CHANGING PATTERN OF OXYTOCIN-INDUCED EXCITATION OF NEURONS IN THE BEDNUCLEI OF THE STRIA TERMINALIS AND VENTROLATERAL SEPTUM IN THE PERIPARTUM PERIOD

Oxytocin acts within the limbic system (bed nuclei of the stria termin alis and ventrolateral septum) to induce maternal behaviour and to fac ilitate neuroendocrine activity at specific times during the peripartu m period. Studies were undertaken to determine whether the timing of t hese effects arises from modulation of the oxytocin-induced excitation of limbic neurons. Extracellular activity of single units was recorde d on urethane-anaesthetized rats and neurons were tested for responses to intracerebroventricular injection of 1.1 ng oxytocin. in the first part, animals were recorded on days 19 and 22 of pregnancy and on day s 3 and 5 of lactation. No significant differences in the basal firing rates or in the proportion of oxytocin-responsive neurons were detect ed, but responses by neurons on day 22 of pregnancy occurred after a s ignificant delay (10.7 +/- 2.0 min), resulting in a smaller overall re sponse compared to the ether groups. These differences in the pattern of response were not due to changes in density of oxytocin binding in the limbic areas studied, since autoradiographic detection of oxytocin binding sites using the iodinated antagonist 2)(5)[Tyr(Me)(2),Thr(4), Orn(8),Tyr-NH29]-vasotocin showed no differences between the pregnant and postpartum animals. In the second part, parturient animals (day 22 of pregnancy) received intravenous injection of the long-acting opioi d antagonist naltrexone, or unilateral knife-cut lesions to the stria terminalis, a source of inhibitory inputs (including enkephalinergic) to the bed nuclei of the stria terminalis and ventrolateral septum. Bo th treatments abolished the characteristic delay of oxytocin-induced e xcitation in non-treated animals on day 22 of pregnancy, and increased the overall excitatory response.Thus, during the peripartum period, a population of limbic neurons sensitive to oxytocin display a dynamica lly changing pattern of excitatory responses, apparently modulated by an endogenous opioid tone and independent of changes in oxytocin recep tor expression. The attenuated neuronal response to central oxytocin s een on the day of parturition could account for the absence of a facil itatory effect of oxytocin on neuroendocrine activity at this time. (C ) 1997 IBRO. Published by Elsevier Science Ltd.