Achieving pharmacologically relevant IV alcohol self-administration in the rat

Abstract

Alcohol consumption produces a complex array of effects that can be divided into two types: the explicit pharmacological effects of ethanol (which can be quite separate temporally from time of intake) and the more temporally “relevant” effects (primarily olfactory and taste) that bridge the time from intake to the onset of the pharmacological effects. Dissociating these effects is essential to untangling the neurologic underpinnings of alcohol abuse and dependence. Intravenous self-administration of ethanol allows for controlled and precise dosing, bypasses first order absorption kinetics allowing for a faster onset of pharmacologic effects, and eliminates the confounding “non-pharmacological” effects associated with oral consumption. Intravenous self-administration of ethanol has been reliably demonstrated in both mouse and human experimental models; however, consistent intravenous self-administration of pharmacologically relevant levels of ethanol remains elusive in the rat. Previous work has demonstrated reliable elevated intravenous ethanol self administration using a compound reinforcer of oral sucrose and intravenous ethanol. The present study sought to elucidate the role of each component of this reinforcer complex using a multiple schedule study design. Male P rats had free access to both food and water during all intravenous self-administration sessions and all testing was performed in conjunction with the onset of the dark cycle. Once animals achieved stable operant responding on both levers for an orally delivered 1% sucrose solution (1S) on a FR4 schedule, surgery was conducted to implant an indwelling jugular catheter. Animals were habituated to the attachment of infusion apparatus and received twice daily sessions for four days to condition each lever to its associated schedule. Animals were then trained to respond on a multiple FR4-FR4 schedule composed of alternating 2.5 minute components. During one component only oral 1S was presented, while in the second component a compound reinforcer of oral 1S + IV 20% ethanol was presented (25 mg/kg/injection). Both levers were extended into the chamber during the session, with the active lever/schedule alternating as the session progressed across components. Average ethanol intake was 0.47 ± 0.04 g/kg. A significant increase in sucrose only reinforcers and sucrose lever error responding was found suggesting that sucrose not ethanol is responsible for driving overall responding. The current findings suggest that the existing intravenous ethanol self-administration methodology remains aversive in the rat.