Thyrotropin-releasing hormone affects the oxytocin, vasopressin and prolactin release in female rats during midlactation: relation to suckling

Abstract

The effect of thyrotropin-releasing hormone (TRH; 200 ng i.c.v.) on oxytocin (OT), vasopressin (AVP) and prolactin (PRL) release was estimated in female Wistar rats during midlactation. The hypothalamo-neurohypophysial radioimmunoassayed OT and AVP storage as well as blood plasma level of both neurohypophysial hormones and PRL in females suckled or not suckled have been studied. I.c.v. administration of TRH increased AVP content both in the hypothalamus and neurohypophysis of suckled females; however, plasma AVP level did not change. TRH increased the hypothalamic as well as neurohypophysial OT content during suckling. Simultaneously, TRH inhibited OT release into the blood plasma. On the contrary, in not suckled females TRH increased OT plasma concentration. I.c.v. TRH raised the PRL concentration in plasma of lactating but, at the moment, not suckled females. On the contrary, i.c.v. TRH injection into females just suckled was followed by a decrease in PRL plasma level. TRH probably acts in the central nervous system as an inhibitory neuromodulating factor for the vasopressin release. Also, it cannot be excluded that TRH--otherwise known to enhance the PRL release--suppresses the oxytocin-prolactin positive feedback mechanism when activated temporarily by suckling.