Abstract
1. In anaesthetized male rats, the hypothalamus and pituitary stalk were exposed by a transpharyngeal approach. The compound field potential of the supraoptic nucleus evoked by stimulation of the pituitary stalk, was recorded with glass electrodes inserted near the origin of the anterior cerebral artery.2. The mean latency of 169 antidromically evoked action potentials isolated from the field was 9.9 msec with an extreme range of 6-26 msec. Although the wave form of the antidromic action potential showed a variety of shapes and sizes and the initial wave could be of either polarity, the majority were strikingly similar in form. The initial wave was positive with an inflexion on the rising phase and was followed by a shallow rather longer lasting negative potential.3. The antidromic nature of the action potential was confirmed when the action potential evoked at constant latency after the stimulus was observed to be cancelled by another occurring spontaneously. Although the antidromic action potentials followed stimulation frequencies greater than 100 Hz, the response to high frequency stimulation was seldom tested since the amplitude of the action potential was greatly reduced at frequencies above 30 Hz if the number of shocks exceeded a critical number, as few as 3-6 at 100 Hz.4. Stimulation of the pituitary stalk at intensities below and near threshold for antidromic invasion of the cell under study was shown by means of post-stimulus time histograms to be associated with an inhibitory period lasting on average 80 msec (S.D. = 13, N = 30).5. An increase in the intensity and duration of the inhibitory period occurred as the intensity of the stimulation was increased as might be expected if the response was mediated synaptically. The inhibitory pathway is believed to involve the recurrent collateral axons already demonstrated anatomically since the stimulation intensities necessary to produce either a marked inhibitory response or antidromic invasion of the cell in question are in most instances nearly the same.