DISSOCIATION BETWEEN BEHAVIORAL AND HORMONAL RESPONSES TO THE FORCED SWIM STRESS IN LACTATING RATS

Retention of immobility in the Porsolt forced swim test is believed to be dependent upon glucocorticoid secretion in male rats. Because lact ating females exhibit increased basal glucocorticoid secretion and blu nted stress responses, we tested the hypothesis that lactation-induced changes in adrenal glucocorticoid and in circulating estrogen and pro gesterone levels would improve retention and/or acquisition of immobil ity. Immobility was recorded during 3 intervals of 5 min on day 1 (acq uisition) and one 5 min interval 24 h later (retention). Blood samples were collected before the swim test and at various times after the on set of stress for plasma ACTH and corticosterone (B) determinations. M ale rats (young = 200 g, old = 325 g) were compared to virgin females (V) and to lactating females in early (day 8-10, EL) and late (day 17- 19, LL) lactation. Adrenalectomy (ADX) and ovariectomy (OVX) were perf ormed 5 and 10 days prior to testing, respectively. All animals acquir ed immobility at the end of the 15 min swim on day 1, but only the you ng male group exhibited a significant retention of immobility on day 2 . Total immobility was higher in males than females (V) although basal and stress-induced ACTH and B secretion were comparable on both testi ng days. Lactational status did not affect immobility in either the ac quisition or retention phases. However, stress-induced ACTH secretion was greatly diminished in intact and ADX lactating females (EL and LL) compared to virgins (LL < EL < virgin), demonstrating a clear dissoci ation between behavioral and neuroendocrine responses, Following ADX, immobility in the retention phase was either decreased in males or inc reased in lactating females. Finally, OVX decreased immobility in both lactating (EL) and virgin females without significantly altering the magnitude of the ACTH and B responses to stress. In summary, our resul ts demonstrated both sex-related and lactation-related differences in the behavioral and endocrine responses to the forced swim test of Pors olt. Although retention of the immobile response is thought to involve glucocorticoids and/or opioids secreted during the first testing sess ion, we did not find evidence for a direct relationship between basal or stress-induced total corticosterone secretion, the magnitude of ACT H response to stress and behavioral scores in the retention period. Ho wever, experimental variables such as body weight, sex and water depth could significantly modify the outcome of behavioral testing and ques tion the validity of glucocorticoid-mediated retention processes. Sinc e the effect of ADX was reversed in lactating females compared to male rats, we hypothesize that glucocorticoid sensitivity of cognitive pro cesses controlling behavioral reactivity is different from that contro lling hypothalamic-adrenocortical function. Our results also demonstra ted a clear dissociation between behavioral and neuroendocrine respons es to the swim test, in particular during lactation. In early and late lactation, blunted responsiveness to stress was not caused by enhance d glucocorticoid feedback but might result from modifications in the i nhibitory and/or stimulatory inputs to hypothalamic neurons controllin g adrenocortical activity.