Nicotine and Ethanol Activate Protein Kinase A Synergistically via G_iβγ Subunits in Nucleus Accumbens/Ventral Tegmental Cocultures: The Role of Dopamine D₁/D₂and Adenosine A_2AReceptors

Abstract

Tobacco and alcohol are the most commonly used drugs of abuse and show the most serious comorbidity. The mesolimbic dopamine system contributes significantly to nicotine and ethanol reinforcement, but the underlying cellular signaling mechanisms are poorly understood. Nicotinic acetylcholine (nACh) receptors are highly expressed on ventral tegmental area (VTA) dopamine neurons, with relatively low expression in nucleus accumbens (NAcb) neurons. Because dopamine receptors D 1 and D 2 are highly expressed on NAcb neurons, nicotine could influence NAcb neurons indirectly by activating VTA neurons to release dopamine in the NAcb. To investigate this possibility in vitro, we established primary cultures containing neurons from VTA or NAcb separately or in cocultures. Nicotine increased cAMP response element-mediated gene expression only in cocultures; this increase was blocked by nACh or dopamine D 1 or D 2 receptor antagonists. Furthermore, subthreshold concentrations of nicotine with ethanol increased gene expression in cocultures, and this increase was blocked by nACh, D 2 or adenosine A 2A receptor antagonists, G␤␥ or protein kinase A (PKA) inhibitors, and adenosine deaminase. These results suggest that nicotine activated VTA neurons, causing the release of dopamine, which in turn stimulated both D 1 and D 2 receptors on NAcb neurons. In addition, subthreshold concentrations of nicotine and ethanol in combination also activated NAcb neurons through synergy between D 2 and A 2A receptors. These data provide a novel cellular mechanism, involving G␤␥ subunits, A 2A receptors, and PKA, whereby combined use of tobacco and alcohol could enhance the reinforcing effect in humans as well as facilitate long-term neuroadaptations, increasing the risk for developing coaddiction.Alcoholism and nicotine addiction are significant public health problems (see Dani and Harris, 2005). The vast majority of alcoholics also smoke tobacco, with a significantly greater incidence of ethanol dependence in smokers versus nonsmokers (Miller and Gold, 1998). Early onset of smoking carries an increased risk for alcoholism later in life, in part because ethanol consumption is higher in smokers than in nonsmokers (Rimm et al., 1995). Smoking is also associated with increased risk for alcoholism relapse and other substance abuse (Sobell et al., 2002). Moreover, recent studies in PC12 cells show that ethanol up-regulates the expression of nicotinic acetylcholine receptors (nAChR) (Dohrman and Reiter, 2003), considered to reflect physical dependence on nicotine (Dani and Heinemann, 1996). However, despite frequent mutual reinforcement and comorbidity of alcoholism

MCH and apomorphine in combination enhance action potential firing of nucleus accumbens shell neuronsin vitro

“…We also found that the increase in NAshell firing with apomorphine + MCH was inhibited by antagonists of either DA1, DA2, or MCH receptors or by an agent that blocks both DA1 and DA2 receptors. These results concur with our previous findings that DA1 and DA2 receptors interact to enhance NAshell activity (Hopf et al, 2003; Hopf et al, 2005; Chung et al, 2009) (see also Seif et al, 2011 in the NAcb core and Inoue et al, 2007), and suggest that apomorphine enhancement of firing required both DA1 and DA2 receptors in addition to an interaction with MCH receptors. The apomorphine + MCH increase in firing was prevented by a PKA inhibitor, consistent with previous work showing that PKA is an important mediator of NAcb dopaminergic signaling involving DA1 receptors (Svenningsson et al, 2004), including dopamine receptor enhancement of NAshell firing (Hopf et al, 2003).…”

Ethanol Regulation of D1 Dopamine Receptor Signaling is Mediated by Protein Kinase C in an Isozyme-Specific Manner

“…Although gene deletion and pharmacological studies have implicated both D 1 and D 2 receptors in mediating dopaminergic responses to EtOH administration (El-Ghundi et al, 1998;Phillips et al, 1998;Eiler et al, 2003;Inoue et al, 2007), the D 1 subtype appears to be particularly important with respect to the rewarding properties of EtOH and motivation for its consumption. For instance, administration of the D 1 receptor antagonists SCH23390 or ecopipam decreases EtOH consumption in mice, whereas administration of the D 1 agonist SKF81297 with EtOH facilitates alcohol-related behaviors (El-Ghundi et al, 1998;D'Souza et al, 2003;Price and Middaugh, 2004).…”

Nicotinic acetylcholine receptors and the ascending dopamine pathways

“…The key effector protein, downstream of D1 receptors and inhibited by D2 receptor activation, is protein kinase A (PKA) [108,121,122]. Activation of PKA phosphorylates dopamine-and cyclic AMP-regulated phosphoprotein of 32kDa (DARPP-32) at Thr34, making it a potent inhibitor of protein phosphatase 1.…”