Social isolation‐induced increase in α₄ and δ subunit gene expression is associated with a greater efficacy of ethanol on steroidogenesis and GABA_A receptor function

Abstract

Previously we have demonstrated that social isolation of rats reduces both the cerebrocortical and plasma concentrations of 3a-hydroxy-5a-pregnan-20-one (3a,5a-TH PROG), and potentiates the positive effects of acute ethanol administration on the concentrations of this neurosteroid. We now show that the ethanol-induced increase in 3a,5a-TH PROG is more pronounced in the brain than in the plasma of isolated rats. The ability of ethanol to inhibit isoniazid-induced convulsions is greater in isolated rats than in group-housed animals and this effect is prevented by treatment with finasteride. Social isolation modified the effects of ethanol on the amounts of steroidogenic regulatory protein mRNA and protein in the brain. Moreover, ethanol increased the amplitude of GABA A receptor-mediated miniature inhibitory postsynaptic currents recorded from CA1 pyramidal neurones with greater potency in hippocampal slices prepared from socially isolated rats than in those from group-housed rats, an effect inhibited by finasteride. The amounts of the a 4 and d subunits of the GABA A receptor in the hippocampus were increased in isolated rats as were GABA A receptor-mediated tonic inhibitory currents in granule cells of the dentate gyrus. These results suggest that social isolation results in changes in GABA A receptor expression in the brain, and in an enhancement of the stimulatory effect of ethanol on brain steroidogenesis, GABA A receptor function and associated behaviour. Social isolation of rats after weaning results in a decrease in the brain and plasma concentrations of neuroactive steroids such as 3a-hydroxy-5a-pregnan-20-one (allopregnanolone, 3a,5a-TH PROG) and 3a,5a-tetrahydrodeoxycorticosterone . The molecular mechanisms that underlie this effect remain unclear. However, consistent with the hypothesis that ethanol (EtOH) activates the hypothalamicpituitary-adrenal (HPA) axis (Ellis 1966;Rivier et al. 1984;Rivier 1996), we have previously shown that the increases in both the activity of the HPA axis and in the plasma and brain concentrations of neuroactive steroids induced by an acute injection of EtOH are potentiated by social isolation (Serra et al. 2003). EtOH increases the abundance of 3a,5a-TH PROG and 3a,5a-tetrahydrodeoxycorticosterone in the brain and plasma of control rats (Barbaccia et al. 1999;Van Doren et al. 2000), an effect thought to be dependent predominantly on stimulation of the HPA axis, given that it is largely abolished after adrenalectomy (O'Dell et al. 2004). Received November 30, 2005; revised manuscript received February 6, 2006; accepted February 8, 2006.Address correspondence and reprint requests to Mariangela Serra, Department of Experimental Biology, University of Cagliari, Cagliari 09100, Italy. E-mail: mserra@unica.it Abbreviations used: ACSF, artificial cerebrospinal fluid; 3a,5a-TH PROG or AP, 3a-hydroxy-5a-pregnan-20-one; bic, bicuculline; DG, dentate gyrus; eIPSC, synaptically evoked miniature inhibitory postsynaptic current; EtOH, ethanol; GAPDH, glyceraldehyde-3-phopshate ...

Stress-induced plasticity of GABAergic inhibition

“…In contrast, a decrease in the frequency of sIPSCs has been observed following chronic mild stress in DGGCs (Holm et al, 2011), which is consistent with a decrease in the expression of the α1/α2 and γ2 subunits (Matsumoto et al, 2007). Consistent with an upregulation of the GABA A R α4, α5 and δ subunit (Serra et al, 2006; Matsumoto et al, 2007), an increase in tonic GABAergic inhibition has been measured in DGGCs following chronic stress (Serra et al, 2008; Holm et al, 2011). In the PVN, a decrease in the frequency of sIPSCs has been observed (Verkuyl et al, 2004), which can be mimicked with exogenous corticosterone (Verkuyl et al, 2005) and reversed with adrenalectomy (Verkuyl and Joels, 2003).…”

“…Chronic social isolation stress results in a decrease in α1 and α2 expression and an increase in α4 and α5 expression in the frontal cortex (Matsumoto et al, 2007). Similarly, an increase in α4 and δ subunit expression has been observed in the hippocampus following chronic social isolation stress (Serra et al, 2006). These changes are summarized in Table 2, highlighting the brain region-specific alterations in GABA A Rs following chronic stress.…”

Changes in Expression and Function of Extrasynaptic GABAAReceptors in the Rat Hippocampus during Pregnancy and after Delivery

“…Immunoblot analysis showed that these antibodies recognized a single protein of ϳ70 kDa in a crude membrane fraction prepared from rat hippocampal neurons (Sanna et al, 2003). The specificity of this antibody preparation was also confirmed by immunocytochemistry (Sanna et al, 2003;Serra et al, 2006). The antibodies to the ␥ 2 subunit were generated in response to a synthetic peptide corresponding to residues 39 -53 of the rat protein.…”

“…Immunoblot analysis of a crude membrane fraction of the rat hippocampus with these antibodies yielded a single immunoreactive band at a position corresponding to a molecular size of ϳ54 kDa (Follesa et al, 2005). The specificity of the antibody preparation was also confirmed by immunocytochemical analysis (Follesa et al, 2005;Serra et al, 2006). The antibodies to the ␣ 4 subunit were generated in response to a peptide corresponding to the N-terminal region of the rat protein.…”

Ethanol effects on GABA-gated current in a model of increased α4βδ GABAA receptor expression depend on time course and preexposure to low concentrations of the drug

“…In fact, one recent study (Serra et al, 2006) demonstrated that low dose (25 mM) ethanol increased inhibitory current in hippocampal neurons under conditions of high α4βδ GABAR expression. This effect increased progressively over time, with peak level noted by 20 min after ethanol exposure.…”