Journal zyxwvutsrqp of Neuroendocrinology, 1993 zyxwvutsrqp Vol. 5, 365-369 zyxwvutsr Effects of Novelty Stress on Vasopressin and Oxytocin Secretion z by the Pituitary in the Rat Tatsushi Onaka and Kinji Yagi Department of Physiology, Jichi Medical School, Minarnikawachi-machi, Tochigi-ken 329-04, Japan. Key words: vasopressin, oxytocin, adrenocorticotrophic hormone, prolactin, novelty stress Abstract Effects of novel environmental stimuli on vasopressin and oxytocin secretion by the pituitary were studied in dehydrated male rats. As the novel environmental stimuli, rats were transferred to an experimental room, placed in a box painted black and given a pure tone auditory stimulus of 2 kHz. Exposure of rats to the novel environmental stimuli for a period of 2 min decreased plasma concentrations of vasopressin and increased plasma levels of adrenocorticotrophic hormone (ACTH) and prolactin, but did not significantly change the plasma level of oxytocin. The stimuli, however, became ineffective for producing the suppressive vasopressin response as the period of exposure was prolonged to more than 5 and up to 30 min, although the prolonged stimuli were still effective for inducing facilitatory ACTH and prolactin responses. After repeated exposures of rats to the environmental stimuli once a day for 5 or 10 days, the stimuli became disabled from producing the suppressive vasopressin response. However, the rats were still capable of responding to the novel stimuli of another kind. All these data suggest that novelty stress suppresses vasopressin secretion but does not change oxytocin secretion. In order to test the possibility that glucocorticoids expectedly secreted by the adrenals in response to the stress might have suppressed vasopressin secretion, a large amount of dexamethasone was administered to the rat before testing. Dexamethasone pretreatment depressed plasma levels of ACTH and vasopressin as reported previously and blocked the facilitatory ACTH response to the novelty stress. However, dexamethasone treatment did not affect the suppressive vasopressin response to the novelty stress. Thus, it is likely that the suppressive vasopressin response to novelty stress does not primarily depend upon endogenous glucocorticoids. zyxwvu Associatively or non-associatively applied fear-related emotional stress results, in rats, in an increased secretion of oxytocin, adrenocorticotrophic hormone zyxwvutsrq (ACTH) and prolactin, and in a decreased secretion of vasopressin (1). Exposure to novel environ- mental stimuli also results in an increased secretion of ACTH and prolactin (2, 3), and it has been suggested that conditioned fear stimuli and novel stimuli may activate common neural mechanisms to elicit similar neuroendocrine and behavioural responses zyxwvutsrq (4). In the present experiments we tested this hypothesis by measuring plasma concentrations of vasopressin and oxytocin as well as those of ACTH and prolactin in rats exposed to novel environmental stimuli. In addition, we tested the possibility that changes in neurohypophysial hormone release following novel stimuli are secondary to increased secretion of glucocorticoids by prior administration of dexamethasone in rats exposed to novel stimuli. Results Effects of novel environmental stimuli Rats were moved from their home cages into a small black box and exposed to an intermittent pure tone auditory stimulus as a standardized ‘novel environment’ stimulus, and killed immediately after different durations of exposure. Plasma vasopressin concen- trations, measured in trunk blood, were significantly depressed after 2-min exposure, compared to control rats not exposed to the novel stimulus, returned to normal within 10 min of exposure, and remained normal after 30 min of exposure. Plasma oxytocin concentrations were unaffected by the novel environmental stimu- lation of all durations tested. By contrast, plasma concentrations of ACTH were significantly elevated after 2-min exposure and remained elevated after 30-min exposure, whereas prolactin con- centrations were maximal after 10-min exposure and had declined significantly towards normal after 30-min exposure (Fig. 1). Effects of repeated exposure to the environmental stimuli We investigated whether the environmental stimuli itself, or the novelty of the stimulus, was responsible for the observed suppres- sion of vasopressin secretion. Rats were exposed to 5-min environ- mental stimulus daily for 5 or 10 days before a final ‘test’ exposure to 2-min environmental stimulation, after which they were killed. In these rats, the plasma concentration of vasopressin was not significantly different from that in control rats not exposed to Correspondence to: K. Yagi, Department of Physiology, Jichi Medical School, Tochigi-ken 329-04, Japan.