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

The effects of etomidate and midazolam on adipose tissue-derived mesenchymal stem cell proliferation

Background

Stem cell therapy using adipose tissue-derived mesenchymal stem cells (ADSCs), which are capable of multipotent differentiation, is currently being investigated in the field of tissue regeneration and the treatment of patients in intensive care units. It is known that type-A γ-aminobutyric acid (GABA_A) receptor activity has an influence on stem cell proliferation. Thus, we investigated the effects of the clinically available GABA_A receptor agonists, etomidate and midazolam, on ADSC proliferation measured by the cell counting kit-8 assay.

Methods

ADSCs cultured in control medium or adipogenic differentiation medium for 15 days were divided into 5 treatment groups: non-medicated (Control) and 4 groups including treatment with etomidate or midazolam at 1 and 50 µM (n = 3 per group). The cell counting kit-8 assay was performed for determining the cell proliferation in both medium groups at day 0, 3, 6, 9, 12, and 15 in culture. The absorbance values at 450 nm were then measured by enzyme-linked immunosorbent assay reader and statistically compared among groups.

Results

There was no significant difference in cell proliferation profiles among the 5 groups at any time point in both control and adipogenic differentiation media.

Conclusions

Etomidate and midazolam did not influence ADSC proliferation under both media when compared to the non-medicated group and there was no dose-dependent effect of etomidate and midazolam on ADSC viability.

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.

Fast-spiking Cell to Pyramidal Cell Connections Are the Most Sensitive to Propofol-induced Facilitation of GABAergic Currents in Rat Insular Cortex

Background:

Propofol facilitates γ-aminobutyric acid–mediated inhibitory synaptic transmission. In the cerebral cortex, γ-aminobutyric acidergic interneurons target both excitatory pyramidal cells (Pyr) and fast-spiking (FS) and non-FS interneurons. Therefore, the propofol-induced facilitation of inhibitory transmission results in a change in the balance of excitatory and inhibitory inputs to Pyr. However, it is still unknown how propofol modulates γ-aminobutyric acidergic synaptic transmission in each combination of Pyr and interneurons.

Methods:

The authors examined whether propofol differentially regulates inhibitory postsynaptic currents (IPSCs) depending on the presynaptic and postsynaptic cell subtypes using multiple whole cell patch clamp recording from γ-aminobutyric acidergic interneurons and Pyr in rat insular cortex.

Results:

Propofol (10 μM) consistently prolonged decay kinetics of unitary IPSCs (uIPSCs) in all types of inhibitory connections without changing paired-pulse ratio of the second to first uIPSC amplitude or failure rate. The FS→Pyr connections exhibited greater enhancement of uIPSC charge transfer (2.2 ± 0.5 pC, n = 36) compared with that of FS→FS/non-FS connections (0.9 ± 0.2 pC, n = 37), whereas the enhancement of charge transfer in non-FS→Pyr (0.3 ± 0.1 pC, n = 15) and non-FS→FS/non-FS connections (0.2 ± 0.1 pC, n = 36) was smaller to those in FS→Pyr/FS/non-FS. Electrical synapses between FS pairs were not affected by propofol.

Conclusions:

The principal inhibitory connections (FS→Pyr) are the most sensitive to propofol-induced facilitation of uIPSCs, which is likely mediated by postsynaptic mechanisms. This preferential uIPSC enhancement in FS→Pyr connections may result in suppressed neural activities of projection neurons, which in turn reduces excitatory outputs from cortical local circuits.

Expression of the γ2-subunit distinguishes synaptic and extrasynaptic GABA(A) receptors in NG2 cells of the hippocampus

NG2 cells are equipped with transmitter receptors and receive direct synaptic input from glutamatergic and GABAergic neurons. The functional impact of these neuron-glia synapses is still unclear. Here, we combined functional and molecular techniques to analyze properties of GABA(A) receptors in NG2 cells of the juvenile mouse hippocampus. GABA activated slowly desensitizing responses in NG2 cells, which were mimicked by muscimol and inhibited by bicuculline. To elucidate the subunit composition of the receptors we tested its pharmacological properties. Coapplication of pentobarbital, benzodiazepines, and zolpidem all significantly increased the GABA-evoked responses. The presence of small tonic currents indicated the presence of extrasynaptic GABA(A) receptors. To further analyze the subunit expression, single cell transcript analysis was performed subsequent to functional characterization of NG2 cells. The subunits α1, α2, β3, γ1, and γ2 were most abundantly expressed, matching properties resulting from pharmacological characterization. Importantly, lack of the γ2-subunit conferred a high Zn²⁺ sensitivity to the GABA(A) receptors of NG2 cells. Judging from the zolpidem sensitivity, postsynaptic GABA(A) receptors in NG2 cells contain the γ2-subunit, in contrast to extrasynaptic receptors, which were not modulated by zolpidem. To determine the effect of GABA(A) receptor activation on membrane potential, perforated patch recordings were obtained from NG2 cells. In the current-clamp mode, GABA depolarized the cells to approximately -30 mV, indicating a higher intracellular Cl⁻ concentration (∼50 mM) than previously reported. GABA-induced depolarization in NG2 cells might trigger Ca²⁺ influx through voltage-activated Ca²⁺ channels.

ERK/MAPK pathway regulates GABAA receptors

The GABAA receptor is a ligand-gated ion channel whose function and activity can be regulated by ligand binding or alternatively may be influenced indirectly through the phosphorylation of specific subunits that comprise the GABAA receptor pentamer. With respect to phosphorylation, most studies have focused on either beta or gamma subunits, whereas the role of the alpha subunit as a relevant target of signaling kinases is largely unknown. Interestingly, we found a putative phosphorylation site for extracellular-signal regulated kinase (ERK), a key effector of the MAPK pathway, in almost all known alpha subunits of the GABAA receptor, including the ubiquitously expressed alpha1 subunit. To determine whether this putative ERK phosphorylation site was functionally relevant, we evaluated if ERK inhibition (through pharmacological inhibition of its upstream kinase, MEK) altered GABA-gated currents. Using HEK293 cells stably transfected with the alpha1beta2gamma2 form of the GABAA receptor, we found that UO126 reduced basal ERK phosphorylation and resulted in an enhancement of GABA-induced peak current amplitudes. Further, the enhancement of GABA-gated currents required an intact intracellular environment as it was robust in perforated patch recordings (which preserves the intracellular milieu), but absent in conventional whole-cell recordings (which dialyzes the cytosolic contents), supporting the involvement of an intracellular signaling pathway. Finally, mutation of the ERK phosphorylation site (T375-->A) prevented the UO126-induced enhancement of GABA-gated currents. Collectively, our results implicate the MAPK pathway as a negative modulator of GABAA receptor function, whose influence on GABA-gated currents may be mediated by phosphorylation of the alpha subunit.

Selective changes in thalamic and cortical GABAA receptor subunits in a model of acquired absence epilepsy in the rat

Neonatal treatment of Long-Evans Hooded rats with the cholesterol synthesis inhibitor (CSI) AY9944 has been shown to increase occurrence of spike-waves in EEG recordings and decrease benzodiazepines sensitivity of GABA(A) receptor-mediated responses in neurons from the thalamic reticular nuclei (nRt, Wu et al., 2004). The present experiments were designed to investigate the changes in the gamma2 and alpha1 subunits of the GABA(A) receptor in CSI model rats as possible mechanisms of these changes. Western blot, immunohistochemistry and real-time PCR techniques were performed to measure the levels of GABA(A) receptor gamma2 and alpha1 subunit transcripts and protein in the nRt and ventrobasal (VB) relay nuclei of thalamus and in somatosensory cortex. In CSI model animals, Western blot results showed that gamma2 subunit expression significantly decreased in thalamus (control, n=6: 0.17+/-0.02 relative to actin vs. CSI model, n=6: 0.11+/-0.01, P<0.05) but neither in cortex nor in hippocampal tissues. Conversely, alpha1 subunit expression decreased in CSI model somatosensory cortex, but not in nRt and VB. The present results demonstrate that neonatal block of cholesterol synthesis produces region- and subunit-specific decreases in GABA(A) receptor subunits in thalamus and cortex. Selective reductions in GABA(A) receptor subunits in thalamus may play a role in pathophysiology of absence epilepsy.

GABAergic signaling to newborn neurons in dentate gyrus

Neurogenesis in the dentate gyrus begins before birth but then continues into adulthood. Consequently, many newborn granule cells must integrate into a preexisting hippocampal network. Little is known about the timing of this process or the characteristics of the first established synapses. We used mice that transiently express enhanced green fluorescent protein in newborn granule cells to examine their synaptic input. Although newborn granule cells had functional glutamate receptors, evoked and spontaneous synaptic currents were exclusively GABAergic with immature characteristics including slow rise and decay phases and depolarized reversal potentials. Synaptic currents in newborn granule cells were relatively insensitive to the GABA(A) receptor modulator zolpidem compared with neighboring mature granule cells. Consistent with the kinetics and pharmacology, newborn granule cells isolated by fluorescent cell sorting lacked the alpha1 GABA(A) receptor subunit. Our results indicate that newborn granule cells initially receive only GABAergic synapses even in the adult.

Abnormal benzodiazepine and zinc modulation of GABAA receptors in an acquired absence epilepsy model

Brain cholesterol synthesis inhibition (CSI) at a young age in rats has been shown to be a faithful model of acquired absence epilepsy, a devastating condition for which few therapies or models exist. We employed the CSI model to study cellular mechanisms of acquired absence epilepsy in Long-Evans Hooded rats. Patch-clamp, whole-cell recordings were compared from neurons acutely dissociated from the nucleus reticularis of thalamus (nRt) treated and untreated with a cholesterol synthesis inhibitor, U18666A. In U18666A-treated animals, 91% of rats developed EEG spike-waves (SWs). Patchclamp results revealed that although there was no remarkable change in GABAA receptor affinity, both a loss of ability of benzodiazepines to enhance GABAA-receptor responses and an increase of Zn2+ inhibition of GABAA-receptor responses of nRt neurons occurred in Long-Evans Hooded rats previously administered U18666A. This change was specific, since no significant changes were found in neurons exposed to the GABA allosteric modulator, pentobarbital. Taken collectively, these findings provide evidence for abnormalities in benzodiazepine and Zn2+ modulation of GABAA receptors in the CSI model, and suggest that decreased gamma2 subunit expression may underlie important aspects of generation of thalamocortical SWs in atypical absence seizures. The present results are also consistent with recent findings that mutation of the gamma2 subunit of the GABAA receptor changes benzodiazepine modulation in families with generalized epilepsy syndromes.

Biphasic action of midazolam on GABAA receptor-mediated responses in rat sacral dorsal commissural neurons

The effect of the benzodiazepine agonist midazolam on gamma-aminobutyric acid(A) (GABA(A)) receptor-mediated currents was investigated in neurons acutely dissociated from the rat sacral dorsal commissural nucleus (SDCN) using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. Midazolam displayed a biphasic effect on GABA responses. Low concentrations of midazolam (1nM-10 microM) reversibly potentiated GABA (3 microM)-activated Cl(-) currents (I(GABA)) in a bell-shaped manner, with the maximal facilitary effect at 0.1 microM; whereas at higher concentrations (above 10 microM), midazolam had an antagonistic effect on I(GABA). Our further study indicated that midazolam changed GABA(A) receptor affinity to GABA and the effects of midazolam on I(GABA) were voltage-independent. The benzodiazepine receptor antagonist, flumazenil, abolished the facilitary effect of low concentrations of midazolam rather than the antagonism of I(GABA) induced by high doses of midazolam. In addition, activation of protein kinase C prevented the inhibitory effect of midazolam at higher concentrations, but did not influence the effect of midazolam at low concentrations. These results indicate that midazolam interacts with another distinct site other than the central benzodiazepine receptors on GABA(A) receptors as an antagonist at higher concentrations in SDCN neurons.

GABA depolarizes neuronal progenitors of the postnatal subventricular zone via GABAA receptor activation

Previous studies have reported the presence of migrating and dividing neuronal progenitors in the subventricular zone (SVZ) and rostral migratory stream (RMS) of the postnatal mammalian brain. Although the behaviour of these progenitors is thought to be influenced by local signals, the nature and mode of action of the local signals are largely unknown. One of the signalling molecules known to affect the behaviour of embryonic neurons is the neurotransmitter GABA. In order to determine whether GABA affects neuronal progenitors via the activation of specific receptors, we performed cell-attached, whole-cell and gramicidin perforated patch-clamp recordings of progenitors in postnatal mouse brain slices containing either the SVZ or the RMS. Recorded cells displayed a morphology typical of migrating neuronal progenitors had depolarized zero-current resting potentials, and lacked action potentials. A subset of progenitors contained GABA and stained positive for glutamic acid decarboxylase 67 (GAD-67) as shown by immunohistochemistry. In addition, every neuronal progenitor responded to GABA via picrotoxin-sensitive GABAA receptor (GABAAR) activation. GABAARs displayed an ATP-dependent rundown and a low sensitivity to Zn2+. GABA responses were sensitive to benzodiazepine agonists, an inverse agonist, as well as a barbiturate agonist. While GABA was hyperpolarizing at the zero-current resting potentials, it was depolarizing at the cell resting potentials estimated from the reversal potential of K+ currents through a cell-attached patch. Thus, our study demonstrates that neuronal progenitors of the SVZ/RMS contain GABA and are depolarized by GABA, which may constitute the basis for a paracrine signal among neuronal progenitors to dynamically regulate their proliferation and/or migration.

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.

Single cell expression analysis--pharmacogenomic potential

A fundamental challenge in biology is to correlate physiology with gene expression in specific cell types. This can only be achieved by understanding gene expression at the level of the single cell because, in many systems, each cell has the capacity to express a unique set of genes. Therefore, each cell can be considered to be functionally distinct. A clearer understanding of gene expression differences at such a discrete level provides an opportunity to develop drugs with more targeted pharmacologies or with decreased side effects.

GABA(A) receptor heterogeneity in histaminergic neurons

Histaminergic neurons of the tuberomamillary nucleus display pacemaker properties; their firing rate is regulated according to behavioural state by gabaergic inhibition. Whole-cell recordings and single-cell RT-PCR from acutely isolated rat tuberomamillary neurons were used to characterize GABA -evoked currents and to correlate them with the expression pattern of 12 GABAA receptor subunits. We report differences in sensitivity to GABA and zinc as well as in the modulation of IPSC-decay times by zolpidem in histaminergic neurons expressing gamma-subunits at different levels. Immunocytochemistry and pharmacological analysis of whole-cell GABA-currents in these neurons revealed that all carry the gamma2-subunit protein and that all receptors contain at least one gamma-subunit. Neurons with different expression levels of gamma-subunits displayed a difference in cooperativity of GABA and zolpidem binding which we explain by the presence of one vs. two gamma-subunits in one receptor. Thus, we describe here native GABAA receptor function in relation to its stoichiometry.

Allosteric interaction of zinc with recombinant alpha(1)beta(2)gamma(2) and alpha(1)beta(2) GABA(A) receptors

In a recent study we have provided evidence that inhibition of native GABA(A) receptors by zinc depends primarily on the allosteric modulation of receptor gating. Both the kinetics and the sensitivity of the GABA(A) receptor to zinc depend on subunit composition, especially on the presence of the gamma(2) subunit. To analyze the mechanism of action of zinc its effects have been tested on recombinant alpha(1)beta(2)gamma(2) and alpha(1)beta(2) receptors expressed in HEK 293 cells. The currents produced by ultrafast application of GABA have been measured to assess the impact of zinc ions on GABA(A) receptor gating with resolution corresponding to the time scale of synaptic currents. While, as expected, zinc markedly reduced the peak amplitude of alpha(1)beta(2)-mediated currents, its effect on kinetics was significantly different from that observed for alpha(1)beta(2)gamma(2). In particular, unlike alpha(1)beta(2)gamma(2), zinc did not affect the onset of alpha(1)beta(2)-mediated responses. Moreover, zinc increased the extent of desensitisation of alpha(1)beta(2)gamma(2) receptors and reduced desensitisation of alpha(1)beta(2) ones. Quantitative analysis suggests that zinc exerts an allosteric modulation on both alpha(1)beta(2)gamma(2) and alpha(1)beta(2) receptors. Zinc effects on alpha(1)beta(2)gamma(2) were qualitatively similar to those reported for native receptors.

Competitive quantitative measurement of the AMPA receptor gene expression at the single cell level

Our laboratory has developed a competitive reverse transcriptase polymerase chain reaction (RT-PCR) procedure to analyse the mRNA expression of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunits in single cells. By the use of an internal RNA standard competing equally with the four subunit's mRNA, we have analysed 283 whole single hippocampus CA1 cells from adult rat brain. The cells were sampled from three groups of animals: one control group, one group subjected to preconditioning ischemia, and one group subjected to global cerebral ischemia. After reverse-transcription and PCR-amplification of mRNA in the cells, the PCR product was digested using subunit specific endonucleases and quantified by Cy-5 fluorescence. The median mRNA copy numbers achieved from control rats were 290, 247, 207, and 16 GluR1-4, respectively.

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.