Increased GABA Release in the Central Amygdala of Ethanol-Dependent Rats

Abstract

The central nucleus of amygdala (CeA) is important in regulating alcohol consumption and plays a major role in the anxiogenic response to ethanol withdrawal. We showed previously that acute ethanol augments GABA A receptor-mediated IPSPs and IPSCs, possibly by a presynaptic mechanism. Here, we have examined the interaction of acute ethanol with the GABAergic system in chronic ethanol-treated (CET) rats using an in vitro CeA slice preparation and in vivo brain microdialysis. We found that in CeA slices from CET rats, the baseline evoked IPSP and IPSC amplitudes were increased, and paired-pulse facilitation ratios were lower than in naive rats, suggesting an increased GABAergic transmission after chronic ethanol treatment. Interestingly, acute ethanol (5-66 mM) significantly enhanced IPSPs and IPSCs equally in CET and naive rats, indicating a lack of tolerance for this effect of acute ethanol. Analysis of miniature IPSC frequency suggests that the increased GABAergic transmission by both acute and chronic ethanol arises from a presynaptic mechanism involving enhanced vesicular release of GABA. These data are supported by microdialysis studies showing that CET rats presented a fourfold increase in baseline GABA dialysate content compared with naive rats. In vivo administration of ethanol (0.1, 0.3, and 1.0 M) produced a dose-dependent increase in GABA release in the CeA dialysate in both CET and naive rats. These combined findings suggest that acute and chronic ethanol increases GABA release in CeA and support previous reports that the behavioral actions of ethanol are mediated, in part, by increased GABAergic transmission in the CeA.

A Conceptualization of Integrated Actions of Ethanol Contributing to its GABAmimetic Profile: A Commentary

“…Likewise, Ziskind-Conhaim et al (2003) demonstrated that 70 mM ethanol increased the number of mIPSPs related to GABA release from neurons in slices of rat spinal cord. A similar presynaptic action of ethanol to increase the frequency of GABA-related mIPSCs was observed from all neurons in slices containing the central nucleus of the amygdala (Roberto et al, 2003), a finding consistent with the ethanol-induced increase of GABA in microdialysates from this site (Roberto et al, 2004a). Ethanol also increased the frequency of mIPSCs from CA1 pyramidal neurons in slices of hippocampus (Sanna et al, 2004), and from brain stem motor neurons (Sebe et al, 2003).…”

“…The demonstrated presynaptic action of ethanol to increase the frequency of GABA-induced mIPSCs (Criswell et al, 2004;Roberto et al, 2003Roberto et al, , 2004aSiggins et al, 2005;Zhu and Lovinger, 2004) supports the view that ethanol can affect a mechanism related to GABA function distinct from a postsynaptic action on GABA A receptors (see Figure 4). These findings can also explain the positive interaction of ethanol with modulators of GABA A receptor function, such as barbiturates and BZDs (Akaike et al, 1990;Puia et al, 1991), by their enhancing the effect of GABA released by ethanol.…”

“…Ethanol also increased the frequency of mIPSCs from CA1 pyramidal neurons in slices of hippocampus (Sanna et al, 2004), and from brain stem motor neurons (Sebe et al, 2003). In addition to the work on mIPSCs performed in slices (see Roberto et al, 2003Roberto et al, , 2004aNie et al, 2004;Carta et al, 2004), the use of mechanically dissociated neurons that have terminals attached (see Akaike and Moorhouse, 2003) has allowed confirmation of the conclusion that ethanol indeed has the ability to release GABA directly from presynaptic terminals. Utilizing this approach, Criswell et al (2004) provided evidence that ethanol increased the frequency of mIPSCs from cerebellar Purkinje neurons.…”

“…However, various investigations using microdialysis were unable to demonstrate an elevation of GABA by acute ethanol from nucleus accumbens, ventral pallidum, or the ventral tegmental area (Cowen et al, 1998;Dahchour et al, 1994;Heidbreder and De Witte, 1993;Piepponen et al, 2002). On the other hand, Roberto et al (2004a) demonstrated an ethanol dose-related release of GABA from amygdala utilizing this approach. One possibility for this apparent discrepancy among the various dialysis studies would be ethanol having a selective effect on GABA release in some, but not all, brain regionsFa possibility to be delineated in future experiments.…”

“…Consistent with this view, Marszalec et al (1998) found that perfusion of 100 mM ethanol augmented GABA A receptor mediated IPSCs from rat cortical neurons, even though the effect of GABA applied to the neuron was not altered. Subsequently, Roberto et al (2003Roberto et al ( , 2004a provided evidence that ethanol enhanced the effect of stimulusinduced GABA-mediated IPSPs and mIPSCs from neurons in slices from the central nucleus of the amygdala and caused release of GABA into microdialysatesFclear evidence that ethanol could facilitate GABA release. This action of ethanol to release GABA can apparently be modified at presynaptic sites (see designation in Figure 4), by GABA B (Ariwodola and Weiner, 2004;Wan et al, 1996) and corticotrophin releasing factor (CRF; Nie et al, 2004) receptors.…”