Effects of adolescent social instability stress on social reward motivation and dopamine receptor expression in female and male rats

Social experiences in adolescence are known to modify ongoing brain and behavioural development. We previously reported that adolescent social instability stress (SS; daily 1 h isolation and pairing with a new cage partner from postnatal days 30-45) reduced social interaction in female and male rats. Here, we investigated whether adolescent SS reduced social motivation and altered dopamine receptor mRNA expression in brain regions involved in social reward in female and male Long-Evans rats. When tested in a social operant conditioning task in adolescence whereby nose-pokes at one gate provided access to a social stimulus and at the other did not, SS rats made more social nose-pokes during training than did CTL rats. SS rats did not differ from CTLs when increased effort was required on the progressive ratio test (PRT; progressively more nose-pokes required to open gate; measure of social motivation). In adulthood, however, SS and CTL rats did not differ during training, although SS female rats had reduced social motivation on the PRT. Further, social motivation was higher in female than in male rats. Irrespective of age at testing, SS rats had increased dopamine receptor subtype 2 (D2), but not dopamine receptor subtype 1, mRNA expression in the medial prefrontal cortex in both sexes and in the nucleus accumbens in female rats. Given the evidence that D2 signaling in such brain regions inhibits social behaviour, the present research suggests that the increase in D2 mRNA expression may underlie the deficits in social behaviour evident in SS rats.