CENTRAL EFFECTS OF CATECHOLAMINES UPON MAMMARY CONTRACTILITY IN RATS ARE NEURALLY-MEDIATED

We injected, i.e., intracerebroventricularly (ICV) or systematically, small amounts of adrenaline (ADR), noradrenaline (NA), isoproterenol ( ISOP) and dopamine (DA) in urethane-anesthetized lactating rats, and d etermined the effects on isometrically recorded intramammary pressure (IMP) responses to exogenous oxytocin (OXY). While centrally administe red ADR, NA and DA provoked increased IMP responses to OXY, the beta-a drenergic agonist ISOP induced the opposite effect. These effects were reversible, dose related and also occurred in hypophysectomized rats. However, when injected systematically, all adrenergic agonists but DA depressed IMP responses to OXY. Further experiments showed that centr al effects of catecholamines were exerted by regulating ductal tone, t hrough the direct innervation of the mammary glands. Thus, whereas com plete blockage of these effects occurred after selective denervation o f the mammary glands, increased ductal tone resulted from ICV administ ration of ISOP. Finally, evidence was also obtained that antagonistic alpha- and beta-adrenergic mechanisms may interact with each other to regulate milk ejection, and with afferent signals from the mammary gla nds. Thus, beta-adrenergic inhibition upon IMP was counteracted by eit her NA administration or by activation of ductal mechanoreceptors. Tog ether, these results suggest that regulation of milk ejection may invo lve neurally mediated influences on mammary contractility. Such action s would interact closely with afferents from the mammary gland influen cing ductal tone.