Effects of juvenile isolation and morphine treatment on social interactions and opioid receptors in adult rats: behavioural and autoradiographic studies.

The consequences of juvenile isolation and morphine treatment during the isolation period on (social) behaviour and mu-, delta- and kappa-opioid receptors in adulthood were investigated by using a social interaction test and in vitro autoradiography in rats. Juvenile isolation reduced social exploration in adults. Morphine treatment counteracted this reduction in isolated rats, but decreased social exploration in nonisolated rats. Self-grooming and nonsocial exploration were enhanced after juvenile isolation. Morphine treatment had no effect on self-grooming, but suppressed nonsocial exploration in isolated rats. With respect to the opioid receptors, juvenile isolation resulted in regiospecific increases in mu-binding sites with a 58% increase in the basolateral amygdala and a 33% increase in the bed nucleus of stria terminalis. Morphine treatment in isolated rats reversed this upregulation in both areas. The number of delta-binding sites did not differ between the experimental groups. A general upregulation of kappa-binding sites was observed after juvenile isolation, predominantly in the cortical regions, the hippocampus and the substantia nigra. Morphine treatment did not affect the upregulation of kappa-receptors. The results show that juvenile isolation during the play period causes long-term effects on social and nonsocial behaviours and on the number of mu- and kappa- but not delta-opioid receptors in distinct brain areas. The number of mu-receptors in the basolateral amygdala appears to be negatively correlated with the amount of social exploration in adult rats.