Reduced responses of prolactin and catecholamine to stress in the lactating rat.

Prolactin, GH, TSH, adrenaline and noradrenaline responses to the stress of immobilization were compared between lactating and non-lactating dioestrous rats. The concentrations of GH in plasma were reduced to a similar degree by the immobilization of lactating and non-lactating rats, and TSH levels were unchanged in both groups. The increases in plasma concentrations of adrenaline and noradrenaline induced by stress were significantly smaller in lactating than in non-lactating rats. Immobilization caused a marked increase in prolactin levels in the plasma of non-lactating rats but no increase in lactating rats. These changes may help to save energy and maintain milk production during the period of lactation.