SEXUAL SEGREGATION IN INFANT MICE - BEHAVIORAL AND NEUROENDOCRINE RESPONSES TO D-AMPHETAMINE ADMINISTRATION

Individual differences arise from both genetic and epigenetic factors. The aim of this study was to test whether pups raised in distinct soc io-sexual conditions would show different behavioural and neuroendocri ne responses to d-amphetamine (AMPH) administration upon placement in a novel environment. This issue was addressed by testing infant CD-1 m ouse pups ofboth sexes at three different developmental ages [3, 8, or 18 postnatal (PND) days]. These pups were raised from birth in all-ma le, all-female, or mixed-sex litters. AMPH effects were assessed as a function of the hypothalamic-pituitary-adrenal (HPA) axis activational state using litters that were either maternally deprived for 24 h (DE P) or normally kept with the dam (NDEP). A concomitant maternal behavi our score carried out on selected postpartum days showed that mothers laking care of all-male litters were more often involved in Active nur sing than those rearing the mixed-sex ones. whereas the latter were fo und more often Laying still out of the nest. Basal and stress-induced corticosterone (CORT) secretion was increased in unisexually reared pu ps following maternal deprivation, an effect limited to PND 3. In gene ral, neuroendocrine and behavioural responses to AMPH were found to be dissociated and were affected by sexual segregation only in conjuncti on with maternal deprivation. On PND 3, AMPH injection (1 or 3 mg/kg, IP) decreased CORT secretion in deprived unisexually reared subjects w ithout affecting their behaviour. As a whole, behavioural changes due to unisexual rearing were limited to female subjects. On PND 8, unisex ually reared females showed, upon maternal deprivation, a generalized shift to the left in the dose-response curve to AMPH for Crossing beha viour, while on PND 18 AMPH-induced stereotypies were considerably red uced in sexually segregated females, especially following maternal dep rivation. Thus: maternal deprivation appeared to ''sensitize'' the mon oaminergic system to an AMPH challenge. The individual behavioural and neuroendocrine profiles shown in response to a stressful challenge su ggest that changes in social stimulation early during development migh t produce subtle shifts in the function of selected central monoaminer gic systems.