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Review
. 2007 May;41(3):187-99.
doi: 10.1016/j.alcohol.2007.04.004. Epub 2007 May 23.

Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Affiliations
Review

Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Paolo Botta et al. Alcohol. 2007 May.

Abstract

Cerebellar granule neurons (CGNs) receive inhibitory input from Golgi cells in the form of phasic and tonic currents that are mediated by postsynaptic and extrasynaptic gamma-aminobutyric acid type A (GABAA) receptors, respectively. Extrasynaptic receptors are thought to contain alpha6betaxdelta subunits. Here, we review studies on ethanol (EtOH) modulation of these receptors, which have yielded contradictory results. Although studies with recombinant receptors expressed in Xenopus oocytes indicate that alpha6beta3delta receptors are potently enhanced by acute exposure to low (>or=3 mM) EtOH concentrations, this effect was not observed when these receptors were expressed in Chinese hamster ovary cells. Slice recordings of CGNs have consistently shown that EtOH increases the frequency of phasic spontaneous inhibitory postsynaptic currents (sIPSCs), as well as the tonic current amplitude and noise. However, there is a lack of consensus as to whether EtOH directly acts on extrasynaptic receptors or modulates them indirectly; that is, via an increase in spillover of synaptically released GABA. It was recently demonstrated that an R to Q mutation of amino acid 100 of the alpha6 subunit increases the effect of EtOH on both sIPSCs and tonic current. These electrophysiological findings have not been reproducible in our hands. Moreover, it was shown the alpha6-R100Q mutation enhances sensitivity to the motor-impairing effects of EtOH in outbred Sprague-Dawley rats, but this was not observed in a line of rats selectively bred for high sensitivity to EtOH-induced motor alterations (Alcohol Non-Tolerant rats). We conclude that currently there is insufficient evidence conclusively supporting a direct potentiation of extrasynaptic GABAA receptors following acute EtOH exposure in CGNs.

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Figures

Figure 1
Figure 1
Simplified model of the EtOH-induced enhancement of GABAergic transmission to CGNs. Under control conditions, spontaneous action potential firing of Golgi cells evokes synaptic GABA release and activation of postsynaptic GABAA-Rs. Activation of these receptors generates sIPSCs. Extrasynaptic GABAA-Rs are also activated via GABA spillover, contributing to the tonic current generation together with ambient GABA. Application of EtOH (≥ 20 mM) increases spontaneous action potential (AP) firing in Golgi cells, which elevates GABA levels both at synaptic and extrasynaptic sites. This results in a parallel increase in both sIPSC frequency and the tonic GABAergic current.
Figure 2
Figure 2
Chromosome 10 QTL map of tilting plane test variables in F2 progeny derived from an AT X ANT intercross. Symbols indicate marker locations and horizontal lines show empirically-derived significance LOD thresholds. Physical location of Gabra6 is indicated by the arrow.
Figure 3
Figure 3
The slice preparation protocol does not influence EtOH modulation of tonic currents in CGNs. (A) Sample trace illustrating the effect of EtOH on tonic currents recorded at 23 °C from slices prepared using isoflurane and d,l-APV (protocol #1-see text). (B) Sample trace illustrating the effect of EtOH on tonic currents recorded at 23 °C from slices prepared using ketamine (protocol #2-see text). (C) Time course of the effect of 40 mM EtOH on tonic current noise recorded at 23 °C from slices prepared as described in A-B (n = 4); data were normalized with respect to the values obtained during the first 100 s of recording. Bin size = 100 s. (D) Same as in C but for holding current amplitude. (E) Summary of the percent change in tonic current noise induced by 40 mM EtOH in slices obtained using isoflurane/ d,l-APV or ketamine; recordings were obtained at 23 °C and SR954331 (SR; 10 μM) was used to block GABAA-Rs at the end of the experiment (n = 4). For comparison, four recordings from the study reported in (Carta et al., 2004) were reanalyzed as described in the text. Slices for those studies were prepared using ketamine (protocol #1); recordings were obtained at 32 °C using 50 mM EtOH and bicuculine (BIC; 20 μM) was used to block GABAA-Rs at the end of the experiments. (F) Same as in E but for EtOH-induced percent change in tonic current. (G) Same as in E but for the basal magnitude of tonic current (steady state current recorded in the absence minus that recorded in the presence of the indicated GABAA-R antagonist); p< 0.05 by unpaired t-test. See text for explanation of all-point histogram analysis used to calculate tonic current amplitude and tonic current noise. Bars represent the mean ± S.E.M.
Figure 4
Figure 4
The slice preparation protocol does not influence EtOH modulation of sIPSC frequency in CGNs. (A) Left panels: sample traces illustrating the effect of EtOH on sIPSCs recorded at 23 °C from slices prepared using isoflurane and d,l-APV (protocol #1-see text). Right panel: time course of the effect of EtOH corresponding to the same neuron. (B) Same as in A but for slices prepared using ketamine (protocol #2-see text). (C) Summary of the basal sIPSC frequency recorded from neurons in slices obtained using isoflurane/ d,l-APV (n = 4) or ketamine (n = 3). (D) Same as in C but for basal sIPSC amplitude. (E) Summary of the effect of 40 mM EtOH on sIPSC frequency and amplitude in slices prepared using isoflurane/ d,l-APV (n = 4) or ketamine (n = 3). Recordings were obtained at 23 °C in all cases. Bars represent the mean ± S.E.M.

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