Effect of gonadal steroids on the oxytocin-induced excitation of neurons in the bed nuclei of the stria terminalis at parturition in the rat

Experiments were undertaken to examine the role of ovarian steroids in peri partum programming of oxytocin sensitivity of limbic neurons implicated in oxytocin-induced facilitation of the milk-ejection reflex. In vivo recordin gs of neurons in the bed nuclei of the stria terminalis and ventrolateral s eptum of pre-parturient rats which had undergone prior ovariectomy and hyst erectomy showed that oestradiol significantly increased the excitatory resp onses of bed nuclei/ventrolateral septum neurons to intracerebroventricular oxytocin, compared to oil-treated controls. Oestradiol also increased the excitation of bed nuclei neurons to the selective oxytocin agonist, [Thr(4) ,Gly(7)]oxytocin ih brain slices from steroid pre-treated ovariectomized hy sterectomized rats, so that both the proportion of responsive neurons, and the magnitude of their responses were significantly increased. Parallel aut oradiographic studies showed that oxytocin binding in the medial bed nuclei and ventrolateral septum was selectively increased following oestradiol tr eatment. Progesterone pre-treatment had no effect on either oxytocin sensit ivity of bed nuclei/ventrolateral septum neurons recorded in vivo, or on ox ytocin binding in the medial bed nuclei and ventrolateral septum, compared to oil-treated controls. Mean responses to [Thr(4),Gly(7)]oxytocin in bed n uclei neurons recorded in slices from progesterone-treated rats were larger than controls, but this effect was highly variable. These results demonstrate that oestradiol greatly enhances oxytocin recepto r expression and sensitivity of bed nuclei/ventrolateral septum neurons to oxytocin over the peripartum period, consistent with involvement of this st eroid in enhancing oxytocin regulation of neuroendocrine and behavioural ad aptations required for lactation. (C) 1999 IBRO. Published by Elsevier Scie nce Ltd.