Elevated reinforcing and motivational properties of alcohol at the end of the nocturnal period in sP rats

Rationale: Sardinian alcohol-preferring (sP) rats displayed high sensitivity to time schedule and consumed intoxicating amounts of alcohol during the last portion of the dark phase of the light/dark cycle when exposed to daily drinking sessions of 1 h, with concurrent availability of multiple alcohol concentrations and unpredictability of time of alcohol access. Objectives: The present study investigated whether sensitivity of sP rats to time schedule extended to operant procedures of alcohol self-administration. Methods: In experiment 1, three different alcohol solutions (10, 20, and 30 %, v/v) were concurrently available under a fixed ratio 4 schedule of reinforcement and with unpredictable time schedule; water was available uncontingently. Experiments 2 and 3 assessed the sensitivity of the motivational properties of alcohol to time schedule; rats were exposed to (a) self-administration sessions under the progressive ratio (PR) schedule of reinforcement and (b) sessions of alcohol seeking under the extinction responding (ER) schedule. Results: In experiment 1, number of lever responses and amount of self-administered alcohol were positively correlated with time of alcohol access during the dark phase. When the self-administration session occurred at the first and latest hours of the dark phase, the amount of self-administered alcohol averaged 0.95-1.0 and 1.55-1.65 g/kg, respectively. In experiments 2 and 3, values of breakpoint and ER for alcohol were approximately 50 % higher when the sessions occurred at the last than first hour of the dark phase. Conclusions: The reinforcing and motivational properties of alcohol were sensitive to time schedule and stronger at the end of the dark phase.