Developmental alteration in GABAA receptor structure and physiological properties in cultured cerebellar granule neurons

GABA modulation of SVZ-derived progenitor ventral cell migration

The subventricular zone (SVZ) is a proliferative region that provides neurons to olfactory bulb throughout life. The new neurons undergo cell migration from SVZ and travel until they reach their final destination. We previously showed in the early postnatal mouse a ventral migratory subpopulation from SVZ targets the Islands of Calleja (ICC) in the basal forebrain. However, unlike the well-characterized rostral migratory stream, little is known about the guidance mechanisms operating in the ventrally directed migratory pathway. In this study, we examined the role of neurotransmitter γ-aminobutyric acid (GABA) in SVZ-derived progenitor ventral migration and the involvement of this neurotransmitter in the cytoarchitectual organization of dispersed cells into the tight clusters of the ICC. Our results show that the ventral SVZ cell migration rate was enhanced by GABA acting through a GABAA receptor and that GABA acts as a directional guidance cue for ventral migrating cells. Furthermore, disruption of GABA signaling inhibited the formation of Island clusters in vitro. Taken together, these data suggest that GABA is an important guidance and organizational cue for the Island of Calleja.

Pachymic Acid Enhances Pentobarbital-Induced Sleeping Behaviors via GABAA-ergic Systems in Mice

This study was investigated to know whether pachymic acid (PA), one of the predominant triterpenoids in Poria cocos (Hoelen) has the sedative-hypnotic effects, and underlying mechanisms are mediated via γ-aminobutyric acid (GABA)-ergic systems. Oral administration of PA markedly suppressed locomotion activity in mice. This compound also prolonged sleeping time, and reduced sleep latency showing synergic effects with muscimol (0.2 mg/kg) in shortening sleep onset and enhancing sleep time induced by pentobarbital, both at the hypnotic (40 mg/kg) and sub-hypnotic (28 mg/kg) doses. Additionally, PA elevated intracellular chloride levels in hypothalamic primary cultured neuronal cells of rats. Moreover, Western blotting quantitative results showed that PA increased the amount of protein level expression of GAD_65/67 over a broader range of doses. PA increased α- and β-subunits protein levels, but decreased γ-subunit protein levels in GABA_A receptors. The present experiment provides evidence for the hypnotic effects as PA enhanced pentobarbital-induced sleeping behaviors via GABA_A-ergic mechanisms in rodents. Taken together, it is proposed that PA may be useful for the treatment of sleep disturbed subjects with insomnia.

Time-dependent modulation of GABA(A)-ergic synaptic transmission by allopregnanolone in locus coeruleus neurons of Mecp2-null mice

Rett syndrome (RTT) is a neurodevelopmental disorder with symptoms starting 6-18 mo after birth, while what underlies the delayed onset is unclear. Allopregnanolone (Allop) is a metabolite of progesterone and a potent modulator of GABAA-ergic currents whose defects are seen in RTT. Allop changes its concentration during the perinatal period, which may affect central neurons via the GABAA-ergic synaptic transmission, contributing to the onset of the disease. To determine whether Mecp2 disruption affects Allop modulation, we performed studies in brain slices obtained from wild-type (WT) and Mecp2(-/Y) mice. Allop dose dependently suppressed locus coeruleus (LC) neuronal excitability in WT mice, while Mecp2-null neurons showed significant defects. Using optogenetic approaches, channelrhodopsin was specifically expressed in GABA-ergic neurons in which optical stimulation evoked action potentials. In LC neurons of WT mice, Allop exposure increased the amplitude of GABAA-ergic inhibitory postsynaptic currents (IPSCs) evoked by optical stimulation and prolonged the IPSC decay time. Consistently, Allop augmented both frequency and amplitude of GABAA-ergic spontaneous IPSCs (sIPSCs) and extended the decay time of sIPSCs. The Allop-induced potentiation of sIPSCs was deficient in Mecp2(-/Y) mice. Surprisingly, the impairment occurred at 3 wk postnatal age, while no significant difference in Allop modulation was observed in 1-2 wk between WT and Mecp2(-/Y) mice. These results indicate that the modulation of GABAA-ergic synaptic transmission by Allop is impaired in LC neurons of Mecp2-null mice at a time when RTT-like symptoms manifest, suggesting a potential mechanism for the delayed onset of the disease.

Cultures of cerebellar granule neurons

INTRODUCTIONPrimary cultures of granule neurons from the post-natal rat cerebellum provide an excellent model system for molecular and cell biological studies of neuronal development and function. The cerebellar cortex, with its highly organized structure and few neuronal subtypes, offers a well-characterized neural circuitry. Many fundamental insights into the processes of neuronal apoptosis, migration, and differentiation in the mammalian central nervous system have come from investigating granule neurons in vitro. Granule neurons are the most abundant type of neurons in the brain. In addition to the sheer volume of granule neurons, the homogeneity of the population and the fact that they can be transfected with ease render them ideal for elucidating the molecular basis of neuronal development. This protocol for isolating granule neurons from post-natal rats is relatively straightforward and quick, making use of standard enzymatic and mechanical dissociation methods. In a serum-based medium containing an inhibitor of mitosis, cerebellar granule neurons can be maintained with high purity. Axons and dendrites can be clearly distinguished on the basis of morphology and markers. For even greater versatility, this protocol for culturing granule neurons can be combined with knockout or transgenic technologies, or used in cerebellar slice overlay assays.

Rat alpha6beta2delta GABAA receptors exhibit two distinct and separable agonist affinities

The onset of motor learning in rats coincides with exclusive expression of GABAA receptors containing alpha6 and delta subunits in the granule neurons of the cerebellum. This development temporally correlates with the presence of a spontaneously active chloride current through alpha6-containing GABAA receptors, known as tonic inhibition. Here we report that the coexpression of alpha6, beta2, and delta subunits produced receptor-channels which possessed two distinct and separable states of agonist affinity, one exhibiting micromolar and the other nanomolar affinities for GABA. The high-affinity state was associated with a significant level of spontaneous channel activity. Increasing the level of expression or the ratio of beta2 to alpha6 and delta subunits increased the prevalence of the high-affinity state. Comparative studies of alpha6beta2delta, alpha1beta2delta, alpha6beta2gamma2, alpha1beta2gamma2 and alpha4beta2delta receptors under equivalent levels of expression demonstrated that the significant level of spontaneous channel activity is uniquely attributable to alpha6beta2delta receptors. The pharmacology of spontaneous channel activity arising from alpha6beta2delta receptor expression corresponded to that of tonic inhibition. For example, GABAA receptor antagonists, including furosemide, blocked the spontaneous current. Further, the neuroactive steroid 5alpha-THDOC and classical glycine receptor agonists beta-alanine and taurine directly activated alpha6beta2delta receptors with high potency. Specific mutation within the GABA-dependent activation domain (betaY157F) impaired both low- and high-affinity components of GABA agonist activity in alpha6betaY157Fdelta receptors, but did not attenuate the spontaneous current. In comparison, a mutation located between the second and third transmembrane segments of the delta subunit (deltaR287M) significantly diminished the nanomolar component and the spontaneous activity. The possibility that the high affinity state of the alpha6beta2delta receptor modulates the granule neuron activity as well as potential mechanisms affecting its expression are discussed.

Role of glutathione in determining the differential sensitivity between the cortical and cerebellar regions towards mercury-induced oxidative stress

Certain discrete areas of the CNS exhibit enhanced sensitivity towards MeHg. To determine whether GSH is responsible for this particular sensitivity, we investigated its role in MeHg-induced oxidative insult in primary neuronal and astroglial cell cultures of both cerebellar and cortical origins. For this purpose, ROS and GSH were measured with the fluorescent indicators, CMH(2)DCFDA and MCB. Cell associated-MeHg was measured with (14)C-radiolabeled MeHg. The intracellular GSH content was modified by pretreatment with NAC or DEM. For each of the dependent variables (ROS, GSH, and MTT), there was an overall significant effect of cellular origin, MeHg and pretreatment in all the cell cultures. A trend towards significant interaction between originxMeHgxpretreatment was observed only for the dependent variable, ROS (astrocytes p=0.056; neurons p=0.000). For GSH, a significant interaction between originxMeHg was observed only in astrocytes (p=0.030). The cerebellar cell cultures were more vulnerable (astrocytes(mean)=223.77; neurons(mean)=138.06) to ROS than the cortical cell cultures (astrocytes(mean)=125.18; neurons(mean)=107.91) for each of the tested treatments. The cell associated-MeHg increased when treated with DEM, and the cerebellar cultures varied significantly from the cortical cultures. Non-significant interactions between originxMeHgxpretreatment for GSH did not explain the significant interactions responsible for the increased amount of ROS produced in these cultures. In summary, although GSH modulation influences MeHg-induced toxicity, the difference in the content of GSH in cortical and cerebellar cultures fails to account for the increased ROS production in cerebellar cultures. Hence, different approaches for the future studies regarding the mechanisms behind selectivity of MeHg have been discussed.

Positioning of the alpha-subunit isoforms confers a functional signature to gamma-aminobutyric acid type A receptors

Fast synaptic inhibitory transmission in the CNS is mediated by gamma-aminobutyric acid type A (GABA(A)) receptors. They belong to the ligand-gated ion channel receptor superfamily, and are constituted of five subunits surrounding a chloride channel. Their clinical interest is highlighted by the number of therapeutic drugs that act on them. It is well established that the subunit composition of a receptor subtype determines its pharmacological properties. We have investigated positional effects of two different alpha-subunit isoforms, alpha(1) and alpha(6), in a single pentamer. For this purpose, we used concatenated subunit receptors in which subunit arrangement is predefined. The resulting receptors were expressed in Xenopus oocytes and analyzed by using the two-electrode voltage-clamp technique. Thus, we have characterized gamma(2)beta(2)alpha(1)beta(2)alpha(1), gamma(2)beta(2)alpha(6)beta(2)alpha(6), gamma(2)beta(2)alpha(1)beta(2)alpha(6), and gamma(2)beta(2)alpha(6)beta(2)alpha(1) GABA(A) receptors. We investigated their response to the agonist GABA, to the partial agonist piperidine-4-sulfonic acid, to the noncompetitive inhibitor furosemide and to the positive allosteric modulator diazepam. Each receptor isoform is characterized by a specific set of properties. In this case, subunit positioning provides a functional signature to the receptor. We furthermore show that a single alpha(6)-subunit is sufficient to confer high furosemide sensitivity, and that the diazepam efficacy is determined exclusively by the alpha-subunit neighboring the gamma(2)-subunit. By using this diagnostic tool, it should become possible to determine the subunit arrangement of receptors expressed in vivo that contain alpha(1)- and alpha(6)-subunits. This method may also be applied to the study of other ion channels.

Time windows and reverberating loops: a reverse-engineering approach to cerebellar function

We review a reverse-engineering approach to cerebellar function that pays particular attention to temporal aspects of neuronal interactions. This approach offers new vistas on the role of GABAergic synapses and reverberating projections within the olivo-cerebellar system. More specifically, our simulations show that Golgi cells can control the ring time of granule cells rather than their ring rate and that Purkinje cells can trigger precisely timed rebound spikes in neurons of the deep cerebellar nuclei. This rebound activity can reverberate back to the cerebellar cortex giving rise to a complex oscillatory dynamics that may have interesting functional implications for working memory and timed-response tasks.

Loss of GABAergic neuronal phenotype in primary cerebellar cultures following blockade of glutamate reuptake

Prolonged inhibition of glutamate reuptake by L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a specific glutamate transporter blocker, reduced the number of GABA positive neurons in a primary cerebellar culture by 54%. The disappearance of immunostaining for GABA was gradual and was partially prevented by the N-methyl-D-aspartate (NMDA) receptor blocker, MK-801, and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist, NBQX. Combined blockade of NMDA and AMPA receptors restored the original proportion of GABAergic neurons observed in control cultures. Following the PDC exposure, expression of other GABAergic markers, such as glutamic acid decarboxylase (GAD) and vesicular GABA transporter (VGAT) was also dramatically decreased in an AMPA receptor-dependent manner. Loss of GABA or GAD immunostaining is commonly regarded as a sign of degeneration of GABAergic neurons. However, none of the GABAergic neurons were positive for propidium iodide uptake or showed abnormal nuclear morphology. Based on the above data we conclude that prolonged activation of ionotropic glutamate receptors by endogenously released glutamate was not toxic to cerebellar GABAergic neurons, but lead to the loss of their characteristic neurotransmitter phenotype.

Actions and interactions of extracellular potassium and kainate on expression of 13 gamma-aminobutyric acid type A receptor subunits in cultured mouse cerebellar granule neurons

Cerebellar granule neurons in culture are a popular model for studying neuronal signaling and development. Depolarizing concentrations of K(+) are routinely used to enhance cell survival, and kainate is sometimes added to eliminate GABAergic neurons. We have investigated the effect of these measures on expression of mRNA for gamma-aminobutyric acid type A (GABA(A)) receptor alpha1-6, beta1-3, gamma1-3, and delta subunits in cultures of mouse cerebellar granule neurons grown for 7 or 12 days in vitro (DIV) using semiquantitative reverse transcription-PCR. We detected mRNA for the alpha1, alpha2, alpha5, alpha6, beta2, beta3, gamma2, and delta subunits in all the cell cultures, but the expression levels of the alpha5-, alpha6-, and beta2-subunit mRNAs were significantly dependent on the composition of the culture medium. Both an increase of the extracellular K(+) concentration from 5 to 25 mm and the addition of 50 microm kainate immediately depolarized the neurons but prolonged exposure (7-8 DIV)-induced compensatory hyperpolarization. 25 mm K(+) caused a shift from alpha6 to alpha5 expression measured at 7 and 12 DIV, which was mimicked by kainate in 12 DIV cultures. The expression of beta2 was decreased by 25 mm K(+) in 7 DIV cultures and by kainate in 12 DIV cultures. The effects on beta2 expression could not be ascribed to depolarization. Alterations of alpha6 mRNA expression were reflected in altered sensitivity to GABA and furosemide of the resulting receptors. Our study has shown that a depolarizing K(+) concentration as well as kainate in the culture medium significantly disturbs maturation of GABA(A) receptor subunit expression.

Neuronal activity and its influence on developmentally regulated GABA(A) receptor expression in cultured mouse cerebellar granule cells

Mouse cerebellar granule cells (CGCs) were cultured in either non-depolarising (5 mM KCl, '5K') or depolarising (25 mM KCl, '25K') media. CGCs at 5K developed an elaborate network of processes and formed compact cell aggregates, whilst at 25K, cell aggregation was rarely observed. GABA(A) receptor (GABAR) expression was significantly affected by the culture conditions. Depolarisation resulted in a approximately 55% reduction in the expression of total specific [(3)H]Ro15-4513 binding sites, largely due to a >85% loss of the flunitrazepam-insensitive (BZ-IS) subtype of binding sites. The number of flunitrazepam-sensitive (BZ-S) [(3)H]Ro15-4513 binding sites expressed and the K(D) of [(3)H]Ro15-4513 to either GABAR subtype were not significantly affected. The BZ-S subtype expressed by 5K CGCs was essentially all type I, as expected of mature CGCs, however at 25K, these were predominantly type II (approximately 70%) and zolpidem-insensitive (approximately 30%)-pharmacological finger-prints of immature CGCs. By immunoblotting we determined that CGCs cultured at 25K expressed GABAR alpha1, alpha6 and beta3 subunits at 14, 3 and 167% of 5K levels, respectively. GABAR beta2 subunits, however, were barely detectable. The changes in GABAR subunit expression were paralleled by similar changes in the steady-state levels of the subunit mRNAs. The switch from type I to type II BZ-S pharmacology upon depolarisation was mirrored by a switch in gene expression from alpha1 (12% of 5K) to alpha3 (371% of 5K). Interestingly, although depolarisation reduced beta2 subunit mRNA to 25% of 5K, beta2 protein was undetectable.Thus, we have shown that electrically active (5K) mouse CGCs differentiate in vitro to express a GABAR profile expected of adult CGCs in vivo. Chronically depolarised, electrically silent CGCs appear to be developmentally arrested, expressing some GABAR characteristics of prenatal CGCs. This model system should prove invaluable for elucidating signalling pathways orchestrating GABAR differentiation.

The anti-epileptic drug levetiracetam reverses the inhibition by negative allosteric modulators of neuronal GABA- and glycine-gated currents

1. In this study in vitro and in vivo approaches were combined in order to investigate if the anti-epileptic mechanism(s) of action of levetiracetam (LEV; Keppra) may involve modulation of inhibitory neurotransmission. 2. GABA- and glycine-gated currents were studied in vitro using whole-cell patch-clamp techniques applied on cultured cerebellar granule, hippocampal and spinal neurons. Protection against clonic convulsions was assessed in vivo in sound-susceptible mice. The effect of LEV was compared with reference anti-epileptic drugs (AEDs): carbamazepine, phenytoin, valproate, clonazepam, phenobarbital and ethosuximide. 3. LEV contrasted the reference AEDs by an absence of any direct effect on glycine-gated currents. At high concentrations, beyond therapeutic relevance, it induced a small reduction in the peak amplitude and a prolongation of the decay phase of GABA-gated currents. A similar action on GABA-elicited currents was observed with the reference AEDs, except ethosuximide. 4. These minor direct effects contrasted with a potent ability of LEV (EC(50)=1 - 10 microM) to reverse the inhibitory effects of the negative allosteric modulators zinc and beta-carbolines on both GABA(A) and glycine receptor-mediated responses. 5. Clonazepam, phenobarbital and valproate showed a similar ability to reverse the inhibition of beta-carbolines on GABA-gated currents. Blockade of zinc inhibition of GABA responses was observed with clonazepam and ethosuximide. Phenytoin was the only AED together with LEV that inhibited the antagonism of zinc on glycine-gated currents and only clonazepam and phenobarbital inhibited the action of DMCM. 6. LEV (17 mg kg(-1)) produced a potent suppression of sound-induced clonic convulsions in mice. This protective effect was significantly abolished by co-administration of the beta-carboline FG 7142, from a dose of 5 mg kg(-1). In contrast, the benzodiazepine receptor antagonist flumazenil (up to 10 mg kg(-1)) was without any effect on the protection afforded by LEV. 7. The results of the present study suggest that a novel ability to oppose the action of negative modulators on the two main inhibitory ionotropic receptors may be of relevance for the anti-epileptic mechanism(s) of action of LEV.

Organotypic rat cerebellar slice culture as a model to analyze the molecular pharmacology of GABAA receptors

The preservation of the neuronal circuitry in rat cerebellar slice cultures provides an advantage in monitoring the development and characterizing the pharmacology of GABA(A) receptor subtypes. Sprague-Dawley rats, 8-11 days of age, were decapitated, their cerebella were cut into 400-microm slices and transferred into culture dishes. Cell viability and organotypic cerebellar organization of the culture remained well preserved up to 3 weeks. Autoradiographic procedures were introduced in these advanced culture technique and employed [(3)H]Ro 15-4513 in the absence and presence of 10 microM diazepam to visualize all benzodiazepine (BZD) and diazepam-insensitive (DIS) binding sites, respectively. Since expression of the alpha6 subunit variant of the GABA(A)/BZD receptor is restricted to the cerebellar granule cells and the BZD receptor agonist diazepam has very low affinity for this subunit, changes in DIS [(3)H]Ro 15-4513 binding sites during cultivation time can be attributed to changes in alpha6 subunit expression. A time-dependent development of total and DIS [(3)H]Ro 15-4513 binding sites were observed in the culture with a trend towards an increase in GABA(A) receptor alpha6 subunit levels during the first week. These findings suggest that explant preparations can be used to examine morphological changes in rat cerebellar slices. In addition, these preparations can be utilized to study the pharmacological effects of GABA(A)/BZD selective drugs on postnatal development of GABA(A) receptors in rat cerebellum.

Enhanced neurosteroid potentiation of ternary GABA(A) receptors containing the delta subunit

Attenuated behavioral sensitivity to neurosteroids has been reported for mice deficient in the GABA(A) receptor delta subunit. We therefore investigated potential subunit-specific neurosteroid pharmacology of the following GABA(A) receptor isoforms in a transient expression system: alpha1beta3gamma2L, alpha1beta3delta, alpha6beta3gamma2L, and alpha6beta3delta. Potentiation of submaximal GABA(A) receptor currents by the neurosteroid tetrahydrodeoxycorticosterone (THDOC) was greatest for the alpha1beta3delta isoform. Whole-cell GABA concentration--response curves performed with and without low concentrations (30 nm) of THDOC revealed enhanced peak GABA(A) receptor currents for isoforms tested without affecting the GABA EC50. Alpha1beta3delta currents were enhanced the most (>150%), whereas the other isoform currents were enhanced 15-50%. At a higher concentration (1 microm), THDOC decreased peak alpha1beta3gamma2L receptor current amplitude evoked by GABA (1 mm) concentration jumps and prolonged deactivation but had little effect on the rate or extent of apparent desensitization. Thus the polarity of THDOC modulation depended on GABA concentration for alpha1beta3gamma2L GABA(A) receptors. However, the same protocol applied to alpha1beta3delta receptors resulted in peak current enhancement by THDOC of >800% and prolonged deactivation. Interestingly, THDOC induced pronounced desensitization in the minimally desensitizing alpha1beta3delta receptors. Single channel recordings obtained from alpha1beta3delta receptors indicated that THDOC increased the channel opening duration, including the introduction of an additional longer duration open state. Our results suggest that the GABA(A) receptor delta subunit confers increased sensitivity to neurosteroid modulation and that the intrinsic gating and desensitization kinetics of alpha1beta3delta GABA(A) receptors are altered by THDOC.

Pharmacological heterogeneity of gamma-aminobutyric acid receptors during development suggests distinct classes of rat cerebellar granule cells in situ

The gamma-aminobutyric acid receptor (GABA(A)R) represents a ligand-gated Cl(-)-channel assembling as heteropentamere from 19 known subunits. Cerebellar granule cells contain a unique subset, namely the alpha1-, alpha6-, beta2-, gamma2- and delta-subunits. We studied their GABAergic pharmacology in situ using whole-cell patch-clamp recordings in brain slices and a modified Y-tube application system. The distribution of the EC50s for GABA in young (P8-P14) and medium aged animals (P15-P28) could be fitted with the sum of two Gaussian distributions with means of 60 and 185 microM and 27 and 214 microM, respectively. In older animals (P29-P48) the observed homogeneous range of sensitivities fitted a single Gaussian distribution (11 microM). In young animals (< or =P14) GABA-responses were largely insensitive towards 300 microM of the alpha6-specific inhibitor furosemide (82% of control response). The sensitivity increased in older animals at the EC5-20 of GABA (31% of control responses), supporting an increased expression of alpha6-subunits as molecular basis for the observed developmental changes. Approximately 50% of cells in the age range P15-P48 were potentiated by 1 microM diazepam and by 3 microM methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), suggesting the concurrent presence of alpha1- and alpha6-containing receptors, whereas the remaining of cells were neither potentiated by diazepam nor did they show the alpha6-typical DMCM potentiation, though they were potentiated by loreclezole. These properties indicate unknown pharmacological characteristics of cerebellar receptor-subunit combinations in approximately 50% of granule cells in situ.

GABAAreceptor subunit interactions important for benzodiazepine and zinc modulation: a patch-clamp and single cell RT-PCR study

The expression of mRNAs for the GABAA receptor subunits alpha1, alpha6, beta2, beta3, gamma2 and delta in single mouse cerebellar granule cells and cortical interneurons were analysed by RT-PCR and correlated to their midazolam and zinc modulation of agonist-induced receptor currents. The registration of molecular and electrophysiological data from each cell allowed us to estimate the significance of individual subunits and their two-factor interaction for modulation. The presence of alpha6 decreased midazolam modulation, but statistical analysis also suggested interactions of alpha6 with beta3 and gamma2 with respect to midazolam modulation. Zinc modulation was decreased by the presence of gamma2, and analysis points to an beta3 effect as well as an interaction between gamma2 and delta in zinc modulation. Thus, our model confirmed, in single native cells, the known effects of alpha6 in midazolam and gamma2 in zinc modulation, and additionally pointed to significant subunit interactions that need to be further tested in recombinant receptors. The present study offers a method to identify subunit interactions in heteromeric receptor complexes.

gamma-Aminobutyric acid(A) receptor subunit expression predicts functional changes in hippocampal dentate granule cells during postnatal development

Profound alterations in the function of GABA occur over the course of postnatal development. Changes in GABA(A) receptor expression are thought to contribute to these differences in GABAergic function, but how subunit changes correlate with receptor function in individual developing neurons has not been defined precisely. In the current study, we correlate expression of 14 different GABA(A) receptor subunit mRNAs with changes in the pharmacological properties of the receptor in individual hippocampal dentate granule cells over the course of postnatal development in rat. We demonstrate significant developmental differences in GABA(A) receptor subunit mRNA expression, including greater than two-fold lower expression of alpha1-, alpha4- and gamma2-subunit mRNAs and 10-fold higher expression of alpha5-mRNA in immature compared with adult neurons. These differences correlate both with regional changes in subunit protein level and with alterations in GABA(A) receptor function in immature dentate granule cells, including two-fold higher blockade by zinc and three-fold lower augmentation by type-I benzodiazepine site modulators. Further, we find an inverse correlation between changes in GABA(A) receptor zinc sensitivity and abundance of vesicular zinc in dentate gyrus during postnatal development. These findings suggest that developmental differences in subunit expression contribute to alterations in GABA(A) receptor function during postnatal development.

Effects of GABA(A) receptor partial agonists in primary cultures of cerebellar granule neurons and cerebral cortical neurons reflect different receptor subunit compositions

Based on an unexpected high maximum response to piperidine-4-sulphonic acid (P4S) at human alpha1alpha6beta2gamma2 GABA(A) receptors expressed in Xenopus oocytes attempts to correlate this finding with the pharmacological profile of P4S and other GABA(A) receptor ligands in neuronal cultures from rat cerebellar granule cells and rat cerebral cortex were carried out. GABA and isoguvacine acted as full and piperidine-4-sulphonic acid (P4S) as partial agonists, respectively, at alpha1beta2gamma2, alpha6beta2gamma2 and alpha1alpha6beta2gamma2 GABA receptors expressed in Xenopus oocytes with differences in potency. Whole-cell patch-clamp recordings were used to investigate the pharmacological profile of the partial GABA(A) receptor agonists 4,5,6,7-tetrahydroisoxazolo-(5,4-c)pyridin-3-ol (THIP), P4S, 5-(4-piperidyl)isoxazol-3-ol (4-PIOL), and 3-(4-piperidyl)isoxazol-5-ol (iso-4-PIOL), and the competitive GABA(A) receptor antagonists Bicuculline Methbromide (BMB) and 2-(3-carboxypropyl)-3-amino-6-methoxyphenyl-pyridazinium bromide (SR95531) on cerebral cortical and cerebellar granule neurons. In agreement with findings in oocytes, GABA, isoguvacine and P4S showed similar pharmacological profiles in cultured cortical and cerebellar neurones, which are known to express mainly alpha1, alpha2, alpha3, and alpha5 containing receptors and alpha1, alpha6 and alpha1alpha6 containing receptors, respectively. 4-PIOL and iso-4-PIOL, which at GABA(A) receptors expressed in oocytes were weak antagonists, showed cell type dependent potency as inhibitors of GABA mediated responses. Thus, 4-PIOL was slightly more potent at cortical neurones than at granule neurones and iso-4-PIOL was more potent in inhibiting isoguvacine-evoked currents at cortical than at granule neurons. Furthermore the maximum response to 4-PIOL corresponded to that of a partial agonist, whereas that of iso-4-PIOL gave a maximum response close to zero. It is concluded that the pharmacological profile of partial agonists is highly dependent on the receptor composition, and that small structural changes of a ligand can alter the selectivity towards different subunit compositions. Moreover, this study shows that pharmacological actions determined in oocytes are generally in agreement with data obtained from cultured neurons.

Alternative splicing of a Drosophila GABA receptor subunit gene identifies determinants of agonist potency

Alternative splicing of the Drosophila melanogaster Rdl gene yields four ionotropic GABA receptor subunits. The two Rdl splice variants cloned to date, RDL(ac) and RDL(bd) (DRC17-1-2), differ in their apparent agonist affinity. Here, we report the cloning of a third splice variant of Rdl, RDL(ad). Two-electrode voltage clamp electrophysiology was used to investigate agonist pharmacology of this expressed subunit following cRNA injection into Xenopus laevis oocytes. The EC(so) values for GABA and its analogues isoguvacine, muscimol, isonipecotic acid and 3-amino sulphonic acid on the RDL(ad) homomeric receptor differed from those previously described for RDL(ac) and DRC17-1-2 receptors. In addition to providing a possible physiological role for the alternative splicing of Rdl, these data delineate a hitherto functionally unassigned region of the N-terminal domain of GABA receptor subunits, which affects agonist potency and aligns closely with known determinants of potency in nicotinic acetylcholine receptors. Thus, using expression in Xenopus oocytes, we have demonstrated differences in agonist potency for the neurotransmitter GABA (and four analogues) between splice variant products of the Drosophila melanogaster Rdl gene encoding homomer-forming GABA receptor subunits.

Evidence for direct involvement of beta-catenin in phorbol ester-induced neurite outgrowth in GT1-1 hypothalamic neurones

Gonadotropin-releasing hormone (GnRH) is a pivotal neuroendocrine regulator controlling reproductive functions. However, the scattered distribution of GnRH neurones in the mammalian brain has hindered studies on the development and differentiation of GnRH neurones. In the present study, we used the immortalized GnRH-producing GT1-1 cells to examine whether activation of protein kinase C (PKC) pathway with 12-O-tetradecanoyl-13-acetate (TPA) induces morphological and functional differentiation of GnRH neurones. TPA induced neurite outgrowth and inhibited proliferation of GT1-1 cells that were specifically antagonized by cotreatment of PKC inhibitor, calphostin C. The functional significance of TPA-induced differentiation of GT1-1 cells was manifested in part by the changes in the effects of gamma-aminobutyric acid (GABA) on intracellular Ca2+ levels. In untreated GT1-1 cells, activation of GABA-A receptor with 10 microM muscimol increased intracellular Ca2+ levels, whereas such stimulatory effects disappeared in GT1-1 cells bearing neurites. Accordingly, muscimol could not stimulate GnRH release in TPA-treated GT1-1 cells. To elucidate the molecular mechanism underlying TPA-induced neurite outgrowth, we performed differential display reverse transcription-polymerase chain reaction. Among several genes that are affected by TPA treatment, we found a significant induction of beta-catenin mRNA expression. Along with the rapid induction of beta-catenin protein levels, we observed that beta-catenin was reallocated from cell-cell adhesion sites to the growth cones within 3 h of TPA treatment. Transient transfection studies with green fluorescent protein as a reporter gene demonstrated that beta-catenin overexpression alone can promote neurite outgrowth in GT1-1 cells. Moreover, TPA was found to increase the transcription-activational roles of beta-catenin. Together, these data provide evidence that beta-catenin is involved in the TPA-induced functional differentiation of immortalized GnRH neurones.

Proton sensitivity of rat cerebellar granule cell GABAA receptors: dependence on neuronal development

The effect of GABAA receptor development in culture on the modulation of GABA-induced currents by external H+ was examined in cerebellar granule cells using whole-cell and single-channel recording. Equilibrium concentration-response curves revealed a lower potency for GABA between 11 and 12 days in vitro (DIV) resulting in a shift of the EC50 from 10.7 to 2.4 uM. For granule cells before 11 DIV, the peak GABA-activated current was inhibited at low external pH and enhanced at high pH with a pKa of 6.6. For the steady-state response, low pH was inhibitory with a pKa of 5.56. After 11 DIV, the peak GABA-activated current was largely pH insensitive; however, the steady-state current was potentiated at low pH with a pKa of 6.84. Single GABA-activated ion channels were recorded from outside-out patches of granule cell bodies. At pH 5.4-9.4, single GABA channels exhibited multiple conductance states occurring at 22-26, 16-17 and 12-14 pS. The conductance levels were not significantly altered over the time period of study, nor by changing the external H+ concentration. Two exponential functions were required to fit the open-time frequency histograms at both early (< 11 DIV) and late (> 11 DIV) development times at each H+ concentration. The short and long open time constants were unaffected either by the extracellular H+ concentration or by neuronal development. The distribution of all shut times was fitted by the sum of three exponentials designated as short, intermediate and long. At acidic pH, the long shut time constant decreased with development as did the relative contribution of these components to the overall distribution. This was concurrent with an increase in the mean probability of channel opening. In conclusion, this study demonstrates in cerebellar granule cells that external pH can either reduce, have no effect on, or enhance GABA-activated responses depending on the stage of development, possibly related to the subunit composition of the GABAA receptors. The mode of interaction of H+ at the single-channel level and implications of such interactions at cerebellar granule cell GABAA receptors are discussed.

Electrophysiological evidence for the coexistence of alpha1 and alpha6 subunits in a single functional GABA(A) receptor

The subunit combinations alpha1beta2gamma2, alpha6beta2gamma2, and alpha1alpha6beta2gamma2 of the GABA(A) receptor were functionally expressed in Xenopus oocytes. The properties of the resulting ion currents were characterized by using electrophysiological techniques. The concentration-response curve of the channel agonist GABA for alpha1alpha6beta2gamma2 showed a single apparent component characterized by an EC(50) of 107 +/- 26 microM (n = 4). It was different from the one for alpha1beta2gamma2, which had an EC(50) of 41 +/- 9 microM (n = 4), that for alpha6beta2gamma2, with an EC(50) of 6.7 +/- 1.9 microM (n = 5), and those for alpha1beta2 and alpha1alpha6beta2. There was no appreciable functional expression of alpha6beta2. Allosteric responses of alpha1alpha6beta2gamma2 to diazepam were intermediate to those of alpha1beta2gamma2 and alpha6beta2gamma2, and allosteric responses to flumazenil were comparable to the ones for alpha1beta2gamma2. The inhibition by furosemide of the currents elicited by GABA in alpha1alpha6beta2gamma2 [IC(50) = 298 +/- 116 microM (n = 7), assuming only one component] was not identical with inhibition of alpha6beta2gamma2 (IC(50) = 38 +/- 2 microM, n = 4), alpha1beta2gamma2 (IC(50) = 5,610 +/- 910 microM, n = 5), or a mixture of these components (assuming two components). These findings indicate unambiguously the formation of functional GABA(A) receptors containing two different alpha subunits, alpha1 and alpha6, with properties different from those of alpha1beta2gamma2 and alpha6beta2gamma2. Furthermore, we provide evidence for the facts that in the Xenopus oocyte (a) the formation of the different receptor types depends on the relative abundance of cRNAs coding for the different receptor subunits and (b) that functional dual subunit combinations alphabeta do not form in the presence of cRNA coding for the gamma subunit.

Neurosteroids: biosynthesis and function of these novel neuromodulators

Over the past decade, it has become clear that the brain is a steroidogenic organ. The steroids synthesized by the brain and nervous system, given the name neurosteroids, have a wide variety of diverse functions. In general, they mediate their actions, not through classic steroid hormone nuclear receptors, but through ion-gated neurotransmitter receptors. This paper summarizes what is known about the biosynthesis of neurosteroids, the enzymes mediating these reactions, their localization during development and in the adult, and their function and mechanisms of action in the developing and adult central and peripheral nervous systems. The expression of the steroidogenic enzymes is developmentally regulated, with some enzymes being expressed only during development, while others are expressed during development and in the adult. These enzymes are expressed in both neurons and glia, suggesting that these two cell types must work in concert to produce the appropriate active neurosteroid. The functions attributed to specific neurosteroids include modulation of GABA(A) and NMDA function, modulation of sigma receptor function, regulation of myelinization, neuroprotection, and growth of axons and dendrites. Neurosteroids have also been shown to modulate expression of particular subunits of GABA(A) and NMDA receptors, providing additional sites at which these compounds can regulate neural function. The pharmacological properties of specific neurosteroids are described, and potential uses of neurosteroids in specific neuropathologies and during normal aging in humans are also discussed.

Evidence of two populations of GABA(A) receptors in cerebellar granule cells in culture: different desensitization kinetics, pharmacology, serine/threonine kinase sensitivity, and localization

GABA(A) receptors of rat cerebellar granule cells in culture have been studied by the whole cell patch clamp technique. The biphasic desensitization kinetic observed could be due either to different desensitization mechanisms of a single receptor population or to different receptor populations. The overall data indicate that the latter hypothesis is most probably the correct one. In fact, the fast desensitizing component was selectively potentiated by a benzodiazepine agonist and preferentially down-regulated by activation of the protein serine/threonine kinases A and G, as a consequence of the latter characteristic that receptor population was preferentially down-regulated by previous activation of N-methyl-d-aspartate glutamate receptors, via production of nitric oxide and PKG activation, most probably in dendrites. The other population is benzodiazepine insensitive and not influenced by activation of PKA or PKG. This slowly desensitizing population may correspond to the extrasynaptic delta subunit containing GABA(A) receptors described by other authors. Instead, the rapidly desensitizing population appears to represent dendritic synaptic GABA(A) receptors.

GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons

Previous studies have described maturational changes in GABAergic inhibitory synaptic transmission in the rodent somatosensory cortex during the early postnatal period. To determine whether alterations in the functional properties of synaptically localized GABA(A) receptors (GABA(A)Rs) contribute to development of inhibitory transmission, we used the whole cell recording technique to examine GABAergic miniature postsynaptic currents (mPSCs) in developing cortical neurons. Neurons harvested from somatosensory cortices of newborn mice showed a progressive, eightfold increase in GABAergic mPSC frequency during the first 4 wk of development in dissociated cell culture. A twofold decrease in the decay time of the GABAergic mPSCs, between 1 and 4 wk, demonstrates a functional change in the properties of GABA(A)Rs mediating synaptic transmission in cortical neurons during development in culture. A similar maturational profile observed in GABAergic mPSC frequency and decay time in cortical neurons developing in vivo (assessed in slices), suggests that these changes in synaptically localized GABA(A)Rs contribute to development of inhibition in the rodent neocortex. Pharmacological and reverse transcription-polymerase chain reaction (RT-PCR) studies were conducted to determine whether changes in subunit expression might contribute to the observed developmental alterations in synaptic GABA(A)Rs. Zolpidem (300 nM), a subunit-selective benzodiazepine agonist with high affinity for alpha1-subunits, caused a reversible slowing of the mPSC decay kinetics in cultured cortical neurons. Development was characterized by an increase in the potency of zolpidem in modulating the mPSC decay, suggesting a maturational increase in percentage of functionally active GABA(A)Rs containing alpha1 subunits. The relative expression of alpha1 versus alpha5 GABA(A)R subunit mRNA in cortical tissue, both in vivo and in vitro, also increased during this same period. Furthermore, single-cell RT-multiplex PCR analysis revealed more rapidly decaying mPSCs in individual neurons in which alpha1 versus alpha5 mRNA was amplified. Together these data suggest that changes in alpha-subunit composition of GABA(A)Rs contribute to the maturation of GABAergic mPSCs mediating inhibition in developing cortical neurons.

Single-channel properties of synaptic and extrasynaptic GABAA receptors suggest differential targeting of receptor subtypes

Many neurons express a multiplicity of GABAA receptor subunit isoforms. Despite having only a single source of inhibitory input, the cerebellar granule cell displays, at various stages of development, more than 10 different GABAA subunit types. This subunit diversity would be expected to result in significant receptor heterogeneity, yet the functional consequences of such heterogeneity remain poorly understood. Here we have used single-channel properties to characterize GABAA receptor types in the synaptic and extrasynaptic membrane of granule cells. In the presence of high concentrations of GABA, which induced receptor desensitization, extrasynaptic receptors in outside-out patches from the soma entered long-lived closed states interrupted by infrequent clusters of openings. Each cluster of openings, which is assumed to result from the repeated activation of a single channel, was to one of three main conductance states (28, 17, or 12 pS), the relative frequency of which differed between patches. Such behavior indicates the presence of at least three different receptor types. This heterogeneity was not replicated by individual recombinant receptors (alpha1beta2gamma2S or alpha1beta3gamma2S), which gave rise to clusters of a single type only. By contrast, the conductance of synaptic receptors, determined by fluctuation analysis of the synaptic current or direct resolution of channel events, was remarkably uniform and similar to the highest conductance value seen in extrasynaptic patches. These results suggest that granule cells express multiple GABAA receptor types, but only those with a high conductance, most likely containing a gamma subunit, are activated at the synapse.

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.

Cerebellar granule-cell-specific GABAA receptors attenuate benzodiazepine-induced ataxia: evidence from alpha 6-subunit-deficient mice

Benzodiazepine- and alcohol-induced ataxias in rodents have been proposed to be affected by the gamma-aminobutyric acid type A (GABAA) receptor alpha 6 subunit, which contributes to receptors specifically expressed in cerebellar granule cells. We have studied an alpha 6 -/- mouse line for motor performance and drug sensitivity. These mice, as a result of a specific genetic lesion, carry a precise impairment at their Golgi-granule cell synapses. On motor performance tests (rotarod, horizontal wire, pole descending, staircase and swimming tests) there were no robust baseline differences in motor function or motor learning between alpha 6 -/- and alpha 6 +/+ mice. On the rotarod test, however, the mutant mice were significantly more impaired by diazepam (5-20 mg/kg, i.p.), when compared with alpha 6 +/+ control and background C57BL/6J and 129/SvJ mouse lines. Ethanol (2.0-2.5 g/kg, i.p.) produced similar impairment in the alpha 6 -/- and alpha +/+ mice. Diazepam-induced ataxia in alpha 6 -/- mice could be reversed by the benzodiazepine site antagonist flumazenil, indicating the involvement of the remaining alpha 1 beta 2/3 gamma 2 GABAA receptors of the granule cells. The level of activity in this synapse is crucial in regulating the execution of motor tasks. We conclude that GABAA receptor alpha 6 subunit-dependent actions in the cerebellar cortex can be compensated by other receptor subtypes; but if not for the alpha 6 subunit, patients on benzodiazepine medication would suffer considerably from ataxic side-effects.

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.

Astrocytes regulate developmental changes in the chloride ion gradient of embryonic rat ventral spinal cord neurons in culture

1. Embryonic rat ventral spinal cord neurons were dissociated at day 15 and grown on: (i) poly-D-lysine (PDL); (ii) a confluent monolayer of type I astrocytes; or (iii) PDL in astrocyte-conditioned medium (ACM) to examine the influence of astroglia on the regulation of GABAA receptor/Cl- channel properties. 2. Potentiometric oxonol dye recordings of intact cells indicated that embryonic neurons were uniformly depolarized by muscimol. The depolarizing effects disappeared in cells dissociated during the early postnatal period and recovered in culture for 24 h. Similar recordings using the calcium-imaging dye fura-2 AM revealed that GABA or muscimol triggered a sustained rise in cytosolic Ca2+ (Ca2+c ) in embryonic neurons that was dependent on extracellular Ca2+, blocked by bicuculline and nifedipine and sensitive to changes in extracellular chloride. The incidence and amplitude of the Ca2+ response decreased with time in vitro and was accelerated in neurons cultured on astrocytes compared with those on PDL. 3. Perforated patch-clamp recordings revealed that GABA depolarized neurons in a Cl--dependent and bicuculline-sensitive manner. Both the resting membrane potential and the GABA equilibrium potential became more hyperpolarized with time in vitro. 4. Astrocytes and ACM accelerated the transformation of GABAergic potential responses from depolarizing to hyperpolarizing. The change occurred over the first 4 days in co-culture or in ACM but took more than 2 weeks in neurons cultured on PDL alone. 5. The intrinsic, elementary properties of GABAA receptor/Cl- channels including open time and unitary conductance changed independently of the presence of astrocytes or ACM. Mean open time of the dominant kinetic component decreased and conductance increased with time in vitro. 6. In sum, astrocytes accelerate the developmental change in the Cl- ion gradient extrinsic to GABAA receptor/Cl- channels, which is critical for triggering Ca2+ entry, without influencing parallel changes in the intrinsic properties of the channels.

Differential expression and association of calcium channel alpha1B and beta subunits during rat brain ontogeny

Calcium functions as an essential second messenger during neuronal development and synapse acquisition. Voltage-dependent calcium channels (VDCC), which are critical to these processes, are heteromultimeric complexes composed of alpha1, alpha2/delta, and beta subunits. beta subunits function to direct the VDCC complex to the plasma membrane as well as regulate its channel properties. The importance of beta to neuronal functioning was recently underscored by the identification of a truncated beta4 isoform in the epileptic mouse lethargic (lh) (Burgess, D. L., Jones, J. M., Meisler, M. H., and Noebels, J. L. (1997) Cell 88, 385-392). The goal of our study was to investigate the role of individual beta isoforms (beta1b, beta2, beta3, and beta4) in the assembly of N-type VDCC during rat brain development. By using quantitative Western blot analysis with anti-alpha1B-directed antibodies and [125I-Tyr22]omega-conotoxin GVIA (125I-CTX) radioligand binding assays, we observed that only a small fraction of the total alpha1B protein present in embryonic and early postnatal brain expressed high affinity 125I-CTX-binding sites. These results suggested that subsequent maturation of alpha1B or its assembly with auxiliary subunits was required to exhibit high affinity 125I-CTX binding. The temporal pattern of expression of beta subunits and their assembly with alpha1B indicated a developmental pattern of expression of beta isoforms: beta1b increased 3-fold from P0 to adult, beta4 increased 10-fold, and both beta2 and beta3 expression remained unchanged. As the beta component of N-type VDCC changed during postnatal development, we were able to identify both immature and mature forms of N-type VDCC. At P2, the relative contribution of beta is beta1b > beta3 >> beta2, whereas at P14 and adult the distribution is beta3 > beta1b = beta4. Although we observed no beta4 associated with the alpha1B at P2, beta4 accounted for 14 and 25% of total alpha1B/beta subunit complexes in P14 and adult, respectively. Thus, of the beta isoforms analyzed, only the beta4 was assembled with the rat alpha1B to form N-type VDCC with a time course that paralleled its level of expression during rat brain development. These results suggest a role for the beta4 isoform in the assembly and maturation of the N-type VDCC.

Mouse cerebellar granule cell differentiation: electrical activity regulates the GABAA receptor alpha 6 subunit gene

GABAA receptor alpha6 subunit gene expression marks cerebellar granule cell maturation. To study this process, we used the Deltaalpha6lacZ mouse line, which has a lacZ reporter inserted into the alpha6 gene. At early stages of postnatal cerebellar development, alpha6-lacZ expression is mosaic; expression starts at postnatal day 5 in lobules 9 and 10, and alpha6-lacZ is switched on inside-out, appearing first in the deepest postmigratory granule cells. We looked for factors regulating this expression in cell culture. Membrane depolarization correlates inversely with alpha6-lacZ expression: granule cells grown in 25 mM [K+]o for 11-15 d do not express the alpha6 gene, whereas cultures grown for the same period in 5 mM [K+]o do. This is influenced by a critical early period: culturing for >/=3 d in 25 mM [K+]o curtails the ability to induce the alpha6 gene on transfer to 5 mM [K+]o. If the cells start in 5 mM [K+]o, however, they still express the alpha6-lacZ gene in 25 mM [K+]o. In contrast to granule cells grown in 5 mM [K+]o, cells cultured in 25 mM [K+]o exhibit no action potentials, mEPSCs, or mIPSCs. In chronic 5 mM [K+]o, factors may therefore be released that induce alpha6. Blockade of ionotropic and metabotropic GABA and glutamate receptors or L-, N-, and P/Q-type Ca2+ channels did not prevent alpha6-lacZ expression, but inhibition of action potentials with tetrodotoxin blocked expression in a subpopulation of cells.

Changes in the properties of developing glycine receptors in cultured mouse spinal neurons

We studied several neurophysiological properties of in vitro maturing glycine receptors in mouse spinal cord neurons cultured for various times: 3-7 days (early), 10-12 days (intermediate), and 17-24 days (mature), using whole-cell and gramicidin-perforated techniques. The glycine-activated Cl- conductance increased about 6-fold during in vitro development, and the current density increased from 177+/-42 pA/pF in early to 504+/-74 pA/pF in mature neurons. The sensitivity to glycine increased transiently from 39+/-2.8 microM in early neurons to 29+/-1 microM in intermediate neurons. Using whole-cell recordings, we found that ECl did not change during development. With the gramicidin-perforated technique, on the other hand, ECl shifted from -27 to -52 mV with development. Thus, immature neurons were depolarized by the activation of glycine receptors, whereas mature neurons were hyperpolarized. The current decayed (desensitized) during the application of 500 microM glycine. The decay was single exponential and the time constant increased from 2,212+/-139 msec in early neurons to 4,580+/-1,071 msec in mature neurons. Picrotoxin (10 microM) inhibited the current to a larger extent in early neurons (46+/-6% of control), and the sensitivity of these receptors to strychnine (IC50) increased from 23+/-3 nM to 9+/-1 nM in mature neurons. In conclusion, several properties of spinal glycine receptors changed during in vitro neuronal maturation. This indicates that, similar to GABA(A) receptors, the functions of these receptors are developmentally regulated. These changes should affect the excitability of spinal neurons as well as other maturation processes.

Structure-activity studies of fluoroalkyl-substituted gamma-butyrolactone and gamma-thiobutyrolactone modulators of GABA(A) receptor function

Dihydro-2(3H)-furanones (gamma-butyrolactones) and dihydro-2(3H)-thiophenones (gamma-thiobutyrolactones) containing fluoroalkyl groups at positions C-3, C-4, and C-5 of the heterocyclic rings were prepared. The anticonvulsant/convulsant activities of the compounds were evaluated in mice. Brain concentrations of the compounds were determined and the effects of the compounds on [35S]-tert-butylbicyclophosphorothionate ([35S]TBPS) binding to the picrotoxin site on GABAA receptors were investigated. The effects of the compounds on GABAA receptor function were studied using electrophysiological methods and cultured rat hippocampal neurons. Fluorination at C-3 results in either subtle or pronounced effects on the pharmacological activity of the compounds. When hydrogens are replaced with fluorines at the methylene carbon of an ethyl group, as in 3-(1,1-difluoroethyl)dihydro-3-methyl-2(3H)-furanone (1), the anticonvulsant actions of the compound are not much changed from those found for the corresponding alkyl-substituted analogue. In marked contrast, fluorination at the methyl carbon of the ethyl group, as in dihydro-3-methyl-3-(2,2,2-trifluoroethyl)-2(3H)-furanone (3), produces a compound having convulsant activity. This convulsant activity seems to be due to an increased affinity of the compound for the picrotoxin site on GABAA receptors caused by an interaction that involves the trifluoromethyl group. Results obtained with gamma-butyrolactones containing either a 3-(1-trifluoromethyl)ethyl or a 3-(1-methyl-1-trifluoromethyl)ethyl substituent indicate that the interactions of the trifluoromethyl group with the picrotoxin binding site are subject to both stereochemical and steric constraints. Sulfur for oxygen heteroatom substitution, as in the corresponding gamma-thiobutyrolactones, affects the type (competitive, non-competitive, etc.) of binding interactions that these compounds have with the picrotoxin site in a complex manner. Fluorination of alkyl groups at the C-4 and C-5 positions of gamma-butyrolactones having convulsant activity increases convulsant potency.

Plasticity in fast synaptic inhibition of adult oxytocin neurons caused by switch in GABA(A) receptor subunit expression

We found that magnocellular oxytocin neurons in adult female rats exhibit an endogenous GABA(A) receptor subunit switch around parturition: a decrease in alpha1:alpha2 subunit mRNA ratio correlated with a decrease in allopregnanolone potentiation and increase in decay time constant of the GABA(A) receptor-mediated IPSCs in these cells. The causal relationship between changes in alpha1:alpha2 mRNA ratio and the ion channel kinetics was confirmed using in vitro antisense deletion. Further, GABA(A) receptors exhibited a tonic inhibitory influence upon oxytocin release in vivo, and allopregnanolone helped to restrain oxytocin neuron in vitro firing only before parturition, when the alpha1:alpha2 subunit mRNA ratio was still high. Such observations provide evidence for the physiological significance of GABA(A) receptor subunit heterogeneity and plasticity in the adult brain.

Optical characterization of a novel GABA response in early embryonic chick brainstem

To examine the functional expression of embryonic GABA receptors, the inhibitory effects were studied of GABA (GABA responses) on the excitatory postsynaptic potentials evoked by vagal stimulus in seven- to 10-day-old embryonic chick brainstem slice preparations. A multiple-site optical recording technique was used, with a multiple element photodiode array system and a fast voltage-sensitive merocyanine-rhodanine dye (NK2761). First, in the GABA response, three components were pharmacologically identified: component 1, related to GABA(A) receptors; component 2, related to GABA(B) receptors; and component 3 which is insensitive to GABA(A) and GABA(B) antagonists, but is stimulated by both GABA(A) and GABA(B) agonists. Subsequently. the embryogenesis and early development of the three components were investigated, and early developmental maps of regional distribution patterns of the three components were constructed. Components 1 and 3 have already emerged in the seven-day-old embryonic brainstem preparation; component 2 appeared in the eight-day-old preparations. No component related to GABA(C) receptors was observed in the seven- to 10-day-old embryonic stages. From the pharmacological properties of component 3, we suggest that it is related to a new subtype, the GABA(D) receptor.

Slow kinetics of miniature IPSCs during early postnatal development in granule cells of the dentate gyrus

Whole-cell patch-clamp recordings were used to investigate the properties of GABAA receptor-mediated postsynaptic currents during development in dentate gyrus granule cells from neonatal [postnatal day 0 (P0)] to adult rats in brain slices. The frequency of miniature IPSCs (mIPSCs) was low at birth and increased progressively with age. The mIPSCs of all ages could be satisfactorily fitted with the sum of a single exponential rise and single exponential decay. From P0 to P14, both the rise time and the decay time constants were significantly longer than in the adult. The mIPSC rise and decay kinetics did not change during the first 2 postnatal weeks, but during the third week the kinetics sped up and by P21 attained adult values. In contrast, the amplitude of the mIPSCs did not change during development. The synaptic GABAA receptors in immature and adult cells showed differential sensitivity to modulators. The subunit-specific benzodiazepine agonist zolpidem increased the decay time constant of the IPSCs of immature granule cells with a reduced potency compared with the adult. Furthermore, zinc decreased the amplitude and decay time constant of mIPSCs from developing granule cells, whereas it had no effect on mIPSCs in adult neurons. The results reveal for the first time that until the end of the second postnatal week the synaptic GABAA receptor-mediated currents in dentate granule cells display slower rise and decay kinetics but similar amplitudes compared with adult, resulting in a net decrease in synaptic charge transfer during development.

Development of action potential-dependent and independent spontaneous GABAA receptor-mediated currents in granule cells of postnatal rat cerebellum

The postnatal development of spontaneous GABAergic transmission between cerebellar Golgi cells and granule cells was investigated with voltage-clamp recording from rat cerebellar slices, in symmetrical Cl- conditions. Between postnatal days 7 and 14 (P7-14), bicuculline- and TTX (tetrodotoxin)-sensitive spontaneous inhibitory postsynaptic currents (sIPSCs), occurred at high frequency in 56% of granule cells. Between P10 and P14, sIPSCs were superimposed on a tonic current of -12 +/- 1.8 pA at -70 mV, that was accompanied by noise with a variance of 17 +/- 3 pA2. Both the current and noise were inhibited by bicuculline. TTX blocked the bicuculline-sensitive current and noise by approximately 60%. Between P18 and P25, sIPSCs were less frequent; all cells showed tonic, bicuculline-sensitive currents, but these were partially inhibited by TTX (approximately 35%). Between P40 and P53, sIPSCs were rare; tonic, bicuculline-sensitive currents and noise were greater in amplitude, with mean values of -17 pA and 22 pA2 at -70 mV, they were present in all cells but they were not inhibited by TTX. Glycine receptor channels that were expressed in immature, but not adult cells, did not mediate spontaneous currents. Our results indicate that spontaneous transmission onto cerebellar granule cells in immature animals consists primarily of action potential-dependent, phasic release of vesicular GABA. This generates GABAA receptor-mediated sIPSCs. The effects of GABA transporter blockers suggest that it also produces the TTX-sensitive current-noise, as GABA spills out of synapses to activate extrasynaptic receptors or receptors in neighbouring synapses. In older animals, action potential-independent release of transmitter is predominant and results in tonic activation of GABAA receptors. This does not appear to be spontaneous vesicular release of GABA. Neither does it appear to be reversed uptake of GABA, although further work is required to rule out these possibilities.

GABAA receptor subunit expression and assembly in cultured rat cerebellar granule neurons

The assembly of multisubunit GABAA receptors in specific neuronal populations is a complex process which is poorly understood. To begin to examine receptor assembly, alpha 1, beta 2/3, and gamma 2 subunit polypeptide expression and association, as well as receptor binding, were examined in cultured rat cerebellar granule neurons. Western blots revealed two alpha 1-immunoreactive proteins. A 39 kDa species was maximal at 2 days in culture and subsequently declined. In contrast, a 51 kDa polypeptide, the anticipated size of the mature alpha 1 subunit, was first detected at 4 days and increased throughout the culture period. Additional studies demonstrated that the beta 2/3 and gamma 2 subunits were detectable at 2 days and attained maximal levels by 6 days. The level of [3H]Ro15-1788 binding, a measure of assembled receptors, rose in parallel with the increases in the 51 kDa alpha 1, beta 2/3 and gamma 2 subunits. Moreover, the 51 kDa alpha 1, beta 2/3, and gamma 2 subunits were associated in receptor complexes. However, immunohistochemical studies demonstrated the presence of substantial intracellular subunit staining. This finding suggest that only some of the subunits expressed in granule neurons contribute to functional GABAA receptors on the cell surface.

Plasticity in GABAA receptor subunit mRNA expression by hypothalamic magnocellular neurons in the adult rat

The magnocellular hypothalamic neurons exhibit a substantial degree of structural and functional plasticity over the time of pregnancy, parturition, and lactation. This study has used in situ hybridization techniques to examine whether the content of alpha 1, alpha 2, beta 2, gamma 2 GABAA receptor subunit mRNAs expressed by these cells fluctuates over this period. A process of regional, followed by cellular and then topographical, analyses within the supraoptic (SON) and posterior paraventricular (PVN) nuclei revealed that an increase in magnocellular alpha 1 subunit mRNA content occurred during the course of pregnancy up to day 19, after which a decline in expression was detected on the day of parturition. Significant fluctuations of this nature were observed only in the oxytocin neuron-enriched regions of the SON and PVN. The expression of alpha 2, beta 2, and gamma 2 subunit mRNAs in the SON and PVN and of all subunit mRNAs in the cingulate cortex did not change over this period. During lactation, gamma 2 subunit mRNA content within the PVN increased significantly on day 14 of lactation as compared with day 7, and topographical analysis suggested that it involved principally magnocellular vasopressin neurons. These results demonstrate the cell-and subunit-specific regulation of GABAA receptor mRNA expression within the hypothalamic magnocellular system. In particular, they suggest that fluctuations in alpha 1 subunit expression may contribute to the marked variations in electrical activity exhibited by magnocellular oxytocin neurons at the time of parturition. More generally, they provide evidence in support of GABAA receptor plasticity within a physiological context in the adult rat brain.

Excitatory actions of GABA after neuronal trauma

GABA is the dominant inhibitory neurotransmitter in the CNS. By opening Cl- channels, GABA generally hyperpolarizes the membrane potential, decreases neuronal activity, and reduces intracellular Ca2+ of mature neurons. In the present experiment, we show that after neuronal trauma, GABA, both synaptically released and exogenously applied, exerted a novel and opposite effect, depolarizing neurons and increasing intracellular Ca2+. Different types of trauma that were effective included neurite transection, replating, osmotic imbalance, and excess heat. The depolarizing actions of GABA after trauma increased Ca2+ levels up to fourfold in some neurons, occurred in more than half of the severely injured neurons, and was long lasting (>1 week). The mechanism for the reversed action of GABA appears to be a depolarized Cl- reversal potential that results in outward rather than inward movement of Cl-, as revealed by gramicidin-perforated whole-cell patch-clamp recording. The consequent depolarization and resultant activation of the nimodipine sensitive L- and conotoxin-sensitive N-type voltage-activated Ca2+ channel allows extracellular Ca2+ to enter the neuron. The long-lasting capacity to raise Ca2+ may give GABA a greater role during recovery from trauma in modulating gene expression, and directing and enhancing outgrowth of regenerating neurites. On the negative side, by its depolarizing actions, GABA could increase neuronal damage by raising cytosolic Ca2+ levels in injured cells. Furthermore, the excitatory actions of GABA after neuronal injury may contribute to maladaptive signal transmission in affected GABAergic brain circuits.

Delta subunit inhibits neurosteroid modulation of GABAA receptors

Neurosteroid modulation of GABAA receptors has been observed with all subunit combinations investigated; however, hetero-oligomeric GABAA receptors containing delta subunits were not studied previously. We describe the effect of delta subunit expression on 3alpha,21-dihydroxy-5alpha-pregnan-20-1 (THDOC)-induced potentiation of GABA-gated currents in transfected HEK 293 cells and in cerebellar granule cells in vitro. THDOC (100 nM) significantly potentiated GABA-gated currents in cells transfected with combinations of alpha1, alpha6, beta3, and gamma2 subunit cDNAs, whereas cotransfection of delta subunit cDNA inhibited this potentiation. In contrast, the direct Cl- channel activation by THDOC at higher concentrations (1-10 microM) was not significantly dependent on delta subunit cotransfection. These results suggest that the presence of the delta subunit inhibits GABAA receptor modulation but not the direct activation by neurosteroids. Cotransfection with delta subunit also affected the negative allosteric modulation by pregnenolone sulfate. THDOC potentiation of GABA-gated currents was greater in cerebellar granule cell cultures at 4 d in vitro (DIV) compared with those at 14 DIV. Single-cell reverse transcription-PCR analysis of the mRNAs expressed in cultured cerebellar granule cells shows that an increased number of granule cells at 14 DIV express delta subunit mRNAs as compared with 4 DIV granule cells. The presence of delta subunit mRNAs detected in individual cells correlated well with the lack of sensitivity to THDOC. These results suggest that developmental expression of GABAA receptor delta subunits may play an important role in determining the region-specific neurosteroid-induced modification of fast inhibitory synaptic function.

GABAA-receptor-mediated conductance and action potential waveform in cutaneous and muscle afferent neurons of the adult rat: differential expression and response to nerve injury

1. Whole cell patch-clamp recordings were obtained from identified cutaneous and muscle afferent neurons (33-60 microns diam) in dissociated L4 and L5 dorsal root ganglia (DRGs) from normal rats and from rats 2-3 wk after sciatic nerve ligation or crush injury. gamma-Aminobutyric acid (GABA)-induced conductance was compared in normal and injured neurons from both functional classes of sensory neurons. 2. Control cutaneous afferent neurons had a peak GABA-mediated conductance of 287 +/- 27 (SE) nS compared with 457 +/- 42 nS for control muscle afferent neurons. 3. An inflection on the downslope of the action potential was observed in 47% of cutaneous afferent neurons compared with 20% of muscle afferent neurons. 4. After ligation and transection of the sciatic nerve there was no change in the GABA-mediated conductance of muscle afferent neurons or in the action potential waveform (23% inflected). However, the cutaneous afferent neurons displayed a greater than two-fold increase in their GABA-mediated conductance and displayed a prominent reduction in the number of neurons with inflected action potentials (13% inflected). Input resistance was similar in cutaneous and muscle afferent neurons and decreased after ligation in cutaneous but not muscle afferents. Resting potential averaged from -50 to -56 mV in normal and ligated groups for both cutaneous and muscle afferent neurons. 5. After crush injury in cutaneous afferent neurons where the transected axons were allowed to regenerate into the distal nerve stump, GABAA-receptor-mediated conductance was elevated compared with controls. However, action potential waveform was not altered by crush injury, suggesting a differential regulation of these two properties in cutaneous afferent neurons. 6. These data indicate that injury-induced plasticity of GABAA-receptor-mediated conductance and action potential waveform occurs in cutaneous but not muscle afferent DRG neurons. It appears that peripherally derived influences are critical in maintaining the electrophysiological phenotype of cutaneous afferent neurons but not muscle afferent neurons.

Functional evidence for two native GABAA receptor subtypes in adult rat hippocampus and cerebellum

Studies of molecular cloning predict great heterogeneity for the GABAA receptor; however, evidence for functionally and pharmacologically distinct native GABAA receptors is relatively scarce. In this work we have compared some of the functional and pharmacological properties of two GABAA receptors previously shown to be present in the adult rat central nervous system. In superfused hippocampal synaptosomes activation of GABAA receptors increased the basal release of [3H]noradrenaline (EC50 for GABA = 3.2 microM). In contrast, the overflow evoked by depolarization with high-K+ (12 or 35 mM) was not affected. Conversely, GABAA receptor activation led to potentiation of the K(+)-evoked overflow of [3H]D-aspartate from cerebellar synaptosomes (EC50 for GABA = 1.3 microM) whereas the basal release remained unchanged. GABA and muscimol also potentiated the K(+)-evoked overflow of endogenous glutamate in cerebellum. Diazepam enhanced the GABA (3 microM)-evoked [3H]noradrenaline release (EC50 = 65 nM). The diazepam potentiation of the GABA- or muscimol-evoked release of [3H]noradrenaline was inversely related to the agonist concentration. The effect of diazepam was reversed by the benzodiazepine antagonist flumazenil. Zolpidem mimicked diazepam (EC50 = 14 nM). The increase of the K(+)-evoked overflow of [3H]D-aspartate (or of endogenous glutamate) elicited by GABA or muscimol in cerebellar synaptosomes was not affected by benzodiazepines (diazepam or clonazepam) or by zolpidem. On the other hand, Ro 15-4513, an inverse agonist at the benzodiazepine site, strongly inhibited (EC50 = 7 nM) the enhancement by GABA (3 microM) of the K(+)-evoked [3H]D-aspartate overflow in cerebellar synaptosomes; the effect of Ro 15-4513 was reversed by flumazenil. These results suggest the existence in the central nervous system of the adult rat of two native pharmacological-subtypes of the GABAA receptor having different function, regional distribution and neuronal location; the receptors require different membrane potential to be activated and display different sensitivity to benzodiazepines and to drugs acting at benzodiazepine sites.

Molecular mechanisms of benzodiazepine-induced down-regulation of GABAA receptor alpha 1 subunit protein in rat cerebellar granule cells

1. Chronic benzodiazepine treatment of rat cerebellar granule cells induced a transient down-regulation of the gamma-aminobutyric acidA (GABAA) receptor alpha 1 subunit protein, that was dose-dependent (1 nM-1 microM) and prevented by the benzodiazepine antagonist flumazenil (1 microM). After 2 days of treatment with 1 microM flunitrazepam the alpha 1 subunit protein was reduced by 41% compared to untreated cells, which returned to, and remained at, control cell levels from 4-12 days of treatment. Chronic flunitrazepam treatment did not significantly alter the GABAA receptor alpha 6 subunit protein over the 2-12 day period. 2. GABA treatment for 2 days down-regulates the alpha 1 subunit protein in a dose-dependent (10 microM-1 mM) manner that was prevented by the selective GABAA receptor antagonist bicuculline (10 microM). At 10 microM and 1 mM GABA the reduction in alpha 1 subunit expression compared to controls was 31% and 66%, respectively. 3. The flunitrazepam-induced decrease in alpha 1 subunit protein is independent of GABA, which suggests that it involves a mechanism distinct from the GABA-dependent action of benzodiazepines on GABAA receptor channel activity. 4. Simultaneous treatment with flunitrazepam and GABA did not produce an additive down-regulation of alpha 1 subunit protein, but produced an effect of the same magnitude as that of flunitrazepam alone. This down-regulation induced by the combination of flunitrazepam and GABA was inhibited by flumazenil (78%), but unaffected by bicuculline. 5. The flunitrazepam-induced down-regulation of alpha 1 subunit protein at 2 days was completely reversed by the protein kinase inhibitor staurosporine (0.3 microM). 6. This study has shown that both flunitrazepam and GABA treatment, via their respective binding sites, caused a reduction in the expression of the GABAA receptor alpha 1 subunit protein; an effect mediated through the same neurochemical mechanism. The results also imply that the benzodiazepine effect is independent of GABA, and that the benzodiazepine and GABA sites may not be equally coupled to the down-regulation process, with the benzodiazepine site being the more dominant. The biochemical mechanism underlying the benzodiazepine-mediated down-regulation of the alpha 1 subunit protein seems to involve the activity of staurosporine-sensitive protein kinases.