The olfactory bulb: a critical site of action for oxytocin in the induction of maternal behaviour in the rat.

Expanding on research showing that oxytocin originating in the hypothalamic paraventricular nucleus acts to decrease olfactory processing at the level of the olfactory bulb, we explored the importance of oxytocin acting on the olfactory bulb for the onset of maternal behaviour in Wistar rats. Experiment I was designed to test whether spontaneous maternal behaviour following natural delivery is blocked by bilateral infusions of a low dose (5 fmol) of the oxytocin antagonist d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)]ornithine-vasotocin into the olfactory bulb immediately after the delivery of the first pup and again just before a test for maternal behaviour. Intrabulbar infusions of the antagonist markedly delayed the occurrence of all components (retrieval, licking, nest building, crouching) of maternal behaviour, whereas intracerebroventricular infusions of the antagonist were without effect on any component as compared with intrabulbar infusions of saline. Experiment 2 was undertaken to determine whether infusions of oxytocin into the bulb induce a rapid onset of maternal behaviour in virgin rats. Forty-eight hours before pup presentation virgins were ovariectomized and treated with oestradiol benzoate. Immediately before pup presentation a low dose (20 pmol) of oxytocin or saline was infused bilaterally into the bulb or lateral ventricle. Intrabulbar infusions of oxytocin induced full maternal behaviour in half of the animals tested within 2 h of pup exposure, in contrast to the ineffectiveness of intracerebroventricular infusions of oxytocin and intrabulbar infusions of saline. These results suggest that the olfactory bulb is a critical site where oxytocin acts to induce a rapid onset of maternal behaviour.