THE TIME COURSE OF EFFECTS OF SEX HORMONES ON ELECTRICALLY-EVOKED POTENTIALS IN THE FROG FOREBRAIN

In the frog, over a period of up to 12 days, the effects of a single injection of steroid hormone were measured on potentials evoked in the olfactory lobe and telencephalon after electrical stimulation of the olfactory nerve. Response amplitudes in the olfactory lobe and telencephalon were depressed by 10 μg doses of progesterone whether given alone, or in combination with 10 μg of oestradiol. In the olfactory lobe the change in response induced by progesterone was significant by 2 days and reached a peak by 6 to 8 days after injection. In the telencephalon the peak effect of progesterone was also at about 6 days, but no significant change from the control could be found before 4 days. In contrast, oestradiol, when given alone, augmented the response to olfactory stimulation detected in the olfactory lobe and telencephalon. The time relations were about the same as for progesterone. Effects of the two hormones on stimulus threshold were relatively modest and in general were correlated with the effects on response amplitude. Similarity in time course of effects of progesterone and oestradiol, and the dominance of the progesterone effect over that of oestradiol suggests that despite their opposite actions, at least part of their influence is on the same elements in the olfactory response system.