3H]SKF 38393, in the rat brain and spinal cord. Comparison with the distribution of D2 dopamine receptors. Neuroscience 19: 125–137, 1986.[CrossRef][ISI][Medline]
Fleetwood-Walker SM, Hope PJ, and Mitchell R. Antinociceptive actions of descending dopaminergic tracts on cat and rat dorsal horn somatosensory neurons. J Physiol 399: 335–348, 1988.
Fonseca MI, Ni YG, Dunning DD, and Miledi R. Distribution of serotonin 2A, 2C and 3 receptor mRNA in spinal cord and medulla oblongata. Brain Res Mol Brain Res 89: 11–19, 2001.[Medline]
Gabbay H, Delvolve I, and Lev-Tov A. Pattern generation in caudal-lumbar and sacrococcygeal segments of the neonatal rat spinal cord. J Neurophysiol 88: 732–739, 2002.
Garraway SM and Hochman S. Pharmacological characterization of serotonin receptor subtypes modulating primary afferent input to deep dorsal horn neurons in the neonatal rat. Br J Pharmacol 132: 1789–1798, 2001.[CrossRef][ISI]
Gerin C, Becquet D, and Privat A. Direct evidence for the link between monoaminergic descending pathways and motor activity. I. A study with microdialysis probes implanted in the ventral funiculus of the spinal cord. Brain Res 704: 191–201, 1995.[CrossRef][ISI][Medline]
Gerin C, Legrand A, and Privat A. Study of 5-HT release with a chronically implanted microdialysis probe in the ventral horn of the spinal cord of unrestrained rats during exercise on a treadmill. J Neurosci Methods 52: 129–141, 1994.[CrossRef][ISI][Medline]
Gordon IT and Whelan PJ. Dopamine modulates afferent transmission to the spinal central pattern generator of the neonatal mouse. Soc Neurosci Abstr 2004: 883.5, 2004.
Gordon IT and Whelan PJ. Deciphering the organization and modulation of spinal locomotor central pattern generators. J Exp Biol 209: 2007–2014, 2006.
Gosgnach S, Lanuza GM, Butt SJ, Saueressig H, Zhang Y, Velasquez T, Riethmacher D, Callaway EM, Kiehn O, and Goulding M. V1 spinal neurons regulate the speed of vertebrate locomotor outputs. Nature 440: 215–219, 2006.[CrossRef][Medline]
Hammar I, Bannatyne BA, Maxwell DJ, Edgley SA, and Jankowska E. The actions of monoamines and distribution of noradrenergic and serotoninergic contacts on different subpopulations of commissural interneurons in the cat spinal cord. Eur J Neurosci 19: 1305–1316, 2004.[CrossRef][ISI][Medline]
Hammar I and Jankowska E. Modulatory effects of alpha1-, alpha2-, and beta -receptor agonists on feline spinal interneurons with monosynaptic input from group I muscle afferents. J Neurosci 23: 332–338, 2003.
Hellgren J, Grillner S, and Lansner A. Computer simulation of the segmental neural network generating locomotion in lamprey by using populations of network interneurons. Biol Cybern 68: 1–13, 1992.[CrossRef][ISI][Medline]
Holstege JC, Van Dijken H, Buijs RM, Goedknegt H, Gosens T, and Bongers CM. Distribution of dopamine immunoreactivity in the rat, cat, and monkey spinal cord. J Comp Neurol 376: 631–652, 1996.[CrossRef][ISI][Medline]
Jacobs BL and Fornal CA. 5-HT and motor control: a hypothesis. Trends Neurosci 16: 346–352, 1993.[CrossRef][ISI][Medline]
Jankowska E. Spinal interneuronal systems: identification, multifunctional character and reconfigurations in mammals. J Physiol 533: 31–40, 2001.
Jankowska E, Edgley SA, Krutki P, and Hammar I. Functional differentiation and organization of feline midlumbar commissural interneurones. J Physiol 565: 645–658, 2005.
Jankowska E and Hammar I. Spinal interneurones; how can studies in animals contribute to the understanding of spinal interneuronal systems in man? Brain Res Brain Res Rev 40: 19–28, 2002.[CrossRef][Medline]
Jankowska E, Jukes MG, Lund S, and Lundberg A. The effect of DOPA on the spinal cord. V. Reciprocal organization of pathways transmitting excitatory action to alpha motoneurons of flexors and extensors. Acta Physiol Scand 70: 369–388, 1967a.[ISI][Medline]
Jankowska E, Jukes MG, Lund S, and Lundberg A. The effect of DOPA on the spinal cord. VI. Half-center organization of interneurons transmitting effects from the flexor reflex afferents. Acta Physiol Scand 70: 389–402, 1967b.[ISI][Medline]
Jones LS, Gauger LL, Davis JN, Slotkin TA, and Bartolome JV. Postnatal development of brain alpha 1-adrenergic receptors: in vitro autoradiography with [125I]HEAT in normal rats and rats treated with alpha-difluoromethylornithine, a specific, irreversible inhibitor of ornithine decarboxylase. Neuroscience 15: 1195–1202, 1985.[CrossRef][ISI][Medline]
Jordan LM and Schmidt BJ. Propriospinal neurons involved in the control of locomotion: potential targets for repair strategies? Prog Brain Res 137: 125–139, 2002.[ISI][Medline]
Kiehn O, Sillar KT, Kjaerulff O, and McDearmid JR. Effects of noradrenaline on locomotor rhythm-generating networks in the isolated neonatal rat spinal cord. J Neurophysiol 82: 741–746, 1999.
Lauder JM, Wilkie MB, Wu C, and Singh S. Expression of 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptors in the mouse embryo. Int J Dev Neurosci 18: 653–662, 2000.[CrossRef][ISI][Medline]
Lev-Tov A and Delvolve I. Pattern generation in non-limb moving segments of the mammalian spinal cord. Brain Res Bull 53: 671–675, 2000.[CrossRef][ISI][Medline]
Lev-Tov A, Delvolve I, and Kremer E. Sacrocaudal afferents induce rhythmic efferent bursting in isolated spinal cords of neonatal rats. J Neurophysiol 83: 888–894, 2000.
Li H, Kang JF, and Li YQ. Serotonin potentiation of glycine-activated whole-cell currents in the superficial laminae neurons of the rat spinal dorsal horn is mediated by protein kinase C. Brain Res Bull 58: 593–600, 2002.[CrossRef][ISI][Medline]
Liu J and Jordan LM. Stimulation of the parapyramidal region of the neonatal rat brainstem produces locomotor-like activity involving spinal 5-HT7 and 5-HT2A receptors. J Neurophysiol 2005.
Madriaga MA, McPhee LC, Chersa T, Christie KJ, and Whelan PJ. Modulation of locomotor activity by multiple 5-HT and dopaminergic receptor subtypes in the neonatal mouse spinal cord. J Neurophysiol 92: 1566–1576, 2004.
Millan MJ. Descending control of pain. Prog Neurobiol 66: 355–474, 2002.[CrossRef][ISI][Medline]
Mundorf ML, Joseph JD, Austin CM, Caron MG, and Wightman RM. Catecholamine release and uptake in the mouse prefrontal cortex. J Neurochem 79: 130–142, 2001.[CrossRef][ISI][Medline]
Norreel JC, Pflieger JF, Pearlstein E, Simeoni-Alias J, Clarac F, and Vinay L. Reversible disorganization of the locomotor pattern after neonatal spinal cord transection in the rat. J Neurosci 23: 1924–1932, 2003.
Pearlstein E, Mabrouk FB, Pflieger JF, and Vinay L. Serotonin refines the locomotor-related alternations in the in vitro neonatal rat spinal cord. Eur J Neurosci 21: 1338–1346, 2005.[CrossRef][ISI][Medline]
Rajaofetra N, Poulat P, Marlier L, Geffard M, and Privat A. Pre- and postnatal development of noradrenergic projections to the rat spinal cord: an immunocytochemical study. Brain Res Dev Brain Res 67: 237–246, 1992.[Medline]
Ridet JL, Rajaofetra N, Teilhac JR, Geffard M, and Privat A. Evidence for nonsynaptic serotonergic and noradrenergic innervation of the rat dorsal horn and possible involvement of neuron-glia interactions. Neuroscience 52: 143–157, 1993.[CrossRef][ISI][Medline]
Ridet JL, Sandillon F, Rajaofetra N, Geffard M, and Privat A. Spinal dopaminergic system of the rat: light and electron microscopic study using an antiserum against dopamine, with particular emphasis on synaptic incidence. Brain Res 598: 233–241, 1992.[CrossRef][ISI][Medline]
Schmidt BJ and Jordan LM. The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord. Brain Res Bull 53: 689–710, 2000.[CrossRef][ISI][Medline]
Shi TJ, Winzer-Serhan U, Leslie F, and Hokfelt T. Distribution of alpha2-adrenoceptor mRNAs in the rat lumbar spinal cord in normal and axotomized rats. Neuroreport 10: 2835–2839, 1999.[ISI][Medline]
Sillar KT, Reith CA, and McDearmid JR. Development and aminergic neuromodulation of a spinal locomotor network controlling swimming in Xenopus larvae. Ann NY Acad Sci 860: 318–332, 1998.
Smith DO, Lowe D, Temkin R, Jensen P, and Hatt H. Dopamine enhances glutamate-activated currents in spinal motoneurons. J Neurosci 15: 3905–3912, 1995.[Abstract]
Sqalli-Houssaini Y and Cazalets JR. Noradrenergic control of locomotor networks in the in vitro spinal cord of the neonatal rat. Brain Res 852: 100–109, 2000.[CrossRef][ISI][Medline]
Stone LS, Broberger C, Vulchanova L, Wilcox GL, Hokfelt T, Riedl MS, and Elde R. Differential distribution of alpha2A and alpha2C adrenergic receptor immunoreactivity in the rat spinal cord. J Neurosci 18: 5928–5937, 1998.
Strauss I and Lev-Tov A. Neural pathways between sacrocaudal afferents and lumbar pattern generators in neonatal rats. J Neurophysiol 89: 773–784, 2003.
Tucker LB and Stehouwer DJ. L-DOPA-induced air-stepping in the preweanling rat: electromyographic and kinematic analyses. Behav Neurosci 114: 1174–1182, 2000.[CrossRef][ISI][Medline]
Wallis DI, Wu J, and Wang X. Descending inhibition in the neonate rat spinal cord is mediated by 5-hydroxytryptamine. Neuropharmacology 32: 73–83, 1993a.[CrossRef][ISI][Medline]
Wallis DI, Wu J, and Wang XC. Is 5-hydroxytryptamine mediating descending inhibition in the neonatal rat spinal cord through different receptor subtypes? Eur J Pharmacol 250: 371–377, 1993b.[CrossRef][ISI][Medline]
Weil-Fugazza J and Godefroy F. Dorsal and ventral dopaminergic innervation of the spinal cord: functional implications. Brain Res Bull 30: 319–324, 1993.[CrossRef][ISI][Medline]
Whelan P, Bonnot A, and O'Donovan MJ. Properties of rhythmic activity generated by the isolated spinal cord of the neonatal mouse. J Neurophysiol 84: 2821–2833, 2000.
Vinay L, Brocard F, Clarac F, Norreel JC, Pearlstein E, and Pflieger JF. Development of posture and locomotion: an interplay of endogenously generated activities and neurotrophic actions by descending pathways. Brain Res Brain Res Rev. 40: 118–29, 2002.[CrossRef][Medline]
Yomono HS, Suzuki H, and Yoshioka K. Serotonergic fibers induce a long-lasting inhibition of monosynaptic reflex in the neonatal rat spinal cord. Neuroscience 47: 521–531, 1992.[CrossRef][ISI][Medline]
Yoshida M and Tanaka M. Existence of new dopaminergic terminal plexus in the rat spinal cord: assessment by immunohistochemistry using anti-dopamine serum. Neurosci Lett 94: 5–9, 1988.[CrossRef][ISI][Medline]
Zhong G, Diaz-Rios M, and Harris-Warrick RM. Serotonin modulates the properties of ascending commissural interneurons in the neonatal mouse spinal cord. J Neurophysiol 95: 1545–1555, 2006a.
Zhong G, Diaz-Rios M, and Harris-Warrick RM. Intrinsic and functional differences among commissural interneurons during fictive locomotion and serotonergic modulation in the neonatal mouse. J Neurosci 26: 6509–6517, 2006b.
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