Postsynaptic GABA Research Articles

Morphine history sensitizes postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Previous work has shown that the dorsal raphe nucleus (DR)-5-HT system is inhibited by swim stress via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor (CRF). Additionally, the DR 5-HT system is regulated by opioids. The present study tests the hypothesis that the DR 5-HT system regulates stress-induced opioid relapse. In the first experiment, electrophysiological recordings of GABA synaptic activity in 5-HT DR neurons were conducted in brain slices from Sprague-Dawley rats that were exposed to swim stress-induced reinstatement of previously extinguished morphine conditioned place preference (CPP). Behavioral data indicate that swim stress triggers reinstatement of morphine CPP. Electrophysiology data indicate that 5-HT neurons in the morphine-conditioned group exposed to stress had increased amplitude of inhibitory postsynaptic currents (IPSCs), which would indicate greater postsynaptic GABA receptor density and/or sensitivity, compared to saline controls exposed to stress. In the second experiment, rats were exposed to either morphine or saline CPP and extinction, and then 5-HT DR neurons from both groups were examined for sensitivity to CRF in vitro. CRF induced a greater inward current in 5-HT neurons from morphine-conditioned subjects compared to saline-conditioned subjects. These data indicate that morphine history sensitizes 5-HT DR neurons to the GABAergic inhibitory effects of stress as well as to some of the effects of CRF. These mechanisms may sensitize subjects with a morphine history to the dysphoric effects of stressors and ultimately confer an enhanced vulnerability to stress-induced opioid relapse.

Modulation of presynaptic GABAA receptors by endogenous neurosteroids

Although 3α-hydroxy, 5α-reduced pregnane steroids, such as allopregnanolone (AlloP) and tetrahydrodeoxycorticosterone, are endogenous positive modulators of postsynaptic GABA(A) receptors, the functional roles of endogenous neurosteroids in synaptic transmission are still largely unknown. In this study, the effect of AlloP on spontaneous glutamate release was examined in mechanically isolated dentate gyrus hilar neurons by use of the conventional whole-cell patch-clamp technique. AlloP increased the frequency of glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) in a dose-dependent manner. The AlloP-induced increase in sEPSC frequency was completely blocked by a non-competitive GABA(A) receptor blocker, tetrodotoxin or Cd(2+) , suggesting that AlloP acts on presynaptic GABA(A) receptors to depolarize presynaptic nerve terminals to increase the probability of spontaneous glutamate release. On the other hand, γ-cyclodextrin (γ-CD) significantly decreased the basal frequency of sEPSCs. However, γ-CD failed to decrease the basal frequency of sEPSCs in the presence of a non-competitive GABA(A) receptor antagonist or tetrodotoxin. In addition, γ-CD failed to decrease the basal frequency of sEPSCs after blocking the synthesis of endogenous 5α-reduced pregnane steroids. Furthermore, γ-CD decreased the extent of muscimol-induced increase in sEPSC frequency, suggesting that endogenous neurosteroids can directly activate and/or potentiate presynaptic GABA(A) receptors to affect spontaneous glutamate release onto hilar neurons. The modulation of presynaptic GABA(A) receptors by endogenous neurosteroids might affect the excitability of the dentate gyrus-hilus-CA3 network, and thus contribute, at least in part, to some pathological conditions, such as catamenial epilepsy and premenstrual dysphoric disorder.

Estrous Cycle- and Sex-Dependent Changes in Pre- and Postsynaptic GABABControl of GnRH Neuron Excitability

The GnRH neurons are the key neurons controlling fertility in mammals. Although γ-aminobutyric acid (GABA) plays an important role in the regulation of GnRH neurons, the role of GABA(B) receptors is poorly understood. Using GnRH-green fluorescent protein transgenic mice and a parahorizontal brain slice preparation, we have undertaken a series of electrophysiological experiments to examine 1) postsynaptic GABA(B) receptors expressed by GnRH neurons, and 2) presynaptic GABA(B) receptors located on the terminals of an important neural input to GnRH neurons originating from the anteroventral periventricular nucleus (AVPV). The GABA(B) receptor agonist baclofen induced a direct postsynaptic hyperpolarization of GnRH neurons through induction of an outward current blocked by barium. Baclofen also acted presynaptically to suppress AVPV-activated GABA- and glutamate-evoked postsynaptic currents in GnRH neurons. The number of GnRH neurons exhibiting postsynaptic GABA(B) receptors was significantly (P < 0.05) different in males (22%) and females (70%), whereas presynaptic GABA(B) modulation of AVPV afferents was the same in the two sexes. Across the estrous cycle, a striking approximately 70% reduction (P < 0.05) in presynaptic GABA(B) modulation of AVPV afferents to GnRH neurons was found on proestrus compared with diestrus and estrus. In contrast, postsynaptic GABA(B) receptors did not change. Together, these findings show that GABA(B) receptors are active at both pre- and postsynaptic sites to modulate the excitability of GnRH neurons. The balance of this pre- and postsynaptic activity is different between the sexes and changes in a dynamic manner across the estrous cycle.

Rapid Target-Specific Remodeling of Fast-Spiking Inhibitory Circuits after Loss of Dopamine

In Parkinson's disease (PD), dopamine depletion alters neuronal activity in the direct and indirect pathways and leads to increased synchrony in the basal ganglia network. However, the origins of thesechanges remain elusive. Because GABAergic interneurons regulate activity of projection neurons andpromote neuronal synchrony, we recorded from pairs of striatal fast-spiking (FS) interneurons and direct- or indirect-pathway MSNs after dopamine depletion with 6-OHDA. Synaptic properties ofFS-MSN connections remained similar, yet within 3days of dopamine depletion, individual FS cells doubled their connectivity to indirect-pathway MSNs, whereas connections to direct-pathway MSNs remained unchanged. A model of the striatal microcircuit revealed that such increases in FS innervation were effective at enhancing synchrony within targeted cell populations. These data suggest that after dopamine depletion, rapid target-specific microcircuit organization in the striatum may lead to increased synchrony of indirect-pathway MSNs that contributes to pathological network oscillations and motor symptoms of PD.

Inhibitory Synapse Dynamics: Coordinated Presynaptic and Postsynaptic Mobility and the Major Contribution of Recycled Vesicles to New Synapse Formation

Dynamics of GABAergic synaptic components have been studied previously over milliseconds to minutes, revealing mobility of postsynaptic scaffolds and receptors. Here we image inhibitory synapses containing fluorescently tagged postsynaptic scaffold Gephyrin, together with presynaptic vesicular GABA transporter (VGAT) or postsynaptic GABA(A) receptor γ2 subunit (GABA(A)Rγ2), over seconds to days in cultured rat hippocampal neurons, revealing modes of inhibitory synapse formation and remodeling. Entire synapses were mobile, translocating rapidly within a confined region and exhibiting greater nonstochastic motion over multihour periods. Presynaptic and postsynaptic components moved in unison, maintaining close apposition while translocating distances of several micrometers. An observed flux in the density of synaptic puncta partially resulted from the apparent merging and splitting of preexisting clusters. De novo formation of inhibitory synapses was observed, marked by the appearance of stably apposed Gephyrin and VGAT clusters at sites previously lacking either component. Coclustering of GABA(A)Rγ2 supports the identification of such new clusters as synapses. Nascent synapse formation occurred by gradual accumulation of components over several hours, with VGAT clustering preceding that of Gephyrin and GABA(A)Rγ2. Comparing VGAT labeling by active uptake of a luminal domain antibody with post hoc immunocytochemistry indicated that recycling vesicles from preexisting boutons significantly contribute to vesicle pools at the majority of new inhibitory synapses. Although new synapses formed primarily on dendrite shafts, some also formed on dendritic protrusions, without apparent interconversion. Altogether, the long-term imaging of GABAergic presynaptic and postsynaptic components reveals complex dynamics and perpetual remodeling with implications for mechanisms of assembly and synaptic integration.

Differential Regulation of the Postsynaptic Clustering of γ-Aminobutyric Acid Type A (GABAA) Receptors by Collybistin Isoforms

Collybistin promotes submembrane clustering of gephyrin and is essential for the postsynaptic localization of gephyrin and γ-aminobutyric acid type A (GABA(A)) receptors at GABAergic synapses in hippocampus and amygdala. Four collybistin isoforms are expressed in brain neurons; CB2 and CB3 differ in the C terminus and occur with and without the Src homology 3 (SH3) domain. We have found that in transfected hippocampal neurons, all collybistin isoforms (CB2(SH3+), CB2(SH3-), CB3(SH3+), and CB3(SH3-)) target to and concentrate at GABAergic postsynapses. Moreover, in non-transfected neurons, collybistin concentrates at GABAergic synapses. Hippocampal neurons co-transfected with CB2(SH3-) and gephyrin developed very large postsynaptic gephyrin and GABA(A) receptor clusters (superclusters). This effect was accompanied by a significant increase in the amplitude of miniature inhibitory postsynaptic currents. Co-transfection with CB2(SH3+) and gephyrin induced the formation of many (supernumerary) non-synaptic clusters. Transfection with gephyrin alone did not affect cluster number or size, but gephyrin potentiated the clustering effect of CB2(SH3-) or CB2(SH3+). Co-transfection with CB2(SH3-) or CB2(SH3+) and gephyrin did not affect the density of presynaptic GABAergic terminals contacting the transfected cells, indicating that collybistin is not synaptogenic. Nevertheless, the synaptic superclusters induced by CB2(SH3-) and gephyrin were accompanied by enlarged presynaptic GABAergic terminals. The enhanced clustering of gephyrin and GABA(A) receptors induced by collybistin isoforms was not accompanied by enhanced clustering of neuroligin 2. Moreover, during the development of GABAergic synapses, the clustering of gephyrin and GABA(A) receptors preceded the clustering of neuroligin 2. We propose a model in which the SH3- isoforms play a major role in the postsynaptic accumulation of GABA(A) receptors and in GABAergic synaptic strength.

GABA inhibits excitatory neurotransmission in rat pelvic ganglia.

Intravenous GABA inhibited, in a bicuculline-sensitive manner, contractions of the urinary bladder induced by preganglionic nerve stimulation in urethane anaesthetized rats, while it had no significant effect on contractions produced by postganglionic nerve stimulation. In addition, intravenous GABA strongly inhibited DMPP-induced bladder contractions; this effect was also prevented by bicuculline. These experiments suggest that GABA inhibits excitatory neurotransmission in rat pelvic ganglia through a mechanism involving, at least in part, the activation of postsynaptic GABA A receptors.

A question of balance – Positive versus negative allosteric modulation of GABA A receptor subtypes as a driver of analgesic efficacy in rat models of inflammatory and neuropathic pain

After injury GABA A receptor positive allosteric modulators (PAMs) mediate robust analgesia in animals via putative restoration of post-synaptic GABA A-α2 and -α3 receptor function within the spinal cord. GABA can also act at GABA A receptors localized on primary afferent neurones to inhibit presynaptic neurotransmitter release and produce analgesia via a process called primary afferent depolarization (PAD). Some forms of injury might sufficiently enhance PAD to shift it into a net excitatory process. Thus, negative allosteric modulators (NAMs) might also possess analgesic activity. We have compared compounds capable of either positively or negatively modulating GABA A receptors in rat models associated with injury-induced central sensitization. The subtype-selective PAMs NS11394 (1–10 mg/kg) and TPA023 (3–30 mg/kg) attenuated formalin-induced nocifensive behaviours. Similarly, both compounds reversed hindpaw mechanical hypersensitivity and weight bearing deficits in carrageenan-inflamed and nerve-injured rats. The non-selective PAM diazepam (1–5 mg/kg) was ineffective in all models. Surprisingly, both the non-selective NAM FG-7142 (3–30 mg/kg) and the α5-selective NAM α5IA-II (10–60 mg/kg) also attenuated formalin-induced nocifensive behaviours. In carrageenan-inflamed rats α5IA-II reversed mechanical hypersensitivity and weight bearing deficits whilst FG-7142 only attenuated weight bearing deficits. This picture was essentially reversed in nerve-injured rats for these two NAMs. With the exception of NS11394, all compounds attenuated exploratory motility behaviour in rats, either as a consequence of sedative or anxiogenic-like side-effects. These data indicate that the preferred selectivity and activity profiles for mediating analgesia upon activation of GABA A receptors might be more complex than previously anticipated, and is worthy of further exploration.

Prolonged cannabinoid exposure alters GABA A receptor mediated synaptic function in cultured hippocampal neurons

Developing cannabinoid-based medication along with marijuana's recreational use makes it important to investigate molecular adaptations the endocannabinoid system undergoes following prolonged use and withdrawal. Repeated cannabinoid administration results in development of tolerance and produces withdrawal symptoms that may include seizures. Here we employed electrophysiological and immunochemical techniques to investigate the effects of prolonged CB1 receptor agonist exposure on cultured hippocampal neurons. Approximately 60% of CB1 receptors colocalize to GABAergic terminals in hippocampal cultures. Prolonged treatment with the cannabinamimetic WIN 55,212-2 (+ WIN, 1 μM, 24 h) caused profound CB1 receptor downregulation accompanied by neuronal hyperexcitability. Furthermore, prolonged + WIN treatment resulted in increased GABA release as indicated by increased mIPSC frequency, a diminished GABAergic inhibition as indicated by reduction in mIPSC amplitude and a reduction in GABA A channel number. Additionally, surface staining for the GABA A β 2/3 receptor subunits was decreased, while no changes in staining for the presynaptic vesicular GABA transporter were observed, indicating that GABAergic terminals remained intact. These findings demonstrate that agonist-induced downregulation of the CB1 receptor in hippocampal cultures results in neuronal hyperexcitability that may be attributed, in part, to alterations in both presynaptic GABA release mechanisms and postsynaptic GABA A receptor function demonstrating a novel role for cannabinoid-dependent presynaptic control of neuronal transmission.

An Historical Perspective on Gabaergic Drugs

In 1950, γ-aminobutyric acid (GABA) was discovered in the brain and in 1967 it was recognized as an inhibitory neurotransmitter. The discovery of the benzodiazepines Librium® (launched in 1960) and Valium® by Sternbach initiated huge research activities resulting in 50 marketed drugs. In 1975, Haefely found that GABA is involved in the actions of benzodiazepines. The baclofen-sensitive, bicuculline-insensitive GABA(B) receptor was discovered by Bowery in 1980, and the baclofen-insensitive, bicuculline-insensitive GABA(C) receptor by Johnston in 1984. Barnard & Seeburg reported the cloning of the GABA(A) receptor in 1987, Cutting the GABA(C) receptor in 1991 and Bettler the GABA(B1a) and GABA(B1b) receptors in 1997. Six groups cloned the GABA(B2) receptor in 1998/1999 showing that the GABA(B) receptor functions as a heterodimer with GABA(B1b)/GABA(B2) mediating postsynaptic inhibition and GABA(B1a)/GABA(B2) mediating presynaptic inhibition. Möhler and McKernan dissected the pharmacology of the benzodiazepine-receptor subtypes. Antagonists and positive allosteric modulators of GABA(B) receptors were discovered in 1987 and 2001, respectively. GABA transporter inhibitor, tiagabine, was launched in 1996, a GABA aminotransferase inhibitor, vigabatrin, in 1998 and a glutamic acid decarboxylase activator, pregabalin, in 2004. Most recently, brain-penetrating GABA(C)-receptor antagonists were reported in 2009.

Dopamine D3-Like Receptors Modulate Anxiety-Like Behavior and Regulate GABAergic Transmission in the Rat Lateral/Basolateral Amygdala

Central among the brain regions that regulate fear/anxiety behaviors is the lateral/basolateral amygdala (BLA). BLA output is tightly controlled by the relative activity of two populations of inhibitory GABAergic interneurons, local feedback cells distributed throughout the nucleus, and feedforward cells found along the lateral paracapsular border of this subdivision. Recent studies suggest that dopamine (DA) can modulate the BLA GABAergic system, thus linking fear/anxiety states with mesolimbic reward/attentional processes. However, the precise dopaminergic mechanisms regulating the activity of the two BLA GABAergic neuron populations have not been fully explored. We therefore examined the effects of DA D3-like receptors on BLA-dependent anxiety-like behavior and neurophysiology. After confirming the presence of D3-like receptors within the BLA, we found that microinjection of a D3-selective antagonist into the BLA decreased anxiety-like behavior expressed in both the light/dark transition test and the elevated plus maze. Consistent with this, we found that in vitro D3-like receptor activation selectively inhibits synaptic transmission at both BLA feedback and feedforward GABAergic interneuron populations, with no effect on glutamatergic transmission. This inhibition of GABAergic transmission is a result of a D3-like receptor-mediated, dynamin-dependent process that presumably reflects endocytosis of postsynaptic GABA(A) receptors found on principal BLA neurons. Because environmental cues alter both DA release and relative activity states of the BLA, our data strongly suggest that DA, potentially acting through D3-like receptors, may suppress the relative contribution by inhibitory processes in the BLA and modify the expression of BLA-related behaviors.

Dopaminergic Presynaptic Modulation of Nigral Afferents: Its Role in the Generation of Recurrent Bursting in Substantia Nigra Pars Reticulata Neurons

Previous work has shown the functions associated with activation of dopamine presynaptic receptors in some substantia nigra pars reticulata (SNr) afferents: (i) striatonigral terminals (direct pathway) posses presynaptic dopamine D1-class receptors whose action is to enhance inhibitory postsynaptic currents (IPSCs) and GABA transmission. (ii) Subthalamonigral terminals posses D1- and D2-class receptors where D1-class receptor activation enhances and D2-class receptor activation decreases excitatory postsynaptic currents. Here we report that pallidonigral afferents posses D2-class receptors (D3 and D4 types) that decrease inhibitory synaptic transmission via presynaptic modulation. No action of D1-class agonists was found on pallidonigral synapses. In contrast, administration of D1-receptor antagonists greatly decreased striatonigral IPSCs in the same preparation, suggesting that tonic dopamine levels help in maintaining the function of the striatonigral (direct) pathway. When both D3 and D4 type receptors were blocked, pallidonigral IPSCs increased in amplitude while striatonigral connections had no significant change, suggesting that tonic dopamine levels are repressing a powerful inhibition conveyed by pallidonigral synapses (a branch of the indirect pathway). We then blocked both D1- and D2-class receptors to acutely decrease direct pathway (striatonigral) and enhance indirect pathways (subthalamonigral and pallidonigral) synaptic force. The result was that most SNr projection neurons entered a recurrent bursting firing mode similar to that observed during Parkinsonism in both patients and animal models. These results raise the question as to whether the lack of dopamine in basal ganglia output nuclei is enough to generate some pathological signs of Parkinsonism.

Multiple effects of allopregnanolone on GABAergic responses in single hippocampal CA3 pyramidal neurons

3α-Hydroxy, 5α-reduced pregnane steroids, such as allopregnanolone, are potent modulators of GABA A receptors and have many biological responses including sedative, anxiolytic, anticonvulsant and anesthetic actions. In the present study, we have investigated the effects of allopregnanolone on GABA A receptors in acutely isolated single hippocampal CA3 pyramidal neurons using the whole cell patch-clamp technique. Allopregnanolone induced membrane Cl − currents in a concentration-dependent manner, and the allopregnanolone-induced currents (I AlloP) were blocked by noncompetitive GABA A receptor antagonists. The I AlloP was not affected by the intracellular loading of γ-cyclodextrin (γ-CD), which efficiently sequesters several kinds of endogenous neurosteroids including allopregnanolone, suggesting that allopregnanolone accesses extracellular but not intracellular sites to activate GABA A receptors. Allopregnanolone prolonged the decay time constant of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs), suggesting that allopregnanolone modulates the desensitization kinetics of postsynaptic GABA A receptors. The picrotoxin-sensitive tonic currents (I tonic), which were mediated by extrasynaptic GABA A receptors, were recorded from CA3 pyramidal neurons. The intracellular loading of γ-CD or allopregnanolone significantly decreased or increased the amplitude of picrotoxin-sensitive I tonic, respectively, suggesting that endogenous neurosteroids might, at least in part, be involved in the generation of picrotoxin-sensitive I tonic. Allopregnanolone also increased the frequency of GABAergic sIPSCs, in a manner dependent on the integrity of voltage-dependent Na + and Ca 2+ channels, suggesting that allopregnanolone activates presynaptic GABA A receptors to depolarize GABAergic nerve terminals. The present results suggest that allopregnanolone exerts its pharmacological and pathophysiological actions via the modulation of multiple types of GABA A receptor-mediated responses.

Irregular RNA splicing curtails postsynaptic gephyrin in the cornu ammonis of patients with epilepsy

Anomalous hippocampal inhibition is involved in temporal lobe epilepsy, and reduced gephyrin immunoreactivity in the temporal lobe epilepsy hippocampus has been reported recently. However, the mechanisms responsible for curtailing postsynaptic gephyrin scaffolds are poorly understood. Here, we have investigated gephyrin expression in the hippocampus of patients with intractable temporal lobe epilepsy. Immunohistochemical and western blot analyses revealed irregular gephyrin expression in the cornu ammonis of patients with temporal lobe epilepsy and four abnormally spliced gephyrins lacking several exons in their G-domains were isolated. Identified temporal lobe epilepsy gephyrins have oligomerization deficits and they curtail hippocampal postsynaptic gephyrin and GABA(A) receptor α2 while interacting with regularly spliced gephyrins. We found that cellular stress (alkalosis and hyperthermia) induces exon skipping in gephyrin messenger RNA, which is responsible for curtailed postsynaptic gephyrin and GABA(A) receptor α2 scaffolds. Accordingly, we did not obtain evidence for gephyrin gene mutations in patients with temporal lobe epilepsy. Cellular stress such as alkalosis, for example arising from seizure activity, could thus facilitate the development of temporal lobe epilepsy by reducing GABA(A) receptor α2-mediated hippocampal synaptic transmission selectively in the cornu ammonis.

Open-label riluzole in fragile X syndrome

Objective. Glutamatergic dysregulation is implicated in the pathophysiology of fragile X syndrome (FXS). Riluzole is hypothesized to have an inhibitory effect on glutamate release, block excitotoxic effects of glutamate, and potentiate postsynaptic GABA(A) receptor function. Extracellular signal-related kinase (ERK) activation is known to be delayed in humans with FXS and knockout animal models of FXS. Correction of delayed ERK activation is a potential biomarker of treatment response in FXS. We conducted a six-week open-label prospective pilot study of riluzole (100 mg/day) in six adults with FXS. Methods. Riluzole was started at 50 mg every evening and then increased to 50 mg twice daily at week 2. The dose was kept constant for the final 4 weeks of the trial. Clinical response was determined by a score of 1 “very much improved” or 2 “much improved” on the Clinical Global Impressions Improvement (CGI-I) scale and a ≥ 25% improvement on the Children's Yale-Brown Obsessive Compulsive Scale modified for Pervasive Developmental Disorders. The primary target of treatment in this study was repetitive, compulsive behavior that commonly occurs in persons with FXS. The study incorporated an ERK activation biomarker assay. Potential adverse effects were assessed in a systematic manner at all clinic visits and by phone between visits. Results. Riluzole treatment was associated with clinical response in 1 of 6 subjects (17%). Among a number of secondary outcome measures employed, significant improvement was only noted on the ADHD Rating Scale-IV (became non-significant when corrected for multiple comparisons). Riluzole use was associated with significant correction in ERK activation time in all subjects (mean change from 3.82 ± 0.27 (baseline) to 2.99 ± 0.26 (endpoint) minutes; p = 0.007). Riluzole was well tolerated; mean increases in liver function tests occurred but drug discontinuation was not required. Conclusion. Overall, riluzole use was not associated with significant clinical improvement despite uniform correction of peripheral ERK activation. Future directions of study include testing of riluzole in animal models of FXS and assessment of psychotropic monotherapy on ERK activation.

Activation and Potentiation of Human GABAA Receptors by Non-Dioxin–Like PCBs Depends on Chlorination Pattern

The neurotoxic potential of non-dioxin-like polychlorinated biphenyls (NDL-PCBs) is characterized by disruption of presynaptic processes, including calcium homeostasis and neurotransmitter transport. Recently, using a limited set of congeners, we demonstrated that PCB28 and PCB52 can potentiate postsynaptic GABA(A) receptors. In the present study, effects of 20 NDL-PCBs and 2 dioxin-like PCBs, selected based on their chemical variation and abundance in the environment, on human GABA(A) receptors were investigated. GABA(A) receptors were expressed in Xenopus oocytes, and NDL-PCB effects were determined using the two-electrode voltage-clamp technique. Results demonstrate that lower chlorinated PCB19, PCB28, PCB47, PCB51, PCB52, PCB95, and PCB100 act as a partial agonists (at low receptor occupancy), i.e., potentiating the receptor response during coapplication with GABA (at EC(20)). Importantly, PCB19, PCB47, PCB51, and PCB100 can also act as full agonist, i.e., activate the GABA(A) receptor in the absence of GABA. Potentiation and activation of the GABA(A) receptor is concentration dependent and limited to NDL-PCBs that have 3-5 chlorine atoms, 1-3 ortho-substitutions, an equal number (0-1) of meta-substitutions on both phenyl rings, and do not have an adjacent para- and meta-substitution on the same phenyl ring. Activation and potentiation of the GABA(A) receptor by PCB47, the most potent congener (lowest observed effect concentration of 10nM), is attenuated when coapplied with PCB19, PCB28, PCB153, or PCB180, indicative for competitive binding. Considering the importance of GABA-ergic signaling for brain development, motor coordination, learning, and memory, this mode of action can contribute to the previously observed NDL-PCB-induced neurobehavioral and neurodevelopmental effects and should be included in human risk assessment.

Identification and characterization of Cs+‐permeable K+ channel current in mouse cerebellar Purkinje cells in lobules 9 and 10 evoked by molecular layer stimulation

The mouse cerebellum consists of 10 lobules, which are distinguishable by their anatomical and functional properties. However, the differences in the slow postsynaptic currents (sPSCs) of Purkinje cells between lobules have not been well studied. We recorded the sPSCs of lobules 3, 9 and 10 evoked by tetanic stimulation of the molecular layer in cerebellar slices, and found a novel outward sPSC mediated by the GABA(B) receptor in loblues 9 and 10 but hardly at all in lobule 3. We showed that the lobule-specific difference is at least partly attributable to differences in the density of GABAergic neurons (higher in lobule 10 than in lobules 3 and 9), and the functional expression level of postsynaptic GABA(B) receptor currents (larger in lobules 9 and 10 than in lobule 3). The G-protein-coupled inward rectifying K(+) channel (GIRK) is known to be activated by GABA(B) receptors; however, the outward sPSC was not blocked by a GIRK blocker, was not sensitive to Cs(+) block, and was observed when Cs(+) was used as a charge carrier. These results suggest that a K(+) channel other than GIRK could be activated by GABA(B) receptors. KCNK13 is a Cs(+)-permeable K(+) channel that shows intense expression of mRNA in Purkinje cells. KCNK13 current was enhanced by co-expression of G(βγ) subunits and was observed when Cs(+) was used as a charge carrier in heterologous expression systems, and the amino acids critical for these features were identified by mutagenesis. Taken together, these results show that KCNK13 is a legitimate candidate for the Cs(+)-permeable K(+) channel activated by GABA(B) receptors, presumably via G(βγ) subunits in Purkinje cells.

Slow GABA Transient and Receptor Desensitization Shape Synaptic Responses Evoked by Hippocampal Neurogliaform Cells

The kinetics of GABAergic synaptic currents can vary by an order of magnitude depending on the cell type. The neurogliaform cell (NGFC) has recently been identified as a key generator of slow GABA(A) receptor-mediated volume transmission in the isocortex. However, the mechanisms underlying slow GABA(A) receptor-mediated IPSCs and their use-dependent plasticity remain unknown. Here, we provide experimental and modeling data showing that hippocampal NGFCs generate an unusually prolonged (tens of milliseconds) but low-concentration (micromolar range) GABA transient, which is responsible for the slow response kinetics and which leads to a robust desensitization of postsynaptic GABA(A) receptors. This strongly contributes to the use-dependent synaptic depression elicited by various patterns of NGFC activity including the one detected during theta network oscillations in vivo. Synaptic depression mediated by NGFCs is likely to play an important modulatory role in the feedforward inhibition of CA1 pyramidal cells provided by the entorhinal cortex.

Wnt-5a Modulates Recycling of Functional GABAA Receptors on Hippocampal Neurons

GABA(A) receptors (GABA(A)-Rs) play a significant role in mediating fast synaptic inhibition and it is the main inhibitory receptor in the CNS. The role of Wnt signaling in coordinating synapse structure and function in the mature CNS is poorly understood. In previous studies we found that Wnt ligands can modulate excitatory synapses through remodeling both presynaptic and postsynaptic regions. In this current study we provide evidence for the effect of Wnt-5a on postsynaptic GABA(A)-Rs. We observed that Wnt-5a induces surface expression and maintenance of this receptor in the neuronal membrane. The evoked IPSC recordings in rat hippocampal slice indicate that Wnt-5a can regulates postsynaptically the hippocampal inhibitory synapses. We found also that Wnt-5a: (a) induces the insertion and clustering of GABA(A)-Rs in the membrane; (b) increases the amplitude of GABA-currents due exclusively to postsynaptic mechanisms; (c) does not affect the endocytic process, but increases the receptor recycling. Finally, all these effects on the GABA(A)-Rs are mediated by the activation of calcium/calmodulin-dependent kinase II (CaMKII). Therefore, we postulate that Wnt-5a, by activation of CaMKII, induces the recycling of functional GABA(A)-Rs on the mature hippocampal neurons.

Plasticity of postsynaptic, but not presynaptic, GABA B receptors inSSADH deficient mice

Succinic semialdehyde dehydrogenase (SSADH) deficiency is an autosomal-recessively inherited disorder of γ-aminobutyrate (GABA) catabolism characterized by ataxia and epilepsy. Since SSADH is responsible for GABA break-down downstream of GABA transaminase, patients manifest high extracellular levels of GABA, as well as the GABA B receptor (GABA BR) agonist γ-hydroxybutyrate (GHB). SSADH knockout (KO) mice display absence seizures, which progress into lethal tonic–clonic seizures at around 3 weeks of age. It is hypothesized that desensitization of GABA BRs plays an important role in the disease, although detailed studies of pre- and postsynaptic GABA BRs are not available. We performed patch-clamp recordings from layer 2/3 pyramidal neurons in neocortical brain slices of wild-type (WT) and SSADH KO mice. Electrical stimulation of GABAergic fibers during wash in of the GABA BR agonist baclofen revealed no difference in presynaptic GABA BR mediated inhibition of GABA release between WT and SSADH KO mice. In contrast, a significant decrease in postsynaptic baclofen-induced potassium currents was seen in SSADH KO mice. This reduction was unlikely to be caused by accumulation of potassium, GABA or GHB in the brain slices, or an altered expression of regulators of G-protein signaling (RGS) proteins. Finally, adenosine-induced potassium currents were also reduced in SSADH KO mice, which could suggest heterologous desensitization of the G-protein dependent effectors, leading to a reduction in G-protein coupled inwardly rectifying potassium (GIRK) channel responses. Our findings indicate that high GABA and GHB levels desensitize postsynaptic, but not certain presynaptic, GABA BRs, promoting a decrease in GIRK channel function. These changes could contribute to the development of seizures in SSADH KO mice and potentially also in affected patients.