NEUROHYPOPHYSEAL VASOPRESSIN IN THE SYRIAN-HAMSTER - RESPONSE TO SHORT PHOTOPERIOD, PINEALECTOMY, MELATONIN TREATMENT, OR OSMOTIC STIMULATION

In the present study, the effect of photoperiod on vasopressin content in the pituitary neurointermediate lobe (NIL), as well as the ability of pinealectomy to prevent and melatonin to mimic the short photoperi od-induced changes in NIL vasopressin were studied in male Syrian hams ters. The ability of melatonin to modify the hyperosmotically stimulat ed vasopressin release was also determined. Exposure to short photoper iod (SD) for 4 or 10 weeks increased vasopressin content in the hamste r NIL. In long photoperiod (LD)-exposed hamsters, pinealectomy induced a decrease in NIL vasopressin content, whereas no effect of melatonin injections on vasopressin storage in the NIL was detected. In SD-expo sed animals, pineal removal failed to alter vasopressin content in the NIL. Hypertonic saline administration led to the expected decrease in vasopressin content in the NIL both in vehicle- and melatonin-treated animals. The hyperosmotically stimulated release of vasopressin was n ot modified by previous treatment with melatonin. The data from the pr esent study show that, in male Syrian hamsters, exposure of animals to SD increases the vasopressin content in the posterior pituitary, but these changes appear not to be mediated by SD-induced changes in melat onin secretion. Furthermore, the exposure of animals to SD prevents th e pinealectomy-induced changes in NIL vasopressin content. Melatonin d oes not modify the hyperosmotically stimulated vasopressin release in the male Syrian hamster. Copyright (C) 1997 Elsevier Science Inc.