INCREASED BASAL ACTIVITY OF THE HYPOTHALAMO-PITUITARY-ADRENAL AXIS DURING PREGNANCY IN RATS BRED FOR HIGH ANXIETY-RELATED BEHAVIOR

In order to test the hypothesis that prenatal hormones influence the e motional maturation of the offspring, the hypothalamo-pituitary-adrena l (HPA) axis activity was studied at the end of pregnancy in two rat b reeding lines differing consistently in their innate anxiety-related b ehaviour in the elevated plus-maze. Virgin and pregnant rats were fitt ed with a chronic jugular vein catheter and tested 5 days later. The h igh basal level of anxiety-related behaviour (HAB) described in males anti females of the HAB breeding line persists in pregnancy as indicat ed by a significantly reduced number of entries into and time spent on the open arms of the elevated plus-maze between days le and 20 of pre gnancy compared with pregnant rats of the breeding line with low anxie ty-related behaviour (LAB). In general, an increase in anxiety was fou nd in both breeding lines in pregnancy compared with the respective vi rgin controls. With respect to HPA axis activity, increased basal leve ls of adrenocorticotropin (ACTH) and corticosterone have been found in pregnant rats of the HAB line compared with pregnant LAB rats. ACTH a nd corticosterone secretion in response to emotional and complex physi cal stressors (exposure to the elevated plus-maze and forced swimming, respectively) did not differ between virgin and pregnant rats of eith er breeding line. However, independent of the inborn emotionality of t he animals, a general attenuation in the HPA axis response to stressor s and to exogenous CRH could be confirmed in pregnant rats. The basal and stress-induced activity of the hypothalamo-neurohypophysial system secreting oxytocin and vasopressin was also tested, and no difference s were found relating to the emotionality or reproductive state of the animals except for a reduced vasopressin secretion in pregnant HAB ra ts after forced swimming. The elevated basal activity of the HPA axis, including enhanced circulating concentrations of corticosterone in pr egnant HAB rats, may influence both the neuroendocrine and emotional d evelopment of their offspring. Thus, the passing-on of maternal behavi oural characteristics via prenatal, hormonal 'imprinting' has to be co nsidered as a possible contribution to emotional maturation during an individual's development. (C) 1998 Elsevier Science Ltd. All rights re served.