2009 sona meeting program

POSTER #98 Diurnal Variation in Reward, Mesolimbic Tyrosine Hydroxylase and Clock Gene Expression in the Male Rat Ricardo M. Baltazar, MSc Student, Ian C. Webb, PhD, Lique M. Coolen, PhD & Michael N. Lehman, PhD. Anatomy and Cell Biology, University of Western Ontario.

The impact of the circadian timing system upon behavior and physiology is pervasive and previous evidence suggests a circadian modulation of drug-seeking behaviour and responsiveness to drugs of abuse (Baird & Gauvin, 2000; Abarca et al., 2002). To further characterize daily rhythms in reward and to extend these observations to natural reinforcers, diurnal variation in the rewarding value of sex, palatable food, and systemic amphetamine was assessed via the conditioned place preference paradigm. To identify potential mechanisms for rhythmicity in reward, the expression of tyrosine hydroxylase (TH) and core clock genes (period1 and bmal1) were examined across the day in the ventral tegmental area (VTA) and the nucleus accumbens (NAcc). During an initial training period, male rat sexual performance varied diurnally with a nadir during the light-to-dark transition. Diurnal rhythms also were evident for both sexual and amphetamine-related reward with the variation in sexual reward maintained under constant conditions. However, the rhythms for these particular stimuli exhibited a phase disparity. A diurnal variation also was noted for the locomotor activating effect of acute amphetamine administration. In general, western blotting revealed that TH, Period1, and Bmal1 were rhythmic in the NAcc but did not show robust oscillations in the VTA. Thus, it appears that both natural and drug-related reward vary in a diurnal fashion and that this may be mediated, in part, by rhythmic NAcc dopaminergic activity or clock gene expression. However, as disparate rewards are associated with their own unique rhythms, circadian regulation likely occurs at multiple levels of the mesolimbic reward system.