Unit activity in the paraventricular nucleus of female rats at different stages of the reproductive cycle and after ovariectomy, with or without oestrogen or progesterone treatment.

Abstract

SUMMARY Spontaneous firing rates were determined from extracellular recordings made from 878 antidromically identified units in the paraventricular nucleus (PVN) during the reproductive cycle of the female rat and in analytical experiments. In the latter, rats were ovariectomized and subsequently received either no treatment or oestrogen and/or progesterone. Among rats at metoestrus, dioestrus, mid-pregnancy and in ovariectomized progesterone-treated groups there was no significant difference in the firing rates. However, they were significantly lower than the rates recorded during pro-oestrus, oestrus, full-term pregnancy, the day of parturition, during lactation and in ovariectomized, oestrogen-treated rats. In spayed rats the mean firing rate was significantly lower than at pro-oestrus, oestrus, fullterm pregnancy, the 24 h period after parturition, during lactation and after oestrogen treatment. When progesterone was given subcutaneously to oestrogenized rats, the PVN activity, increased by oestrogen, was significantly depressed 4 h after administration. By 8 h the firing rate had completely recovered. The frequency distribution of the firing rates in pro-oestrus and oestrus showed an approximately normal distribution while those in metoestrus and dioestrus and mid-pregnancy had a Poisson distribution. At full term there were two peaks: one in the range of 3–5 spikes/s and the other less than one spike/s. The distribution was approximately normal on the day of parturition and subsequently the pattern became irregular. In ovariectomized rats and those treated with progesterone it was of a Poisson type while there was a distinct shift to higher frequencies after oestrogen treatment. The mean absolute refractory period, measured for each unit, varied and appears to be dependent on hormonal conditions. It was short in oestrus and long in dioestrus and mid-pregnancy. Oestrogen treatment significantly shortened the absolute refractory period of ovariectomized rats.