The effect of 6-hydroxydopamine infusion into the third cerebral ventricle on the plasma prolactin levels in anoestrous ewes subjected to prolonged and repeated stress.

The effect of 6-hydroxydopamine (6-OHDA) infusion into the third cerebral ventricle on the secretion pattern of prolactin in anoestrous sheep, subjected to repeated and prolonged stress-stimuli was followed. On the 12-15 th day after the infusion of 6-OHDA into the third cerebral ventricle and after having tested the effect of this infusion on plasma prolactin concentration, the animals were subjected to mild electric footshocks 9 hrs daily (9 a.m.-6 p.m.) for 3 consecutive days. Infusion of 6-OHDA induced a very high rise of plasma prolactin values by about 3 times as compared to those found before infusion (from 118.2 +/- 39.5 to 338.9 +/- 81.7 ng/ml mesor 24 h). On the other hand, the response of the animals pretreated with 6-OHDA to electric stimulation was characterized by the lack of changes in plasma prolactin concentration; their plasma hormone concentrations were neither raised during stimulation nor decreased after the stimulation and were maintained during all these days nearly at the same level as after the infusion of 6-OHDA. Central mechanisms which could account for the so high and prolonged elevation of plasma prolactin concentration after the infusion of 6-OHDA and for the lack of its rise under conditions of prolonged stress are discussed.