Individual Differences in the Feeding and Locomotor Stimulatory Effects of Acute and Repeated Morphine Treatments

Abstract

Rats with high propensity to ingest sugar (HIGH) show increased responsiveness to amphetamine treatments than rats with low propensity to ingest sugar (LOW). Intrinsic variation in the functioning of the mesolimbic dopamine system has been suggested to account for these individual differences. Morphine has stimulatory effects on feeding and locomotion that are in part mediated by the mesolimbic dopamine system. The present study therefore examined whether LOW and HIGH rats would exhibit differences in the feeding and locomotor stimulating effects of morphine. Morphine (1, 2, and 4 mg/kg) significantly stimulated the intake of chow and sugar in LOW rats without affecting food consumption in HIGH rats. Further, it was found that both groups of rats did most of their feeding in the first 20 min following injection, and that the stimulatory effect of morphine in LOW rats was restricted to the first hour of the 3-h test session. Repeated morphine (2 mg/kg) stimulated sugar intake in LOW but not HIGH rats, and there was no evidence of increased intake across injections. Acute administration of 5.0 mg/kg, but not 2.0 mg/kg, of morphine produced higher levels of locomotor activity in LOW rats compared to HIGH rats; repeated treatment with 5.0 mg/kg morphine produced a sensitized locomotor response in both LOW and HIGH rats. These results indicate that LOW rats exhibit increased responsiveness to the locomotor and feeding stimulatory effects of morphine compared to HIGH rats. One implication arising from these findings is that LOW and HIGH rats may be distinguished by differences in opiatergic function, as well as by differences in dopaminergic function.