Morphine attenuates the effects of juvenile isolation in rats

The acute effects of juvenile isolation on sucrose intake and its long-term consequences on adult social behavior were investigated. Additionally, the role of the endogenous opioid systems was studied. Juvenile rats were housed in one of three conditions: in groups or in isolation with (partial isolation, PI) or without 30 min of daily social contact from 22 to 35 days-of-age. During this period the rats were treated daily with saline or morphine. Juvenile isolated rats showed an increased sucrose intake as compared to non-isolated controls, with PI-rats somewhere in-between, suggesting a negative correlation between the amount of social contact and sucrose consumption. Morphine treatment during the isolation period enhanced the sucrose intake in non-isolated rats, whereas it decreased sucrose consumption in (partial) isolated rats. With regard to the long-term effects, (partial) isolated rats decreased social activity as compared to non-isolated controls which was reversed by morphine treatment during the isolation period. In non-isolated rats, morphine treatment caused an opposite effect: it decreased social activity as compared to the saline treated controls. The data suggest that stimulation of endogenous opioid systems in the juvenile phase may have an important modulatory role in the expression of adult social behavior. The results are discussed in relation to a possible function of morphine as a substitute for the release of endogenous opioid peptides during social play.