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1Fondazione Santa Lucia, Istituto di Ricovero e Cura a Carattere Scientifico, 00179 Rome; and 2Clinica Neurologica, Università di Tor Vergata, 00133 Rome, Italy
Submitted 14 June 2004; accepted in final form 6 July 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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; Schmidt et al. 1982; Smith et al. 1990; Triarhou et al. 1988)—and represent a genetic animal model of nigro-striatal deficiency (Simon and Ghetti 1994).
The wv mutation affects the putative pore-forming H5 region of the type 2 subunit of a G-protein-sensitive inwardly rectifying potassium channel (GIRK2) (Patil et al. 1995), resulting in the functional loss of selectivity for potassium ions, and insensitivity to G protein 
dimers. Various studies using heterologous expression systems have reported that wvGIRK2 channels allow sodium and calcium ions to enter the cell (Kofuji et al. 1996; Navarro et al. 1996; Silverman et al. 1996) and are constitutively active (Navarro et al. 1996). Within the brain, GIRK channels consist of homo (GIRK2 and GIRK4)- or hetero-tetramers (GIRK1/2 or GIRK1/4) (Kofuji et al. 1996; Yang et al. 1995). Thus their properties in native neurons of the wv mouse can be diverse depending on the type of GIRK subunits expressed in different brain areas (Liao et al. 1996). Indeed, wv cerebellar granule cells, expressing both GIRK1 and GIRK2 subunits, show a constitutive inward rectifier current (Rossi et al. 1998) with properties similar to those of the wvGIRK2 expressed in heterologous systems (Kofuji et al. 1996; but see Surmeier et al. 1996 for evidence for a loss of function rather than a gain of function). Hippocampal CA3 neurons also express heteromultimers GIRK1/2, but their wvGIRK1/2 channels are neither G protein regulated nor constitutively active (Jarolimek et al. 1998). The dopaminergic neurons of the substantia nigra pars compacta mainly express GIRK2 subunits compared with GIRK1 (Davila et al. 2003; Liao et al. 1996). Thus they most likely express homomeric wvGIRK2 channels that show a loss of potassium selectivity but no loss in their regulation by G-protein-coupled D2 and GABAB receptors (Guatteo et al. 2000). However, in a subpopulation of dopaminergic neurons with a low content of ATP, wvGIRK2 channels were reported to be constitutively active (Liss et al. 1999).
A detailed study on the ion selectivity of wvGIRK channels (and particularly of the homomeric wvGIRK2) in native neurons is still lacking. Indirect evidence supports the hypothesis that cerebellar wvGIRK1/2 channels are permeable to divalent and monovalent cations, thus rendering neurons leaky to calcium (Fox et al. 1998; Harkins et al. 2000; Rossi et al. 1998) as well as sodium ions (Kofuji et al. 1996). Cerebellar granule cells of weaver mice were rescued when cultured in the presence of the wvGIRK1/2 blockers QX-314 and verapamil but not in the presence of the voltage-gated sodium channel blocker TTX. Conflicting reports have indicated the protective (Liesi and Wright 1996) or nonprotective (Kofuji et al. 1996) effects of calcium channels blockers on these cells. On the other hand, it has been reported that homomeric wvGIRK2 channels transiently expressed in mammalian neurons are not permeable to divalent cations (Hou et al. 2000). Thus it is still not clear whether the weaver mutation introduces a new routefor calcium entry into mammalian cells or if the intracellular accumulation of calcium (Fox et al. 1998; Harkins et al. 2000) results from the opening of voltage-gated calcium channels (VGCC) activated by a wvGIRK-dependent membrane depolarization. Our group has previously shown that dopamine (DA) and baclofen excite wv/wv dopaminergic neurons, causing membrane depolarization in a G-protein-regulated manner, and we suggested that the distorted function of receptor-operated wvGIRK2 channels could contribute to cell death (Guatteo et al. 2000). By simultaneously recording microfluorometric and electrophysiological signals, here we present evidence that an intracellular calcium accumulation occurs when D2 or GABAB receptor activation depolarize the wv/wv dopaminergic neurons. This accumulation is due to a calcium influx through VGCC that are activated by wvGIRK2-dependent depolarization. This receptor-mediated phenomenon could further account for the death of these cells.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Heterozygous breeder weaver mice were purchased from Jackson Laboratories (Bar Harbor, ME) and housed in our animal facilities. Homozygous wv/wv mice were identified by their poor motor function as described previously (Guatteo et al. 2000). All experiments were carried out according to guidelines of the Comitato Etico of the Tor Vergata University on the use of animals in research. Mice 16–20 days old were used. They were anesthetized with ketamine and killed by decapitation. The brain was rapidly removed, and horizontal slices (thickness 250 µm) were cut in artificial cerebrospinal fluid solution (ACSF) at 10–14°C using a vibratome and starting from the ventral surface of the midbrain. The ACSF contained (in mM) 126 NaCl, 2.5 KCl, 1.2 MgCl2, 1.2 NaH2PO4, 2.4 CaCl2, 10 Glucose, and 24 NaHCO3 gassed with 95% O2-5% CO2. After incubation in a reservoir (1–2 h, 34°C), a single slice was transferred into a recording chamber and completely submerged in continuously flowing (2.5 ml/min) ACSF at 33–34°C (pH 7.4). The chamber was mounted on the stage of an upright microscope (Axioskop, Carl Zeiss, Oberkochen, Germany) equipped for infrared video microscopy (Hamamatsu, Hamamatsu City, Japan) and video microfluorometry (ImproVision, Coventry, UK).
Electrophysiology
Whole cell patch-clamp recordings were made both in current- and voltage-clamp (holding potential = –60 mV) mode with an Axopatch 1D amplifier (Axon Instruments, Foster City, CA), from visually identified dopaminergic neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) (Guatteo et al. 1998). Pipettes were made from borosilicate glass (1.5 mm, WPI, Sarasota, FL), were pulled with a PP 83 Narishige puller (Tokyo) and had a resistance of 4 M
when filled with a standard solution containing (in mM) 145 K+-gluconate, 0.1 CaCl2, 2 MgCl2, 10 HEPES, 0.75 EGTA, 2 Mg-ATP, an 0.3 Na3GTP (pH 7.35). For microfluorometry (see following text), fura-2 pentapotassium salt (250 µM) (Molecular Probes, Leiden, the Netherlands) was added to the pipette solution. Series resistance was partially compensated. The membrane voltage and current were digitized at 5 kHz (Digidata 1200B A/D converter), acquired, and analyzed using pClamp (Axon Instruments) and Origin (Microcal Software, Northampton, MA) software.
Microfluorometry
The fluorescent ion indicator fura-2 pentapotassium salt was loaded into the cell via the patch pipette and excited via a x40 water-immersion objective (Olympus, Hamburg, Germany) by epi-illumination with light provided by a 75-W Xenon lamp. Excitation light was band-pass filtered alternatively at 340 and 380 nm. Emission light passed a barrier filter (500 nm) and was detected by a CCD camera (Photonic Science, Millham, UK). Image pairs were acquired at 6- or 12-s intervals using IonVision software (ImproVision, Coventry, UK). Fluorescence values were evaluated over selected regions of the neuron ("regions of interest," defined as those pixels that exhibit 
20–30% of maximal specific fluorescence) and were transformed into ion concentrations using the method of Grynckiewicz (Grynckiewicz et al. 1985) as previously reported (Guatteo et al. 1998). The fluorescent signal was relatively homogenous over the cell body and initial portion of the proximal dendrites and the analysis was restricted to this part of the neuron. Data analysis was done using Origin 6.1 software (Microcal, Northampton, MA).
Drug application
All drugs were purchased from Sigma (Milan, Italy). The VGCC blockers CoCl2 (1 mM) and nifedipine (50 µM), were dissolved in a phosphate-free, bicarbonate-free HEPES-buffered solution with the following composition (in mM) 141 NaCl, 9.5 NaOH, 2.5 KCl, 2.5 CaCl2, 1.2 MgCl2, 20 HEPES, and 10 glucose. This solution was applied to the slice 10 min before adding CoCl2 and nifedipine. In most dopamine neurons, application of calcium channel blockers prevented action potential discharge as previously reported (Durante et al. 2004; Mercuri et al. 1994; Nedergaard et al. 1993). Drugs were dissolved at the final concentration in ACSF solution and were applied by means of three-way taps in the perfusion system.
Statistical analysis
Values in the text are expressed as mean ± SE. Student's t-test, one-way ANOVA, and Dunnet's test for post hoc analysis were used. P values of 0.01–0.05 were considered as significant (**P < 0.01, *P < 0.05).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Whole cell patch-clamp recordings in current-clamp mode combined with microfluorometry from +/+ and wv/wv dopaminergic neurons revealed no significant differences in the spontaneous activity (wv/wv: 0.62 ± 0.3 Hz, n = 9, +/+: 0.81 ± 0.1 Hz, n = 7; t-test, P = 0.61) or in the basal [Ca2+]i (wv/wv: 116 ± 22 nM, n = 11, +/+: 148 ± 22.7 nM, n = 7; t-test, P = 0.34; Fig. 1A). Both parameters were averaged over 2 min of simultaneous recordings of action potentials and fura-2 fluorescence. This supports previous data from our group showing no differences in resting membrane potential, h-current conductance and responses to current injection to indicate that +/+ and wv/wv dopamine cells do not differ in their intrinsic membrane properties.
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We previously reported that DA and baclofen hyperpolarized and inhibited +/+ neurons but depolarized and transiently excited wv/wv dopaminergic neurons (Guatteo et al. 2000). Here we have investigated the calcium dynamics associated with the electrophysiological responses of dopamine and GABAB receptor stimulation in wv/wv neurons. Figure 2A illustrates the effects of dopamine (DA,100 µM) on the firing rate and [Ca2+]i of dopaminergic neurons from wild-type (+/+, Fig. 2A1) and weaver mutants (wv/wv, Fig. 2A, 2 and 3). In +/+ neurons, the known hyperpolarization (Lacey et al. 1987) of the membrane potential (Fig. 2A1, top) in response to dopamine application was associated with a decrease in [Ca2+]i (Fig. 2A1, bottom). By contrast, in wv/wv neurons, the dopamine-induced excitation (Fig. 2A2, top) and increase of the firing frequency (Guatteo et al. 2000) are associated with a transient rise in [Ca2+]i (Fig. 2A2, bottom). In some wv/wv neurons, dopamine silenced the spontaneous discharge at depolarized potentials (approximately –30 mV, not shown). In the presence of TTX (1 µM), which prevented action potential discharge, dopamine produced similar membrane depolarization (Fig. 2A3, top, t-test, P = 0.14) and calcium accumulation in wv/wv neurons (Fig. 2A3, bottom, t-test, P = 0.8, n = 3). Dopamine induced a hyperpolarization in +/+ neurons to –53 ± 2.6 mV (
mV = –9.35 ± 1.1 mV, n = 7, Fig. 1B) and caused [Ca2+]i to drop to 54 ± 13 nM [1-way ANOVA, F(2,17) = 8.7, P < 0.01, Dunnet's test for post hoc analysis, n = 7, Fig. 1A]. In wv/wv neurons, dopamine induced a depolarization to –32.4 ± 2.7 mV (mV = 9 ± 2.7 mV, 1-way ANOVA, P < 0.01, n = 8, Fig. 1B) and an increase in the [Ca2+]i to 519 ± 110 nM {[Ca2+]i = 393 ± 111 nM, Fig. 1A, [1-way ANOVA, F(3,25) = 7.2, P < 0.01, Dunnet's test for post hoc analysis, n = 8]}. Similar responses as those evoked by dopamine were elicited by application of the D2 selective agonist quinpirole to wv/wv neurons (Fig. 1, A and B). Quinpirole induced a mean membrane depolarization of 7.4 ± 2.3 mV (n = 3, Fig. 1B) and a mean increase in [Ca2+]i to 504 ± 60 nM [1-way ANOVA, F(3,25) = 7.2, P < 0.01, Dunnet's test for post hoc analysis, n = 3, Fig. 1A]. Thus the depolarizing responses to dopamine in wv/wv neurons were mediated by activation of D2 receptors.
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mV = –22.91 ± 1.69 mV, n = 6, Figs. 1B and 3A1, top) associated with a decrease in the [Ca2+]i to 67 ± 14 nM, [[Ca2+]i = 84 ± 19 nM, 1-way ANOVA, F(2,17) = 8.7, P < 0.01, Dunnet's test for post hoc analysis, n = 6, Figs. 1A and 3A1, bottom], whereas wv/wv neurons responded to baclofen with a depolarization to –33.6 ± 2.4 mV (mV = 10.5 ± 2.46 mV, 1-way ANOVA, P < 0.01, n = 5, Fig. 1B) that caused an increase of firing frequency (Fig. 3A2, top) and was associated with a transient increase in the [Ca2+]i to 278 ± 33 nM [[Ca2+]i = 194 ± 29 nM, 1-way ANOVA, F(3,25) = 7.2, P < 0.05, Dunnet's test for post hoc analysis, n = 7, Figs. 1A and Fig. 3A2, bottom]. In the presence of TTX (1 µM), baclofen evoked membrane depolarization and calcium accumulation (Fig. 3A3) similar to those obtained without TTX (t-test, P = 0.17 and P = 0.19, n = 3, respectively).
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It has been proposed that the potassium permeable GIRK channel is altered to form a sodium/calcium permeable wvGIRK channel in the weaver mouse (Silverman et al. 1996; Slesinger et al. 1997), thus accounting for the depolarizing rather than hyperpolarizing effect of wvGIRK activation. To assess whether calcium influx after D2- and GABAB receptor activation in wv/wv dopamine neurons was directly mediated by wvGIRK2 channels or was instead mediated by the indirect activation of VGCCs by depolarization, we performed two sets of experiments. First, we measured [Ca2+]i in voltage-clamped neurons during application of either dopamine or baclofen. At a holding potential of –60 mV, both agonists elicited inward currents in wv/wv neurons (Figs. 2B1 and 3B1) as previously described (Guatteo et al. 2000) but caused no changes in [Ca2+]i (Figs. 2B, bottom, and 3B, bottom, n = 4). Second, we performed current-clamp recordings in wv/wv neurons hyperpolarized by current injection to approximately –80 mV, well below the activation threshold of VGCCs. Under these conditions, application of dopamine and baclofen (Figs. 2B2 and 3B2) induced a depolarization of 6 ± 2.1 and of 7 ± 1.8 mV (n = 4), respectively, that was not associated with any changes in [Ca2+]i (Figs. 2B and 3B, bottom panels).
To further investigate the putative involvement of VGCCs in the influx of calcium induced by D2 and GABAB receptor stimulation, we measured [Ca2+]i in current-clamp mode at the same membrane potential at which dopamine and baclofen usually generated a calcium increase but in the presence of a cocktail of VGCC blockers (1 mM Co2+ and 50 µM nifedipine, see METHODS) (Nedergaard et al. 1993; Ping and Shepard 1999; Shepard and Stump 1999). A relatively high concentration of nifedipine was necessary to block calcium-mediated events in dopamine neurons (Bonci et al. 1998; Mercuri et al. 1994; Nedergaard et al. 1993; Ping and Shepard 1999; Shepard and Stump 1999) and to affect to the presynaptic release of neurotransmitters and GABAA and glycine receptor-mediated postsynaptic responses in these neurons (Chesnoy-Marchais and Cathala 2001; Das et al. 2004; Hirasawa and Pittman 2003). In the presence of Co2+/nifedipine, neither agonist produced any change in [Ca2+]i (Fig. 4A, 1–3), despite causing membrane depolarizations (Fig. 4B, 1–3, 
) comparable to those obtained in the absence of VGCC blockers (—).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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We have previously shown that wvGIRK2 channels are coupled to D2 and GABAB receptors in a G-protein-dependent manner in the dopaminergic neurons of the weaver mouse and that the activation of these receptors mediates excitation/depolarization in wv/wv cells (Guatteo et al. 2000); this is opposite to the inhibition/hyperpolarization observed in control (+/+) neurons (Lacey et al. 1987). The blockade of spontaneous firing activity and the membrane hyperpolarization caused by DA and baclofen in +/+ cells accounts for the reduction of the [Ca2+]i. By contrast, in wv/wv dopaminergic cells the excitation/depolarization causes an accumulation of [Ca2+]i. Here we investigated the putative source of the calcium accumulation, trying to discriminate between two possibilities, the calcium-permeability of the wvGIRK2 and the involvement of VGCCs activated by the wvGIRK2-induced depolarization. Intracellular stores represent another putative source for calcium accumulation. However, this seems unlikely because here we show evidence that in wv/wv dopamine neurons the calcium signal in response to D2 and GABAB receptor activation strongly depends on membrane potential changes.
We excluded the contribution of VGCC either pharmacologically or by preventing their activation with membrane hyperpolarization. Both strategies revealed that if we prevent VGCC activation, calcium accumulation is abolished or is greatly reduced in wv/wv neurons during D2 and GABAB receptor activation of wvGIRK2 channels. This implies that wvGIRK2 channels in native dopamine neurons are not permeable to calcium ions. This differs from what has been shown in native cerebellar granule neurons from heterozygous (Fox et al. 1998) and homozygous (Harkins et al. 2000) weaver mice. These authors have hypothesized that a leak of calcium ions may occur through the wvGIRK1/2 channels, initiating a cell-death cascade in the cerebellar neurons.
On the other hand, in line with our results, a recent study has shown that wvGIRK2 channels are not permeable to divalent cations in mammalian neurons and are only weakly permeable to divalent cations when expressed in Xenopus oocytes (Hou et al. 2000).
We propose that the susceptibility to cell death of wvGIRK2-expressing dopaminergic neurons might be due to a wvGIRK2-mediated depolarization evoked by D2 and GABAB receptor activation. This depolarization activates VGCCs by itself and does not require action potential discharge to produce calcium accumulation. This is supported by the observation that in the presence of TTX, wvGIRK2 activation resulted in a membrane depolarization and calcium rise similar to those obtained in control. This calcium rise was, however, blocked by a combination of the VGCC antagonists, Co2+ and nifedipine. The high concentration of nifedipine (50 µM) did not affect the depolarizing effects of DA and baclofen, indicating that the VGCC blockers do not affect wvGIRK2 channels. Because neither TTX-sensitive Na+ channels nor the wvGIRK2 channels directly mediated the dopamine or baclofen induced calcium increases, we conclude that the Co2+/nifedipine-sensitive VGCCs mediated the calcium influx secondary to depolarizations caused by wvGIRK2 channel activation with agonists. Thus a VGCC-regulated influx of Ca2+ ions could become harmful during a sustained receptor-mediated depolarization. Indeed, although the D2- and GABAB-induced calcium increases are transient during exogenous application of agonists, a tonic release of endogenous dopamine and GABA could occur onto dopamine neurons of the weaver mouse, and this could cause a sustained D2- and GABAB-induced calcium rise. A reduced GABAB-mediated presynaptic inhibition of GABA and dopamine release (Radnikow et al. 2001) might also favor a tonic depolarization of dopaminergic cells in weaver mice. Thus a dendritic release of dopamine and an upregulation of presynaptic GABA release may result in a vicious circle of depolarization and consequent Ca2+ influx in wv/wv dopamine cells. This may lead to Ca2+ overload and contribute to the death of dopamine cells in weaver mutant mice.
The present data also suggest that the initial pathogenetic mechanisms that underlie the neuronal death observed in the cerebellum and in the ventral mesencephalon of the weaver mouse are different. The contribution of VGCCs to weaver granule cell death is still controversial (Kofuji et al. 1996; Liesi and Wright 1996), and these cells show an elevated resting [Ca2+]i due to a chronic leak of calcium ions through wvGIRK1/2 channels (Harkins et al. 2000). Differences in the calcium permeability of wvGIRK channels in cerebellar and dopamine neurons may be due to differences in their subunit composition. Cerebellar granule co-express GIRK1 and GIRK2 subunits that form heteromultimers (Liao et al. 1996), whereas midbrain dopamine neurons mainly express GIRK2 subunits that form homomultimers. Thus the effects of the wv mutation may vary with the type of GIRK channel subunits expressed by the neuron. A key difference reported here in wv/wv dopaminergic cells is that [Ca2+]i is normal in resting conditions and rises after receptor stimulation, and this is strongly attenuated by VGCCs antagonists. On the basis of our experimental data presented here, we suggest that a functional reversal of the GABAB and dopamine receptor-mediated inhibitory inputs to the dopaminergic neurons produces a depolarization-excitation of these cells and subsequent calcium entry. This intracellular calcium accumulation could lead to progressive energy depletion, toxicity, and cell death, which occurs within the first 3 wk of life in the wv/wv mice.
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ACKNOWLEDGMENTS |
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Present address of C. P. Bengston: Interdisciplinary Centre for Neurosciences (IZN), University of Heidelberg, Heidelberg 69120, Germany.
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FOOTNOTES |
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Address for reprint requests and other correspondence: N. B. Mercuri, Laboratory of Experimental Neurology, Fondazione Santa Lucia IRCCS, Via Ardeatina 306, 00179 Roma, Italy (E-mail: mercurin{at}uniroma2.it).
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REFERENCES |
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Chesnoy-Marchais D and Cathala L. Modulation of glycine responses by dihydropyridines and verapamil in rat spinal neurons. Eur J Neurosci 13: 2195–2204, 2001.[CrossRef][ISI][Medline]
Das P, Bell-Horner CL, Huang RQ, Raut A, Gonzales EB, Chen ZL, Covey DF, and Dillon GH. Inhibition of type A GABA receptors by L-type calcium channel blockers. Neuroscience 124: 195–206, 2004.[CrossRef][ISI][Medline]
Davila V, Yan Z, Craciun LC, Logothetis D, and Sulzer D. D3 dopamine autoreceptors do not activate G-protein-gated inwardly rectifying potassium channel currents in substantia nigra dopamine neurons. J Neurosci 23: 5693–5697, 2003.
Durante P, Cardenas CG, Whittaker JA, Kitai ST, and Scroggs RS. Low-threshold L-type calcium channels in rat dopamine neurons. J Neurophysiol 91: 1450–1454, 2004.
Fox AP, Dlouhy S, Ghetti B, Hurley JH, Nucifora PGP, Nelson DJ, Won L, and Heller A. Altered responses to potassium in cerebellar neurons from weaver heterozygote mice. Exp Brain Res 123: 298–306, 1998.[CrossRef][ISI][Medline]
Grynkiewicz G, Poenie M, and Tsien RY. A new generation of calcium indicators with greatly improved fluorescence properties. J Biol Chem 260: 3440–3450, 1985.
Guatteo E, Fusco FR, Giacomini P, Bernardi G, and Mercuri NB. The weaver mutation reverses the function of dopamine and GABA in mouse dopaminergic neurons. J Neurosci 20: 6013–6020, 2000.
Guatteo E, Mercuri NB, Bernardi G, and Knopfel T. Intracellular sodium and calcium homeostasis during hypoxia in dopamine neurons of rat substantia nigra pars compacta. J Neurophysiol 80: 2237–2243, 1998.
Gupta AM, Felten DL, and Ghetti B. Selective loss of monoaminergic neurons in the weaver mutant mice: an immunocytochemical study. Brain Res 402: 379–382, 1987.[CrossRef][ISI][Medline]
Harkins AB, Dlouhy S, Ghetti B, Cahill AL, Won L, Heller B, Heller A, and Fox AP. Evidence for elevated intracellular calcium levels in weaver homozygote mice. J Physiol 524: 447–455, 2000.
Hirasawa M, and Pittman QJ. Nifedipine facilitates neurotransmitter release independently of calcium channels. Proc Natl Acad Sci USA 100: 6139–6144, 2003.
Hou P, Di A, Huang P, Hansen CB, and Nelson DJ. Impermeability of the GIRK2 weaver channel to divalent cations. Am J Physiol Cell Physiol 278: C1038–C1046, 2000.
Jarolimek W, Bäurle J, and Misgeld U. Pore mutation in a G-protein-gated inwardly rectifying K+ channel subunit causes loss of K+-dependent inhibition in weaver hippocampus. J Neurosci 18: 4001–4007, 1998.
Kofuji P, Hofer M, Millen KJ, Millonig JH, Davidson N, Lester HA, and Hatten ME. Functional analysis of the weaver mutant GIRK2 K+ channel and rescue of weaver granule cells. Neuron 16: 941–952, 1996[CrossRef][ISI][Medline]
Lacey MG, Mercuri NB, and North RA. Dopamine acts on D2 receptors to increase potassium conductance in neurons of the rat substantia nigra zona compacta. J Physiol 392: 397–416, 1987.
Liesi P and Wright JM. Weaver granule neurons are rescued by calcium channel antagonists and antibodies against a neurite outgrowth domain of the B2 chain of laminin. J Cell Biol 134: 477–486, 1996.
Liao YJ, Jan YN, and Jan YJ. Heteromultimerization of G-protein-gated inwardly rectifying K+channel proteins GIRK1 and GIRK2 and their altered expression in weaver brain. J Neurosci 16: 7137–7150, 1996.
Liss B, Neu A, and Roeper J. The weaver mouse gain-of-function phenotype of dopaminergic neurons is determined by coactivation of wvGIRK2 and K-ATP channels. J Neurosci 19: 8839–8848, 1999.
Mercuri NB, Bonci A, Calabresi P, Stratta F, Stefani A, and Bernardi G. Effects of dihydropyridine calcium antagonists on rat midbrain dopaminergic neurones. Br J Pharmacol 113: 831–838, 1994.[ISI][Medline]
Navarro B, Kennedy ME, Velimirovic B, Bhat D, Peterson AS, and Clapham DE. Nonselective and G -insensitive weaver K+ channels. Science 272: 1950–1953, 1996.[Abstract]
Nedergaard S, Flatman JA, and Engberg I. Nifedipine- and omega-conotoxin-sensitive Ca2+ conductances in guinea pig substantia nigra pars compacta neurones. J Physiol 466: 727–747, 1993.
Patil N, Cox DR, Bhat D, Faham M, Myers RM, and Peterson AS. A potassium channel mutation in weaver mice implicates membrane excitability in granule cell differentiation. Nat Genet 11: 126–129, 1995.[CrossRef][ISI][Medline]
Ping HX and Shepard PD. Bolckade of SK-type Ca2+-activated K+ channels uncovers a Ca2+-dependent slow afterdepolarization in nigral dopamine neurons. J Neurophysiol 81: 977–984, 1999.
Radnikow G, Titz S, Mades S, Baurle J, and Misgeld U. Gamma-aminobutyric acid(B) autoreceptors in substantia nigra and neostriatum of the weaver mutant mouse. Neurosci Lett 299: 81–84, 2001.[CrossRef][ISI][Medline]
Rakic P and Sidman RL. Sequence of developmental abnormalities leading to granule cell deficit in cerebellar cortex of weaver mutant mice. J Comp Neurol 152: 103–132, 1973.[CrossRef][ISI][Medline]
Rossi P, De Filippi G, Armano S, Taglietti V, and D'Angelo E. The weaver mutation causes a loss of inward rectifier current regulation in premigratory granule cells of the mouse cerebellum. J Neurosci 18: 3537–3547, 1998.
Schmidt MJ, Sawyer BD, Perry KW, Foreman MM, and Ghetti B. Dopamine deficiency in the weaver mutant mouse. J Neurosci 2: 376–380, 1982.[Abstract]
Shepard PD and Stump D. Nifedipine blocks apamin-induced bursting activity in nigral dopamine-containing neurons. Brain Res 817: 104–109, 1999.[CrossRef][ISI][Medline]
Silverman SK, Kofuji P, Dougherty DA, Davidson N, and Lester HA. A regenerative link in the ionic fluxes through the weaver potassium channel underlies the pathophysiology of the mutation. Proc Natl Acad Sci USA 93: 15429–15434, 1996.
Simon JR and Ghetti B. The weaver mutant mouse as a model of nigrostriatal dysfunction. Mol Neurobiol 9: 183–189, 1994.[ISI][Medline]
Slesinger PA, Stoffel M, Jan YN, and Jan LY. Defective gammaaminobutyric acid type B receptor-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from weaver and Girk2 null mutant mice. Proc Natl Acad Sci USA 94: 12210–12217, 1997.
Smith MW III, Cooper TR, Joh TH, and Smith DE. Cell loss and class distribution of TH-I cells in the substantia nigra of the neurological mutant, weaver. Brain Res 510: 242–250, 1990.[CrossRef][ISI][Medline]
Surmeier DJ, Merlmestein PG, and Goldowitz D. The weaver mutation of GIRK2 results in a loss of inwardly rectifying K+ current in cerebellar granule cells. Proc Natl Acad Sci USA 93: 11191–11195, 1996.
Triarhou LC, Norton J, and Ghetti B. Mesencephalic dopamine cell deficit involves areas A8, A9 and A10 in weaver mice. Exp Brain Res 70: 256–265, 1988.[ISI][Medline]
Yang J, Jan YN, and Jan LY. Determination of the subunit stoichiometry of an inwardly rectifying potassium channel. Neuron 14: 1047–1054, 1995.[CrossRef][ISI][Medline]
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) and weaver (wv/wv, 
) neurons. The difference in basal calcium concentration between the 2 genotypes was not significant (t-test, P = 0.34). All 3 agonists significantly increased [Ca2+]i in wv/wv neurons [1-way ANOVA, F(3,25) = 7.2 and Dunnet's test for post hoc analysis, P < 0.01 for DA and Quinp, P < 0.05 for Baclo] and decreased [Ca2+]i in +/+ neurons [1-way ANOVA, F(2,17) = 8.7 and Dunnet's test for post hoc analysis, P < 0.01 for both agonists]. B: dopamine and baclofen induce hyperpolarization (
