SUMMARY1. Isolated rat neurohypophyses were studied in vitro and the hormones released on electrical stimulation of the pituitary stalk or on exposure to excess potassium were estimated by a milk-ejection assay.2. The stalk was stimulated with trains of 500 stimuli, or multiples thereof, applied at different frequencies. Below frequencies of ca. 35 c/s, hormone release was found to depend on the total number of stimuli applied as well as on the frequency of stimulation. Above ca. 35 c/s, identical numbers of stimuli were progressively less effective as the frequency of stimulation was increased, and the dependence of the hormone output on the total number of stimuli was less apparent.3. The amplitude of the compound action potential recorded from the neurohypophysis following electrical stimulation of the stalk was found to decrease as a function of the frequency of stimulation. Stimulation at 50 c/s reduced its amplitude about sevenfold within 30 sec.4. The addition oftetrodotoxin (TTX) to the incubation media abolished the compound action potential recorded from the neural lobe as well as the release of hormones evoked by electrical stimulation. Resting release, however, was unaffected by TTX.5. In TTX-treated neural lobes, excess potassium was still effective in eliciting graded secretary responses. This indicates the independence of the release process from the action potential generating mechanism and suggests that TTX-paralysed preparations represent a useful model for the study of hormone release in the absence of conducted action potentials.6. The release of hormones from the neurohypophysis and the release of neurotransmitters at chemical synapses both depend on the entry of calcium into the nerve terminals following their depolarization by invading
SUMMARY1. The milk-ejection response in lactating rabbits has been used to study the effect of electrical stimuli of different types applied to the supraopticohypophysial tract in the pituitary stalk.2. Sine-wave alternating-current pulses were compared with balanced biphasic square-wave pulses of the same frequency and peak-to-peak current strength. At a pulse duration of 2-4 msec the square-wave stimulation was less effective than the sine wave, but at a pulse duration of 8 msec and over, more effective.3. Above threshold levels of 0-12 mA for the current strength, and of 0 5 msec for the pulse duration, the response increased with increasing current strengths to 2-4 mA and increasing pulse durations to 10 msec.4. With constant and effective strength and duration of the pulse a slight diminution of the response was seen as the frequency was diminished from 100/sec to 50/sec. Further diminution in the frequency revealed that at some point between 50 and 10 c/s a sudden abrupt diminution in the magnitude of the responses occurred over a small range of frequency. This was a reversible phenomenon and it is suggested that it may be related to neurosecretory events occurring in the nerve terminals.5. Stimuli with parameters within the ranges mentioned above are effective in eliciting an oxytocic response on the uterus in the rabbit and ovulation responses (when applied to the hypothalamus) in rats and rabbits. It appears likely that stimuli with such parameters are suitable for experiments concerned with stimulation of the hypothalamus.
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