POSTNATAL MATERNAL SEPARATION DURING THE STRESS HYPORESPONSIVE PERIODENHANCES THE ADRENOCORTICAL-RESPONSE TO NOVELTY IN ADULT RATS BY AFFECTING FEEDBACK-REGULATION IN THE CA1 HIPPOCAMPAL FIELD

The aim of the present experiment was to study the effects of early po stnatal maternal separation on behavioural and adrenocortical response s to novelty in rats tested as adults. Sprague-Dawley rat pups were ex posed to daily maternal separation (5 h/day) from postnatal day 2 to 6 , during the stress hyporesponsive period. Since this procedure requir es physical contact with the animals, a first control group of daily h andled pups was introduced. A second control group, consisting of pups never handled or separated from the mother, was also considered. At p ostnatal day 45, the rats were tested in a two-compartment exploratory apparatus: the maternally separated and the non-handled rats, whose b ehavioural performance did not differ, showed higher emotional behavio ur when compared with the handled rats (P < 0.05), suggesting that the handling procedure hut not maternal separation improved the capacity to cope with novelty. Corticosterone plasma levels were found to be hi gher in the maternally separated rats than in the other two groups (P < 0.05), either at resting conditions or at 30 min after novelty expos ure (P < 0.05). Levels of nuclear glucocorticoid receptor immunoreacti vity in the CA1 hippocampal field were shown to be regulated by novelt y exposure, as expected, in both the handled and the non-handled rats but not in the maternally separated rats. In conclusion, repeated mate rnal separation periods of 5 h/day during the first week of life produ ced long-lasting effects on the hippocampal regulation of the hypothal amic-pituitary-adrenocortical axis, which appear to be associated with increased responsiveness to stress stimuli in adulthood. (C) 1998 ISD N. Published by Elsevier Science Ltd.