Free-choice responding for ethanol versus water in alcohol preferring (P) and unselected Wistar rats is differentially modified by naloxone, bromocriptine, and methysergide

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Abstract
The role of opioids, dopamine and serotonin in ethanol (EtOH) reward and preference was investigated in non-deprived, Alcohol-Preferring (P), and genetically heterogenous Wistar rats. Operant responding for ethanol was initiated using sweet-solution substitution procedures. The rats were then trained in 30-min daily sessiosn to respond for ethanol (10% v/v) versus water under a two-lever, free-choice contingency. All testing was conducted in the absence of water and food deprivation or addition of sweeteners to the ethanol drinking solution. Rats of both strains developed stable preferences in responding for ethanol over water and consumed ethanol at quantities sufficient to produce pharmacologically relevant mean blood alcohol concentrations (P-Rats: 98±19.6 mg%; unselected Wistars: 41.7±8.5 mg%). InP-rats, systemic naloxone (NAL; 0.125, 0.25 and 0.5 mg/kg) pretreatments resulted in a dose-dependent suppression in responding for both ethanol and water, but did not alter ethanol preference (expressed as percent ethanol of total intake). In contrast, bromocriptine (BRO; 1.0, 2.0 and 4.0 mg/kg) produced a significant, dose-dependent shift in preference from ethanol toward water by inhibiting responding for ethanol while enhancing water consumption. Inunselected Wistar rats, NAL and BRO treatments produced changes in ethanol preference patterns similar to those observed in P-rats. However, compared to P-rats, these changes were smaller and not consistently dose dependent. No changes in ethanol preference and water or ethanol intake were observed with methysergide (MET; 2.5, 5.0, 10.0 mg/kg) in either strain of rat. Together, the results suggest a possible involvement of dopaminergic mechanisms in the reinforcing properties of ethanol. In contrast, the response decrements observed with naloxone may reflect a more general depression in consummatory behavior.