Received for publication February 8, 2007.
Revised April 24, 2007.
Accepted for publication April 25, 2007.

Nicotine and ethanol activate PKA synergistically via Gi subunits in nucleus accumbens/ventral tegmental co-cultures: the role of dopamine D₁/D₂ and adenosine A_2A receptors

Abstract

Tobacco and alcohol are the most commonly used drugs of abuse and show the most serious co-morbidity. The mesolimbic dopamine system contributes significantly to nicotine and ethanol reinforcement, but the underlying cellular signaling mechanisms are poorly understood. Nicotinic acetylcholine receptors (nAChR) are highly expressed on ventral tegmental area (VTA) dopamine neurons, with relatively low expression in nucleus accumbens (NAcb) neurons. Since dopamine receptors (D₁ and D₂) 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 co-cultures. Nicotine increased CRE-mediated gene expression only in co-cultures; this increase was blocked by nACh or dopamine D₁ or D₂ receptor antagonists. Further, sub-threshold concentrations of nicotine with ethanol increased gene expression in co-cultures, and this increase was blocked by nACh, D₂, 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 release of dopamine, which in turn stimulated both D₁ and D₂ receptors on NAcb neurons. In addition, sub-threshold concentrations of nicotine and ethanol in combination also activated NAcb neurons through synergy between D₂ 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 co-addiction.

Key words: addiction, dopamine, nicotine, nucleus accumbens, synergy, ventral tegmental area