Immunocytochemical localization of the alpha 6 subunit of the gamma-aminobutyric acidA receptor in the rat nervous system

Estradiol modulates the role of the spinal α6-subunit containing GABAA receptors in female rats with neuropathic pain

The purpose of this study was to investigate the mechanisms underlying sex differences in the role of spinal α6-subunit containing GABAA (α6GABAA) receptors in rats with neuropathic pain. Intrathecal 2,5-dihydro-7-methoxy-2-(4-methoxyphenyl)-3H-pyrazolo [4,3-c] quinoline-3-one (PZ-II-029, positive allosteric modulator of α6GABAA receptors) reduced tactile allodynia in female but not in male rats with neuropathic pain. PZ-II-029 was also more effective in females than males in inflammatory and nociplastic pain. Ovariectomy abated the antiallodynic effect of PZ-II-029 in neuropathic rats, whereas 17β-estradiol or 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol (PPT), estradiol receptor-α agonist, restored the effect of PZ-II-029 in ovariectomized rats. Blockade of estradiol receptor-α, using MPP (1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy) phenol]-1H-pyrazole dihydrochloride), prevented the effect of 17β-estradiol on PZ-II-029-induced antiallodynia in ovariectomized neuropathic females. Nerve injury reduced α6GABAA receptor protein expression at the dorsal root ganglia (DRG) and spinal cord of intact and ovariectomized female rats. In this last group, reconstitution with 17β-estradiol fully restored its expression in DRG and spinal cord. In male rats, nerve injury reduced α6GABAA receptor protein expression only at the spinal cord. Nerve injury enhanced estradiol receptor-α protein expression at the DRG in intact non-ovariectomized rats. However, ovariectomy decreased estradiol receptor-α protein expression at the DRG. In the spinal cord there were no changes in estradiol receptor-α protein expression. 17β-estradiol restored estradiol receptor-α protein expression at the DRG and increased it at the spinal cord of neuropathic rats. These data suggest that 17β-estradiol modulates the expression and function of the α6GABAA receptor through its interaction with estradiol receptor-α in female rats.

A Role for GABAA Receptor β3 Subunits in Mediating Harmaline Tremor Suppression by Alcohol: Implications for Essential Tremor Therapy

Background

Essential tremor patients may find that low alcohol amounts suppress tremor. A candidate mechanism is modulation of α6β3δ extra-synaptic GABAA receptors, that in vitro respond to non-intoxicating alcohol levels. We previously found that low-dose alcohol reduces harmaline tremor in wild-type mice, but not in littermates lacking δ or α6 subunits. Here we addressed whether low-dose alcohol requires the β3 subunit for tremor suppression.

Methods

We tested whether low-dose alcohol suppresses tremor in cre-negative mice with intact β3 exon 3 flanked by loxP, and in littermates in which this region was excised by cre expressed under the α6 subunit promotor. Tremor in the harmaline model was measured as a percentage of motion power in the tremor bandwidth divided by overall motion power.

Results

Alcohol, 0.500 and 0.575 g/kg, reduced harmaline tremor compared to vehicle-treated controls in floxed β3 cre- mice, but had no effect on tremor in floxed β3 cre+ littermates that have β3 knocked out. This was not due to potential interference of α6 expression by the insertion of the cre gene into the α6 gene since non-floxed β3 cre+ and cre- littermates exhibited similar tremor suppression by alcohol.

Discussion

As α6β3δ GABAA receptors are sensitive to low-dose alcohol, and cerebellar granule cells express β3 and are the predominant brain site for α6 and δ expression together, our overall findings suggest alcohol acts to suppress tremor by modulating α6β3δ GABAA receptors on these cells. Novel drugs that target this receptor may potentially be effective and well-tolerated for essential tremor.

Highlights

We previously found with the harmaline essential tremor model that GABAA receptors containing α6 and δ subunits mediate tremor suppression by alcohol. We now show that β3 subunits in α6-expressing cells, likely cerebellar granule cells, are also required, indicating that alcohol suppresses tremor by modulating α6β3δ extra-synaptic GABAA receptors.

Activation of α 6 -containing GABA A receptors induces antinociception under physiological and pathological conditions

ABSTRACT

The loss of GABAergic inhibition is a mechanism that underlies neuropathic pain. Therefore, rescuing the GABAergic inhibitory tone through the activation of GABA A receptors is a strategy to reduce neuropathic pain. This study was designed to elucidate the function of the spinal α 6 -containing GABA A receptor in physiological conditions and neuropathic pain in female and male rats. Results show that α 6 -containing GABA A receptor blockade or transient α 6 -containing GABA A receptor knockdown induces evoked hypersensitivity and spontaneous pain in naive female rats. The α 6 subunit is expressed in IB4 + and CGRP + primary afferent neurons in the rat spinal dorsal horn and dorsal root ganglia but not astrocytes. Nerve injury reduces α 6 subunit protein expression in the central terminals of the primary afferent neurons and dorsal root ganglia, whereas intrathecal administration of positive allosteric modulators of the α 6 -containing GABA A receptor reduces tactile allodynia and spontaneous nociceptive behaviors in female, but not male, neuropathic rats and mice. Overexpression of the spinal α 6 subunit reduces tactile allodynia and restores α 6 subunit expression in neuropathic rats. Positive allosteric modulators of the α 6 -containing GABA A receptor induces a greater antiallodynic effect in female rats and mice compared with male rats and mice. Finally, α 6 subunit is expressed in humans. This receptor is found in CGRP + and P2X3 + primary afferent fibers but not astrocytes in the human spinal dorsal horn. Our results suggest that the spinal α 6 -containing GABA A receptor has a sex-specific antinociceptive role in neuropathic pain, suggesting that this receptor may represent an interesting target to develop a novel treatment for neuropathic pain.

Neuroimmune interactions with binge alcohol drinking in the cerebellum of IL-6 transgenic mice

The neuroimmune system of the brain, which is comprised primarily of astrocytes and microglia, regulates a variety of homeostatic mechanisms that underlie normal brain function. Numerous conditions, including alcohol consumption, can disrupt this regulatory process by altering brain levels of neuroimmune factors. Alcohol and neuroimmune factors, such as proinflammatory cytokines IL-6 and TNF-alpha, act at similar targets in the brain, including excitatory and inhibitory synaptic transmission. Thus, alcohol-induced production of IL-6 and/or TNF-alpha could be important contributing factors to the effects of alcohol on the brain. Recent studies indicate that IL-6 plays a role in alcohol drinking and the effects of alcohol on the brain activity following the cessation of alcohol consumption (post-alcohol period), however information on these topics is limited. Here we used homozygous and heterozygous female and male transgenic mice with increased astrocyte expression of IL-6 to examined further the interactions between alcohol and IL-6 with respect to voluntary alcohol drinking, brain activity during the post-alcohol period, IL-6 signal transduction, and expression of synaptic proteins. Wildtype littermates (WT) served as controls. The transgenic mice model brain neuroimmune status with respect to IL-6 in subjects with a history of persistent alcohol use. Results showed a genotype dependent reduction in voluntary alcohol consumption in the Drinking in the Dark protocol and in frequency-dependent relationships between brain activity in EEG recordings during the post-alcohol period and alcohol consumption. IL-6, TNF-alpha, IL-6 signal transduction partners pSTAT3 and c/EBP beta, and synaptic proteins were shown to play a role in these genotypic effects.

High quality mapping of chromatin at or near the nuclear lamina from small numbers of cells reveals cell cycle and developmental changes of chromatin at the nuclear periphery

The chromatin associated with the nuclear lamina (NL) is referred to as lamina-associated domains (LADs). Here, we present an adaptation of the tyramide-signal amplification sequencing (TSA-seq) protocol, which we call chromatin pull down-based TSA-seq (cTSA-seq), that can be used to map chromatin regions at or near the NL from as little as 50 000 cells. The cTSA-seq mapped regions are composed of previously defined LADs and smaller chromatin regions that fall within the Hi-C defined B-compartment containing nuclear peripheral heterochromatin. We used cTSA-seq to map chromatin at or near the assembling NL in cultured cells progressing through early G1. cTSA-seq revealed that the distal ends of chromosomes are near or at the reassembling NL during early G1, a feature similar to those found in senescent cells. We expand the use of cTSA-seq to the mapping of chromatin at or near the NL from fixed-frozen mouse cerebellar tissue sections. This mapping reveals a general conservation of NL-associated chromatin and identifies global and local changes during cerebellar development. The cTSA-seq method reported here is useful for analyzing chromatin at or near the NL from small numbers of cells derived from both in vitro and in vivo sources.

Targeting α6GABAA receptors as a novel therapy for schizophrenia: A proof-of-concept preclinical study using various animal models

GABA_A receptors containing α6 subunits (α6GABA_ARs) in the cerebellum have -been implicated in schizophrenia. It was reported that the GABA synthesizing enzymes were downregulated whereas α6GABA_ARs were upregulated in postmortem cerebellar tissues of patients with schizophrenia and in a rat model induced by chronic phencyclidine (PCP). We have previously demonstrated that pyrazoloquinolinone Compound 6, an α6GABA_AR-highly selective positive allosteric modulator (PAM), can rescue the disrupted prepulse inhibition (PPI) induced by methamphetamine (METH), an animal model mimicking the sensorimotor gating deficit based on the hyper-dopaminergic hypothesis of schizophrenia. Here, we demonstrate that not only Compound 6, but also its structural analogues, LAU463 and LAU159, with similarly high α6GABA_AR selectivity and their respective deuterated derivatives (DK-I-56-1, DK-I-58-1 and DK-I-59-1) can rescue METH-induced PPI disruption. Besides, Compound 6 and DK-I-56-I can also rescue the PPI disruption induced by acute administration of PCP, an animal model based on the hypo-glutamatergic hypothesis of schizophrenia. Importantly, Compound 6 and DK-I-56-I, at doses not affecting spontaneous locomotor activity, can also rescue impairments of social interaction and novel object recognition in mice induced by chronic PCP treatments. At similar doses, Compound 6 did not induce sedation but significantly suppressed METH-induced hyperlocomotion. Thus, α6GABA_AR-selective PAMs can rescue not only disrupted PPI but also hyperlocomotion, social withdrawal, and cognitive impairment, in both METH- and PCP-induced animal models mimicking schizophrenia, suggesting that they are a potential novel therapy for the three core symptoms, i.e. positive symptoms, negative symptoms, and cognitive impairment, of schizophrenia.

Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABAA receptors of A17 amacrine cells in the rat retina

Amacrine cells constitute a large and heterogenous group of inhibitory interneurons in the retina. The A17 amacrine plays an important role for visual signaling in the rod pathway microcircuit of the mammalian retina. It receives excitatory input from rod bipolar cells and provides feedback inhibition to the same cells. However, from ultrastructural investigations, there is evidence for input to A17s from other types of amacrine cells, presumably inhibitory, but there is a lack of information about the identity and functional properties of the synaptic receptors and how inhibition contributes to the integrative properties of A17s. Here, we studied the biophysical and pharmacological properties of GABAergic spontaneous inhibitory postsynaptic currents (spIPSCs) and GABA_A receptors of A17 amacrines, using whole-cell and outside-out patch recordings from rat retinal slices. The spIPSCs displayed fast onsets (10-90% rise time ~740 μs) and double-exponential decays (τ_fast ~4.5 ms [43% of amplitude]; τ_slow ~22 ms). Ultrafast application of brief pulses of GABA (3 mM) to patches evoked responses with deactivation kinetics best fitted by a triple-exponential function (τ₁ ~5.3 ms [55% of amplitude]; τ₂ ~48 ms [32% amplitude]; τ₃ ~187 ms). Non-stationary noise analysis of spIPSCs and patch responses yielded single-channel conductances of ~21 and ~25 pS, respectively. Pharmacological analysis suggested that the spIPSCs are mediated by receptors with an α1βγ2 subunit composition and the somatic receptors have an α2βγ2 and/or α3βγ2 composition. These results demonstrate the presence of synaptic GABA_A receptors on A17s, which may play an important role in signal integration in these cells.

α6-Containing GABAA Receptors: Functional Roles and Therapeutic Potentials

GABAA receptors containing the α6 subunit are highly expressed in cerebellar granule cells and less abundantly in many other neuronal and peripheral tissues. Here, we for the first time summarize their importance for the functions of the cerebellum and the nervous system. The cerebellum is not only involved in motor control but also in cognitive, emotional, and social behaviors. α6βγ2 GABAA receptors located at cerebellar Golgi cell/granule cell synapses enhance the precision of inputs required for cerebellar timing of motor activity and are thus involved in cognitive processing and adequate responses to our environment. Extrasynaptic α6βδ GABAA receptors regulate the amount of information entering the cerebellum by their tonic inhibition of granule cells, and their optimal functioning enhances input filtering or contrast. The complex roles of the cerebellum in multiple brain functions can be compromised by genetic or neurodevelopmental causes that lead to a hypofunction of cerebellar α6-containing GABAA receptors. Animal models mimicking neuropsychiatric phenotypes suggest that compounds selectively activating or positively modulating cerebellar α6-containing GABAA receptors can alleviate essential tremor and motor disturbances in Angelman and Down syndrome as well as impaired prepulse inhibition in neuropsychiatric disorders and reduce migraine and trigeminal-related pain via α6-containing GABAA receptors in trigeminal ganglia. Genetic studies in humans suggest an association of the human GABAA receptor α6 subunit gene with stress-associated disorders. Animal studies support this conclusion. Neuroimaging and post-mortem studies in humans further support an involvement of α6-containing GABAA receptors in various neuropsychiatric disorders, pointing to a broad therapeutic potential of drugs modulating α6-containing GABAA receptors. SIGNIFICANCE STATEMENT: α6-Containing GABAA receptors are abundantly expressed in cerebellar granule cells, but their pathophysiological roles are widely unknown, and they are thus out of the mainstream of GABAA receptor research. Anatomical and electrophysiological evidence indicates that these receptors have a crucial function in neuronal circuits of the cerebellum and the nervous system, and experimental, genetic, post-mortem, and pharmacological studies indicate that selective modulation of these receptors offers therapeutic prospects for a variety of neuropsychiatric disorders and for stress and its consequences.

GABAA Receptor Subunit Composition Drives Its Sensitivity to the Insecticide Fipronil

Fipronil (FPN) is a worldwide-used neurotoxic insecticide, targeting, and blocking GABA_A receptors (GABA_ARs). Beyond its efficiency on insect GABA_ARs, FPN causes neurotoxic effects in humans and mammals. Here, we investigated the mode of action of FPN on mammalian α6-containing GABA_ARs to understand its inhibitory effects on GABA-induced currents, as a function of the synaptic or extrasynaptic localization of GABA_ARs. We characterized the effects of FPN by electrophysiology using Xenopus oocytes which were microtransplanted with cerebellum membranes or injected with α6β3, α6β3γ2S (synaptic), and α6β3δ (extrasynaptic) cDNAs. At micromolar concentrations, FPN dose-dependently inhibited cerebellar GABA currents. FPN acts as a non-competitive antagonist on ternary receptors. Surprisingly, the inhibition of GABA-induced currents was partial for extra-synaptic (α6β3δ) and binary (α6β3) receptors, while synaptic α6β3γ2S receptors were fully blocked, indicating that the complementary γ or δ subunit participates in FPN-GABA_AR interaction. FPN unexpectedly behaved as a positive modulator on β3 homopentamers. These data show that FPN action is driven by the subunit composition of GABA_ARs-highlighting the role of the complementary subunit-and thus their localization within a physiological synapse. We built a docking model of FPN on GABA_ARs, which reveals two putative binding sites. This is consistent with a double binding mode of FPN on GABA_ARs, possibly one being of high affinity and the other of low affinity. Physiologically, the γ/δ subunit incorporation drives its inhibitory level and has important significance for its toxicity on the mammalian nervous system, especially in acute exposure.

The α6 GABAA Receptor Positive Allosteric Modulator DK-I-56-1 Reduces Tic-Related Behaviors in Mouse Models of Tourette Syndrome

Tourette syndrome (TS) is a disabling neurodevelopmental disorder characterized by multiple, recurrent tics. The pharmacological treatment of TS is currently based on dopaminergic antagonists; however, these drugs are associated with extrapyramidal symptoms and other serious adverse events. Recent evidence suggests that positive allosteric modulators (PAMs) of GABA_A receptors containing α6 subunits (α6 GABA_ARs) oppose the behavioral effects of dopamine. Building on this evidence, in the present study, we tested the efficacy of DK-I-56-1, a highly selective PAM for α6 GABA_ARs, in mouse models of TS exhibiting tic-related responses. DK-I-56-1 significantly reduced tic-like jerks and prepulse inhibition (PPI) deficits in D1CT-7 transgenic mice, a well-documented mouse model of TS. DK-I-56-1 also prevented the exacerbation of spontaneous eyeblink reflex induced by the potent dopamine D₁ receptor agonist SKF 82958, a proxy for tic-like responses. We also showed that both systemic and prefrontal cortical administration of DK-I-56-1 countered the PPI disruption caused by SKF 82958. Although the effects of DK-I-56-1 were akin to those elicited by dopaminergic antagonists, this drug did not elicit extrapyramidal effects, as measured by catalepsy. These results point to α6 GABA_AR PAMs as promising TS therapies with a better safety profile than dopaminergic antagonists.

Trigeminal neuropathic pain development and maintenance in rats are suppressed by a positive modulator of α6 GABAA receptors

γ-Aminobutyric acid type A (GABAA ) receptors containing the α6 subunit are located in trigeminal ganglia, and their reduction by small interfering RNA increases inflammatory temporomandibular and myofascial pain in rats. We thus hypothesized that enhancing their activity may help in neuropathic syndromes originating from the trigeminal system. Here, we performed a detailed electrophysiological and pharmacokinetic analysis of two recently developed deuterated structurally similar pyrazoloquinolinone compounds. DK-I-56-1 at concentrations below 1 µM enhanced γ-aminobutyric acid (GABA) currents at recombinant rat α6β3γ2, α6β3δ and α6β3 receptors, whereas it was inactive at most GABAA receptor subtypes containing other α subunits. DK-I-87-1 at concentrations below 1 µM was inactive at α6-containing receptors and only weakly modulated other GABAA receptors investigated. Both plasma and brain tissue kinetics of DK-I-56-1 were relatively slow, with half-lives of 6 and 13 hr, respectively, enabling the persistence of estimated free brain concentrations in the range 10-300 nM throughout a 24-hr period. Results obtained in two protocols of chronic constriction injury of the infraorbital nerve in rats dosed intraperitoneally with DK-I-56-1 during 14 days after surgery or with DK-I-56-1 or DK-I-87-1 during 14 days after trigeminal neuropathy were already established, demonstrated that DK-I-56-1 but not DK-I-87-1 significantly reduced the hypersensitivity response to von Frey filaments. SIGNIFICANCE: Neuropathic pain induced by trigeminal nerve damage is poorly controlled by current treatments. DK-I-56-1 that positively modulates α6 GABAA receptors is appropriate for repeated administration and thus may represent a novel treatment option against the development and maintenance of trigeminal neuropathic pain.

The α6 subunit-containing GABAA receptor: A novel drug target for inhibition of trigeminal activation

Novel treatments against migraine are an urgent medical requirement. The α6 subunit-containing GABA_A receptors (α6GABA_ARs) are expressed in trigeminal ganglia (TG), the hub of the trigeminal vascular system (TGVS) that is involved in the pathogenesis of migraine. Here we reveal an unprecedented role of α6GABA_ARs in ameliorating TGVS activation using several pharmacological approaches in an animal model mimicking pathological changes in migraine. TGVS activation was induced by intra-cisternal (i.c.) instillation of capsaicin in Wistar rats. Centrally, i.c. capsaicin activated the trigeminal cervical complex (TCC) measured by the increased number of c-Fos-immunoreactive (c-Fos-ir) TCC neurons. Peripherally, it elevated calcitonin gene-related peptide immunoreactivity (CGRP-ir) in TG and depleted CGRP-ir in the dura mater. Pharmacological approaches included a recently identified α6GABA_AR-selective positive allosteric modulator (PAM), the pyrazoloquinolinone Compound 6, two α6GABA_AR-active PAMs (Ro15-4513 and loreclezole), an α6GABA_AR-inactive benzodiazepine (diazepam), an α6GABA_AR-selective antagonist (furosemide), and a clinically effective antimigraine agent (topiramate). We examined effects of these compounds on both central and peripheral TGVS responses induced by i.c. capsaicin. Compound 6 (3-10 mg/kg, i.p.) significantly attenuated the TCC neuronal activation and TG CGRP-ir elevation, and dural CGRP depletion induced by capsaicin. All these effects of Compound 6 were mimicked by topiramate, Ro15-4513 and loreclezole, but not by diazepam. The brain-impermeable furosemide antagonized the peripheral, but not central, effects of Compound 6. These results suggest that the α6GABA_AR in TG is a novel drug target for TGVS activation and that α6GABA_AR-selective PAMs have the potential to be developed as a novel pharmacotherapy for migraine.

Cerebellar α6 -subunit-containing GABAA receptors: a novel therapeutic target for disrupted prepulse inhibition in neuropsychiatric disorders

BACKGROUND AND PURPOSE

The pathophysiological role of α₆ -subunit-containing GABA_A receptors, which are mainly expressed in cerebellar granule cells, remains unclear. Recently, we demonstrated that hispidulin, a flavonoid isolated from a local herb that remitted a patient's intractable motor tics, attenuated methamphetamine-induced hyperlocomotion in mice as a positive allosteric modulator (PAM) of cerebellar α₆ GABA_A receptors. Here, using hispidulin and a selective α₆ GABA_A receptor PAM, the pyrazoloquinolinone Compound 6, we revealed an unprecedented role of cerebellar α₆ GABA_A receptors in disrupted prepulse inhibition of the startle response (PPI), which reflects sensorimotor gating deficits manifested in several neuropsychiatric disorders.

EXPERIMENTAL APPROACH

PPI disruptions were induced by methamphetamine and NMDA receptor antagonists in mice. Effects of the tested compounds were measured in Xenopus oocytes expressing recombinant α₆ β₃ γ_2S GABA_A receptors.

KEY RESULTS

Hispidulin given i.p. or by bilateral intracerebellar (i.cb.) injection rescued PPI disruptions induced by methamphetamine, ketamine, MK-801 and phencyclidine. Intracerebellar effects of hispidulin were mimicked by Ro15-4513 and loreclezole (two α₆ GABA_A receptor PAMs), but not by diazepam (an α₆ GABA_A receptor-inactive benzodiazepine) and were antagonized by furosemide (i.cb.), an α₆ GABA_A receptor antagonist. Importantly, Compound 6 (i.p.) also rescued methamphetamine-induced PPI disruption, an effect prevented by furosemide (i.cb.). Both hispidulin and Compound 6 potentiated α₆ β₃ γ_2S GABA_A receptor-mediated GABA currents.

CONCLUSIONS AND IMPLICATIONS

Positive allosteric modulation of cerebellar α₆ GABA_A receptors rescued disrupted PPI by attenuating granule cell activity. α₆ GABA_A receptor-selective PAMs are potential medicines for treating sensorimotor gating deficits in neuropsychiatric disorders. A mechanistic hypothesis is based on evidence for cerebellar contributions to cognitive functioning including sensorimotor gating.

Design and Synthesis of Novel Deuterated Ligands Functionally Selective for the γ-Aminobutyric Acid Type A Receptor (GABAAR) α6 Subtype with Improved Metabolic Stability and Enhanced Bioavailability

Recent reports indicate that α6β2/3γ2 GABA_AR selective ligands may be important for the treatment of trigeminal activation-related pain and neuropsychiatric disorders with sensori-motor gating deficits. Based on 3 functionally α6β2/3γ2 GABA_AR selective pyrazoloquinolinones, 42 novel analogs were synthesized, and their in vitro metabolic stability and cytotoxicity as well as their in vivo pharmacokinetics, basic behavioral pharmacology, and effects on locomotion were investigated. Incorporation of deuterium into the methoxy substituents of the ligands increased their duration of action via improved metabolic stability and bioavailability, while their selectivity for the GABA_AR α6 subtype was retained. 8b was identified as the lead compound with a substantially improved pharmacokinetic profile. The ligands allosterically modulated diazepam insensitive α6β2/3γ2 GABA_ARs and were functionally silent at diazepam sensitive α1β2/3γ2 GABA_ARs, thus no sedation was detected. In addition, these analogs were not cytotoxic, which render them interesting candidates for treatment of CNS disorders mediated by GABA_AR α6β2/3γ2 subtypes.

Towards functional selectivity for α6β3γ2 GABAA receptors: a series of novel pyrazoloquinolinones

BACKGROUND AND PURPOSE

The GABAA receptors are ligand-gated ion channels, which play an important role in neurotransmission. Their variety of binding sites serves as an appealing target for many clinically relevant drugs. Here, we explored the functional selectivity of modulatory effects at specific extracellular α+/β- interfaces, using a systematically varied series of pyrazoloquinolinones.

EXPERIMENTAL APPROACH

Recombinant GABAA receptors were expressed in Xenopus laevis oocytes and modulatory effects on GABA-elicited currents by the newly synthesized and reference compounds were investigated by the two-electrode voltage clamp method.

KEY RESULTS

We identified a new compound which, to the best of our knowledge, shows the highest functional selectivity for positive modulation at α6β3γ2 GABAA receptors with nearly no residual activity at the other αxβ3γ2 (x = 1-5) subtypes. This modulation was independent of affinity for α+/γ- interfaces. Furthermore, we demonstrated for the first time a compound that elicits a negative modulation at specific extracellular α+/β- interfaces.

CONCLUSION AND IMPLICATIONS

These results constitute a major step towards a potential selective positive modulation of certain α6-containing GABAA receptors, which might be useful to elicit their physiological role. Furthermore, these studies pave the way towards insights into molecular principles that drive positive versus negative allosteric modulation of specific GABAA receptor isoforms.

Hispidulin alleviated methamphetamine-induced hyperlocomotion by acting at α6 subunit-containing GABAA receptors in the cerebellum

RATIONALE

Hispidulin is a flavonoid we isolated from Clerodendrum inerme, an herb that effectively remitted a case of intractable motor tic disorders. Hispidulin was shown to be a positive allosteric modulator (PAM) of GABAA receptors, including the α6 subunit-containing subtype (α6GABAAR) that is predominantly expressed in cerebellar granule cells and insensitive to diazepam.

OBJECTIVES

We explored the action mechanism(s) of hispidulin using hyperdopaminergic mouse models induced by methamphetamine and apomorphine, based on the hyperdopaminergic nature of tic disorders.

RESULTS

Hispidulin significantly inhibited methamphetamine-induced hyperlocomotion (MIH) at i.p. doses without affecting apomorphine-induced hyperlocomotion and stereotypy behaviors or having significant benzodiazepine-like effects (BZLE), including sedation, anxiety, and motor impairment. When given by intracerebellar (i.c.b.) microinjection, hispidulin also alleviated MIH and this effect was prevented by i.c.b. coadministration of furosemide, an α6GABAAR antagonist, and mimicked by i.c.b. Ro 15-4513, an α6GABAAR PAM. Conversely, i.c.b. diazepam did not affect MIH while it reduced MIH at i.p. doses having significant BZLE. In a screening assay for 92 neurotransmitter receptors/degradation enzymes/transporters, hispidulin displayed significant (>50 % inhibition of radiolabeled ligand binding at 10 μM) binding affinity only at the benzodiazepine binding site of GABAARs (IC50 0.73∼1.78 μM) and catecholamine-o-methyl-transferase (COMT) (IC50 1.32 μM). OR-486, a more potent COMT inhibitor than hispidulin, did not affect MIH.

CONCLUSIONS

It is suggested that hispidulin alleviates MIH via acting as a PAM of cerebellar α6GABAARs, but not through COMT inhibition or affecting dopamine receptor responsiveness. Thus, selective α6GABAAR PAMs may have the potential to be a novel treatment for hyperdopaminergic disorders.

Subtype selectivity of α+β- site ligands of GABAA receptors: identification of the first highly specific positive modulators at α6β2/3γ2 receptors

BACKGROUND AND PURPOSE

GABAA receptors are the major inhibitory neurotransmitter receptors in the mammalian brain and the target of many clinically important drugs interacting with different binding sites. Recently, we demonstrated that CGS 9895 (2-(4-methoxyphenyl)-2H-pyrazolo[4,3-c]quinolin-3(5H)-one) elicits a strong and subtype-dependent enhancement of GABA-induced currents via a novel drug-binding site at extracellular αx+βy- (x = 1-6, y = 1-3) interfaces. Here, we investigated 16 structural analogues of CGS 9895 for their ability to modulate GABA-induced currents of various GABAA receptor subtypes.

EXPERIMENTAL APPROACH

Recombinant GABAA receptor subtypes were expressed in Xenopus laevis oocytes and investigated by the two-electrode voltage clamp method.

KEY RESULTS

Most of the compounds investigated were able to modulate GABA-induced currents of αβ and αβγ receptors to a comparable extent, suggesting that the effect of these drugs is not dependent on the benzodiazepine site of GABAA receptors. Steric hindrance experiments demonstrated that these compounds exert their action predominantly via the αx+βy- (x = 1-6, y = 1-3) interfaces. Whereas some compounds are unselectively modulating a broad range of receptor subtypes, other compounds feature remarkable functional selectivity for the α6β3γ2 receptor, or behave as null modulators at some receptor subtypes investigated.

CONCLUSION AND IMPLICATIONS

Pyrazoloquinolinones and pyrazolopyridinones represent the first prototypes of drugs exerting benzodiazepine-like modulatory effects via the α+β- interface of GABAA receptors. The discovery of modulators with functional subtype selectivity at this class of binding sites provides a highly useful tool for the investigation of α6β2/3γ2 receptor function, and may lead to novel therapeutic principles.

Regulation of GABA(A) receptor subunit expression by pharmacological agents

The gamma-aminobutyric acid (GABA) type A receptor system, the main fast-acting inhibitory neurotransmitter system in the brain, is the pharmacological target for many drugs used clinically to treat, for example, anxiety disorders and epilepsy, and to induce and maintain sedation, sleep, and anesthesia. These drugs facilitate the function of pentameric GABA(A) receptors that exhibit widespread expression in all brain regions and large structural and pharmacological heterogeneity as a result of composition from a repertoire of 19 subunit variants. One of the main problems in clinical use of GABA(A) receptor agonists is the development of tolerance. Most drugs, in long-term use and during withdrawal, have been associated with important modulations of the receptor subunit expression in brain-region-specific manner, participating in the mechanisms of tolerance and dependence. In most cases, the molecular mechanisms of regulation of subunit expression are poorly known, partly as a result of neurobiological adaptation to altered neuronal function. More knowledge has been obtained on the mechanisms of GABA(A) receptor trafficking and cell surface expression and the processes that may contribute to tolerance, although their possible pharmacological regulation is not known. Drug development for neuropsychiatric disorders, including epilepsy, alcoholism, schizophrenia, and anxiety, has been ongoing for several years. One key step to extend drug development related to GABA(A) receptors is likely to require deeper understanding of the adaptational mechanisms of neurons, receptors themselves with interacting proteins, and finally receptor subunits during drug action and in neuropsychiatric disease processes.

Endogenous steroid production in the spinal cord and potential involvement in neuropathic pain modulation

It has recently been demonstrated that the spinal cord (SC) is an active production center of neuroactive steroids including pregnenolone, dehydroepiandrosterone, progesterone and allopregnanolone. Indeed, anatomical, cellular and biochemical investigations have shown that the SC dorsal horn (DH), a pivotal structure in nociception, contains various active steroidogenic enzymes such as cytochrome P450side-chain-cleavage, cytochrome P450c17, 3beta-hydroxysteroid dehydrogenase, 5alpha-reductase and 3alpha-hydroxysteroid oxido-reductase. Reviewed here are several data obtained with in vitro and vivo experiments showing that endogenous steroids synthesized in the SC are involved in the modulation of nociceptive mechanisms. Various approaches were used as the real-time polymerase chain reaction after reverse transcription to determine the effects of neuropathic pain on the expression of genes encoding steroidogenic enzymes in the DH. Combination of the pulse-chase technique with high performance liquid chromatography and continuous flow scintillation detection allowed investigations of the impact of noxious signals on the activity of steroid-producing enzymes in the SC in vitro. Radioimmunological analyses of spinal tissue extracts contributed to determine the link between the painful state and endogenous steroid secretion in the SC in vivo. Finally, the physiological relevance of the modification of endogenous steroid formation in the SC during painful situation was discussed.

Clustered and non-clustered GABAA receptors in cultured hippocampal neurons

In cultured hippocampal neurons, gamma2 subunit-containing GABA(A) Rs form large postsynaptic clusters at GABAergic synapses and small clusters outside GABAergic synapses. We now show that a pool of non-clustered gamma2 subunit-containing GABA(A) Rs are also present at the cell surface. We also demonstrate that myc- or EGFP-tagged gamma2, alpha2, beta3 or alpha1 subunits expressed in these neurons assemble with endogenous subunits, forming GABA(A) Rs that target large postsynaptic clusters, small clusters outside GABAergic synapses or a pool of non-clustered surface GABA(A) Rs. In contrast, myc- or EGFP-tagged delta subunits only form non-clustered GABA(A) Rs, which can be induced to form clusters by antibody capping. A myc-tagged chimeric gamma2 subunit possessing the large intracellular loop (IL) of the delta-subunit IL (myc gamma2S/delta-IL) assembled into GABA(A) Rs, but it did not form clusters, therefore behaving like the delta subunit. Thus, the large intracellular loops of gamma2 and delta play an important role in determining the synaptic clustering/non-clustering capacity of the GABA(A) Rs.

A conceptualization of integrated actions of ethanol contributing to its GABAmimetic profile: a commentary

Early behavioral investigations supported the contention that systemic ethanol displays a GABAmimetic profile. Microinjection of GABA agonists into brain and in vivo electrophysiological studies implicated a regionally specific action of ethanol on GABA function. While selectivity of ethanol to enhance the effect of GABA was initially attributed an effect on type-I-benzodiazepine (BZD)-GABA(A) receptors, a lack of ethanol's effect on GABA responsiveness from isolated neurons with this receptor subtype discounted this contention. Nonetheless, subsequent work identified GABA(A) receptor subtypes, with limited distribution in brain, sensitive to enhancement of GABA at relevant ethanol concentrations. In view of these data, it is hypothesized that the GABAmimetic profile for ethanol is due to activation of mechanisms associated with GABA function, distinct from a direct action on the majority of postsynaptic GABA(A) receptors. The primary action proposed to account for ethanol's regional specificity on GABA transmission is its ability to release GABA from some, but not all, presynaptic GABAergic terminals. As systemic administration of ethanol increases neuroactive steroids, which can enhance GABA responsiveness, this elevated level of neurosteroids is proposed to magnify the effect of GABA released by ethanol. Additional factors contributing to the degree to which ethanol interacts with GABA function include an involvement of GABA(B) and other receptors that influence ethanol-induced GABA release, an effect of phosphorylation on GABA responsiveness, and a regional reduction of glutamatergic tone. Thus, an integration of these consequences induced by ethanol is proposed to provide a logical basis for its in vivo GABAmimetic profile.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Anatomical and cellular localization of neuroactive 5?/3?-reduced steroid-synthesizing enzymes in the spinal cord

The complementary activities of 5 alpha-reductase (5 alpha-R) and 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) are crucial for the synthesis of neuroactive 5 alpha/3 alpha-reduced steroids, such as 3 alpha-androstanediol, allopregnanolone, and tetrahydrodeoxycorticosterone, which control several important neurophysiological mechanisms through allosteric modulation of gamma-aminobutyric acid type A receptors. Immunocytochemical localization of 3 alpha-HSD in the central nervous system (CNS) has never been determined. The presence and activity of 5 alpha-R have been investigated in the CNS, but only the brain was considered; the spinal cord (SC) received little attention, although this structure is crucial for many sensorimotor activities. We have determined the first cellular distribution of 5 alpha-reductase type 1 (5 alpha-R1) and type 2 (5 alpha-R2) and 3 alpha-HSD immunoreactivities in adult rat SC. 5 alpha-R1 immunostaining was detected mainly in the white matter (Wm). In contrast, intense 5 alpha-R2 labeling was observed in dorsal (DH) and ventral horns of gray matter (Gm). 3 alpha-HSD immunoreactivity was largely distributed in the Wm and Gm, but the highest density was found in sensory areas of the DH. Double-labeling experiments combined with confocal analysis revealed that, in the Wm, 5 alpha-R1 was localized in glial cells, whereas 35% of 5 alpha-R2 and 3 alpha-HSD immunoreactivities were found in neurons. In the DH, 60% of 5 alpha-R2 immunostaining colocalized with oligodendrocyte, 25% with neuron, and 15% with astrocyte markers. Similarly, 45% of 3 alpha-HSD immunoreactivity was found in oligodendrocytes, 35% in neurons, and 20% in astrocytes. These results are the first demonstrating that oligodendrocytes and neurons of the SC possess the key enzymatic complex for synthesizing potent neuroactive steroids that may control spinal sensorimotor processes.

Cre-mediated recombination in rhombic lip derivatives

To study the development of the cerebellum, we generated a transgenic mouse line Tg(malpha6-cre)B1LFR that expresses CRE recombinase under the GABA(A) receptor alpha6 subunit promoter. In this line, recombination of an R26R reporter allele occurred postnatally in granule cells of the cerebellum and dorsal cochlear nucleus, as well as in a subset of precerebellar nuclei in the brainstem. All neurons in which recombination occurred originated during embryogenesis from the rhombic lip. This might be explained by a very early specification event at the rhombic lip that primes cells derived from this structure to express the transgene during neuronal maturation. As no recombination occurred in the inferior olive, it may be derived from a distinct subset of precursors at the rhombic lip. No recombination occurred in any of the interneurons in the cerebellum (stellate cells, basket cells, and Golgi cells), consistent with the hypothesis that they are not derived from the same embryonic tissue as the granule cells.

GABA(A) receptors: immunocytochemical distribution of 13 subunits in the adult rat brain

GABA(A) receptors are ligand-operated chloride channels assembled from five subunits in a heteropentameric manner. Using immunocytochemistry, we investigated the distribution of GABA(A) receptor subunits deriving from 13 different genes (alpha1-alpha6, beta1-beta3, gamma1-gamma3 and delta) in the adult rat brain. Subunit alpha1-, beta1-, beta2-, beta3- and gamma2-immunoreactivities were found throughout the brain, although differences in their distribution were observed. Subunit alpha2-, alpha3-, alpha4-, alpha5-, alpha6-, gamma1- and delta-immunoreactivities were more confined to certain brain areas. Thus, alpha2-subunit-immunoreactivity was preferentially located in forebrain areas and the cerebellum. Subunit alpha6-immunoreactivity was only present in granule cells of the cerebellum and the cochlear nucleus, and subunit gamma1-immunoreactivity was preferentially located in the central and medial amygdaloid nuclei, in pallidal areas, the substantia nigra pars reticulata and the inferior olive. The alpha5-subunit-immunoreactivity was strongest in Ammon's horn, the olfactory bulb and hypothalamus. In contrast, alpha4-subunit-immunoreactivity was detected in the thalamus, dentate gyrus, olfactory tubercle and basal ganglia. Subunit alpha3-immunoreactivity was observed in the glomerular and external plexiform layers of the olfactory bulb, in the inner layers of the cerebral cortex, the reticular thalamic nucleus, the zonal and superficial layers of the superior colliculus, the amygdala and cranial nerve nuclei. Only faint subunit gamma3-immunoreactivity was detected in most areas; it was darkest in midbrain and pontine nuclei. Subunit delta-immunoreactivity was frequently co-distributed with alpha4 subunit-immunoreactivity, e.g. in the thalamus, striatum, outer layers of the cortex and dentate molecular layer. Striking examples of complementary distribution of certain subunit-immunoreactivities were observed. Thus, subunit alpha2-, alpha4-, beta1-, beta3- and delta-immunoreactivities were considerably more concentrated in the neostriatum than in the pallidum and entopeduncular nucleus. In contrast, labeling for the alpha1-, beta2-, gamma1- and gamma2-subunits prevailed in the pallidum compared to the striatum. With the exception of the reticular thalamic nucleus, which was prominently stained for subunits alpha3, beta1, beta3 and gamma2, most thalamic nuclei were rich in alpha1-, alpha4-, beta2- and delta-immunoreactivities. Whereas the dorsal lateral geniculate nucleus was strongly immunoreactive for subunits alpha4, beta2 and delta, the ventral lateral geniculate nucleus was predominantly labeled for subunits alpha2, alpha3, beta1, beta3 and gamma2; subunit alpha1- and alpha5-immunoreactivities were about equally distributed in both areas. In most hypothalamic areas, immunoreactivities for subunits alpha1, alpha2, beta1, beta2 and beta3 were observed. In the supraoptic nucleus, staining of conspicuous dendritic networks with subunit alpha1, alpha2, beta2, and gamma2 antibodies was contrasted by perykarya labeled for alpha5-, beta1- and delta-immunoreactivities. Among all brain regions, the median emminence was most heavily labeled for subunit beta2-immunoreactivity. In most pontine and cranial nerve nuclei and in the medulla, only subunit alpha1-, beta2- and gamma2-immunoreactivities were strong, whereas the inferior olive was significantly labeled only for subunits beta1, gamma1 and gamma2. In this study, a highly heterogeneous distribution of 13 different GABA(A) receptor subunit-immunoreactivities was observed. This distribution and the apparently typical patterns of co-distribution of these GABA(A) receptor subunits support the assumption of multiple, differently assembled GABA(A) receptor subtypes and their heterogeneous distribution within the adult rat brain.

Diversity and plasticity in amino acid receptor subunits in the rat auditory brain stem

Glutamate, gamma aminobutyric acid (GABA) and glycine receptors have different properties depending on the specific subunit combination utilized. The subunit composition of amino acid receptors may help to shape the responses of neurons and can provide a diversity of response properties in different neuronal types and regions. This allows a synaptic fine tuning for an optimization of processing requirements and may also allow for changes in response to changes in input. This article reviews the diversity that has been found in the subunit composition of GABA, glycine, alpha amino-3-hydroxy-5-methyl-4 isoxazole propionic acid and N-Methyl, D-aspartate (NMDA) receptors in the mammalian auditory brain stem and provides new data on how the NMDAR1 glutamate receptor subunit changes as a consequence of deafness. In the latter study, quantitative in situ hybridization was used to assess NMDAR1 mRNA expression in six cell types of the rat cochlear nucleus. A unilateral cochlear ablation was performed and expression determined in the ipsilateral and contralateral cochlear nucleus 5 and 20 days later. Significantly decreased expression, compared to normal, was found 5 days following deafness, in ipsilateral spherical bushy cells, octopus cells and shell neurons, but not in fusiform cells, corn cells or granule cells. At 20 days the expression was not significantly different from normal in any of the six cell types.

Distribution of GABA and glycine receptors on bipolar and ganglion cells in the mammalian retina

The amino acids GABA and glycine mediate synaptic transmission via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of GABA and glycine receptors. In the present article, the distribution of GABA and glycine receptors on identified bipolar and ganglion cell types in the mammalian retina is reviewed. Immunofluorescence obtained with antibodies against GABA and glycine receptors is punctate. Electron microscopy shows that the puncta represent a cluster of receptors at synaptic sites. Bipolar cell types were identified with immunohistochemical markers. Double immunofluorescence with subunit-specific antibodies was used to analyze the distribution of receptor clusters on bipolar axon terminals. The OFF cone bipolar cells seem to be dominated by glycinergic input, whereas the ON cone bipolar and rod bipolar cells are dominated by GABAergic input. Ganglion cells were intracellularly injected with Neurobiotin, visualized with Streptavidin coupled to FITC, and subsequently stained with subunit specific antibodies. The distribution and density of receptor clusters containing the alpha1 subunit of the GABA(A) receptor and the alpha1 subunit of the glycine receptor, respectively, were analyzed on midget and parasol cells in the marmoset (a New World monkey). Both GABA(A) and glycine receptors are distributed uniformly along the dendrites of ON and OFF types of parasol and midget ganglion cells, indicating that functional differences between these subtypes of ganglion cells are not determined by GABA or glycinergic input.

Distribution of GABAA and glycine receptors in the mammalian retina

1. GABA and glycine mediate synaptic inhibition via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of receptors for these two neurotransmitters. In the present paper, the distribution of GABAA and glycine receptors in the mammalian retina is reviewed. 2. In situ hybridization, immunocytochemistry with subunit-specific antibodies and single cell injection were used to analyse the localization of receptor subunits. Specific subunits are expressed in characteristic strata of the inner plexi-form layer, suggesting that different functional circuits involve specific subtypes of neurotransmitter receptors. 3. Different cell types express different combinations of receptor subunits and an individual neuron can express several receptor isoforms at distinct post-synaptic sites.

Alterations in the expression of GABAA receptor subunits in cerebellar granule cells after the disruption of the alpha6 subunit gene

Any given subunit of the heteromultimeric type-A gamma-aminobutyric acid (GABA) GABAA receptor may be present in several receptor subtypes expressed by individual neurons. Changes in the expression of a subunit may result in differential changes in the expression of other subunits depending on the subunit composition of the receptor subtype, leading to alterations in neuronal responsiveness to GABA. We used the targeted disruption of the alpha6 subunit gene to test for changes in the expression of other GABAA receptor subunits. Immunoprecipitation and ligand binding experiments indicated that GABAA receptors were reduced by approximately 50% in the cerebellum of alpha6 -/- mice. Western blot experiments indicated that the alpha6 subunit protein completely disappeared from the cerebellum of alpha6 -/- mice, which resulted in the disappearance of the delta subunit from the plasma membrane of granule cells. The amount of beta2, beta3 and gamma2 subunits was reduced by approximately 50%, 20% and 40%, respectively, in the cerebella of alpha6 -/- mice. A comparison of the reduction in the level of alpha1, beta2, beta3, gamma2, or delta-subunit-containing receptors in alpha6 -/- cerebellum with those observed after removal of alpha6-subunit-containing receptors from the cerebella of alpha6 +/+ mice by immuno-affinity chromatography demonstrated the presence of a significantly higher than expected proportion of receptors containing beta3 subunits in alpha6 -/- mice. The receptors containing alpha1, beta2, beta3 and gamma2 subunits were present in the plasma membrane of granule cells of alpha6 -/- mice at both synaptic and extrasynaptic sites, as shown by electron microscopic immunocytochemistry. Despite the changes, the alpha1 subunit content of Golgi-cell-to-granule-cell synapses in alpha6 -/- animals remained unaltered, as did the frequency of alpha1 immunopositive synapses in the glomeruli. Furthermore, no change was apparent in the expression of the alpha1, beta2 and gamma2 subunits in Purkinje cells and interneurons of the molecular layer. These results demonstrate that in alpha6 -/- mice, the cerebellum expresses only half of the number of GABAA receptors present in wild-type animals. Since these animals have no gross motor deficits, synaptic integration in granule cells is apparently maintained by alpha1-subunit-containing receptors with an altered overall subunit composition, and/or by changes in the expression of other ligand and voltage gated channels.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Pharmacological and functional implications of developmentally-regulated changes in GABA(A) receptor subunit expression in the cerebellum

The cerebellum undergoes many morphological, pharmacological, and electrophysiological changes during the first 3 weeks of postnatal development. The purpose of this review is to present the most up to date synopsis of the pharmacological and functional changes in, gamma-aminobutyric acid (GABA) type A receptors during this time of cerebellar maturation. Since most of the diversity in cerebellar, GABA(A) receptor pharmacology lies within the granule cell layer, research groups have focused on this area of the cerebellum to study the developmental changes in GABA(A) receptor subunit expression and the neurodifferentiating factors involved in regulating this expression. Thus, it is important to note that developmental changes in GABA(A) receptor composition and its corresponding pharmacology will be essential for determining the type of GABA-mediated transmission that occurs between neuronal contacts in the neonatal and subsequently in the mature cerebellum.

Differential localization of GABA(A) receptor alpha and beta subunits in the hamster retina and relationship with glutamic acid decarboxylase immunoreactivity

In order to determine the cellular localization of GABA(A)alpha and beta subunits in the hamster retina, single and double immunocytochemistry was performed in perfuse-fixed hamster retina using commercially-available antibodies against the two receptor subunits and glutamic acid decarboxylase. Strong GABA(A)beta immunoreactivity was found in two strata of the inner plexiform layer and in perikarya of amacrine cells and bipolar cells in the inner nuclear layer. In contrast, no GABA(A)alpha immunoreactivity was detected. All but a few of the GABA(A)beta-immunoreactive amacrine cells were found not to display glutamic acid decarboxylase immunoreactivity. The present results indicate that there is a differential localization of GABA(A)alpha and beta subunits in different neuronal subpopulations in the hamster retina.

Glycine and GABA receptors in the mammalian retina

Molecular cloning has introduced an unexpected diversity of neurotransmitter receptors. In this study we review the types, the localization and possible synaptic function of the inhibitory neurotransmitter receptors in the mammalian retina. Glycine receptors (GlyRs) and their localization in the mammalian retina were analyzed immunocytochemically. Specific antibodies against the alpha 1 subunit of the GlyR (mAb2b) and against all subunits of the GlyR (mAb4a) were used. Both antibodies produced a punctate immunofluorescence, which was shown by electron microscopy to represent clustering of GlyRs at synaptic sites. Synapses expressing the alpha 1 subunit of the GlyR were found on ganglion cell dendrites and on bipolar cell axons. GlyRs were also investigated in the oscillator mutant mouse. The complete loss of the alpha 1 subunit was compensated for by an apparent upregulation of the other subunits of the GlyR. GABAA receptors (GABAARs) and their retinal distribution were studied with specific antibodies that recognize the alpha 1, alpha 2, alpha 3, beta 1, beta 2, beta 3, gamma 2 and delta subunits. Most antibodies produced a punctate immunofluorescence in the inner plexiform layer (IPL) which was shown by electron microscopy to represent synaptic clustering of GABAARs. The density of puncta varied across the IPL and different subunits were found in characteristic strata. This stratification pattern was analyzed with respect to the ramification of cholinergic amacrine cells. Using intracellular injection with Lucifer yellow followed by immunofluorescence, we found that GABAARs composed of different subunits were expressed by the same ganglion cell, however, they were clustered at different synaptic sites. The distribution of GABAC receptors was studied in the mouse and in the rabbit retina using an antiserum that recognizes the rho 1, rho 2 and rho 3 subunits. GABAC receptors were found to be clustered at postsynaptic sites. Most, if not all of the synapses were found on rod and cone bipolar axon terminals. In conclusion we find a great diversity of glycine and GABA receptors in the mammalian retina, which might match the plethora of morphological types of amacrine cells. This may also point to subtle differences in synaptic function still to be elucidated.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

GABAA receptor subunit expression changes in the rat cerebellum and cerebral cortex during aging

Significant aging-related decreased expression of various GABAAR subunit mRNAs (alpha 1, gamma 2, beta 2, beta 3 and sigma) was found in both cerebellum and cerebral cortex using quantitative dot blot and in situ hybridization techniques. Contrary to the other subunits, the alpha 6 mRNA expression was significantly increased in the aged cerebellum. Parallel age-related changes in protein expression for gamma 2 and beta 2/3 (decrease) and alpha 6 (increase) were revealed in cerebellum by quantitative immunocytochemistry. However, no significant changes in alpha 1 protein expression nor in the number or affinity of [3H]zolpidem binding sites were detected in cerebellum even though alpha 1 mRNA expression was significantly decreased in the aged rat. Age-related increased expression of alpha 6 mRNA and protein in the cerebellum was accompanied by no significant changes in the number of diazepam-insensitive [3H]Ro15-4513 binding sites. In the cerebral cortex, no changes in the protein expression of the main GABAA receptor subunits (alpha 1, gamma 2 and beta 2/3) were observed which contrasted with the age-related decreased expression of the corresponding mRNAs. No significant changes in the number or affinity of [3H]zolpidem binding sites were observed in the cerebral cortex. Thus, age-related changes in the mRNA expression of a particular subunit does not necessarily lead to similar changes in protein or assembly into mature GABAA receptors. The results reveal the existence of complex regulatory mechanisms of GABAA receptor expression, at the transcriptional, translational and post-translational and/or assembly levels, which vary with the subunit and brain area.

Ligand-gated ion channel subunit partnerships: GABAA receptor alpha6 subunit gene inactivation inhibits delta subunit expression

Cerebellar granule cells express six GABAA receptor subunits abundantly (alpha1, alpha6, beta2, beta3, gamma2, and delta) and assemble various pentameric receptor subtypes with unknown subunit compositions; however, the rules guiding receptor subunit assembly are unclear. Here, removal of intact alpha6 protein from cerebellar granule cells allowed perturbations in other subunit levels to be studied. Exon 8 of the mouse alpha6 subunit gene was disrupted by homologous recombination. In alpha6 -/- granule cells, the delta subunit was selectively degraded as seen by immunoprecipitation, immunocytochemistry, and immunoblot analysis with delta subunit-specific antibodies. The delta subunit mRNA was present at wild-type levels in the mutant granule cells, indicating a post-translational loss of the delta subunit. These results provide genetic evidence for a specific association between the alpha6 and delta subunits. Because in alpha6 -/- neurons the remaining alpha1, beta2/3, and gamma2 subunits cannot rescue the delta subunit, certain potential subunit combinations may not be found in wild-type cells.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

The alpha 4 subunit of the GABAA receptors from rat brain and retina

A novel anti-alpha 4 antibody has been used for the purification and characterization of the alpha 4-containing GABAA receptors in the rat brain and for studying the immunocytochemical distribution of the alpha 4 subunit peptide in rat brain and retina. The anti-alpha 4 antibody recognized a 66 kDa peptide in brain membranes and immunoprecipitated 10-28% of the brain GABAA receptors in various brain regions as determined by [3H]muscimol binding. The highest immunoprecipitation values were obtained in the thalamus and the lowest in the cerebellum. Surprisingly, the receptors immunoprecipitated by anti-alpha 4 showed little or no diazepam-insensitive or diazepam-sensitive [3H]Ro15-4513 binding sites in any brain region. In the cerebellum, where 25% of the [3H]Ro15-4513 binding is diazepam-insensitive, much of the latter was immunoprecipitated by an anti-alpha 6 antibody but not by the anti-alpha 4 antibody. Immunoblots of immunoaffinity-purified GABAA receptors from the cerebral cortex on immobilized anti-alpha 4 revealed molecular colocalization of alpha 4 and gamma 2. However, the absence of significant benzodiazepine binding in these GABAA receptors suggests that the assembly of the alpha 4 and gamma 2 subunits in the cerebral cortex and in other brain regions is such that they do not normally form diazepam-insensitive [3H]Ro15-4513 binding sites. This result contrasts with the presence of diazepam-insensitive [3H]Ro15-4513 binding sites in the GABAA receptors expressed in heterologous systems resulting from the combination of alpha 4, gamma 2 and beta 2 subunits. Immunocytochemistry has revealed the abundance of alpha 4 peptide immunoreactivity in the thalamus and dentate gyrus (mainly in the hilar neurons and the inner third of the granule cell layer). The alpha 4 immunoreactivity is also present in the external plexiform layer of the olfactory bulb and in all layers of the neocortex and pyriform cortex. In the retina, alpha 4 is concentrated on ganglion cells (including some giant ganglion cells), the inner plexiform layer and to a lesser extent in the outer plexiform layer.

The cerebellum: a model system for studying GABAA receptor diversity

The basic unsolved questions concerning GABAA receptors are: "How many receptor subtypes exist?", "What subtypes are used by which types of neuron and where are they located on the cell?", and "What are the functions of the different subtypes?" As described in this Review, the cerebellum is an ideal vertebrate brain region for investigating these issues.