Differential Effects of Chronic Ethanol Consumption and Withdrawal on Homer/Glutamate Receptor Expression in Subregions of the Accumbens and Amygdala of P Rats

Abstract

Background Homer proteins are constituents of scaffolding complexes that regulate the trafficking and function of central Group1 metabotropic glutamate receptors (mGluRs) and N-methyl-D-aspartate (NMDA) receptors. Research supports the involvement of these proteins in ethanol-induced neuroplasticity in mouse. In this study, we examined the effects of short versus long-term withdrawal from chronic ethanol consumption on Homer and glutamate receptor protein expression within striatal and amygdala subregions of selectively bred, alcohol-preferring P rats. Methods For 6 months, male P rats had concurrent access to 15% and 30% ethanol solutions under intermittent (IA: 4 d/wk) or continuous (CA: 7 d/wk) access conditions in their home cage. Rats were killed 24 hours (short withdrawal: SW) or 4 weeks (long withdrawal: LW) after termination of ethanol access, subregions of interest were micropunched and tissue processed for detection of Group1 mGluRs, NR2 subunits of the NMDA receptor and Homer protein expression. Results Within the nucleus accumbens (NAC), limited changes in NR2a and NR2b expression were detected in the shell (NACsh), whereas substantial changes were observed for Homer2a/b, mGluRs as well as NR2a and NR2b subunits in the core (NACc). Within the amygdala, no changes were detected in the basolateral subregion, whereas substantial changes, many paralleling those observed in the NACc, were detected in the central nucleus (CeA) subregion. In addition, most of the changes observed in the CeA, but not NACc, were present in both SW and LW rats. Conclusions Overall, these subregion specific, ethanol-induced increases in mGluR/Homer2/NR2 expression within the NAC and amygdala suggest changes in glutamatergic plasticity had taken place. This may be a result of learning and subsequent memory formation of ethanol’s rewarding effects in these brain structures, which may, in part, mediate the chronic relapsing nature of alcohol abuse.

Binge Alcohol Drinking by Mice Requires Intact Group1 Metabotropic Glutamate Receptor Signaling Within the Central Nucleus of the Amygdale

“…Consistent with studies conducted in alcohol vapor-exposed (Roberto et al, 2006) or chronic, continuous alcohol accessexposed rats (Obara et al, 2009), a month-long history of binge alcohol consumption by B6 mice increased CeA levels of Homer2, mGluR1, GluN2B and PLCb3 proteins. Thus, the capacity of ingested alcohol to elevate CeA mGluR1 expression does not appear to depend highly upon the species studied, the scheduling of alcohol availability or the chronicity of drinking.…”

“…Within the basal forebrain, a neural circuitry forms a separate entity termed the extended amygdala (Alheid and Heimer, 1988) composed of: the bed nucleus of the stria terminalis, the central nucleus of the amygdala (CeA), the shell of the nucleus accumbens (NACshell) and the sublenticular substantia innominata (Alheid and Heimer, 1988). Alcohol increases indices of glutamate neurotransmission within the NAC, CeA and bed nucleus of the stria terminalis (c.f., Gass and Olive, 2008;Siggins et al, 2003; see also Kash et al, 2009;Melendez et al, 2005;Obara et al, 2009;Roberto et al, 2004Roberto et al, , 2006Szumlinski et al, 2008;Wills et al, 2012). Of relevance to this report, repeated bouts of binge alcohol intake can sensitize glutamate release, as well as increase the expression of certain postsynaptic glutamate receptors and their intracellular scaffolding protein Homer2 and downstream effectors within the NACshell (Cozzoli et al, 2009(Cozzoli et al, , 2012Szumlinski et al, 2007); however, very little experimental attention has focused on the role for glutamatergic neurotransmission within other extended amygdala regions vis-à-vis binge drinking.…”

“…Of relevance to this report, repeated bouts of binge alcohol intake can sensitize glutamate release, as well as increase the expression of certain postsynaptic glutamate receptors and their intracellular scaffolding protein Homer2 and downstream effectors within the NACshell (Cozzoli et al, 2009(Cozzoli et al, , 2012Szumlinski et al, 2007); however, very little experimental attention has focused on the role for glutamatergic neurotransmission within other extended amygdala regions vis-à-vis binge drinking. This being said, a chronic history of alcohol experience, produced by either continuous-access procedures or vapor inhalation, elevates CeA indices of both Group1 metabotropic glutamate receptor (mGluR1/5) and ionotropic NMDA receptor signaling (Obara et al, 2009;Roberto et al, 2004Roberto et al, , 2006. Moreover, systemic pretreatment with an mGluR5 antagonist attenuates the reinstatement of alcohol seeking in an operant paradigm and reduces the concomitant elevation in biochemical indices of CeA activation (Schroeder et al, 2008).…”

Synaptic Effects Induced by Alcohol

“…While some investigators have reported increases in NR2B mRNA expression following chronic alcohol exposure in vitro (Hu et al 1996;Snell et al 1996), and in vivo (Follesa and Ticku 1995;Kash et al 2009;) such increases have not been observed in every brain region (Cebere et al 1999;Floyd et al 2003;Läck et al 2005). Increases in NR2B, and to a lesser extent NR2A, protein expression have also been observed using immunologic techniques after both in vitro and in vivo EtOH exposure (Kash et al 2005;Obara et al 2009;Snell et al 1996). However, other investigators did not observe increased expression of this protein.…”

Down-Regulation of Synaptic GluN2B Subunit-Containing N-methyl-d-aspartate Receptors: A Physiological Brake on CA1 Neuron α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Hyperexcitability during Benzodiazepine Withdrawal

“…Previous (Xiang and Tietz, 2007) and current findings indicated that NMDAR depression was secondary to AMPAR potentiation and counteracted the overactivation of AMPAR-mediated glutamatergic transmission during FZP withdrawal. These findings may explain why in most cases benzodiazepine withdrawal is of relatively short duration and self-limited with less severe consequences (O'Brien, 2005) in contrast to the life-threatening withdrawal symptoms that may accompany nonselective central nervous system depressants such as ethanol and barbiturates, which may enhance NMDAR function upon drug withdrawal (Short and Tabakoff, 1993;Kash et al, 2009; see also Obara et al, 2009).…”