EFFECTS OF CENTRALLY ADMINISTERED PROLACTIN UPON MAMMARY CONTRACTILITY IN ANESTHETIZED LACTATING RATS

The possibility was investigated that centrally administered rat prola ctin (PRL), i.e., intracerebroventricularly (ICV) or intrathecally (IT ), may influence mammary contractility in urethane-anesthetized lactat ing rats. In addition to the monomer (23 kD PRL), the 16- and 7-kD PRL fragments were also tested and their effects were determined on isome trically recorded intramammary pressure (IMP) responses to exogenous o xytocin (OXY). ICV injection of 23 kD PRL provoked increased IMP respo nses; similar, but inconsistent effects were obtained after 16 kD PRL, and no effect occurred after either saline or the 7 KD PRL fragment. The 23 kD PRL, effect had a latency of about 5 min, reached plateau at 10-15 min and its magnitude was dose-dependent. Also, a stronger dose -related effect was shown by 23 kD PRL after IT than after ICV adminis tration, which suggests that lower threshold for the PRL actions exist at the spinal level. On the other hand, ICV or IT injections of antis erum to PRL, but not of normal rabbit serum, fully prevented the respe ctive facilitatory effects of 23 kD PRL on mammary contractility. Furt her experiments into the mechanisms involved showed that neither adren alectomy nor hypophysectomy prevented the PRL, effect whereas complete blockage occurred after either spinal cord transection, ventral root section (T8-T10 level), or IV or ICV administration of the adrenergic beta-blocker propranolol. These results indicate that central effects of PRL upon mammary contractility were neurally mediated and may have resulted from depressed beta-adrenergic control of ductal tone.