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

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 estrogen and/or progesterone. Among rats at metestrus, diestrus, midpregnancy 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 proestrus, estrus, full term pregnancy, the day of parturition, during lactation and in ovariectomized, estrogen treated rats. In spayed rats the mean firing rate was significantly lower than at proestrus, estrus, fullterm pregnancy, the 24 hr period after parturition, during lactation and after estrogen treatment. When progesterone was given subcutaneously to estrogenized rats, the PVN activity, increased by estrogen, was significantly depressed 4 hr after administration. By 8 hr the firing rate had completely recovered. The frequency distribution of the firing rates in proestrus and estrus showed an approximately normal distribution while those in metestrus and diestrus and midpregnancy had a Poisson distribution. At full term there were 2 peaks: one in the range of 3-5 spikes/sec and the other less than one spike/sec. 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 estrogen treatment. The mean absolute refractory period, measured for each unit, varied and appears to be dependent on hormonal conditions. It was short in estrus and long in diestrus and midpregnancy. Estrogen treatment significantly shortened the absolute refractory period of ovariectomized rats.