Xiao-Tao Jin

Localization and function of pre‐ and postsynaptic kainate receptors in the rat globus pallidus

Kainate receptors (KARs) are widely expressed the basal ganglia. In this study, we used electron microscopic immunocytochemistry and whole‐cell recording techniques to examine the localization and function of KARs in the rat globus pallidus (GP). Dendrites were the most common immunoreactive elements, while terminals forming symmetric or asymmetric synapses and unmyelinated axons comprised most of the presynaptic labeling. To determine whether synaptically released glutamate activates KARs, we recorded excitatory postsynaptic currents (EPSCs) in the GP following single‐pulse stimulation of the internal capsule. 4‐(8‐Methyl‐9H‐1,3‐dioxolo[4,5 h]{2,3}benzodiazepine‐5‐yl)‐benzenamine hydrochloride (GYKI 52466, 100 µm), an α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole propionic acid (AMPA) receptor antagonist, reduced but did not completely block evoked EPSCs. The remaining EPSC component was mediated through activation of KARs because it was abolished by 6‐cyano‐7‐nitroquinoxaline‐2, 3‐dione (CNQX), an AMPA/KAR antagonist. The rise time (10–90%) and decay time constant (τ) for those EPSCs were longer than those of AMPA‐mediated EPSCs recorded before GYKI 52466 application. KAR activation inhibited EPSCs. This inhibition was associated with a significant increase in paired‐pulse facilitation ratio, suggesting a presynaptic action of KAR. KAR inhibition of EPSCs was blocked by the G‐protein inhibitor, N‐ethylmaleimide (NEM), or the protein kinase C (PKC) inhibitor calphostin C. Our results demonstrate that KAR activation has dual effects on glutamatergic transmission in the rat GP: (1) it mediates small‐amplitude EPSCs; and (2) it reduces glutamatergic synaptic transmission through a presynaptic G‐protein coupled, PKC‐dependent, metabotropic mechanism. These findings provide evidence for the multifarious functions of KARs in regulating synaptic transmission, and open up the possibility for the development of pharmacotherapies to reduce the hyperactive subthalamofugal projection in Parkinsons disease.

Differential localization and function of GABA transporters, GAT-1 and GAT-3, in the rat globus pallidus

GABA transporter subtype 1 (GAT‐1) and GABA transporter subtype 3 (GAT‐3) are the main transporters that regulate inhibitory GABAergic transmission in the mammalian brain through GABA reuptake. In this study, we characterized the ultrastructural localizations and determined the respective roles of these transporters in regulating evoked inhibitory postsynaptic currents (eIPSCs) in globus pallidus (GP) neurons after striatal stimulation. In the young and adult rat GP, GAT‐1 was preferentially expressed in unmyelinated axons, whereas GAT‐3 was almost exclusively found in glial processes. Except for rare instances of GAT‐1 localization, neither of the two transporters was significantly expressed in GABAergic terminals in the rat GP. 1‐(4,4‐Diphenyl‐3‐butenyl)‐3‐piperidinecarboxylic acid hydrochloride (SKF 89976A) (10 μm), a GAT‐1 inhibitor, significantly prolonged the decay time, but did not affect the amplitude, of eIPSCs induced by striatal stimulation (15–20 V). On the other hand, the semi‐selective GAT‐3 inhibitor 1‐(2‐[tris(4‐methoxyphenyl)methoxy]ethyl)‐(S)‐3‐piperidinecarboxylic acid (SNAP 5114) (10 μm) increased the amplitude and prolonged the decay time of eIPSCs. The effects of transporter blockade on the decay time and amplitude of eIPSCs were further increased when both inhibitors were applied together. Furthermore, SKF 89976A or SNAP 5114 blockade also increased the amplitude and frequency of spontaneous IPSCs, but did not affect miniature IPSCs. Significant GABAA receptor‐mediated tonic currents were induced in the presence of high concentrations of both SKF 89976A (30 μm) and SNAP 5114 (30 μm). In conclusion, these data indicate that GAT‐1 and GAT‐3 represent different target sites through which GABA reuptake may subserve complementary regulation of GABAergic transmission in the rat GP.

Localization and Function of GABA Transporters GAT-1 and GAT-3 in the Basal Ganglia

GABA transporter type 1 and 3 (GAT-1 and GAT-3, respectively) are the two main subtypes of GATs responsible for the regulation of extracellular GABA levels in the central nervous system. These transporters are widely expressed in neuronal (mainly GAT-1) and glial (mainly GAT-3) elements throughout the brain, but most data obtained so far relate to their role in the regulation of GABAA receptor-mediated postsynaptic tonic and phasic inhibition in the hippocampus, cerebral cortex and cerebellum. Taking into consideration the key role of GABAergic transmission within basal ganglia networks, and the importance for these systems to be properly balanced to mediate normal basal ganglia function, we analyzed in detail the localization and function of GAT-1 and GAT-3 in the globus pallidus of normal and Parkinsonian animals, in order to further understand the substrate and possible mechanisms by which GABA transporters may regulate basal ganglia outflow, and may become relevant targets for new therapeutic approaches for the treatment of basal ganglia-related disorders. In this review, we describe the general features of GATs in the basal ganglia, and give a detailed account of recent evidence that GAT-1 and GAT-3 regulation can have a major impact on the firing rate and pattern of basal ganglia neurons through pre- and post-synaptic GABAA- and GABAB-receptor-mediated effects.

Activation of presynaptic kainate receptors suppresses GABAergic synaptic transmission in the rat globus pallidus

The globus pallidus (GP) plays a central integrative role in the basal ganglia circuitry. It receives strong GABAergic inputs from the striatum (Str) and significant glutamatergic afferents from the subthalamic nucleus (STN). The change in firing rate and pattern of GP neurons is a cardinal feature of Parkinsons disease pathophysiology. Kainate receptor (KAR) GluR6/7 subunit immunoreactivity is expressed presynaptically in GABAergic striatopallidal terminals which provides a substrate for regulation of GABAergic transmission in GP. To test this hypothesis, we recorded GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) in the GP following electrical stimulation of the Str. Following blockade of AMPA and N-methyl-d-aspartate receptors with selective antagonists, bath application of kainate (KA) (0.3-3 microM) reduced significantly the amplitude of evoked IPSCs. This inhibition was associated with a significant increase in paired-pulse facilitation ratio and a reduction of the frequency, but not amplitude, of miniature inhibitory postsynaptic currents (mIPSCs), suggesting a presynaptic site of KA action. The KA effects on striatopallidal GABAergic transmission were blocked by the G-protein inhibitor, N-ethylmaleimide (NEM), or protein kinase C (PKC) inhibitor calphostin C. Our results demonstrate that KAR activation inhibits GABAergic transmission through a presynaptic G protein-coupled, PKC-dependent metabotropic mechanism in the rat GP. These findings open up the possibility for the development of KA-mediated pharmacotherapies aimed at decreasing the excessive and abnormally regulated inhibition of GP neurons in Parkinsons disease.

A role for 5HT3 receptors in visual processing in the mammalian retina

We investigated the role of 5HT3 receptors in the mammalian retina using electrophysiological techniques to monitor ganglion cell activity. Activation of 5HT3 receptors with the selective agonist 1-phenylbiguanide (PBG) increased the ON responses of ON-center ganglion cells, while decreasing the OFF responses of OFF-center cells. The application of a selective 5HT3 antagonist had a reciprocal effect, namely it reduced the center response in ON-center cells and concomitantly increased the center responses in OFF-center cells. Since putative serotoninergic amacrine cells in the retina are connected specifically to the rod bipolar cell, these agents most likely affect the rod bipolar terminal. These data, together with previous studies, suggest that both 5HT2 and 5HT3 receptors mediate an excitatory influence which serves to facilitate the output from rod bipolar cells, the former via a phosphatidyl inositol second-messenger system, and the latter via a direct ion channel.

A differential effect of APB on ON- and OFF-center ganglion cells in the dark adapted rabbit retina.

The glutamate analog, 2-amino-4-phosphonobutyric acid (APB) is a proven tool in exploring the retinal circuit; it has been shown to interfere specifically with the transmission from photoreceptor to depolarizing bipolar cell. Consequently, in photopic retinae, the application of APB disrupts the ON-channel leaving the OFF-channel undisturbed; on the other hand, in the scotopic state, APB application blocks all ganglion cell responses. In this paper, we will show that the ON- and OFF-channels have a differential sensitivity to application of APB. That is to say, APB blocks center responses in ON-ganglion cells at mean concentration of 22 +/- 5.1 microM (mean +/- standard error of the mean; n = 15) and in OFF-ganglion cells at mean concentration of 91 +/- 15.5 microM (n = 16). Since considerable data rule out direct effects of APB on ganglion cells, we hypothesize that this effect is due to a difference in the synaptic gain of ON and OFF pathways in the inner retina.

GABA transporter subtype 1 and GABA transporter subtype 3 modulate glutamatergic transmission via activation of presynaptic GABA(B) receptors in the rat globus pallidus.

The intra‐pallidal application of γ‐aminobutyric acid (GABA) transporter subtype 1 (GAT‐1) or GABA transporter subtype 3 (GAT‐3) transporter blockers [1‐(4,4‐diphenyl‐3‐butenyl)‐3‐piperidinecarboxylic acid hydrochloride (SKF 89976A) or 1‐[2‐[tris(4‐methoxyphenyl)methoxy]ethyl]‐(S)‐3‐piperidinecarboxylic acid (SNAP 5114)] reduces the activity of pallidal neurons in monkey. This effect could be mediated through the activation of presynaptic GABAB heteroreceptors in glutamatergic terminals by GABA spillover following GABA transporter (GAT) blockade. To test this hypothesis, we applied the whole‐cell recording technique to study the effects of SKF 89976A and SNAP 5114 on evoked excitatory postsynaptic currents (eEPSCs) in the presence of gabazine, a GABAA receptor antagonist, in rat globus pallidus slice preparations. Under the condition of postsynaptic GABAB receptor blockade by the intra‐cellular application of N‐(2,6‐dimethylphenylcarbamoylmethyl)‐triethylammonium bromide (OX314), bath application of SKF 89976A (10 μm) or SNAP 5114 (10 μm) decreased the amplitude of eEPSCs, without a significant effect on its holding current and whole cell input resistance. The inhibitory effect of GAT blockade on eEPSCs was blocked by (2S)‐3‐[[(1S)‐1‐(3,4‐dichlorophenyl)ethyl]amino‐2‐hydroxypropyl](phenylmethyl)phosphinic acid, a GABAB receptor antagonist. The paired‐pulse ratio of eEPSCs was increased, whereas the frequency, but not the amplitude, of miniature excitatory postsynaptic currents was reduced in the presence of either GAT blocker, demonstrating a presynaptic effect. These results suggest that synaptically released GABA can inhibit glutamatergic transmission through the activation of presynaptic GABAB heteroreceptors following GAT‐1 or GAT‐3 blockade. In conclusion, our findings demonstrate that presynaptic GABAB heteroreceptors in putative glutamatergic subthalamic afferents to the globus pallidus are sensitive to increases in extracellular GABA induced by GAT inactivation, thereby suggesting that GAT blockade represents a potential mechanism by which overactive subthalamopallidal activity may be reduced in parkinsonism.

The GABA Transporters GAT-1 and GAT-3 modulate glutamatergic transmission via activation of presynaptic GABAB receptors in the rat globus pallidus

The intra‐pallidal application of γ‐aminobutyric acid (GABA) transporter subtype 1 (GAT‐1) or GABA transporter subtype 3 (GAT‐3) transporter blockers [1‐(4,4‐diphenyl‐3‐butenyl)‐3‐piperidinecarboxylic acid hydrochloride (SKF 89976A) or 1‐[2‐[tris(4‐methoxyphenyl)methoxy]ethyl]‐(S)‐3‐piperidinecarboxylic acid (SNAP 5114)] reduces the activity of pallidal neurons in monkey. This effect could be mediated through the activation of presynaptic GABAB heteroreceptors in glutamatergic terminals by GABA spillover following GABA transporter (GAT) blockade. To test this hypothesis, we applied the whole‐cell recording technique to study the effects of SKF 89976A and SNAP 5114 on evoked excitatory postsynaptic currents (eEPSCs) in the presence of gabazine, a GABAA receptor antagonist, in rat globus pallidus slice preparations. Under the condition of postsynaptic GABAB receptor blockade by the intra‐cellular application of N‐(2,6‐dimethylphenylcarbamoylmethyl)‐triethylammonium bromide (OX314), bath application of SKF 89976A (10 μm) or SNAP 5114 (10 μm) decreased the amplitude of eEPSCs, without a significant effect on its holding current and whole cell input resistance. The inhibitory effect of GAT blockade on eEPSCs was blocked by (2S)‐3‐[[(1S)‐1‐(3,4‐dichlorophenyl)ethyl]amino‐2‐hydroxypropyl](phenylmethyl)phosphinic acid, a GABAB receptor antagonist. The paired‐pulse ratio of eEPSCs was increased, whereas the frequency, but not the amplitude, of miniature excitatory postsynaptic currents was reduced in the presence of either GAT blocker, demonstrating a presynaptic effect. These results suggest that synaptically released GABA can inhibit glutamatergic transmission through the activation of presynaptic GABAB heteroreceptors following GAT‐1 or GAT‐3 blockade. In conclusion, our findings demonstrate that presynaptic GABAB heteroreceptors in putative glutamatergic subthalamic afferents to the globus pallidus are sensitive to increases in extracellular GABA induced by GAT inactivation, thereby suggesting that GAT blockade represents a potential mechanism by which overactive subthalamopallidal activity may be reduced in parkinsonism.

GABA transporter subtype 1 and GABA transporter subtype 3 modulate glutamatergic transmission via activation of presynaptic GABAB receptors in the rat globus pallidus: GABA transporter function in globus pallidus

The intra‐pallidal application of γ‐aminobutyric acid (GABA) transporter subtype 1 (GAT‐1) or GABA transporter subtype 3 (GAT‐3) transporter blockers [1‐(4,4‐diphenyl‐3‐butenyl)‐3‐piperidinecarboxylic acid hydrochloride (SKF 89976A) or 1‐[2‐[tris(4‐methoxyphenyl)methoxy]ethyl]‐(S)‐3‐piperidinecarboxylic acid (SNAP 5114)] reduces the activity of pallidal neurons in monkey. This effect could be mediated through the activation of presynaptic GABAB heteroreceptors in glutamatergic terminals by GABA spillover following GABA transporter (GAT) blockade. To test this hypothesis, we applied the whole‐cell recording technique to study the effects of SKF 89976A and SNAP 5114 on evoked excitatory postsynaptic currents (eEPSCs) in the presence of gabazine, a GABAA receptor antagonist, in rat globus pallidus slice preparations. Under the condition of postsynaptic GABAB receptor blockade by the intra‐cellular application of N‐(2,6‐dimethylphenylcarbamoylmethyl)‐triethylammonium bromide (OX314), bath application of SKF 89976A (10 μm) or SNAP 5114 (10 μm) decreased the amplitude of eEPSCs, without a significant effect on its holding current and whole cell input resistance. The inhibitory effect of GAT blockade on eEPSCs was blocked by (2S)‐3‐[[(1S)‐1‐(3,4‐dichlorophenyl)ethyl]amino‐2‐hydroxypropyl](phenylmethyl)phosphinic acid, a GABAB receptor antagonist. The paired‐pulse ratio of eEPSCs was increased, whereas the frequency, but not the amplitude, of miniature excitatory postsynaptic currents was reduced in the presence of either GAT blocker, demonstrating a presynaptic effect. These results suggest that synaptically released GABA can inhibit glutamatergic transmission through the activation of presynaptic GABAB heteroreceptors following GAT‐1 or GAT‐3 blockade. In conclusion, our findings demonstrate that presynaptic GABAB heteroreceptors in putative glutamatergic subthalamic afferents to the globus pallidus are sensitive to increases in extracellular GABA induced by GAT inactivation, thereby suggesting that GAT blockade represents a potential mechanism by which overactive subthalamopallidal activity may be reduced in parkinsonism.