Central inhibitory effects of relaxin on the milk ejection reflex of the rat depends upon the site of injection into the cerebroventricular system.

The effects of centrally administered porcine relaxin on the milk-ejection reflex were studied in anaesthetized, lactating rats. Injection of relaxin into either the lateral ventricles (10-1000 ng) or third ventricle (500 ng) of the brain significantly inhibited suckling-induced reflex milk-ejection. Following injection of relaxin into the third ventricle, the onset of inhibition was immediate and more profound compared to injection into the lateral ventricle, after which there was a delay to the onset of inhibition. Relaxin was without effect when injected into the fourth ventricle or cisterna magna, thus suggesting that the site of action was within the diencephalon and the mesencephalon. Because these effects were reversed by naloxone, the central inhibitory action of relaxin appears to be mediated through endogenous opioid peptides.