REFLEX ACTIVATION OF PARAVENTRICULAR NUCLEUS UNITS DURING THE REPRODUCTIVE CYCLE AND IN OVARIECTOMIZED RATS TREATED WITH OESTROGEN OR PROGESTERONE

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Abstract
SUMMARY Three hundred and seventy-seven neurones in the paraventricular nucleus (PVN) were antidromically identified in female rats during various stages of the reproductive cycle and in ovariectomized rats with or without oestrogen or progesterone pretreatment. The units were tested for their responses to vaginal distension and pinching of the foot. Unit responses to vaginal distension varied during the oestrous cycle, pregnancy and lactation. The percentages of PVN units recorded at each stage which increased their firing rate in response to vaginal distension were 64·1% in pro-oestrus, 56·7% in oestrus, 33·3% in metoestrus, 42·4% in dioestrus, 6·9% in mid-pregnancy (day 12) which was significantly lower than in any other stage or condition, 72·0% at full term, 55·5% immediately ( The percentage of units which increased their firing rate during vaginal distension in ovariectomized rats was 42·4% and in ovariectomized, oestrogen-treated rats was 69·2% and thus significantly higher. While the percentage in progesterone-treated rats (35·1%) was not significantly different from that in ovariectomized rats, it was significantly lower than in the oestrogen-treated group. In addition, 18·9% of the units had a period of interruption of spontaneous firing in response to vaginal distension constituting the only group besides the mid-pregnancy group in which this was observed. Pinching the foot also excited PVN unit activity in some cases. It was more frequently evoked by pinching a foot contralateral to the PVN nucleus from which the recording was taken. The response to stimulation of the ipsilateral foot was weaker or absent. The responsiveness of PVN units to pinching varied throughout the reproductive cycle as well as in ovariectomized rats with or without treatment with oestrogen or progesterone. The pattern of variation was dissimilar from that observed after vaginal distension and the magnitude of variation was smaller. Interruption of spontaneous firing was observed in seven different groups.