Central nervous system mediation of positive and negative reinforcement in neonatal albino rats

Two classes of affective behaviors served as assays for central opioid mediation of affect during development. On the positive side, we found that 5-day-old rats came to prefer a novel odor that predicted the delivery of morphine intracerebroventricularly, thereby replicating previous findings with peripheral morphine. Moreover, central naltrexone blocked the odor preference formation associated with peripheral morphine. In examining responses to pain and stress, we found that central morphine decreased sensitivity to pain, as measured by paw withdrawal to heat and decreased ultrasonic vocalizations during isolation. Conversely, naltrexone delivered intracerebrally increased sensitivity to a painful stimulus and markedly increased distress vocalizations relative to untreated isolates. Thus, the present results imply that central opioid systems are functional behaviorally in neonatal rats mediating affective responses.