Electrical stimulation of the ventral part of the midbrain central gray (CG) elicited antidromic action potentials in 136 neurons in the ventral tegmental area (VTA) of 23 urethan-anesthetized ovariectomized female rats. 2. Antidromic action potentials were positive-negative biphasic and completed mostly within 2.4 ms. Many had a notch in their initial positive deflection, at which antidromic potentials often failed to propagate into the neuronal soma. 3. The latency for antidromic activation ranged 1.0-10.5 ms, and the threshold was 100-1,700 microA. The relative refractory period was 0.8-2.8 ms. In 23 neurons (17%) gradual changes in stimulus intensity caused latency jumps, suggesting that their axons terminate or spread into branches in stimulation sites. 4. The parameters for antidromic activation were compared among 12 animals with a subcutaneous Silastic capsule of estrogen and 11 others with a blank capsule. Estrogen raised the mean threshold from 675 +/- 40 (SE) microA (n = 69) to 908 +/- 40 microA (n = 67). At the same time, estrogen significantly decreased the probability of the antidromic propagation from 63 to 41%. No changes were detected in the latency or the refractory period. 5. The probability distribution of the threshold was different in ovariectomized animals from that in estrogen-treated animals. Estrogen increased the number of cells with thresholds in the 1,300- to 1,500-microA range. 6. The probability of antidromic propagation was stable in each neuron to stimuli repeated at 1.0 Hz. Electrical stimulation of the preoptic area (POA) with a 30-s train of 50-microA pulses at 100 Hz increased the probability of antidromic propagation into the soma.(ABSTRACT TRUNCATED AT 250 WORDS)
