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

Molecular and Regulatory Mechanisms of Desensitization and Resensitization of GABAA Receptors with a Special Reference to Propofol/Barbiturate

It is known that desensitization of GABA_A receptor (GABA_AR)-mediated currents is paradoxically correlated with the slowdown of their deactivation, i.e., resensitization. It has been shown that an upregulation of calcineurin enhances the desensitization of GABA_AR-mediated currents but paradoxically prolongs the decay phase of inhibitory postsynaptic currents/potentials without appreciable diminution of their amplitudes. The paradoxical correlation between desensitization and resensitization of GABA_AR-mediated currents can be more clearly seen in response to a prolonged application of GABA to allow more desensitization, instead of brief pulse used in previous studies. Indeed, hump-like GABA_AR currents were produced after a strong desensitization at the offset of a prolonged puff application of GABA in pyramidal cells of the barrel cortex, in which calcineurin activity was enhanced by deleting phospholipase C-related catalytically inactive proteins to enhance the desensitization/resensitization of GABA_AR-mediated currents. Hump-like GABA_AR currents were also evoked at the offset of propofol or barbiturate applications in hippocampal or sensory neurons, but not GABA applications. Propofol and barbiturate are useful to treat benzodiazepine/alcohol withdrawal syndrome, suggesting that regulatory mechanisms of desensitization/resensitization of GABA_AR-mediated currents are important in understanding benzodiazepine/alcohol withdrawal syndrome. In this review, we will discuss the molecular and regulatory mechanisms underlying the desensitization and resensitization of GABA_AR-mediated currents and their functional significances.

GABAA Receptors of Cerebellar Granule Cells in Culture: Interaction with Benzodiazepines

GABAA receptor mediated inhibition plays an important role in modulating the input/output dynamics of cerebellum. A characteristic of cerebellar GABAA receptors is the presence in cerebellar granule cells of subunits such as α6 and δ which give insensitivity to classical benzodiazepines. In fact, cerebellar GABAA receptors have generally been considered a poor model for testing drugs which potentially are active at the benzodiazepine site. In this overview we show how rat cerebellar granule cells in culture may be a useful model for studying new benzodiazepine site agonists. This is based on the pharmacological separation of diazepam-sensitive α1 β2/3 γ2 receptors from those which are diazepam-insensitive and contain the α6 subunit. This is achieved by utilizing furosemide/Zn2+ which block α6 containing and incomplete receptors.

New 1,5-benzodiazepine compounds: activity at native GABA(A) receptors

Various new 1,5-benzodiazepine compounds were synthesized and tested for their biological activity in terms of effects on GABA(A) receptors of rat cerebellar granules in culture. Their effects were compared to those of a 1,4-benzodiazepine agonist, flunitrazepam and the already known 1,5-benzodiazepine antiepileptic clobazam. The effects were evaluated for the two different GABA(A) receptor populations present in these neurons, one mediating phasic inhibition and the other one mediating tonic inhibition. Many such compounds display a profile of inverse agonist to both GABA(A) receptor populations. One of them presents a profile of full agonist at the component mediating phasic inhibition. Interestingly, substitution of just one oxygen atom in that compound with sulphur in a specific position of a morpholine ring resulted in a remarkable change of activity from full agonist to a probable inverse agonist. This indicates such a position as a proton accepting one for the ligand within the benzodiazepine binding pocket of the relevant GABA(A) receptors. In addition, that position appears to be critical for the pharmacological activity.

GABA(A) receptors of cerebellar granule cells in culture: explanation of overall insensitivity to ethanol

GABA-activated chloride currents were studied in cerebellar granule cells put in culture from neonatal rats. As previously described, 10 microM GABA perfusion of these cells recorded by whole cell patch-clamp elicits chloride currents displaying a peak and a steady-state component. The two components were studied in the presence of 1 mM furosemide, 1 microM Zn(2+) and a combination of the two in order to evaluate the contribution of the different types of GABA(A) receptors. Furosemide inhibits alpha(6) containing receptors whereas low levels of Zn(2+) specifically block incomplete GABA(A) receptors made up of alpha and beta subunits only. The results show that the peak component involves the following receptors: alpha(x) beta(y), 25%; alpha(1) beta(y) gamma(2), 45%; alpha(6) beta(y) gamma(2) plus alpha(1) alpha(6) beta(y) gamma(2), 30%. The steady state component is made up by alpha(x) beta(y), 38%; alpha(1) beta(y) delta, 62%. Ethanol at relatively high concentration, 100 mM, slows further down the desensitization of alpha(1) beta(y) delta receptors. The results indicate that the relative insensitivity to ethanol of GABA(A) receptors of neonatal cerebellar granule cells in culture is due to the absence of mature alpha(6) beta(y) delta receptors, a major receptor brand involved in tonic inhibition.

Activation of cerebellar granule cells GABA(A) receptors by guanidinoacetate

The extracellular concentration of guanidinoacetate (GAA) in the brain increases in guanidino acetate methyl transferase (GAMT) deficiency, an inherited disorder. We tested whether the levels which this substance can reach in the brain in GAMT deficiency are able to activate GABA(A) receptors in key cerebellar neurons such as the cerebellar granules. GAA in fact activates these receptors in rat cerebellar granules in culture although at quite high concentrations, in the millimolar range. However, these millimolar GAA levels are not reached extracellularly in the brain in GAMT deficiency. In addition, GAA does not act as a partial agonist on granules' GABA(A) receptors. This appears to deny an effect by this molecule on cerebellar function in the disease via interference with granule cells' GABA(A) receptors. Study of partial blockage by furosemide of chloride currents activated by GABA and GAA in granule cells allowed us to distinguish two populations of GABA(A) receptors presumably involved in granule cells' tonic inhibition. One is devoid of alpha6 subunit and another one contains it. The latter when activated by GABA has a decay kinetics much slower than the former. GAA does not distinguish between these two populations. In any case, the very high extracellular GAA concentrations able to activate them are not likely to be reached in GAMT deficiency.

A Quantum-Dot Nanocrystal Study of GABAA Receptor Subunits in Living Cerebellar Granule Cells in Culture

Quantum dots (QDs) are semiconductor nanocrystals emerging as a new class of fluorescent labels with large brightness, multi color fluorescence emission and resistance against photobleaching. Here we have used QDs as biological markers in an immunofluorescence approach. In this work GABA(A )receptors of rat cerebellar granule cells have been studied and in particular we have visualized the beta(2/3) and delta subunits in live cells. The results obtained were compared to those gathered with conventional probes. The images of the delta subunit in living cells appear to correspond to those expected for a subunit part of GABA(A )receptors mediating tonic inhibition in the granules cell bodies.

Only high concentrations of ethanol affect GABAA receptors of rat cerebellum granule cells in culture

In the experiments described in the present report, we evaluated the effects of ethanol on the activity of GABAA receptors of cerebellar granule cells in culture. Only very high ethanol concentrations (100-300 mM) showed a clear and significant stimulatory effect on the activity of such receptors. This result was unexpected. In fact, previous reports from other groups would have suggested high ethanol sensitivity of at least one population of GABAA receptors expressed by granule cells.

Molecular basis for the GABAA receptor-mediated tonic inhibition in rat somatosensory cortex

Fast inhibitory synaptic transmission is primarily mediated by synaptically released gamma-aminobutyric acid (GABA) acting on postsynaptic GABA(A) receptors. GABA acting on GABA(A) receptors produces not only phasic but also tonic inhibitions by persistent activation of extrasynaptic receptors. However, the mechanistic characteristics of tonic inhibition in the neocortex are not well-understood. To address this, we studied pharmacologically isolated GABA(A) receptor-mediated currents in neocortical pyramidal neurons in rat brain slices. Bath application of bicuculline blocked miniature inhibitory postsynaptic currents (mIPSCs) and produced an outward shift in baseline holding current (I(hold)). Low concentrations of SR95531, a competitive GABA(A) receptor antagonist, abolished mIPSCs but had no significant effect on I(hold). The benzodiazepine midazolam produced an inward shift in I(hold) by augmenting tonic GABA(A) receptor-mediated currents, which were significantly greater in layer V neurons than in layer II/III. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) revealed a relatively higher expressions of alpha1 and alpha5 subunit mRNA in layer V neurons. L-655708, an alpha5 subunit-specific inverse agonist, reduced tonic currents in layer V but not in layer II/III neurons, whereas zolpidem, an alpha1-subunit agonist, exerted equivalent effects in both layers. These data suggest that the alpha1 GABA(A) receptor subunit is generally involved in tonic inhibition in pyramidal neurons of the neocortex, whereas the alpha5 subunit is specifically involved in layer V neurons.

The alpha 1 and alpha 6 subunit subtypes of the mammalian GABA(A) receptor confer distinct channel gating kinetics

The GABA(A) receptors show a large degree of structural heterogeneity, with seven different subunit families, and 16 different subtypes in mammalian species. The alpha family is the largest, with six different subtypes. The alpha1 and alpha6 subtypes are among the most diverse within this family and confer distinct pharmacological properties to recombinant and neuronal receptors. To determine whether different single channel and macroscopic kinetic properties were also associated with these subtypes, the alpha1 or alpha6 subunit was expressed in mammalian cells along with beta3 and gamma2L subunits and the kinetic properties examined with outside-out patch recordings. The alpha1 beta3 gamma2L receptors responded to GABA with long-duration openings organized into multi-opening bursts. In contrast, channel openings of the alpha6 beta3 gamma2L receptors were predominately short in duration and occurred as isolated, single openings. The subunit subtype also affected the deactivation rate of the receptor, which was almost 2-fold slower for alpha6 beta3 gamma2L, compared with the alpha1 beta3 gamma2L isoform. Onset of fast desensitization did not differ between the isoforms. To determine the structural domains responsible for these differences in kinetic properties, we constructed six chimeric subunits, combining different regions of the alpha1 and alpha6 subunits. The properties of the chimeric subunits indicated that structures within the third transmembrane domain (TM3) and the TM3-TM4 intracellular loop conferred differences in single channel gating kinetics that subsequently affected the deactivation rate and GABA EC50. The effect of agonist concentration on the rise time of the current showed that the extracellular N-terminal domain was largely responsible for binding characteristics, while the transmembrane domains determined the activation rate at saturating GABA concentrations. This suggests that subunit structures outside of the agonist binding and pore-lining domains are responsible for the kinetic differences conferred by the alpha1 and alpha6 subtypes. Structural heterogeneity within these transmembrane and intracellular regions can therefore influence the characteristics of the postsynaptic response of GABA(A) receptors with different subunit composition.

Altered Pharmacology of Synaptic and Extrasynaptic GABAA Receptors on CA1 Hippocampal Neurons Is Consistent with Subunit Changes in a Model of Alcohol Withdrawal and Dependence

Previously, we reported (Cagetti, Liang, Spigelman, and Olsen, 2003) that chronic intermittent ethanol (CIE) treatment leads to signs of alcohol dependence, including anxiety and hyperactivity, accompanied by reduced synaptic gamma-aminobutyric acid (A) receptor (GABAAR) function and altered sensitivity to its allosteric modulators consistent with a measured switch in subunit composition. In this study, we separated the synaptic and extrasynaptic components of GABAAR activation in recordings from pyramidal CA1 cells of hippocampal slices and demonstrated marked differences in the responsiveness of synaptic and extrasynaptic GABAARs to agonists and allosteric modulators in control rats, and in the way they are altered following CIE treatment. Notably, tonic inhibition mediated by extrasynaptic GABAARs was differentially sensitive to the partial agonist gaboxadol (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; THIP) and the allosteric modulator zolpidem, compared with the miniature inhibitory synaptic currents (mIPSCs) in the same cells from saline-treated rats. After CIE treatment, potentiation of tonic currents by diazepam and zolpidem was lost, whereas potentiation by the alpha4 subunit-preferring benzodiazepine Ro15-4513 (ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a]-[1,4]benzodiazepine-3-carboxylate) and THIP was only partially reduced. Potentiation of synaptic GABAAR currents by zolpidem was eliminated after CIE, whereas THIP slightly inhibited mIPSCs from control rats and greatly enhanced them after CIE treatment. These results are consistent with alpha1 subunit decreases at synaptic and extrasynaptic GABAARs, whereas alpha4 subunits are increased at synaptic and decreased at extrasynaptic GABAARs. Behaviorally, THIP was active as a hypnotic and anxiolytic but not as an anti-convulsant against pentylenetetrazol seizures in control rats. Only slight tolerance was observed to the sleep time, but not to the anxiolytic, effect of THIP after CIE. Thus, differential alterations in synaptic and extrasynaptic GABAARs appear to play an important role in the brain plasticity of alcohol dependence, and withdrawal signs may be profitably treated with GABAergic drugs such as THIP, which does not show cross-tolerance with ethanol.

Role of GABA abnormalities in the inferior colliculus pathophysiology - audiogenic seizures

gamma-Aminobutyric acid (GABA), acting at GABA(A) receptors, mediates inhibition in inferior colliculus (IC) central nucleus (ICc) neurons and plays a prominent role in mediating acoustically evoked non-monotonicity, offset inhibition, and binaural inhibition, and is also important in tonic inhibition. The IC plays an important role in a number of pathophysiological conditions that involve hearing, including tinnitus, age-related hearing loss, and audiogenic seizures (AGS). AGS are a major form of rodent neurological disorder that can be genetically mediated and can also be readily induced in both young and mature animals. A deficit in GABA-mediated inhibition in IC neurons has been shown to be a critical mechanism in genetic and induced forms of AGS. Thus, both endogenously evoked GABA-mediated inhibition and exogenously applied GABA are reduced in efficacy in IC neurons of rats that are susceptible to AGS. GABA-mediated inhibition in IC neurons is significantly more easily blocked by a GABA(A) antagonist in genetic and induced forms of AGS in vivo and in vitro. AGS can be induced in normal animals by treatments that reduce the effectiveness of GABA in the IC. Glutamate-mediated excitation is a critical element of neurotransmission in IC neurons, and excessive activation of glutamate receptors in the IC is also strongly implicated as the other major mechanism in the pathophysiology of AGS. These neurotransmitter abnormalities result in excessive firing of ICc neurons that acts as the critical initiation mechanism for triggering seizures in response to intense acoustic stimuli.

Modulation by lanthanum ions of gamma-aminobutyric acid(A) receptors of rat cerebellum granule cells in culture: clues on their subunit composition

Gamma-aminobutyric acid (GABA) activated chloride currents were studied in rat cerebellum granule cells in culture by the whole cell patch-clamp technique. Both the peak and steady state currents were inhibited by 100 microM lanthanum. In the first case, inhibition is due to an increase of the EC50 for GABA. The inhibitory effect of lanthanum on the peak current at 3 microM GABA increased with the cation concentration. A tendency towards the same behavior was found also for the inhibition of the steady state current, at 3 microM GABA, as a function of lanthanum concentration, although inhibition in this case was lower. The comparison of the results with published data about the effects of lanthanum on recombinant GABA(A) receptors likely to occur in granule cells allows suggestions about the receptor types giving, respectively, the peak and the steady state component.