DEVELOPMENTAL EFFECT OF TESTOSTERONE ON ESTROGEN-SENSITIVITY OF THE RAT PREOPTIC NEURONS WITH AXONS TO THE VENTRAL TEGMENTAL AREA

The neonatal effect of testosterone on neuronal sensitivity to estrogen was examined in the medial preoptic area of the rat when adult. Electrical stimulation of the ventral tegmental area induced antidromic action potentials in 683 neurons in 106 urethane-anesthetized animals which consisted of 38 males castrated on the day of birth (day 1), 30 females androgenized by testosterone injection on day 5, and 38 normal females. All females were used after ovariectomy when adult. Histological localization of the antidromically activated neurons was similar in all the animal groups. The latency jump in 40% of the potentials indicated that activated preoptic axons terminate in the stimulation site. The latency for activation was in the range 1.6-43.7 ms and the threshold was as low as 60 muA. The absolute refractory period did not exceed 1700 mus. Estrogen increased the threshold among 278 cells in the normal females and 181 cells in the neonatally castrated males. Preoptic neurons in the androgenized females (n = 224) differ from those in others in that their threshold, regardless of estrogen treatment, was at the ceiling values seen in the other groups in the presence of estrogen. Estrogen also prolonged the absolute refractory period in the females but not in the androgenized females. The reduced excitability and the lack of estrogen sensitivity may be responsible for the insensitivity of the androgenized females to feminine actions of estrogen, that may presumably underlie a sex difference in certain behavioral functions.