Transient expression of a novel type of GABA response in rat CA3 hippocampal neurones during development

Enzymatic Degradation of Cortical Perineuronal Nets Reverses GABAergic Interneuron Maturation

Perineuronal nets (PNNs) are specialised extracellular matrix structures which preferentially enwrap fast-spiking (FS) parvalbumin interneurons and have diverse roles in the cortex. PNN maturation coincides with closure of the critical period of cortical plasticity. We have previously demonstrated that BDNF accelerates interneuron development in a c-Jun-NH₂-terminal kinase (JNK)–dependent manner, which may involve upstream thousand-and-one amino acid kinase 2 (TAOK2). Chondroitinase-ABC (ChABC) enzymatic digestion of PNNs reportedly reactivates ‘juvenile-like’ plasticity in the adult CNS. However, the mechanisms involved are unclear. We show that ChABC produces an immature molecular phenotype in cultured cortical neurons, corresponding to the phenotype prior to critical period closure. ChABC produced different patterns of PNN-related, GABAergic and immediate early (IE) gene expression than well-characterised modulators of mature plasticity and network activity (GABA_A-R antagonist, bicuculline, and sodium-channel blocker, tetrodotoxin (TTX)). ChABC downregulated JNK activity, while this was upregulated by bicuculline. Bicuculline, but not ChABC, upregulated Bdnf expression and ERK activity. Furthermore, we found that BDNF upregulation of semaphorin-3A and IE genes was TAOK mediated. Our data suggest that ChABC heightens structural flexibility and network disinhibition, potentially contributing to ‘juvenile-like’ plasticity. The molecular phenotype appears to be distinct from heightened mature synaptic plasticity and could relate to JNK signalling. Finally, we highlight that BDNF regulation of plasticity and PNNs involves TAOK signalling.

The Subiculum: A Potential Site of Ictogenesis in a Neonatal Seizure Model

Studies have reported that the subiculum is one origin of interictal-like discharges in adult patients with temporal lobe epilepsy; however, whether the subiculum represents a site of ictogenesis for neonatal seizures remains unclear. In this study, multi-electrode recording techniques were used to record epileptiform discharges induced by low-Mg²⁺ or high-K⁺ artificial cerebrospinal fluid in neonatal mouse hippocampal slices, and the spatiotemporal dynamics of the epileptiform discharges were analyzed. The Na⁺–K⁺–2Cl⁻ cotransporter 1 (NKCC1) blocker, bumetanide, was applied to test its effect upon epileptiform discharges in low-Mg²⁺ model. The effect of N-methyl-d-aspartate receptors (NMDARs) antagonist, d-AP5, upon the epileptiform discharges in high-K⁺ model was examined. We found that the neonatal subiculum not only relayed epileptiform discharges emanating from the hippocampus proper (HP) but also initiated epileptiform discharges (interictal- and ictal-like discharges) independently. The latency to onset of the first epileptiform discharge initiated in the subiculum was similar to that initiated in the HP. Bumetanide efficiently blocked seizures in the neonatal HP, but was less effectively in suppressing seizures initiated in the subiculum. In high-K⁺ model, d-AP5 was more effective in blocking seizures initiated in the subiculum than that initiated in the HP. Furthermore, Western blotting analysis showed that NKCC1 expression was lower in the subiculum than that in the HP, whereas the expression of NMDAR subunits, NR2A and NR2B, was higher in the subiculum than that in the HP. Our results revealed that the subiculum was a potential site of ictogenesis in neonatal seizures and possessed similar seizure susceptibility to the HP. GABAergic excitation resulting from NKCC1 may play a less dominant role during ictogenesis in the subiculum than that in the HP. The subicular ictogenesis may be related to the glutamatergic excitation mediated by NMDARs.

Inhibitory Effects of Ginsenoside Metabolites, Compound K and Protopanaxatriol, on GABAC Receptor-Mediated Ion Currents

Ginsenosides, one of the active ingredients of Panax ginseng, show various pharmacological and physiological effects, and they are converted into compound K (CK) or protopanaxatriol (M4) by intestinal microorganisms. CK is a metabolite derived from protopanaxadiol (PD) ginsenosides, whereas M4 is a metabolite derived from protopanaxatriol (PT) ginsenosides. The γ-aminobutyric acid receptor_C (GABA_C) is primarily expressed in retinal bipolar cells and several regions of the brain. However, little is known of the effects of ginsenoside metabolites on GABA_C receptor channel activity. In the present study, we examined the effects of CK and M4 on the activity of human recombinant GABA_C receptor (ρ1) channels expressed in Xenopus oocytes by using a 2-electrode voltage clamp technique. In oocytes expressing GABA_C receptor cRNA, we found that CK or M4 alone had no effect in oocytes. However, co-application of either CK or M4 with GABA inhibited the GABA-induced inward peak current (I_GABA). Interestingly, pre-application of M4 inhibited I_GABA more potently than CK in a dose-dependent and reversible manner. The half-inhibitory concentration (IC₅₀) values of CK and M4 were 52.1±2.3 and 45.7±3.9 µM, respectively. Inhibition of I_GABA by CK and M4 was voltage-independent and non-competitive. This study implies that ginsenoside metabolites may regulate GABA_C receptor channel activity in the brain, including in the eyes.

Resveratrol Inhibits GABAC ρ Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

Resveratrol is a phytoalexin found in grapes, red wine, and berries. Resveratrol has been known to have many beneficial health effects, such as anti-cancer, neuroprotective, anti-inflammatory, and life-prolonging effects. However, relatively little is known about the effects of resveratrol on the regulation of ligand-gated ion channels. We have previously reported that resveratrol regulates subsets of homomeric ligand-gated ion channels such as those of 5-HT_3A receptors. The γ-aminobutyric acid_C (GABA_C) receptor is mainly expressed in retinal bipolar cells and plays an important role in visual processing. In the present study, we examined the effects of resveratrol on the channel activity of homomeric GABA_C receptor expressed in Xenopus oocytes injected with cRNA encoding human GABA_C ρ subunits. Our data show that the application of GABA elicits an inward peak current (I_GABA) in oocytes that express the GABA_C receptor. Resveratrol treatment had no effect on oocytes injected with H₂O or with GABA_C receptor cRNA. Co-treatment with resveratrol and GABA inhibited I_GABA in oocytes with GABA_C receptors. The inhibition of I_GABA by resveratrol was in a reversible and concentration-dependent manner. The IC₅₀ of resveratrol was 28.9±2.8 µM in oocytes expressing GABA_C receptor. The inhibition of I_GABA by resveratrol was in voltage-independent and non-competitive manner. These results indicate that resveratrol might regulate GABA_C receptor expression and that this regulation might be one of the pharmacological actions of resveratrol on the nervous system.

Excitatory actions of GABA in developing chick vestibular afferents: effects on resting electrical activity

The aim of this study was to characterize the effect of γ-aminobutyric acid (GABA) in the resting multiunit activity of the vestibular afferents during development using the isolated inner ear of embryonic and postnatal chickens (E15-E21 and P5). GABA (10(-3) to 10(-5) M; n = 133) and muscimol (10(-3) M) elicited an increase in the frequency of the basal discharge of the vestibular afferents. We found that GABA action was dose-dependent and inversely related to animal age. Thus, the largest effect was observed in embryonic ages such as E15 and E17 and decreases in E21 and P5. The GABAA receptor antagonists, bicuculline (10(-5) M; n = 10) and picrotoxin (10(-4) M; n = 10), significantly decreased the excitatory action of GABA and muscimol (10(-3) M). Additionally, CNQX 10(-6) M, MCPG 10(-5) M and 7ClKyn 10(-5) M (n = 5) were co-applied by bath substitution (n = 5). Both the basal discharge and the GABA action significantly decreased in these experimental conditions. The chloride channel blocker 9-AC 0.5 mM produced an important reduction in the effect of GABA 10(-3) (n = 5) and 10(-4) M (n = 5). Thus, our results suggest an excitatory role of GABA in the resting activity of the vestibular afferents that can be explained by changes in the gradient of concentration of Cl(-) during development. We show for the first time that the magnitude of this GABA effect decreases at later stages of embryonic and early postnatal development. Taking into account the results with glutamatergic antagonists, we conclude that GABA has a presynaptic action but is not the neurotransmitter in the vestibular afferent synapses, although it could act as a facilitator of the spontaneous activity and may regulate glutamate release.

Expression of GABAρ receptors in the neostriatum: localization in aspiny, medium spiny neurons and GFAP-positive cells

GABAergic transmission in the neostriatum plays a central role in motor coordination, in which a plethora of GABA-A receptor subunits combine to modulate neural inhibition. GABAρ receptors were originally described in the mammalian retina. These receptors possess special electrophysiological and pharmacological properties, forming a characteristic class of ionotropic receptors. In previous studies, we suggested that GABAρ receptors are expressed in the neostriatum, and in this report we show that they are indeed present in all the calretinin-positive interneurons of the neostriatum. In addition, they are located in calbindin-positive interneurons and projection neurons that express the dopamine D(2) receptor. GABAρ receptors were also located in 30% of the glial fibrillary acidic protein-positive cells, and may therefore also contribute to gliotransmission. Quantitative reverse transcription-PCR suggested that the mRNAs of this receptor do not express as much as in the retina, and that GABAρ2 is more abundant than GABAρ1. Electrophysiological recordings in brain slices provided evidence of neurons expressing a cis-4-aminocrotonic acid-activated, 1,2,5,6-tetrahydropyridine-4-yl methylphosphinic acid-sensitive ionotropic GABA receptor, indicating the presence of functional GABAρ receptors in the neostriatum. Finally, electron-microscopy and immunogold located the receptors mainly in perisynaptic as well as in extrasynaptic sites. All these observations reinforce the importance of GABAρ receptors in the neostriatum and contribute to the diversity of inhibitory regulation in this area.

Phenobarbital but Not Diazepam Reduces AMPA/kainate Receptor Mediated Currents and Exerts Opposite Actions on Initial Seizures in the Neonatal Rat Hippocampus

Diazepam (DZP) and phenobarbital (PB) are extensively used as first and second line drugs to treat acute seizures in neonates and their actions are thought to be mediated by increasing the actions of GABAergic signals. Yet, their efficacy is variable with occasional failure or even aggravation of recurrent seizures questioning whether other mechanisms are not involved in their actions. We have now compared the effects of DZP and PB on ictal-like events (ILEs) in an in vitro model of mirror focus (MF). Using the three-compartment chamber with the two immature hippocampi and their commissural fibers placed in three different compartments, kainate was applied to one hippocampus and PB or DZP to the contralateral one, either after one ILE, or after many recurrent ILEs that produce an epileptogenic MF. We report that in contrast to PB, DZP aggravated propagating ILEs from the start, and did not prevent the formation of MF. PB reduced and DZP increased the network driven giant depolarizing potentials suggesting that PB may exert additional actions that are not mediated by GABA signaling. In keeping with this, PB but not DZP reduced field potentials recorded in the presence of GABA and NMDA receptor antagonists. These effects are mediated by a direct action on AMPA/kainate receptors since PB: (i) reduced AMPA/kainate receptor mediated currents induced by focal applications of glutamate; (ii) reduced the amplitude and the frequency of AMPA but not NMDA receptor mediated miniature excitatory postsynaptic currents (EPSCs); (iii) augmented the number of AMPA receptor mediated EPSCs failures evoked by minimal stimulation. These effects persisted in MF. Therefore, PB exerts its anticonvulsive actions partly by reducing AMPA/kainate receptors mediated EPSCs in addition to the pro-GABA effects. We suggest that PB may have advantage over DZP in the treatment of initial neonatal seizures since the additional reduction of glutamate receptors mediated signals may reduce the severity of neonatal seizures.

An Update on GABAρ Receptors

The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.

Redox modulation of homomeric rho1 GABA receptors

The activity of many receptors and ion channels in the nervous system can be regulated by redox-dependent mechanisms. Native and recombinant GABA(A) receptors are modulated by endogenous and pharmacological redox agents. However, the sensitivity of GABA(C) receptors to redox modulation has not been demonstrated. We studied the actions of different reducing and oxidizing agents on human homomeric GABArho(1) receptors expressed in Xenopus laevis oocytes. The reducing agents dithiothreitol (2 mM) and N-acetyl-L-cysteine (1 mM) potentiated GABA-evoked Cl(-) currents recorded by two-electrode voltage-clamp, while the oxidants 5-5'-dithiobis-2-nitrobenzoic acid (500 microM) and oxidized dithiothreitol (2 mM) caused inhibition. The endogenous antioxidant glutathione (5 mM) also enhanced GABArho(1) receptor-mediated currents while its oxidized form GSSG (3 mM) had inhibitory effects. All the effects were rapid and easily reversible. Redox modulation of GABArho(1) receptors was strongly dependent on the GABA concentration; dose-response curves for GABA were shifted to the left in the presence of reducing agents, whereas oxidizing agents produced the opposite effect, without changes in the maximal response to GABA and in the Hill coefficient. Our results demonstrate that, similarly to GABA(A) receptors and other members of the cys-loop receptor superfamily, GABA(C) receptors are subjected to redox modulation.

GABA(C) receptors in retina and brain

The expression of GABA(C) receptors has long been regarded as a specific property of bipolar cells in the inner retina where they control the information transfer from bipolar to retinal ganglion cells. A number of recent anatomical and physiological studies, however, have provided evidence that GABA(C) receptors are also expressed in many brain structures apart from the retina. The presence of GABA(C) receptors in many GABAergic neurons suggests that this receptor type may be involved in the regulation of local inhibition. This chapter focuses on the distribution of GABA(C) receptors and their possible function in various brain areas.

Prenatal GABAB1 and GABAB2 receptors: cellular and subcellular organelle localization in early fetal rat cortical neurons

Gamma-aminobutyric acid (GABA)(B) receptors appear to influence developmental events, depending on whether they are found at a synapse or in extrasynaptic areas. Little, if anything, is known as to the cellular and subcellular localization of GABA(B1) and GABA(B2) receptors during early fetal development. We used Western blots, immunohistochemistry, and postembedding immunoelectronmicroscopy to investigate fetal rat brain expression and distribution of these receptor proteins. GABA(B1) is expressed as early as gestational day (GD) 11.5 and 12.5, with immunoreactivity found in the all neuroepithelium, and a high expression in the mantel zone and the cortical area's plate; no immunolabeling for GABA(B2) receptor was observed. Our immunogold studies define a pattern of early GABA(B1) receptor protein in dendrite processes, endoplasmic reticulum, and axon terminals of the cortical neuroepithelium on GD 11.5. On GD 12.5, GABA(B1) receptor immunogold was found in dendrite processes, spines and tree, axon terminals, mitochondria, and intracellular organelles of the cortical neuroepithelium. No synapse formation was apparent as no synaptophysin could be found on either GD 11.5 or 12.5. We suggest that GABA(B1) has a functional role in the early fetal brain during neuronal proliferation and migration, and that it is different from the established functional GABA(B) receptor.

Mixed antagonistic effects of bilobalide at ρ1 GABAC receptor

Bilobalide was found to be a moderately potent antagonist with a weak use-dependent effect at recombinant human rho(1) GABA(C) receptors expressed in Xenopus oocytes using two-electrode voltage clamp methodology. Antagonism of bilobalide at homomeric rho(1) GABA(C) receptors appeared to be mixed. At low concentration, bilobalide (3 microM) caused a parallel right shift and surmountable GABA maximal response of the GABA dose-response curve characteristic of a competitive antagonist. At high concentrations, bilobalide (10-100 microM) caused nonparallel right shifts and reduced maximal GABA responses of GABA dose-response curves characteristic of a noncompetitive antagonist. The potency of bilobalide appears to be dependent on the concentrations of GABA and was more potent at lower GABA concentrations. The mechanism of action of bilobalide at rho(1) GABA(C) receptors appears to be similar to that of the chloride channel blocker picrotoxinin.

Ambient GABA promotes cortical entry of tangentially migrating cells derived from the medial ganglionic eminence

During corticogenesis, cells from the medial ganglionic eminence (MGE) migrate tangentially into the neocortical anlage. Here we report that gamma-aminobutyric acid (GABA), via GABAA receptors, regulates tangential migration. In embryonic telencephalic slices, bicuculline produced an outward current in migrating MGE-derived cells in the neocortex, suggesting the presence of and tonic activation by ambient GABA. Ambient GABA was also present in the MGE, although this required demonstration using as bioassay HEK293 cells expressing high-affinity alpha6/beta2/gamma2s recombinant GABAA receptors. The concentration of ambient GABA was 0.5+/-0.1 microM in both regions. MGE-derived cells before the corticostriate juncture (CSJ) were less responsive to GABA than those in the neocortex, and profiling of GABAA receptor subunit transcripts revealed different expression patterns in the MGE vis-à-vis the neocortex. These findings suggest a dynamic expression of GABAA receptor number or isoform as MGE-derived cells enter the neocortex and become tonically influenced by ambient GABA. Treatment with bicuculline or antibody against GABA did not affect migration of MGE-derived cells before the CSJ but decreased "crossing index," reflecting impeded migration past the CSJ into the neocortex. Treatment with diazepam or addition of exogenous GABA increased crossing index. We conclude that ambient GABA promotes cortical entry of tangentially migrating MGE-derived cells.

Functional characterization of rat ρ2 subunits expressed in HEK 293 cells

GABA(C) receptors are thought to be homo- or heteropentamers composed of rho1, rho2 and rho3 subunits. Previous work on rat rho2 subunits expressed in Xenopus oocytes has suggested that they do not form functional homo-oligomeric GABA(C) receptors, but do combine with rho1 or rho3 subunits to form hetero-oligomers. These findings are difficult to interpret because both human and mouse rho2 subunits do form functional homo-oligomeric receptors. Also, many regions of the rat brain express solely rho2 subunit transcripts which, according to presently available evidence, would not result in expression of functional GABA(C) receptors. We show here that homomeric rat rho2 receptors can be expressed in HEK 293 cells. Homo-oligomeric rat rho2 receptors expressed in mammalian cells matured slowly and displayed small but detectable GABA-induced currents with slow kinetics. Rat rho2 receptors also had a decreased sensitivity to picrotoxin and a marked sensitivity to the GABA(C) receptor agonist cis-aminocrotonic acid. Our results demonstrate for the first time the expression of functional homomeric rat rho2 receptors, and suggest that rho(2) subunits may contribute to brain function, including in areas not expressing other rho subunits.

GABA receptor rho subunit expression in the developing rat brain

Ionotropic GABA(C) receptors are composed of rho1, rho2 and rho3 subunits. Although the distribution of rho subunit mRNAs in the adult brain has been studied, information on the developmental regulation of different rho subunits in the brain is scattered and incomplete. Here, GABA(C) receptor rho subunit expression was studied in the developing rat brain. In situ hybridization on postnatal brain slices showed rho2 mRNA expression from newborn in superficial gray layer (SGL) of superior colliculus (SuC), and from the first postnatal week in the hippocampal CA1 region and pretectal nucleus of the optic tract. rho2 mRNA was also expressed in the adult dorsal lateral geniculate nucleus. Quantitative RT-PCR revealed expression of all three rho subunits in the hippocampus and superior colliculus from the first postnatal day. In the hippocampus, rho2 mRNA expression clearly dominated over rho1 and rho3, whereas in the superior colliculus, rho1 mRNA expression levels were similar to rho2. In both areas, a clear up-modulation of rho2 and rho3 mRNA during the first postnatal week was detected. GABA(C) receptor protein expression was confirmed in adult hippocampus, superior colliculus and dorsal lateral geniculate nucleus by immunohistochemistry. Our results demonstrate for the first time the expression of all three rho subunit mRNAs in several regions of the developing and adult rat brain. Our quantitative data allows assessment of putative subunit combinations in the superior colliculus and hippocampus. From the selective distribution of rho subunits, it may be hypothesized that GABA(C) receptors are specifically involved in aspects of visual image motion processing in the rat brain.

Evidence for inhibition mediated by coassembly of GABAA and GABAC receptor subunits in native central neurons

Fast inhibition in the nervous system is commonly mediated by GABA(A) receptors comprised of 2alpha/2beta/1gamma subunits. In contrast, GABA(C) receptors containing only rho subunits (rho1-rho3) have been predominantly detected in the retina. However, here using reverse transcription-PCR and in situ hybridization we show that mRNA encoding the rho1 subunit is highly expressed in brainstem neurons. Immunohistochemistry localized the rho1 subunit to neurons at light and electron microscopic levels, where it was detected at synaptic junctions. Application of the GABA(C) receptor agonist cis-4-aminocrotonic acid (100-800 microM) requires the rho1 subunit to elicit responses, which surprisingly are blocked independently by antagonists to GABA(A) (bicuculline, 10 microM) and GABA(C) [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA); 40-160 microM] receptors. Responses to GABA(C) agonists were also enhanced by the GABA(A) receptor modulator pentobarbitone (300 microM). Spontaneous and evoked IPSPs were reduced in amplitude but never abolished by TPMPA, but were completely blocked by bicuculline. We therefore tested the hypothesis that GABA(A) and GABA(C) subunits formed a heteromeric receptor. Immunohistochemistry indicated that rho1 and alpha1 subunits were colocalized at light and electron microscopic levels. Electrophysiology revealed that responses to GABA(C) receptor agonists were enhanced by the GABA(A) receptor modulator zolpidem (500 nm), which acts on the alpha1 subunit when the gamma2 subunit is also present. Finally, coimmunoprecipitation indicated that the rho1 subunit formed complexes that also containedalpha1 and gamma2 subunits. Taken together these separate lines of evidence suggest that the effects of GABA in central neurons can be mediated by heteromeric complexes of GABA(A) and GABA(C) receptor subunits.

Ionotropic GABA receptors with mixed pharmacological properties of GABAA and GABAC receptors

Ionotropic gamma-aminobutyric acid (GABA) receptors form a large family of molecular isoforms with distinct properties. We have characterized a distinct new type of GABA receptors in CA1 pyramidal cells in rat hippocampal slices. Somatic application of GABA induced currents which were partially suppressed by (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), a specific antagonist of GABA(C) receptors. This sensitivity was enhanced when we evoked the currents by the GABA(C) receptor agonist cis-4-aminocrotonic acid (CACA). However, both GABA- and CACA-evoked currents were sensitive towards bicuculline and thus lack the defining feature of GABA(C) receptors, which are insensitive towards this antagonist. Spontaneous miniature post-synaptic currents (mIPSCs) revealed a similar pharmacological behaviour. We conclude that juvenile CA1 pyramidal cells express a fraction of ionotropic GABA receptors with mixed pharmacological properties of both, GABA(A) and GABA(C) receptors.

Prenatal alteration and distribution of the GABA(B1) and GABA(B2) receptor subunit mRNAs during rat central nervous system development

The prenatal developmental expression changes and distribution of the gamma-aminobutyric acid (GABA)(B1) and GABA(B2) receptor subunit were investigated using in situ hybridization and RNase protection assay (RPA). We defined a different expression pattern of GABA(B1) subunit mRNA to that of GABA(B2) subunit. GABA(B1) subunit mRNA signals were moderately expressed in the cerebral cortex neuroepithelium of discrete brain regions on gestational day (GD) 11.5 and 12.5 and were highly expressed in the brain and spinal cord on GD 13.5 and 15.5. However, GABA(B2) subunit mRNAs were not detected on GD 11.5 and 12.5 and were first weakly detected on GD 13.5. On GD 15.5, 17.5, and 19.5, these subunit mRNAs were found in the retina, hippocampus, cerebral cortex, spinal cord, and cerebellum area. On GD 19.5 and 21.5, these subunits mRNA signals increased in the cerebral cortex, hippocampus, thalamus, and cerebellum, but decreased in the spinal cord, spinal ganglion, and midbrain, reaching similar levels to that of the adult brain. On GD 21.5, these subunit mRNAs were similarly expressed in almost all brain areas with a higher expression level of GABA(B1) subunit mRNA than GABA(B2) subunit mRNA. Our results found that GABA(B1) and GABA(B2) subunit mRNAs showed different expression patterns, with the GABA(B1) subunit expressed earlier and higher. We suggest that GABA(B1) and GABA(B2) subunits might have a role in the fetal brain during the gestational period for pre- and post-synaptogenesis, proliferation, differentiation, and neuronal maturation, and GABA(B1) subunit may be more important than GABA(B2) subunit during rat prenatal development.

GABAC receptor agonist suppressed ammonia-induced apoptosis in cultured rat hippocampal neurons by restoring phosphorylated BAD level

Ammonia-induced apoptosis and its prevention by GABAC receptor stimulation were examined using primary cultured rat hippocampal neurons. Ammonia (0.5-5 mm NH4Cl) dose-dependently induced apoptosis in pyramidal cell-like neurons as assayed by double staining with Hoechst 33258 and anti-neurofilament antibody. A GABAC receptor agonist, cis-4-aminocrotonic acid (CACA, 200 microm), but not GABAA and GABAB receptor agonists, muscimol (10 micro m) and baclofen (50 microm), respectively, inhibited the ammonia (2 mm)-induced apoptosis, and this inhibition was abolished by a GABAC receptor antagonist (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA, 15 microm). Expression of all three GABAC receptor subunits was demonstrated in the cultured neurons by RT-PCR. The ammonia-treatment also activated caspases-3 and -9 as observed in immunocytochemistry for PARP p85 and western blot. Such activation of the caspases was again inhibited by CACA in a TPMPA-sensitive manner. The anti-apoptotic effect of CACA was blocked by inhibitors for MAP kinase kinase and cAMP-dependent protein kinase, PD98059 (20 microm) and KT5720 (1 microm), suggesting possible involvement of an upstream pro-apoptotic protein, BAD. Levels of phospho-BAD (Ser112 and Ser155) were decreased by the ammonia-treatment and restored by coadministration of CACA. These findings suggest that GABAC receptor stimulation protects hippocampal pyramidal neurons from ammonia-induced apoptosis by restoring Ser112- and Ser155-phospho-BAD levels.

Early fetal expression of GABA(B1) and GABA(B2) receptor mRNAs on the development of the rat central nervous system

GABA(B) receptors are G-protein-coupled receptors that mediate slow onset and prolonged effects of GABA in the central nervous system (CNS). While they appear to influence developmental events, depending on where they are found at a synapse, little, if anything, is known as to the expression of GABA(B1) and GABA(B2) receptor mRNAs during the early developmental stages. We used in situ hybridization and RNase protection assays (RPA) to investigate the early fetal expression of GABA(B1) and GABA(B2) receptor mRNAs on the development of the rat CNS. Our in situ studies defined a pattern of early and strong GABA(B1) receptor mRNA expression in the spinal cord, medullar and cerebral cortex neuroepithelium of discrete brain regions on gestational day (GD) 11.5. On GD 12.5, GABA(B1) receptor mRNAs were found in the hippocampal formation, cerebral cortex, intermediate and posterior neuroepithelium, and the pontine neuroepithelium of whole brain. RPA results showed GABA(B1) receptor mRNA was intensely expressed on GD 11.5 and GD 12.5, when it was first detected in the ganglia, thalamus, and cerebellum. However, GABA(B2) receptor mRNA was not detected on GD 10.5, 11.5, or 12.5. We suggest that GABA(B1) receptor might have a role in the early fetal brain and spinal cord during pre- and post-synaptogenesis, neuronal maturation, proliferation, and migration, and may be more important than the GABA(B2) receptor in the early development of the rat CNS.

ZIP3, a new splice variant of the PKC-zeta-interacting protein family, binds to GABAC receptors, PKC-zeta, and Kv beta 2

The correct targeting of modifying enzymes to ion channels and neurotransmitter receptors represents an important biological mechanism to control neuronal excitability. The recent cloning of protein kinase C-zeta interacting proteins (ZIP1, ZIP2) identified new scaffolds linking the atypical protein kinase PKC-zeta to target proteins. GABA(C) receptors are composed of three rho subunits (rho 1-3) that are highly expressed in the retina, where they are clustered at synaptic terminals of bipolar cells. A yeast two-hybrid screen for the GABA(C) receptor rho 3 subunit identified ZIP3, a new C-terminal splice variant of the ZIP protein family. ZIP3 was ubiquitously expressed in non-neuronal and neuronal tissues, including the retina. The rho 3-binding region of ZIP3 contained a ZZ-zinc finger domain, which interacted with 10 amino acids conserved in rho 1-3 but not in GABA(A) receptors. Consistently, only rho 1-3 subunits bound to ZIP3. ZIP3 formed dimers with ZIP1-3 and interacted with PKC-zeta and the shaker-type potassium channel subunit Kv beta 2. Different domains of ZIP3 interacted with PKC-zeta and the rho 3 subunit, and simultaneous assembly of ZIP3, PKC-zeta and rho 3 was demonstrated in vitro. Subcellular co-expression of ZIP3 binding partners in the retina supported the proposed protein interactions. Our results indicate the formation of a ternary postsynaptic complex containing PKC-zeta, ZIP3, and GABA(C) receptors.

Gamma-aminobutyric acid-induced calcium signalling in rat superior collicular neurones

Ionotropic gamma-aminobutyric acid (GABA) receptors are known to mediate excitation in neonatal neurones as a crucial developmental factor. In the present study we employed calcium imaging techniques with the calcium indicator Fura-2-AM to study the pharmacology of GABA-induced calcium responses in cultures prepared from neonatal rat superficial superior colliculus (SC), after immunocytochemical labelling confirmed the presence of GABA(C) rho(1) subunits in 35% of neurones. Rises in neuronal intracellular calcium were obtained in response to GABA and also to the subtype-specific GABA(A) agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. However, the GABA(C) agonist cis-4-aminocrotonic acid induced calcium response only at unspecifically high concentrations (500 microM). Co-application of GABA antagonists revealed that both GABA(A&C) agonists' actions could be blocked by the GABA(A) antagonist bicuculline but not the GABA(C) antagonists 1,2,5,6-tetrahydro-(pyridin-4-yl) methylphosphinic acid. This suggests that activation of GABA(A) but not GABA(C) receptors contributes to excitatory GABA responses and related calcium signals in neonatal SC neurones.

Relative picrotoxin insensitivity distinguishes ionotropic GABA receptor-mediated IPSCs in hippocampal interneurons

Inhibitory GABAergic signalling in the hippocampus plays an important role in synchronizing principal cells and regulating the excitability of this seizure-prone structure. Distinct mechanisms modulate release from GABAergic terminals in the hippocampus, depending on whether the postsynaptic partner is an interneuron or a principal cell. Here, we report that postsynaptic ionotropic GABA receptors in principal cells and interneurons also show a striking pharmacological difference. The broad-spectrum antagonist picrotoxin (PTX) was less potent at blocking IPSCs evoked in stratum radiatum interneurons than in pyramidal neurons in the CA1 region. GABA-evoked currents in membrane patches from interneurons showed a smaller mean unitary conductance than in patches from pyramidal neurons. Because retinal GABA(C) receptors show decreased picrotoxin sensitivity and conductance, we examined the effect of the GABA(C) receptor agonist cis-aminocrotonic acid (CACA). Although this agent evoked picrotoxin-resistant currents in interneurons, these were enhanced by the GABA(A) allosteric modulator pentobarbital. Moreover, both picrotoxin-resistant IPSCs and CACA-evoked currents were blocked by the GABA(A) receptor-selective antagonist bicuculline. The presence of relatively picrotoxin-resistant GABA(A) receptors in interneurons provides a potential target for agents to modulate the activity of sub-populations of hippocampal neurons.

Phosphorylation of the recombinant rho1 GABA receptor

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian brain. While a growing body of literature indicates that postsynaptic GABA receptors are regulated by phosphorylation, there is discrepancy as to the specific effects of phosphorylation on GABA receptor function. Here, we have identified phosphorylation sites on the human rho1 GABA receptor for six protein kinases widely expressed in the brain: protein kinase C (PKC); cAMP-dependent protein kinase (PKA); calmodulin-dependent kinase (CaMKII); casein kinase (CKII); mitogen-activated protein kinase (MAPK); and cGMP-dependent protein kinase (PKG). We demonstrate that in nearly all cases, the consensus sites and actual phosphorylation sites do not agree supporting the risk of relying on a sequence analysis to identify potential phosphorylation sites. In addition, of the six kinases examined, only CKII phosphorylated the human rho2 subunit. Site-directed mutagenesis of the phosphorylation sites, or activation/inhibition of select kinase pathways, did not alter the receptor sensitivity or maximal GABA-activated current of the rho1 GABA receptor expressed in Xenopus laevis oocytes suggesting phosphorylation of rho1 does not directly alter receptor properties. This study is a first and necessary step towards elucidating the regulation of rho1 GABA receptors by phosphorylation.

Expression and dendritic mRNA localization of GABAC receptor rho1 and rho2 subunits in developing rat brain and spinal cord

The cellular distribution of GABAC receptor rho1 and rho2 subunits in the rat central nervous system remains controversial. We investigated how these subunits were distributed in cerebellum, hippocampus and spinal cord at postnatal day 1, 7 or in adult life. We found that in the adult cerebellum rho1 and rho2 mRNAs were expressed in Purkinje cells and basket-like cells only. In the hippocampus both subunits were expressed throughout the CA1 pyramidal layer, dentate gyrus and scattered interneurons with maximum staining intensity at P7. In the adult hippocampus in situ staining was predominantly found on interneurons. GABAC antibody labelling in P7 and adult hippocampus was largely overlapping with the in situ staining. Western blot analysis showed GABAC receptor in retina, ovary and testis. In the spinal cord the rho2 signal was consistently stronger than rho1 with overlapping expression patterns. At P1, the most intensely labelled cells were the motoneurons while on P7 and adult sections, interneurons and motoneurons were likewise labelled. On spinal neurons both rho1 and rho2 mRNAs showed somatodendritic localization, extending out for >100 microm with punctate appearance especially in adult cells. A similar spinal distribution pattern was provided with polyclonal antibody labelling, suggesting close correspondence between mRNA and protein compartmentalization. Electrophysiological experiments indicated that P1 spinal motoneurons did possess functional GABAC receptors even though GABAC receptors played little role in evoked synaptic transmission. Our results suggest a pattern of rho1 and rho2 subunit distribution more widespread than hitherto suspected with strong developmental regulation of subunit occurrence.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

gamma-Aminobutyric acidA rho receptor subunits in the developing rat hippocampus

The RT-PCR approach was used to estimate the expression of gamma-aminobutyric acid (GABA)(A) rho receptor subunits in the hippocampus of neonatal and adult rats. All three rho subunits were detected at postnatal day (P) 2, the rho3 subunit being expressed at an extremely low level. The rho1 and rho2 products appeared to be developmentally regulated; they were found to be more pronounced in adulthood. In another set of experiments, to correlate gene expression with receptor function, GABA(A) rho subunit mRNAs were detected with single-cell RT-PCR in CA3 pyramidal cells (from P3-P4 hippocampal slices), previously characterized with electrophysiological experiments for their bicuculline-sensitive or -insensitive responses to GABA. In 6 of 19 cells (31%), pressure application of GABA evoked at -70 mV inward currents that persisted in the presence of 100 microM bicuculline (314 plus minus 129 pA). RT-PCR performed in two of these neurons revealed the presence of rho1 and rho2 subunits, the latter being present with the alpha2 subunit. A rho2 subunit was also found in 1 neuron (among 9) exhibiting a response to GABA, which was completely abolished by bicuculline. This might be due to the lack of putative accessory GABA(A) subunits that can coassemble with rho2 to make functional receptors. Similar experiments from 10 P15 CA3 pyramidal cells failed to reveal any rho1-3 transcripts. However, these neurons abundantly express alpha3 subunits. It is likely that in CA3 pyramidal cells of neonatal and adult hippocampus GABA(A) rho subunits are present but at very low levels of expression.

Analysis of GABA(A)- and GABA(B)-receptor mediated effects on intracellular Ca(2+) in DRG hybrid neurones

1. Using pharmacological analysis and fura-2 spectrofluorimetry, we examined the effects of gamma-aminobutyric acid (GABA) and related substances on intracellular Ca(2+) concentration ([Ca(2+)]i) of hybrid neurones, called MD3 cells. The cell line was produced by fusion between a mouse neuroblastoma cell and a mouse dorsal root ganglion (DRG) neurone. 2. MD3 cells exhibited DRG neurone-like properties, such as immunoreactivity to microtubule-associated protein-2 and neurofilament proteins. Bath applications of capsaicin and alpha, beta-methylene adenosine triphosphate reversibly increased [Ca(2+)]i. However, repeated applications of capsaicin were much less effective. 3. Pressure applications of GABA (100 microM), (Z)-3-[(aminoiminomethyl) thio] prop-2-enoic acid sulphate (ZAPA; 100 microM), an agonist at low affinity GABA(A)-receptors, or KCl (25 mM), transiently increased [Ca(2+)]i. 4. Bath application of bicuculline (100 nM - 100 microM), but not picrotoxinin (10 - 25 microM), antagonized GABA-induced increases in [Ca(2+)]i in a concentration-dependent manner (IC(50)=9.3 microM). 5. Ca(2+)-free perfusion reversibly abolished GABA-evoked increases in [Ca(2+)]i. Nifedipine and nimodipine eliminated GABA-evoked increases in [Ca(2+)]i. These results imply GABA response dependence on extracellular Ca(2+). 6. Baclofen (500 nM - 100 microM) activation of GABA(B)-receptors reversibly attenuated KCl-induced increases in [Ca(2+)]i in a concentration-dependent manner (EC(50)=1.8 microM). 2-hydroxy-saclofen (1 - 20 microM) antagonized the baclofen-depression of the KCl-induced increase in [Ca(2+)]i. 7. In conclusion, GABA(A)-receptor activation had effects similar to depolarization by high external K(+), initiating Ca(2+) influx through high voltage-activated channels, thereby transiently elevating [Ca(2+)]i. GABA(B)-receptor activation reduced Ca(2+) influx evoked by depolarization, possibly at Ca(2+)-channel sites in MD3 cells.

Postnatal development of inhibitory synaptic transmission in the rostral nucleus of the solitary tract

To explore the postnatal development of inhibitory synaptic activity in the rostral (gustatory) nucleus of the solitary tract (rNST), whole cell and gramicidin perforated patch-clamp recordings were made in five age groups of rats [postnatal day 0-7 (P0-7), P8-14, P15-21, P22-30, and P >55]. The passive membrane properties of the developing rNST neurons as well as the electrophysiological and pharmacological characteristics of single and tetanic stimulus-evoked inhibitory postsynaptic potentials (IPSPs) were studied in brain slices under glutamate receptor blockade. During the first postnatal weeks, significant changes in resting membrane potential, spontaneous activity, input resistance, and neuron membrane time constant of the rNST neurons occurred. Although all the IPSPs recorded were hyperpolarizing, the rise and decay time constants of the single stimulus shock-evoked IPSPs decreased, and the inhibition response-concentration function to the gamma-aminobutyric acid (GABA) receptor antagonist bicuculline methiodide (BMI) shifted to the left during development. In P0-7 and P8-14, but not in older animals, the IPSPs had a BMI-insensitive component that was sensitive to block by picrotoxin, suggesting a transient expression of GABA(C) receptors. Tetanic stimulation resulted in both short- and long-term changes of inhibitory synaptic transmission in the rNST. For P0-7 and P8-14 animals tetanic stimulation resulted in a sustained hyperpolarization that was maintained for some time after termination of the tetanic stimulation. In contrast, tetanic stimulation of neurons in P15-21 and older animals resulted in hyperpolarization that was not sustained but decayed back to a more positive level with an exponential time course. Tetanic stimulation resulted in potentiation of single stimulus shock-evoked IPSPs in ~50% of neurons in all age groups. These developmental changes in inhibitory synaptic transmission in the rNST may play an important role in shaping synaptic activity in early development of the rat gustatory system during a time of maturation of taste preferences and aversions.

Recombinant GABA(C) receptors expressed in rat hippocampal neurons after infection with an adenovirus containing the human rho1 subunit

1. A recombinant adenovirus was generated with the human rho1 GABA(C) receptor subunit (adeno-rho). Patch-clamp and antibody staining were employed to confirm functional expression of recombinant rho1 receptors after infection of human embryonic kidney cells (HEK293 cell line), human embryonic retinal cells (911 cell line), dissociated rat hippocampal neurons and cultured rat hippocampal slices. 2. Standard whole-cell recording and Western blot analysis using rho1 GABA(C) receptor antibodies revealed that recombinant rho1 receptors were expressed in HEK293 and 911 cells after adeno-rho infection and exhibited properties similar to those of rho1 receptors after standard transfection. 3. Cultured rat hippocampal neurons (postnatal day (P)3-P5) did not show a native GABA(C)-like current. After adeno-rho infection, however, a GABA(C)-like current appeared in 70-90 % of the neurons. 4. Five days after infection, expression of GABA(C) receptors in hippocampal neurons significantly decreased native GABA(A) receptor currents from 1200 +/- 300 to 150 +/- 70 pA (n = 10). The native glutamate-activated current was unchanged. 5. Hippocampal slices (P8) did not show a native GABA(C)-like current, although recombinant rho1 receptors could be expressed in cultured hippocampal slices after adeno-rho infection. 6. These data indicate that an adenovirus can be used to express recombinant GABA(C) receptors in hippocampal neurons. This finding could represent an important step towards the gene therapy of CNS receptor-related diseases.

GABA(C) receptors: a molecular view

In the central nervous system inhibitory neurotransmission is primarily achieved through activation of receptors for gamma-aminobutyric acid (GABA). Three types of GABA receptors have been identified on the basis of their pharmacological and electrophysiological properties. The predominant type, termed GABA(A), and a recently identified GABA(C) type, form ligand-gated chloride channels, whereas GABA(B) receptors activate separate cation channels via G proteins. Based on their homology to nicotinic acetylcholine receptors, GABA(C) receptors are believed to be oligomeric protein complexes composed of five subunits in a pentameric arrangement. To date up to five different GABA(C) receptors subunits have been identified in various species. Recent studies have shed new light on the biological characteristics of GABA(C) receptors, including the chromosomal localization of its subunit genes and resulting links to deseases, the cloning of new splice variants, the identification of GABA(C) receptor-associated proteins, the identification of domains involved in subunit assembly, and finally structure/function studies examining functional consequences of introduced mutations. This review summarizes recent data in view of the molecular structure of GABA(C) receptors and presents new insights into the biological function of this protein in the retina.

Suppression of neuronal hyperexcitability and associated delayed neuronal death by adenoviral expression of GABA(C) receptors

The excessive neuronal excitation underlying several clinically important diseases is often treated with GABA allosteric modulators in an attempt to enhance inhibition. An alternative strategy would be to enhance directly the sensitivity of postsynaptic neurons to GABA. The GABA(C) receptor, normally found only in the retina, is more sensitive to GABA and demonstrates little desensitization compared with the GABA(A) receptor. We constructed an adenovirus vector that expressed cDNA for both the GABA(C) receptor rho(1) subunit and a green fluorescent protein (GFP) reporter and used it to transduce cultured hippocampal neurons. Transduced neurons were identified by fluorescence, double immunocytochemistry proved colocalization of the rho(1) protein and the reporter, Western blot verified the expected molecular masses, and electrophysiological and pharmacological properties confirmed the presence of functional GABA(C) receptors. rho(1)-GFP transduction resulted in an increased density of GABA(A) receptors as well as expression of novel GABA(C) receptors. This effect was not reproduced by addition of TTX or Mg(2+) to the culture medium to reduce action potentials or synaptic activity. In a model of neuronal hyperexcitability induced by chronic blockade of glutamate receptors, expression of GABA(C) receptors abolished the hyperactivity and the consequent delayed neuronal death. Adenovirus-mediated neuronal GABA(C) receptor engineering, via its dual mechanism of inhibition, may offer a way of inhibiting only those hyperexcitable neurons responsible for clinical problems, avoiding the generalized nervous system depression associated with pharmacological therapy.

Suppression of neuronal hyperexcitability and associated delayed neuronal death by adenoviral expression of GABA(C) receptors

The excessive neuronal excitation underlying several clinically important diseases is often treated with GABA allosteric modulators in an attempt to enhance inhibition. An alternative strategy would be to enhance directly the sensitivity of postsynaptic neurons to GABA. The GABA(C) receptor, normally found only in the retina, is more sensitive to GABA and demonstrates little desensitization compared with the GABA(A) receptor. We constructed an adenovirus vector that expressed cDNA for both the GABA(C) receptor rho(1) subunit and a green fluorescent protein (GFP) reporter and used it to transduce cultured hippocampal neurons. Transduced neurons were identified by fluorescence, double immunocytochemistry proved colocalization of the rho(1) protein and the reporter, Western blot verified the expected molecular masses, and electrophysiological and pharmacological properties confirmed the presence of functional GABA(C) receptors. rho(1)-GFP transduction resulted in an increased density of GABA(A) receptors as well as expression of novel GABA(C) receptors. This effect was not reproduced by addition of TTX or Mg(2+) to the culture medium to reduce action potentials or synaptic activity. In a model of neuronal hyperexcitability induced by chronic blockade of glutamate receptors, expression of GABA(C) receptors abolished the hyperactivity and the consequent delayed neuronal death. Adenovirus-mediated neuronal GABA(C) receptor engineering, via its dual mechanism of inhibition, may offer a way of inhibiting only those hyperexcitable neurons responsible for clinical problems, avoiding the generalized nervous system depression associated with pharmacological therapy.

Postnatal alterations of GABA receptor profiles in the rat superior colliculus

Midbrain sections taken from Sprague-Dawley rats of varying ages within the first four postnatal weeks were used to determine, immunocytochemically, putative changes of GABA(A) receptor beta2/3 subunits, GABA(B) receptor (R1a and R1b splice variants), and GABA(C) receptor rho1 subunit expression and distribution in the superficial, visual layers of the superior colliculus. Immunoreactivity for the GABA(A) receptor beta2/3 subunits was found in the superficial grey layer from birth. The labelling changed with age, with an overall continuous reduction in the number of cells labelled and a significant increase in the labelling intensity distribution (neuropil vs soma). Further analysis revealed an initial increase in the labelling intensity between postnatal days 0 and 7 in parallel with an overall reduction of labelled neurones. This was followed by a significant decrease in labelling intensity distribution between postnatal days 7 and 16, and a subsequent increase in intensity between postnatal days 16 and 28. The labelling profiles for GABA(B) receptors (R1a and R1b splice variants) and GABA(C) receptors (rho1 subunit) showed similar patterns. Both receptors could be found in the superficial layers of the superior colliculus from birth, and the intensity and distribution of labelling remained constant during the first postnatal month. However, the cell body count showed a significant decrease between postnatal days 7 and 16. These changes may be related to the time-point of eye opening, which occurred approximately two weeks after birth. For all three receptor types, the cell body count remained constant after postnatal day 16. By four weeks of age, there was no significant difference between the cell numbers obtained for the different receptors. Both GABA itself and neurofilament labelling were also obtained in the superficial superior colliculus at birth. Neurofilament, although found at birth, showed very little ordered arrangement until 16days after birth. When slices were double labelled for GABA(C) receptors and neurofilament, some overlap was observed. Double labelling for the presynaptic protein synaptophysin and GABA(C) receptors showed proximity in some places, indicative of a partly synaptic location of GABA(C) receptors. When GABA(C) and GABA(A) receptors were labelled simultaneously, some but not all neurones showed immunoreactivity for both receptor types. In conclusion, all three GABA receptor types were found to be present in the superior colliculus from birth, and all show some form of postnatal modification, with GABA(A) receptors demonstrating the most dramatic changes. However, GABA(B) and GABA(C) receptors are modified significantly around the onset of input-specific activity. Together, this points towards a contribution of the GABAergic system to processes of postnatal maturation in the superficial superior colliculus.

Evidence for coassembly of mutant GABAC rho1 with GABAA gamma2S, glycine alpha1 and glycine alpha2 receptor subunits in vitro

Functional coassembly of gamma-aminobutyric acid (GABA)C rho1 subunits with GABAA (alpha1, beta2, and gamma2S) or glycine (alpha1, alpha2, and beta) subunits was examined using two-electrode voltage-clamp recordings in the Xenopus laevis oocyte expression system. To facilitate this study, we took advantage of the unique gating and pharmacological properties of two mutant rho1 subunits, rho1(T314A) and rho1(T314A/L317A). When the rho1(T314A) subunit was coexpressed with GABA gamma2S, glycine alpha1 or glycine alpha2 subunits, GABA response properties were different from those of homomeric rho1(T314A) receptors. Additionally, the sensitivity of heteromeric rho1(T314A) and gamma2S receptors to picrotoxinin (PTX) blockade of GABA-evoked responses was altered compared to that of homomeric rho1(T314A) receptors. Changes in GABA response properties and picrotoxinin sensitivity were also observed when rho1(T314A) subunits were coexpressed with wild-type rho1 subunits. When rho1(T314A/L317A) subunits were coexpressed with GABA gamma2S, glycine alpha1 or glycine alpha2 subunits, suppression by GABA of spontaneously active current was reduced compared to that of homomeric rho1(T314A/L317A) receptors. Recovery of the spontaneous current from inhibition by GABA for GABA rho1(T314A/L317A)/gamma2S heteromeric receptors displayed an additional component. Coinjection of wild-type rho1 with gamma2S cRNAs at a ratio of 1 : 1 resulted in a > 10-fold reduction in GABA-evoked current. Furthermore, coexpression of wild-type rho1 and gamma2S subunits was found to shift the GABA dose-response curve. Our results provide functional evidence that the GABAC rho1 subunit can coassemble with the GABAA gamma2S subunit, and, at least in its mutated form, rho1 can also form heteromeric receptors with glycine alpha1 or alpha2 subunits in vitro.

'Specific' oligonucleotides often recognize more than one gene: the limits of in situ hybridization applied to GABA receptors

As exquisite probes for gene sequences, oligonucleotides are one of the most powerful tools of recombinant molecular biology. In studying the GABA receptor subunits in the neonatal hippocampus we have used oligonucleotide probes in in situ hybridization and cloning techniques. The oligonucleotides used and assumed to be specific for the target gene, actually recognized more than one gene, leading to surprising and contradictory results. In particular, we found that a GABA(A)-rho specific oligonucleotide recognized an abundant, previously unknown, transcription factor in both in situ and library screening, while oligos 'specific' for GABA(A) subunits were able to recognize 30 additional unrelated genes in library screening. This suggests that positive results obtained with oligonucleotides should be interpreted with caution unless confirmed by identical results with oligonucleotides from different parts of the same gene, or cDNA library screening excludes the presence of other hybridizing species.

GABA receptors inhibited by benzodiazepines mediate fast inhibitory transmission in the central amygdala

The amygdala is intimately involved in emotional behavior, and its role in the generation of anxiety and conditioned fear is well known. Benzodiazepines, which are commonly used for the relief of anxiety, are thought to act by enhancing the action of the inhibitory transmitter GABA. We have examined the properties of GABA-mediated inhibition in the amygdala. Whole-cell recordings were made from neurons in the lateral division of the central amygdala. Application of GABA evoked a current that reversed at the chloride equilibrium potential. Application of the GABA antagonists bicuculline or SR95531 inhibited the GABA-evoked current in a manner consistent with two binding sites. Stimulation of afferents to neurons in the central amygdala evoked an IPSC that was mediated by the release of GABA. The GABA(A) receptor antagonists bicuculline and picrotoxin failed to completely block the IPSC. The bicuculline-resistant IPSC was chloride-selective and was unaffected by GABA(B)-receptor antagonists. Furthermore, this current was insensitive to modulation by general anesthetics or barbiturates. In contrast to their actions at GABA(A) receptors, diazepam and flurazepam inhibited the bicuculline-resistant IPSC in a concentration-dependent manner. These effects were fully antagonized by the benzodiazepine site antagonist Ro15-1788. We conclude that a new type of ionotropic GABA receptor mediates fast inhibitory transmission in the central amygdala. This receptor may be a potential target for the development of new therapeutic strategies for anxiety disorders.

Permeability and single channel conductance of human homomeric rho1 GABAC receptors

1. Homomeric human rho1 GABAC receptors were expressed in Xenopus oocytes and in human embryonic kidney cells (HEK293) in order to examine their conductance and permeability. 2. Reversal potentials of currents elicited by gamma-aminobutyric acid (GABA) were measured in extracellular solutions of various ionic composition to determine relative permeability of homomeric rho1 receptors. The rank order of anionic permeability was: SCN- > I- > NO3- > Br- > Cl- > formate (For-) > HCO3- > acetate (Ac-) approximately proprionate (Prop-) approximately isethionate (Ise-) approximately F- approximately PO4-. 3. In the oocyte expression system, relative permeabilities to SCN-, I-, NO3-, Br- and HCO3- were higher for rho1 GABAC receptors than alpha1beta2gamma2L GABAA receptors. 4. Expression of rho1 GABAC receptors in Xenopus oocytes and in HEK293 cells gave similar relative permeabilities for selected anions, suggesting that the expression system does not significantly alter permeation properties. 5. The pore diameter of the homomeric rho1 GABAC receptor expressed in oocytes was estimated to be 0.61 nm, which is somewhat larger than the 0.56 nm pore diameter estimated for alpha1beta2gamma2L GABAA receptors. 6. Homomeric rho1 GABA receptors expressed in oocytes had a single channel chord conductance of 0.65 +/- 0.04 pS (mean +/- s.e.m.) when the internal chloride concentration ([Cl-]i) was 20 mM. With a [Cl-]i of 100 mM, the single channel chord conductance was 1.59 +/- 0.24 pS. 7. The mean open time directly measured from 43 GABA-induced channel openings in six patches was 3. 2 +/- 0.8 s. The mean open time in the presence of 100 microM picrotoxin was 0.07 +/- 0.01 s (77 openings from 3 patches). 8. The differences observed in ionic permeabilities, pore size, single channel conductance and mean open time suggest that the rho1 homomeric receptor may not be the native retinal GABAC receptor reported previously.

GABA receptors inhibited by benzodiazepines mediate fast inhibitory transmission in the central amygdala

The amygdala is intimately involved in emotional behavior, and its role in the generation of anxiety and conditioned fear is well known. Benzodiazepines, which are commonly used for the relief of anxiety, are thought to act by enhancing the action of the inhibitory transmitter GABA. We have examined the properties of GABA-mediated inhibition in the amygdala. Whole-cell recordings were made from neurons in the lateral division of the central amygdala. Application of GABA evoked a current that reversed at the chloride equilibrium potential. Application of the GABA antagonists bicuculline or SR95531 inhibited the GABA-evoked current in a manner consistent with two binding sites. Stimulation of afferents to neurons in the central amygdala evoked an IPSC that was mediated by the release of GABA. The GABA(A) receptor antagonists bicuculline and picrotoxin failed to completely block the IPSC. The bicuculline-resistant IPSC was chloride-selective and was unaffected by GABA(B)-receptor antagonists. Furthermore, this current was insensitive to modulation by general anesthetics or barbiturates. In contrast to their actions at GABA(A) receptors, diazepam and flurazepam inhibited the bicuculline-resistant IPSC in a concentration-dependent manner. These effects were fully antagonized by the benzodiazepine site antagonist Ro15-1788. We conclude that a new type of ionotropic GABA receptor mediates fast inhibitory transmission in the central amygdala. This receptor may be a potential target for the development of new therapeutic strategies for anxiety disorders.

Changing properties of GABA(A) receptor-mediated signaling during early neocortical development

Evidence from several brain regions suggests gamma-aminobutyric acid (GABA) can exert a trophic influence during development, expanding the role of this amino acid beyond its function as an inhibitory neurotransmitter. Proliferating precursor cells in the neocortical ventricular zone (VZ) express functional GABA(A) receptors as do immature postmigratory neurons in the developing cortical plate (CP); however, GABA(A) receptor properties in these distinct cell populations have not been compared. Using electrophysiological techniques in embryonic and early postnatal neocortex, we find that GABA(A) receptors expressed by VZ cells have a higher apparent affinity for GABA and are relatively insensitive to receptor desensitization compared with neurons in the CP. GABA-induced current magnitude increases with maturation with the smallest responses found in recordings from precursor cells in the VZ. No evidence was found that GABA(A) receptors on VZ cells are activated synaptically, consistent with previous data suggesting that these receptors are activated in a paracrine fashion by nonsynaptically released ligand. After neurons are born and migrate to the CP, they begin to demonstrate spontaneous synaptic activity, the majority of which is GABA(A) mediated. These spontaneous GABA(A) postsynaptic currents (sPSCs) first were detected at embryonic day 18 (E18). At birth, approximately 50% of recordings from cortical neurons demonstrated GABA(A)-mediated sPSCs, and this value increased with age. GABA(A)-mediated sPSCs were action potential dependent and arose from local GABAergic interneurons. GABA application could evoke action potential-dependent PSCs in neonatal cortical neurons, suggesting that during the first few postnatal days, GABA can act as an excitatory neurotransmitter. Finally, N-methyl-D-aspartate (NMDA)- but not non-NMDA-mediated sPSCs were also present in early postnatal neurons. These events were not observed in cells voltage clamped at negative holding potentials (-60 to -70 mV) but were evident when the holding potential was set at positive values (+30 to +60 mV). Together these results provide evidence for the early maturation of GABAergic communication in the neocortex and a functional change in GABA(A)-receptor properties between precursor cells and early postmitotic neurons. The change in GABA(A)-receptor properties may reflect the shift from paracrine to synaptic receptor activation.

Building blocks of pain: the regulation of key molecules in spinal sensory neurones during development and following peripheral axotomy

The pathways, synapses and molecules involved in pain processing in the newborn are not only required to trigger repair and recuperation but are also involved in the process of forming a mature nervous system. Sensory neurons in the dorsal root ganglion and dorsal horn express a phenomenal array of molecules which contribute to their structural and functional characteristics and many of these are developmentally regulated both pre- and postnatally. In order to understand nociceptive signalling and pain in the neonate we need a clear picture of that regulation. This review concentrates on the changing expression of selected key molecules, receptors and channels in the embryo, neonate and adult, which both characterise the sensory neuron and contribute to its response to painful stimuli in normal and pathological conditions.

Evidence for phosphorylation-dependent internalization of recombinant human rho1 GABAC receptors

1. Recombinant wild-type or mutant human rho1 GABA receptors were expressed in human embryonic kidney (HEK) 293 or monkey COS-7 cells and studied using the patch clamp technique. 2. Standard whole-cell recordings with 4 mM Mg-ATP in the patch pipette induced a time-dependent decrease in the GABA-activated current (IGABA) amplitude that was not the result of a decrease in GABA sensitivity. In contrast, IGABA remained stable when recordings were obtained using the perforated patch configuration or with standard whole-cell recording and no Mg-ATP in the patch pipette. 3. The inhibitors of serine/threonine protein kinases KN-62 (20 microM) or staurosporine (20 nM) prevented the time-dependent decrease in the amplitude of IGABA seen in the presence of ATP. Alkaline phosphatase (220 U ml-1), when added to the patch pipette in the absence of ATP, induced a transient potentiation of IGABA. Although the protein kinase C (PKC) activator 4beta-phorbol 12-myristate, 13-acetate (PMA) did not reduce the amplitude of IGABA, inclusion of the catalytic domain of PKC in the recording pipette accelerated the time-dependent decrease in current amplitude. These data suggest that phosphorylation is involved in the regulation of the amplitude of IGABA. 4. Mutation of the three PKC consensus sequences of the rho1 receptor had no significant effect on the decline in IGABA, indicating that direct phosphorylation of these putative sites on the rho1 receptor does not underlie the time-dependent decrease in amplitude. 5. In COS-7 cells transfected with wild-type rho1 receptors, the amplitude of IGABA had completely recovered to the original value when the same cells were repatched after 30-40 min, indicating that the decline in IGABA was a reversible process. 6. The inhibitor of actin filament formation cytochalasin B, when added to the patch pipette in the absence of ATP, induced a time-dependent inactivation suggesting that the actin cytoskeleton may play a role in the regulation of the amplitude. 7. Coincident with the decrease in the amplitude of IGABA, the cell capacitance significantly decreased in the presence of ATP in the patch pipette. This decrease in capacitance was not observed in the absence of Mg-ATP. The decrease in the membrane surface area suggests that receptor internalization could be a potential mechanism for the observed inactivation. 8. At 32 C, compared with 22 C, the rate and magnitude of the decline was increased dramatically. In contrast, at 16 C, no significant change in IGABA was observed over the 20 min recording time. This marked temperature sensitivity is consistent with receptor internalization as a mechanism for the time-dependent decline in IGABA. 9. The specificity of the decrease in IGABA was assessed by coexpressing the voltage-dependent potassium channel Kv1.4 along with the rho1 receptor in HEK293 cells. The amplitude of the potassium current (IKv1.4) exhibited very little decrement in comparison to IGABA suggesting that the putative GABA receptor internalization was not the consequence of a non-specific membrane retrieval.

GABA-Induced Cl- current in cultured embryonic human dorsal root ganglion neurons

gamma-Aminobutyric acid (GABA)-activated channels in embryonic (5-8 wk old) human dorsal root ganglion (DRG) neurons in dissociated culture were characterized by whole cell and single-channel techniques. All DRG neurons when held at negative holding membrane potentials displayed inward current to micromolar concentrations of GABA applied by pressure pulses from closely positioned micropipettes. The current was directly proportional to the concentration of GABA (EC50, 111 microM; Hill coefficient, 1.7). DRG neurons also responded to micromolar concentrations of pentobarbital and alphaxalone but not to cis-4-aminocrotonic acid (CACA), glycine, or taurine. Baclofen (100 microM) affected neither the holding currents nor K+ conductance (when patch pipettes were filled with 130 mM KCl) caused by depolarizing pulses. Whole cell GABA-currents were blocked by bicuculline, picrotoxin, and t-butylbicyclophosphorothionate (TBPS; all at 100 microM). The reversal potential of whole cell GABA-currents was close to the theoretical Cl- equilibrium potential, shifting with changes in intracellular Cl- concentration in a manner expected for Cl--selective channels. The whole cell I-V curve for GABA-induced currents demonstrated slight outward rectification with nearly symmetrical outside and inside Cl- concentrations. Spectral analysis of GABA-induced membrane current fluctuations showed that the kinetic components were best fitted by a triple Lorentzian function. The apparent elementary conductance for GABA-activated Cl- channels determined from the power spectra was 22.6 pS. Single-channel recordings from cell-attached patches with pipettes containing 10 microM GABA indicated that GABA-activated channels have a main and a subconductance level with values of 30 and 19 pS, respectively. Mean open and closed times of the channel were characterized by two or three exponential decay functions, suggesting two or three open channel states and two closed states. Single channels showed a lack of rectification. The actions of GABA on cultured human embryonic DRG neurons are mediated through the activation of GABAA receptors with properties corresponding to those found in the CNS of human and other mammalian species but differing from those of cultured human adult DRG neurons.

Effects of the conformationally restricted GABA analogues, cis- and trans-4-aminocrotonic acid, on GABA neurotransmission in primary neuronal cultures

The effects of the GABA analogues, cis- and trans-4-aminocrotonic acid (ACA) on GABA(A) receptor function and GABA uptake, together with the presence of p-1 subunit mRNA and putative GABAc receptors, were studied in primary cultures of neocortical neurons and cerebellar granule cells. Both isomers induced a Cl- influx, which was inhibited by bicuculline, t-butylbicyclophosphorothionate (TBPS), picrotoxinin (PTX), and gamma-hexachlorocyclohexane (gamma-HCH or lindane). [3H]-flunitrazepam binding was also increased by both isomers and this increase was inhibited by bicuculline. In neocortical neurons, the transisomer completely inhibited the [3H]GABA uptake, whereas the cis-isomer produced only a 25% inhibition at the highest concentration used. The possible presence of GABAc receptors was investigated only in neocortical cultures by using RT-PCR in order to detect the presence of the mRNA encoding the p-1 subunit which assembles to form homooligomeric Cl-channels. The results presented here show that p-1 subunits, and thus GABAc receptors, may represent a very minor population of GABA receptors in these neuronal preparations. We conclude that both GABA analogues may act as agonists at the GABA(A) receptors, although with very different potencies.

Noninvasive measurements of the membrane potential and GABAergic action in hippocampal interneurons

Neurotransmitters affect the membrane potential (Vm) of target cells by modulating the activity of receptor-linked ion channels. The direction and amplitude of the resulting transmembrane current depend on the resting level of Vm and the gradient across the membrane of permeant ion species. Vm, in addition, governs the activation state of voltage-gated channels. Knowledge of the exact level of Vm is therefore crucial to evaluate the nature of the neurotransmitter effect. However, the traditional methods to measure Vm, with microelectrodes or the whole-cell current-clamp technique, have the drawback that the recording pipette is in contact with the cytoplasm, and dialysis with the pipette solution alters the ionic composition of the interior of the cell. Here we describe a novel technique to determine the Vm of an intact cell from the reversal potential of K+ currents through a cell-attached patch. Applying the method to interneurons in hippocampal brain slices yielded more negative values for Vm than subsequent whole-cell current-clamp measurements from the same cell, presumably reflecting the development of a Donnan potential between cytoplasm and pipette solution in the whole-cell mode. Cell-attached Vm measurements were used to study GABAergic actions in intact CA1 interneurons. In 1- to 3-week-old rats, bath-applied GABA inhibited these cells by stabilizing Vm at a level depending on contributions from both GABAA and GABAB components. In contrast, in 1- to 4-d-old animals, only GABAA receptors were activated resulting in a depolarizing GABA response.

Developmental study of miniature IPSCs of CA3 hippocampal cells: modulation by midazolam

Maturation of GABAA/benzodiazepine receptors is associated with changes in their subunit composition. We have investigated whether these changes are accompanied by a developmental modification in the kinetic properties of miniature IPSCs (mIPSCs) and sensitivity to midazolam, a benzodiazepine agonist. In the presence of TTX (10 microM) and excitatory amino acid antagonists, AP5 (50 microM) and CNQX (50 microM), we whole-cell recorded mIPSCs in CA3 cells of hippocampal slices of adult and young (4-8 days) rats. mIPSCs were mediated by GABAA receptors as they were suppressed by bicuculline (10 microM). In both the adult and young rats, mIPSCs were similar in amplitude and kinetic properties. However, the mIPSCs frequency markedly increased with age from 4+/-3 Hz in the young rats to 20+/-9 Hz in the adult rats. In both age groups, midazolam (0.01 microM(-10) microM) and pentobarbital (30 microM) did not affect the amplitude, frequency and rise time of the mIPSCs but they increased to a similar extent their decay time constant. The current responses to isoguvacine, a GABAA agonist, were potentiated by 0.1 microM midazolam in both age groups. It is concluded that in immature and adult rats, synaptic GABAA receptors of CA3 were not different in their kinetic properties and sensitivity to midazolam.

Noninvasive measurements of the membrane potential and GABAergic action in hippocampal interneurons

Neurotransmitters affect the membrane potential (Vm) of target cells by modulating the activity of receptor-linked ion channels. The direction and amplitude of the resulting transmembrane current depend on the resting level of Vm and the gradient across the membrane of permeant ion species. Vm, in addition, governs the activation state of voltage-gated channels. Knowledge of the exact level of Vm is therefore crucial to evaluate the nature of the neurotransmitter effect. However, the traditional methods to measure Vm, with microelectrodes or the whole-cell current-clamp technique, have the drawback that the recording pipette is in contact with the cytoplasm, and dialysis with the pipette solution alters the ionic composition of the interior of the cell. Here we describe a novel technique to determine the Vm of an intact cell from the reversal potential of K+ currents through a cell-attached patch. Applying the method to interneurons in hippocampal brain slices yielded more negative values for Vm than subsequent whole-cell current-clamp measurements from the same cell, presumably reflecting the development of a Donnan potential between cytoplasm and pipette solution in the whole-cell mode. Cell-attached Vm measurements were used to study GABAergic actions in intact CA1 interneurons. In 1- to 3-week-old rats, bath-applied GABA inhibited these cells by stabilizing Vm at a level depending on contributions from both GABAA and GABAB components. In contrast, in 1- to 4-d-old animals, only GABAA receptors were activated resulting in a depolarizing GABA response.