Local stimulation induced GABAergic response in rat striatal slice preparations: Intracellular recordings on QX-314 injected neurons

Gamma-aminobutyric acid (GABA)ergic responses evoked by electrical stimulation in the neostriatal slice preparation were studied in neurons injected intracellularly with Na-conductance blocker QX-314. Local stimulation elicited depolarizing postsynaptic potentials (DPSPs) in the QX-314-injected neurons when the membrane potential was morenegative than −60 mV. When DPSPs were minimized by depolirizing current injection in the QX-314-injected neuron, hyperpolarization was clearly observed following local stimulation. The maximum duration of the hyperpolarizing response to strong local stimulation was about 130 ms. The hyperpolarizing response was blocked by the addition of bicuculine or picrotoxin to the Ringer solution. Intracellular Cl^- injections produced changes in the pattern of the local stimulations-induced responses; the initial depolarizing response was followed by a relatively large amplitude long duration depolarization. The polarity of the long duration of depolarizing response could not be reversed by depolarizing currents which were normally sufficient to reverse the polarity of DPSPs in the neurons without Cl^- injection. The application of pentobarbital enhanced the amplitude and the duration of the hyperpolarizing responses. The revealed potential of the pentobarbital-enhanced response was estimated to be −60 mV. On the basis of their reversal potential, sensitivity to injected Cl^-, sensitivity to GABA blockers picrotoxin and bicuculine, and the effect of pentobarbital, these hyperpolarizing responses are shown to be GABAergic Cl^- mediated inhibitory postsynaptic potentials (IPSPs).     

Receptor subtype-specific modulation by dopamine of glutamatergic responses in striatal medium spiny neurons

The output of GABAergic medium-sized spiny neurons in the dorsal striatum is controlled in part by glutamatergic input from the neocortex and the thalamus, and dopaminergic input from ventral midbrain. We acutely isolated these neurons from juvenile (P14-24) rats to study the consequences of the interaction between glutamate and dopamine for neuronal excitability. Single-cell RT-PCR analysis was used to identify the expression patterns of dopamine receptors. D1 and D2 dopamine receptor mRNA was detected in 11/22 and 3/22 of isolated neurons, respectively. Receptor mRNA co-expression was detected in 1/22 cells tested. Whole-cell voltage clamp recording (V(h)=-70 mV) was combined with local or bath application of dopaminergic and glutamatergic agonists to explore dopamine receptor modulation of glutamatergic excitation. Glutamate-evoked inward currents (5 microM, Mg(2+)-free, 1 microM glycine) were attenuated by dopamine (5 microM) to 83.2+/-3.6% (n=31). NMDA-evoked (20 microM), APV-sensitive currents were attenuated by dopamine to 80.9+/-4.5% (n=24). NMDA-induced responses were also attenuated by the D1 receptor agonist SKF 38393 (1 microM; n=28), while the D2/3 receptor agonist quinpirole (10 microM) had no effect. The currents evoked by application of AMPA (5 microM) displayed a steady rundown. Application of dopamine abolished or significantly reduced the rundown in the cells tested (n=17). A similar effect was observed after the application of SKF 38393 (1 microM), while quinpirole (10 microM) had no significant effect. Our results provide direct evidence for modulation by dopamine of glutamatergic responses of striatal medium spiny neurons, and demonstrate that the effects of this neuromodulator are receptor subtype specific. Disruption of this modulatory effect is likely to contribute to movement disorders associated with Parkinson's disease.     

Acetylcholine attenuates synaptic GABA release to supraoptic neurons through presynaptic nicotinic receptors

Both inhibitory GABAergic and excitatory glutamatergic inputs to supraoptic nucleus (SON) neurons can influence the release of vasopressin and oxytocin. Acetylcholine is known to excite SON neurons and to increase vasopressin release. The functional significance of cholinergic receptors, located at the presynaptic nerve terminals, in the regulation of the excitability of SON neurons is not fully known. In this study, we determined the role of presynaptic cholinergic receptors in regulation of the inhibitory GABAergic inputs to the SON neurons. The magnocellular neurons in the rat hypothalamic slice were identified microscopically, and the spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the whole-cell voltage-clamp technique. The mIPSCs were abolished by the GABA_A receptor antagonist, bicuculline (10 μM). Acetylcholine (100 μM) significantly reduced the frequency of mIPSCs of SON neurons from 3.59±0.36 to 1.62±0.20 Hz (n=37), but did not alter the amplitude and the decay time constant of mIPSCs. Furthermore, the nicotinic receptor antagonist, mecamylamine (10 μM, n=13), eliminated the inhibitory effect of acetylcholine on mIPSCs of SON neurons. The muscarinic receptor antagonist, atropine (100 μM), did not alter significantly the effect of acetylcholine on mIPSCs in most of the 17 SON neurons studied. These results suggest that the excitatory effect of acetylcholine on the SON neurons is mediated, at least in part, by inhibition of presynaptic GABA release. Activation of presynaptic nicotinic receptors located in the GABAergic terminals plays a major role in the cholinergic regulation of the inhibitory GABAergic input to SON neurons.     

Cortical stimulation induces Fos expression in striatal neurons via NMDA glutamate and dopamine receptors

Cortical electrical stimulation has been shown to induce dense and widespread Fos expression throughout the ipsilateral and contralateral striatum. This raises interest for studying the mechanisms underlying the regulation of striatal neuron activity by cortical afferents, and for elucidating the interactions with other systems. However, the receptors mediating cortical-stimulation-induced expression of Fos in striatal neurons have not been identified. This was studied in the work reported here by stimulating the cortex after administration of glutamate or dopamine receptor antagonists, or after 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal dopaminergic system. Pretreatment with the non-competitive N-methyl- d-aspartate (NMDA) glutamate receptor antagonist MK-801 led to a marked reduction in the stimulation-induced density of Fos-immunoreactive nuclei in both the medial (about 80% reduction) and lateral (about 50–60% reduction) striatum. Preadministration of the D₁-selective dopamine antagonist SCH-23390 alone or in combination with the D₂-selective dopamine antagonist eticlopride led to a reduction in the stimulation-induced density of Fos-positive nuclei of about 60–65% in the lateral striatum, but no significant change in the medial region. The effects of 6-OHDA lesion were less pronounced, and the stimulation-induced density of Fos-immunoreactive nuclei decreased by only about 25% in the lateral region. These results indicate that both dopamine and NMDA glutamate receptors are involved in the induction of Fos by cortical stimulation, and support the hypothesis that cortex-dopamine interactions in the lateral striatum may be functionally different from those in the medial striatum.     

Presynaptic and postsynaptic GABAB receptors of neocortical neurons of the rat in vitro: Differences in pharmacology and ionic mechanisms

The properties of pre- and postsynaptic GABA_B receptors were investigated with intracellular recordings from rat neocortical neurons in vitro. An antagonist of the GABA_B receptor (CGP 35348) and ions or drugs interfering with GABA_B receptor-mediated K⁺ conductance (Ba²⁺, QX 314) were employed to delineate possible differences. CGP 35348 reduced the conductance of the late inhibitory postsynaptic potential (IPSP_B) in a dose-dependent manner. Neither the early GABA_A receptor-mediated inhibitory postsynaptic potential (IPSP_A), nor resting membrane potential or direct excitability, were consistently affected by CGP 35348. Bath application of 100 μmol/l Ba²⁺ decreased IPSP_B conductance to about 40% and increased IPSP_A conductance to 130% of control. The depression of a second IPSP by a pair of stimuli (paired pulse depression, or PPD) was used as an index for presynaptic GABA_B receptor activation. Neither CGP 35348 nor Ba²⁺ exerted significant effects on the PPD at intervals of 400 msec. The dependence of PPD on the latency of the interval of the stimulus pair was investigated after intracellular application of QX 314 had virtually abolished the IPSP_B. Decreasing the stimulus interval from 500 msec to 100 msec revealed a stronger depression of the second IPSP_A. Application of CGP 35348 alleviated PPD for stimulus intervals below 300 msec. The data indicate a distinct pharmacological difference between pre- and postsynaptic GABA_B receptors. Moreover, we suggest that two temporally distinct presynaptic GABA_B receptor effects contribute to PPD: a short-lasting effect, sensitive to CGP 35348, and a long-lasting effect, insensitive to CGP 35348. The latter is insensitive to Ba²⁺, implying that this component is not associated with a K⁻ conductance mechanism. Synapse 25:62–72, 1997. © 1997 Wiley-Liss, Inc.     

D1 and D2 dopamine-receptor modulation of striatal glutamatergic signaling in striatal medium spiny neurons

Dopamine shapes a wide variety of psychomotor functions. This is mainly accomplished by modulating cortical and thalamic glutamatergic signals impinging upon principal medium spiny neurons (MSNs) of the striatum. Several lines of evidence suggest that dopamine D1 receptor signaling enhances dendritic excitability and glutamatergic signaling in striatonigral MSNs, whereas D2 receptor signaling exerts the opposite effect in striatopallidal MSNs. The functional antagonism between these two major striatal dopamine receptors extends to the regulation of synaptic plasticity. Recent studies, using transgenic mice in which cells express D1 and D2 receptors, have uncovered unappreciated differences between MSNs that shape glutamatergic signaling and the influence of DA on synaptic plasticity. These studies have also shown that long-term alterations in dopamine signaling produce profound and cell-type-specific reshaping of corticostriatal connectivity and function.     

幼年非洲爪蟾视顶盖区突触前、后膜上受体间的相互作用

目的　 记录幼年非洲爪蟾视顶盖区第六层神经元的自发性微突触后电流 (mPSCs)。 方法 　应用盲法电压膜片钳全细胞记录技术。 结果　 观察到用谷氨酸受体激动剂NMDA灌流脑片后先引起mIPSCs的频率明显增加并出现内向膜电流及高频的mEPSCs,经一段时间洗脱后mIPSCs和mEPSCs又均完全消失 ,而膜电流恢复到原来未加NMDA前的水平。GABA受体的激动剂GABA可诱发明显的外向膜电流。GABAa受体拮抗剂荷包牡丹碱 (bicucullin ,BM)不仅能将mIPSCs全部抑制掉 ,并且还可以诱发mEPSCs。谷氨酸受体的拮抗剂APV对mPSCs亦有类似的作用 ,不仅可以抑制顶盖神经元的mEPSCs,而且可以使原有的mIPSCs的频率和振幅均增加。 结论　幼年期的突触前、后膜上既有兴奋性谷氨酸能受体也有抑制性γ 氨基丁酸能受体 ,而且在突触前膜上受体可以调制突触末梢神经递质的释放。因此突触前、后膜上的受体间存在相互作用 ,以确保突触前后活动和功能上的稳定 ,从而达到神经网络的平衡。     

Muscarinic presynaptic inhibition of neostriatal glutamatergic afferents is mediated by Q-type Ca channels

Cholinergic presynaptic inhibition was investigated on neostriatal glutamatergic transmission. Paired pulse facilitation (PPF) of orthodromic population spikes (PS) were used to construct a concentration-response relationship for muscarine on presynaptic inhibition. Muscarine had an effect proportional to its extracellular concentration with an EC50 (mean +/- standard estimation error) of: 2.5 +/- 1.5 nM, and a maximal effect (saturation) of 245 +/- 16%. Several peptidic toxins against some voltage-gated Ca2+-channels increased PPF indicating that the Ca2+-channels they block participate in transmitter release. However, neither 1 microM omega-conotoxin GVIA, a specific blocker of N-type Ca2+-channels, nor 10-30 nM omega-agatoxinTK, a selective blocker of P-type Ca2+-channels, were able to occlude muscarine's effect on presynaptic inhibition. Nevertheless, 100-400 nM omega-agatoxinTK occluded muscarine's action on PPF in a dose-dependent manner. These results are consistent with Q-type Ca2+-channels mediating muscarinic presynaptic inhibition of neostriatal afferents.     

Adenosine A1 receptor-mediated depression of corticostriatal and thalamostriatal glutamatergic synaptic potentials in vitro

Electrophysiological recordings in rat brain slices have been used to study the actions of adenosine on striatal neurons and striatal excitatory amino acid neurotransmission originating in the cortex or the thalamus. Adenosine had no effects on membrane properties of striatal neurons. Adenosine and the A₁ agonist N⁶-Cyclopentyl adenosine reduced EPSPs of both cortical and thalamic origin by more than 50%. Depression of EPSPs was associated with an increase in paired-pulse facilitation, suggesting a presynaptic locus of action. EPSP depression was blocked by the A₁ antagonist, 8-Cyclopentyl-1,3-dipropyl xanthine. The A₂ agonist 5′-(N-cyclopropyl)-carboxamidoadenosine had no effect on excitatory amino acid neurotransmission. The A₁ antagonist alone enhanced the synaptic component of the evoked field potential (23±12%). These results indicate that endogenous adenosine, acting via A₁ receptors, limits striatal glutamatergic neurotransmission, serving a modulatory and neuroprotective role.     

GABAB受体对大鼠海马CA1区锥体细胞突触传递的作用

目的研究激活GABA_B受体对大鼠海马CA1区锥体细胞突触传递的影响。方法对成年大鼠海马脑片CA1区锥体细胞采用“盲法”全细胞电压钳记录,分别检测和分析巴氯芬(10μmol/L)对自发性的兴奋性突触后电流(EPSCs)和抑制性突触后电流(IPSCs)的影响。结果巴氯芬可显著降低符氨酸能EPSCs和γ-氨基丁酸能IPSCs的频率(P＜0．01),各自达58%±7%(n=17)和42%±10%(n=15),而对它们的幅度无显著性影响。结论巴氯芬对海马CA1区锥体细胞EPSCs和IPSCs的抑制作用属于突触前抑制,推测GABA_B受体所介导的这种抑制作用对CA1区神经元兴奋性的传出具有抑制作用,从而对癫痫的产生有控制作用。     

Depression of retinogeniculate synaptic transmission by presynaptic D(2)-like dopamine receptors in rat lateral geniculate nucleus

Extraretinal projections onto neurons in the dorsal lateral geniculate nucleus (dLGN) play an important role in modifying sensory information as it is relayed from the visual thalamus to neocortex. The dLGN receives dopaminergic innervation from the ventral tegmental area; however, the role of dopamine in synaptic transmission in dLGN has not been explored. In the present study, whole cell recordings were obtained to examine the actions of dopamine on glutamatergic synaptic transmission. Dopamine (2-100 microm) strongly suppressed excitatory synaptic transmission in dLGN relay neurons that was evoked by optic tract stimulation and mediated by both N-methyl-d-aspartate and non-N-methyl-d-aspartate glutamate receptors. In contrast, dopamine did not alter inhibitory synaptic transmission arising from either dLGN interneurons or thalamic reticular nucleus neurons. The suppressive action of dopamine on excitatory synaptic transmission was mimicked by the D(2)-like dopamine receptor agonist bromocriptine (2-25 microm) but not by the D(1)-like receptor agonist SKF38393 (10-25 microm). In addition, the dopamine-mediated suppression was antagonized by the D(2)-like receptor antagonist sulpiride (10-20 microm) but not by the D(1)-like receptor antagonist SCH23390 (5-25 microm). The dopamine-mediated decrease in evoked excitatory postsynaptic current amplitude was accompanied by an increase in the magnitude of paired-pulse depression. Furthermore, dopamine also reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. Taken together, these data suggest that dopamine may act presynaptically to regulate the release of glutamate at the retinogeniculate synapse and modify transmission of visual information in the dLGN.     

Involvement of corticostriatal glutamatergic terminals in striatal dopamine release elicited by stimulation of delta-opioid receptors

We have previously shown that striatal dopamine release induced locally by a delta-opioid receptor agonist was totally inhibited by a glutamate N-methyl-D-aspartate receptor antagonist, indicating the involvement of glutamatergic receptors in this effect. The aim of the present study was to specify this mechanism. Firstly, we investigated the effect of [D-Pen2,D-Pen5]-enkephalin (DPDPE) on glutamate release in rats by intrastriatal microdialysis. The infusion of DPDPE (10 microm) enhanced the glutamate content in dialysate by approximately 34%, an effect which did not appear to result from inhibition of glutamate uptake. We then considered the consequences of a unilateral thermocoagulation of the frontal cortex on either glutamate or dopamine release induced by stimulation of delta-opioid receptors 2 days later. This lesion, which decreased the glutamate content in ipsilateral striatum by approximately 30%, totally prevented the increase in dialysate levels of glutamate induced by DPDPE. Moreover, whereas DPDPE (10 microm) was found to increase the striatal dopamine release in intact animals by approximately 59%, this effect was also completely suppressed by the cortical lesion. Finally, we studied the effect of the lesion on the [3H]-DPDPE binding to striatal membranes prepared from the whole striatum. In the ipsilateral striatum a significant decrease in this [3H]-DPDPE binding (by approximately 18%) was found 2 days after the lesion. Our results indicate that the increase in striatal dopamine release induced by DPDPE probably depends on glutamate release from corticostriatal glutamatergic afferents in response to the stimulation of delta-opioid receptors located on terminals of these neurons.     

Suppression of presynaptic calcium influx by metabotropic glutamate receptor agonists in neonatal rat hippocampus

Mechanisms of presynaptic inhibition by metabotropic glutamate receptor (mGluR) agonists were investigated in neonatal rat hippocampal CA1 region using the optical recording technique recently developed. Following selective loading of presynaptic terminals with a fluorescent Ca²⁺ indicator dye rhod-2 AM, changes in Ca²⁺ signals and the corresponding field excitatory postsynaptic potentials (EPSPs) induced by single electrical stimuli to the Schaffer collateral-commissural (SCC) pathway were recorded simultaneously. Application of a mGluR agonist, 1 S,3 R-1-aminocyclopentane-1,3-dicar?ylic acid (1 S,3 R-ACPD; 100 μM) or (±)-1-aminocyclopentane-trans-1,3-dicar?ylic acid (trans-ACPD; 100 μM), reversibly reduced both the field EPSP and the presynaptic Ca²⁺ transient, and the quantitative relationship between them was quite similar to that observed during application of Cd²⁺, a non-selective Ca²⁺ channel blocker, or in a Ca²⁺-free solution. Application of 4-aminopyridine (4-AP; 1 mM), a blocker of certain subtypes of voltage-dependent K⁺ channels, significantly inhibited the 1 S,3 R-ACPD effect. Application of DCG-IV, a novel mGluR2/mGluR3-selective agonist, suppressed field EPSPs only slightly even at a high dose (3 μM). These results suggest that activation of presynaptic mGluR different from mGluR2/mGluR3 suppresses the action potential-triggered Ca²⁺ influx, probably via 4-AP-sensitive mechanisms, and thereby reduces glutamate release in neonatal rat hippocampal CA1 region.     

Subpopulations of GABA-mediated synaptic potentials in slices of rat dorsal striatum are differentially modulated by presynaptic GABAB receptors.

gamma-Aminobutyric acid (GABA)-mediated synaptic potentials in rat dorsal striatum in vitro were reduced in amplitude by the GABAB receptor agonists baclofen and 3-aminopropyl-(methyl)-phosphinic acid (SK&F 97541), without any detectable postsynaptic effect. Synaptic potentials in 40% of neurones were distinctly multiphasic, the components of which exhibited a differential sensitivity to GABAB agonists. One population of GABA-releasing neurones within the striatum had presynaptic GABAB autoreceptors, whereas others were not directly affected by GABAB agonists.     

Role of GABAB receptors in GABA and baclofen-induced inhibition of adult rat cerebellar interpositus nucleus neurons in vitro

Previous studies suggested that the postsynaptic GABA(B) receptors of deep cerebellar nuclear neurons of adult rats were not activated by selective GABA(B) receptor agonist baclofen or endogenous GABA released by cerebellar cortical Purkinje cells, although the receptors have been demonstrated to exist in the deep cerebellar nuclei. In this study, cerebellar slices of adult rats were prepared for testing effects of GABA, baclofen and muscimol (selective GABA(A) receptor agonist) on cerebellar interpositus nucleus (IN) neurons. Perfusing slices with GABA (10-1000 microM), baclofen (1-30 microM) and muscimol (1-100 microM) respectively produced a dose-dependent inhibitory response on the IN neurons (n = 39, 62 and 50), which was not blocked by low-Ca(2+)/high-Mg(2+) medium (n = 5, 6 and 6), supporting a direct postsynaptic action of these GABAergic agonists. Moreover, both selective GABA(B) receptor antagonist CGP35348 and selective GABA(A) receptor antagonist bicuculline were capable of partially blocking the inhibitory response of IN neurons to GABA (n = 14 and 11), suggesting that the GABA-induced inhibition may contain two components, a GABA(B) receptors-mediated component and a GABA(A) receptors-mediated one. Further experiments revealed that not only muscimol (n = 50) but also baclofen (n = 62) suppressed IN cells' activity. The baclofen-induced inhibition was selectively blocked by CGP35348 (n = 12) but not by bicuculline (n = 8), whereas the muscimol-induced inhibition was selectively antagonized by bicuculline (n = 8) instead of CGP35348 (n = 9). These results indicate that GABA(B) receptors in the IN neurons can be activated not only by GABA but also by baclofen, suggesting that besides GABA(A) receptors, GABA(B) receptors may also be involved in mediating the inhibitory effect of GABA on cerebellar IN neurons of adult rats.     

Modulation of monosynaptic transmission by presynaptic inhibition during fictive locomotion in the cat

The effect of multisensory inputs onto the presynaptic inhibitory pathways affecting IA terminals was studied during fictive locomotion in decerebrated cats. The effect was evaluated from changes in amplitude of the monosynaptic excitatory postsynaptic potential (EPSP) measured in lumbosacral motoneurones. Responses were grouped and averaged according to their timing within the step cycle divided into five bins. Presynaptic inhibition was evoked by stimulating group I afferents from the posterior biceps-semitendinosus (PBSt) muscles and one of three cutaneous nerves: superficial peroneal (SP), sural and saphenous. Statistical analysis was applied to compare (1) EPSPs conditioned by PBSt input alone and those conditioned by the combined PBSt and cutaneous inputs, and (2) each bin dividing the step cycle to disclose phase-dependent changes. Results from 19 motoneurones showed that: (1) there was a significant phase-dependent modulation in EPSP amplitude (by 25%) with the maximum usually occurring during the depolarized phase; (2) PBSt alone reduced the EPSP amplitude (by 21%) in 3.2 bins on average; (3) combined PBSt and cutaneous stimuli further modified (up or down) the EPSP amplitude in half the trials but only in one to two bins; and (4) the most efficient cutaneous nerve (SP) usually decreased the PBSt-evoked reduction in EPSP size. Minimal changes in membrane input resistance suggest that the EPSP modifications were mostly due to presynaptic inhibition. Results indicate that muscle afferents can induce an important phase-dependent presynaptic inhibition of monosynaptic transmission and that concomitant activation of cutaneous afferents can alter this inhibition but only for a restricted part of the step cycle.     

Serotonin inhibits synaptic glutamate currents in rat nucleus accumbens neurons via presynaptic 5‐HT1B receptors

Neurons in the nucleus accumbens septi in brain slices from adult male rats were studied with patch clamp recording in the whole-cell conformation. Cells filled with Lucifer Yellow were identified as medium spiny neurons. Electrical stimulation close to the recorded cell evoked excitatory and inhibitory synaptic currents. In the presence of picrotoxin or bicuculline, stimulation at a holding potential of −90 mV evoked an inward excitatory current that was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 μm ), identifying it as an excitatory postsynaptic current (EPSC) mediated by glutamate acting at AMPA/kainate receptors. Serotonin (5-hydroxytryptamine, 5-HT; 3–100 μm in the bath) decreased the EPSC in about 90% of the cells. The action of 5-HT was mimicked by N-(3-trifluoromethylphenyl)-piperazine HCl (TFMPP), but not by (±)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT) or (±)-2,5-dimethoxy-4-iodoamphetamine HCl (DOI). The 5-HT effect was antagonized by pindolol or cyanopindolol, but not by spiperone, ketanserin or tropisetron. Taken together, these results indicate that 5-HT acts at 5-HT_1B receptors. The effect of 5-HT was potentiated by cocaine (0.3–3 μm ) or the selective serotonin reuptake inhibitor citalopram. Miniature synaptic currents recorded in the presence of tetrodotoxin were inhibited by CNQX, identifying them as spontaneous miniature EPSCs. 5-HT reduced the frequency of these miniature EPSCs without affecting their amplitude, which indicates a presynaptic site of action. This presynaptic inhibition by 5-HT might be involved in the behavioural effects of cocaine.     

Role of GABA transporter 3 in GABAergic synaptic transmission at striatal output neurons

Striatal GABAergic signaling has been shown to be essential for basal ganglia output and proper motor performance. In the mouse neostriatum GABA transporter 1 (GAT‐1) was previously found to assist in the clearance of GABA from the extracellular space and influence both phasic and tonic GABAergic inhibition of medium‐sized striatal output neurons (SONs). It currently remains unknown whether GAT subtypes other than GAT‐1 participate in the modulation of GABAergic transmission in this brain structure. In this study, we aimed at assessing the role of GAT‐3 in the mouse neostriatum. To this end, we recorded GABAergic inhibitory postsynaptic currents (IPSCs) from SONs in brain slices at different developmental stages (postnatal days (P) 7‐9, 12‐14, and 28‐34) using the whole‐cell patch‐clamp technique. When applied under control conditions, SNAP‐5114 (40 μM), a specific GAT‐3 blocker, did not affect miniature or evoked IPSCs (m/eIPSCs) and produced no significant effect on tonic GABAA receptor‐mediated conductances in SONs. However, in the presence of NO‐711 (10 μM), a specific GAT‐1 blocker, SNAP‐5114 reduced mIPSC frequencies without affecting mIPSC amplitudes or kinetics. In addition, SNAP‐5114 reduced the mean amplitude of eIPSCs and increased the paired‐pulse ratio. These effects were entirely abolished by CGP55845 (1 μM), a specific GABAB receptor blocker, indicating that they were mediated by presynaptic GABAB receptors. Similar results were obtained from all age groups. We conclude that GAT‐3 is functionally expressed in the mouse neostriatum. Whereas an acute block of GAT‐3 under resting conditions is fully compensated for by GAT‐1, GAT‐3 might provide an additional uptake capacity when neuronal activity and GABA release are increased. Synapse 63:921–929, 2009. © 2009 Wiley‐Liss, Inc.     

GABAB receptors in neocortical and hippocampal pyramidal neurons are coupled to different potassium channels

Classically, GABA_B receptors are thought to regulate neuronal excitability via G‐protein‐coupled inwardly rectifying potassium (GIRK) channels. Recent data, however, indicate that GABA_B receptors can also activate two‐pore domain potassium channels. Here, we investigate which potassium channels are coupled to GABA_B receptors in rat neocortical layer 5 and hippocampal CA1 pyramidal neurons. Bath application of the non‐specific GIRK channel blocker barium (200 μm ) abolished outward currents evoked by GABA_B receptors in CA1 pyramidal, but only partially blocked GABA_B responses in layer 5 neurons. Layer 5 and CA1 pyramidal neurons also showed differential sensitivity to tertiapin‐Q, a specific GIRK channel blocker. Tertiapin‐Q partially blocked GABA_B responses in CA1 pyramidal neurons, but was ineffective in blocking GABA_B responses in neocortical layer 5 neurons. Consistent with the idea that GABA_B receptors are coupled to two‐pore domain potassium channels, the non‐specific blockers quinidine and bupivacaine partially blocked GABA_B responses in both layer 5 and CA1 neurons. Finally, we show that lowering external pH, as occurs in hypoxia, blocks the component of GABA_B responses mediated by two‐pore domain potassium channels in neocortical layer 5 pyramidal neurons, while at the same time revealing a GIRK channel component. These data indicate that GABA_B receptors in neocortical layer 5 and hippocampal CA1 pyramidal neurons are coupled to different channels, with this coupling pH dependent on neocortical layer 5 pyramidal neurons. This pH dependency may act to maintain constant levels of GABA_B inhibition during hypoxia by enhancing GIRK channel function following a reduction in two‐pore domain potassium channel activity.     

Biphasic modulation of evoked [3H]D-aspartate release by D-2 dopamine receptors in rat striatal slices

It has been hypothesized that dopamine (DA) inhibits glutamate release from corticostriatal fibers via presynaptically located D-2 DA receptors although the evidence presented in the literature has not been conclusive. In the present experiments, the effect of D-2 receptor ligands on K+-stimulated tritium release from rat striatal slices preloaded with the nonmetabolizable glutamate analog [3H]D-aspartate ([3H]ASP was measured. The D-2 receptor antagonist S-sulpiride increased stimulated [3H]ASP release by 75% (EC50 value = 240 nM) and the biologically less-active isomer R-sulpiride, although equally effective, was tenfold less potent. The D-2 receptor agonists pergolide and (+)-4-propyl-9-hydroxynapthoxazine (+PHNO) inhibited [3H]ASP release at nM concentrations; however, this effect was small (20%). This low efficacy of the exogenous agonists was apparently due to competition by high concentrations of endogenous DA since the effect of pergolide was increased in rats whose striatal DA levels were decreased by 97%. These data support the hypothesis that D-2 DA receptors modulate [3H]ASP release in an inhibitory fashion. However, when the agonists were tested at lower concentrations, [3H]ASP release was increased significantly by 20% in control rats and 60% in DA-depleted rats. Both the facilitory and inhibitory effects of pergolide were blocked by 10 microM S-sulpiride, suggesting D-2 receptor mediation. In addition, the facilitory effect of pergolide was blocked by tetrodotoxin (TTX) and by the GABAA antagonist bicuculline, implying mediation of this D-2 effect by an inhibitory GABAergic interneuron. The inhibitory effect of pergolide was decreased by the muscarinic antagonist atropine.(ABSTRACT TRUNCATED AT 250 WORDS)