Inputs to testosterone-sensitive stria terminalis neurones in the rat brain and the effects of castration.

1. The inputs to cortico-medial amygdala neurones which project directly to the area of the medial preoptic/anterior hypothalamic junction were studied electrophysiologically in urethane anaesthetized male rats. 2. In experiments with fifteen male rats it was found that none of these neurones was responsive to electrical stimulation of the ipsilateral olfactory bulb or accessory olfactory bulb or odour stimulation. 3. Experiments with four rats showed that electrical stimulation of the lateral portion of the contralateral fimbria excited 81% of these cortico-medial amygdala neurones. Their typical response to stimulation of the contralateral fimbria was a single action potential followed by an inhibitory period (20-100 ms). 4. Analysis of the polarity of evoked waves in the amygdala suggested that the fimbria input terminated in the basolateral nucleus of the amygdala and that this nucleus subsequently projected to the cortico-medial amygdala. The fimbria input was found to be contralateral in origin, crossing the mid line in the anterior fornical commissure. 5. In a further experiment 118 identified cortico-medial amygdala neurones were recorded from twelve rats (six gonadally intact and six castrated). Castration significantly decreased the percentage of these neurones responding to stimulation of the ipsilateral fimbria (20 vs. 97%) and lengthened post-excitatory inhibitory periods. 6. Results are discussed with respect to the initial finding by Kendrick & Drewett (1979) of testosterone-sensitive absolute refractory periods in cortico-medial amygdala neurones.