Dopamine (DA) signaling in the nucleus accumbens (NAc) increases in response to natural reward and is thought to encode the incentive salience of environmental stimuli, which motivates goal-directed behaviors. All drugs of abuse, including alcohol, increase DA signaling in the NAc, implicating this system in the development of addiction. In contrast to acute effects, DA signaling is reduced during withdrawal from chronic alcohol exposure. It has been proposed that the endogenous kappa opioid receptor (KOR)/dynorphin system is upregulated in the NAc during chronic alcohol administration, and may contribute to reduced DA signaling via inhibitory KORs located on DA terminals. Modulation of DA release through various heteroreceptors (including KOR) located on DA terminals has been investigated in NAc slices using fast scan cyclic voltammetry (FSCV) to measure DA release and application of electrical stimulation to excite DA terminals. However, electrical stimulation of the tissue results in simultaneous excitation of all neuronal processes in the stimulation field. The NAc integrates inputs from multiple brain regions, resulting in a high level of neuronal heterogeneity in the tissue. Release of neurotransmitters from non-DAergic processes provide, individually and together, local regulation of DA terminal release. Thus, pharmacological effects on DA terminal release may occur through direct (receptors on DA terminals) and/or indirect (receptors on other terminals or cells) mechanisms. This level of complexity confounds analysis of KOR effects on DA terminals. In this proposal we introduce a novel model for in vitro investigations of DA terminal function by optogenetically targeting DA neuron stimulation in the NAc and measuring release using FSCV. We will inject the ventral tegmental area with a viral construct encoding expression of channelrhodopsin-2 (ChR2) to induce ChR2 expression in DA neurons. This will allow selective stimulation of DA terminals in NAc slices using blue light. We propose to develop this technique by targeting ChR2 expression specifically to DA neurons utilizing a Cre-inducible viral construct in TH:Cre transgenic mice. We will assess the specificity of expression using immunohistochemistry and confocal microscopy. Also, we will characterize the evoked signal to validate its identity as DA and demonstrate that release is action potential dependent. Furthermore, we will use the model's improved sensitivity for measures of direct actions of KOR on DA terminal release. Finally, we will assess the changes in DA terminal KOR sensitivity following chronic ethanol administration in mice.
Alcohol dependence and addiction is a progressive disease with a lifetime prevalence of 12.5% and an estimated annual economic cost in the United States of $223 billion dollars (Hasin et al. 2007~ Bouchery et al. 2011). Most addictive substances affect dopamine signaling in the nucleus accumbens, critically implicating this area in the development of addiction. Unraveling the mechanisms of dopamine signaling in the nucleus accumbens, and how it is affected by alcohol abuse, will provide insights into the prevention and treatment of addiction.