Changes induced by morphine administration in the hypothalmic neurosecretory system of the spotted owlet, Athene...

The effect of chronic administration of morphine, in gradually increasing doses, on the histology of the HNS of the spotted owlet was investigated. Administration of morphine in low doses (0.25-0.75 mg/day), for 9 days, induced marked depletion of NSM from all regions of the HNS, except the zona externa of the AME. Maximum depletion of NSM was noticed in the HNS of birds treated with morphine for 9 days. Beyond the 9th day, the HNS did not exhibit further depletion of its NSM. Administration of morphine in high doses (1.5-4 mg/day), for 18 days, did not cause depletion of NSM from the HNS. On the other hand there was noticeable increase of the NSM in the HNS over that of the controls. The histological changes in the HNS induced by low doses of morphine are comparable to the changes induced by hypertonic saline administration and are presumably indicative of augmented secretion of the ADH. The accumulation of NSM in the HNS of birds receiving high doses of morphine suggests a decrease in neurohypophysial activity. The results therefore indicate that the effect of morphine on the HNS of the spotted owlet is dose-dependent. The mechanism of action of morphine on the neurohypophysis is briefly discussed.