The effect of ovarian steroids on hypothalamic norepinephrine neuronal activity.

The concentration of norepinephrine and the apparent turnover of norepinephrine in the hypothalamus was studied in castrated rats and after the injection of ovarian steroids. Both parameters were found to vary not only with the hormone treatment, but also with the time of the day. Norepinephrine concentration in the anterior hypothalamus of castrated male and female rats was significantly higher in the afternoon than in the morning, but no statistical difference was observed in the concentration of norepinephrine in the posterior hypothalamus. Three days after the injection of a single dose of 20 microgram oestradiol benzoate, norepinephrine concentration and apparent turnover were found with decreased significant values in the anterior hypothalamus of castrated females, killed in the afternoon. Progesterone 2 mg injected three days after the priming dose of oestradiol benzoate, produced 4 h later in the afternoon a significant rise in norepinephrine concentration and apparent turnover. These results suggest that the apparent turnover of hypothalamic norepinephrine in the afternoon is reduced by oestrogen and increased by oestrogen plus progesterone.