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 is olation period on (social) behaviour and mu-, delta- and kappa-opioid recep tors in adulthood were investigated by using a social interaction test and in vitro autoradiography in rats. Juvenile isolation reduced social explora tion in adults. Morphine treatment counteracted this reduction in isolated rats, but decreased social exploration in nonisolated rats. Self-grooming a nd nonsocial exploration were enhanced after juvenile isolation. Morphine t reatment had no effect on self-grooming, but suppressed nonsocial explorati on in isolated rats. With respect to the opioid receptors, juvenile isolati on resulted in regiospecific increases in m-binding sites with a 58% increa se in the basolateral amygdala and a 33% increase in the bed nucleus of str ia terminalis. Morphine treatment in isolated rats reversed this upregulati on 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, t he hippocampus and the substantia nigra. Morphine treatment did not affect the upregulation of kappa-receptors. The results show that juvenile isolati on 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 recepto rs in distinct brain areas. The number of m-receptors in the basolateral am ygdala appears to be negatively correlated with the amount of social explor ation in adult rats.