ANALYSIS OF PITUITARY PROLACTIN AND ADRENOCORTICAL-RESPONSE TO ETHER,FORMALIN OR RESTRAINT IN LACTATING RATS - RISE IN CORTICOSTERONE, BUTNO INCREASE IN PLASMA PROLACTIN LEVELS AFTER EXPOSURE TO STRESS

It is well established that stress causes a rise in plasma prolactin ( PRL) levels of male or cycling female rats. In lactating animals, the pituitary PRL response to stress is not well understood. Therefore, th e purpose of the present study was to analyze this question in lactati ng rats having low or elevated prestress plasma PRL levels. The animal s were exposed to ether, formalin or restraint, and plasma PRL and cor ticosterone levels were determined. In mothers continually together wi th their pups, plasma PRL levels decreased significantly after exposur e to ether vapor or injection of formalin under the skin. At the same time, both agents caused a significant rise in blood corticosterone co ncentrations. Lactating rats isolated for 4 h had very low levels of P RL before application of stress. However, neither formalin nor restrai nt caused any elevation in their plasma PRL. levels although both inte rventions increased blood corticosterone concentrations. Lactating mot hers receiving formalin after a 30-min suckling stimulus preceded by 4 h isolation did not show appreciable changes in pituitary PRL secreti on following the administration of formalin. For information on the me chanism of the effect of stress on PRL, lactating rats were pretreated with the dopamine receptor antagonist domperidone (injecting 80 mu g/ kg body weight) or were adrenalectomized 7 days prior to exposure to s tress. The very high levels of PRL caused by domperidone decreased mar kedly in animals subjected to restraint stress. Administration of form alin to adrenalectomized lactating rats continually together with thei r litter caused a slight immediate decrease, followed by a transitory elevation and a subsequent small second decrease in blood PRL concentr ation. The depression was significantly less than in nonadrenalectomiz ed animals receiving formalin. If formalin was administered 2 days aft er isolation of the mothers it caused an elevation of plasma PRL conce ntrations indicating that the PRL stress response pattern characterist ic of nonlactating animals returns within 48 h of isolation. On the ba sis of the present findings we conclude that in lactating animals (a) stress causes an inhibition of PRL release from the pituitary; (b) the pituitary-adrenocortical system responds to stress, (c) the inhibitio n of stress-induced pituitary PRL release disappears within 48 h of se paration of the mother from her litter, and (d) a dopaminergic mechani sm is not involved, but the adrenocortical system may participate in t he stress-induced inhibition of PRL release.