Study of the neurophysiological properties of Ventral Tegmental Area dopamine neurons following moderate alcohol drinking in the mouse

Alcohol is the most commonly used substance of abuse worldwide, with 250 million people worldwide affected by an alcohol use disorder (AUD). Dopaminergic (DA) projections from the ventral tegmental area (VTA) to the ventral striatum are strongly implicated in the reward and reinforcing properties of alcohol and other drugs of abuse. Permanent changes in this pathway underlie the formation of compulsive drug-seeking behavior. Unfortunately, neural alterations induced by voluntary ethanol (EtOH) consumption are not yet entirely understood, and current treatments for AUD are hardly available and not very effective. In this respect, a better understanding of the neural modifications marking the early phases of alcohol experience may unravel mechanisms that lead to the development of dependence in advanced stages. On these premises, the main goal of the present project was to investigate the functional remodeling of the reward system induced by EtOH in an animal paradigm of voluntary drinking. To achieve this objective, we first implemented and characterized the 2-bottle choice paradigm (3, 6, 12% v/v EtOH vs water) with the assessment of preference and total intake dynamics, and we evaluated anxiety levels and relapse after EtOH deprivation. We then investigated persistent alterations in VTA DA neurons induced by voluntary alcohol intake and following withdrawal, and the response to acute 40 mM EtOH application, by performing electrophysiological recordings in acute slices obtained from both EtOH-exposed and naive TH (tyrosine hydroxylase)–GFP (green fluorescence protein) mice. Finally, nociceptin/orphanin FQ (N/OFQ), applied in the bath during recordings, was tested on electrophysiological alterations in the VTA identified in this project. Our results demonstrated that voluntary drinking does not lead to behavioral escalation, and EtOH-exposed mice do not show withdrawal syndrome, such as increased anxiety and alcohol deprivation effect (ADE), following EtOH deprivation. However, we found a significant reduction in the basal firing rate of VTA DA neurons from exposed mice, accompanied by a greater response to acute application. In line with this, chronic EtOH increases the frequency of GABAergic, but not glutamatergic, inputs onto VTA DA neurons, while reducing the response to acute EtOH perfusion. Nevertheless, after 5 days of EtOH deprivation, the strength of the inhibitory synaptic activity onto DA neurons and response to acute EtOH is normalized. Together these changes may precede the onset of a hypodopaminergic condition and an addictive state. Finally, we obtained evidence that N/OFQ, long studied as an endogenous able to control addictive behavior in animal models, abolishes the acute EtOH-induced enhancing effect on the firing rate of VTA DA neurons in both groups without affecting their basal activity.