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Neurosecretory cells of preoptic nuclei of bullfrogs were studied in isolated hypothalamo-hypophysial preparations under constant perfusion with oxygenated Ringer solution at 15-17 degress C. Antidromic potentials were recorded following stimuli applied to the posterior lobe of the pituitary or the stalk. 2. Intracellularly and extracellularly recorded potentials resembled those obtained in vivo from neurosecretory cells of the mammalian hypothalamus. They were unique in that the antidromic potential had a long duration (10-20 msec) and a distinct notch on the rising phase (between A and B spikes). The conduction velocity of the stalk fibres in vitro at this temperature was 0-1--0-2 m/sec. 3. When two successive stimuli were given to the posterior lobe or to the stalk separated by intervals of between 30 and 65 msec, the test (second) response showed a longer delay of the B spike. This delay between the A and B components was as long as 10 msec. Further shortening of stimulus intervals produced block of B spikes in the test response. A complete separation of A and B spikes occur spontaneously in a few instances. 4. Evidence indicated that inhibitory recurrent axon collaterals play a role in the control of bullfrog neurosecretory cells. Antidromic potentials were inhibited by a 'conditioning' stimulus for as long as 300-400 msec, even when the stimulus did not evoke an antidromic potential. 5. It was found that in addition to the inhibitory interaction there is a facilitatory recurrent axon collateral system which operates within the nuclei. The evidence for this is: (1) stimulation of the posterior lobe, with single subthreshold pulses evoked an action potential if preceded by another stimulus of subthreshold or just threshold intensity. The durations of such facilitatory effects were found to be 20--400 msec; (2) a single pulse given to the posterior lobe did occasionally evoke two spikes from neurosecretory cells; the second spike which occurred 15-30 msec after the first had the characteristics of a trans-synaptically produced potential; (3) gradual changes in the intensity of stimuli applied to the neural lobe produced a sudden shift in latencies ranging between 15 and 30 msec. The potentials having long latency also showed characteristics of those transsynaptically excited. In addition, an increase in excitability of neurosecretory cells by antidromic stimulation was confirmed by using orthodromically induced action potentials in in vivo studies. Possible functional significance of inhibitory and excitatory recurrent collateral system in neurosecretory cells was discussed. 6. Two to threefold increase in NaCl concentration of a perfusate slightly increased the latency and refractory period of antidromic potential but not the shape of the potential. Norepinephrine added to a perfusate (1 mug/ml). augmented the separation of A and B spikes of the antidromic potential. Acetylcholine at a concentration of 1 mug/ml. did not have an appreciable effect on the antidromic potential.

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