Extracellular recordings from oxytocin neurones during the expulsive phase of birth in unanaesthetized rats.

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Abstract
Extracellular electrical recordings were taken from 9 antidromically identified paraventricular units in unanesthetized, unrestrained rats. Neuronal activity was correlated with the observed events of parturition, i.e., abdominal contractions and delivery of young or placentae. The level of spontaneous activity (0.15-3.2 spikes s-1) of all 9 units began to increase 15 min before the 1st signs of abdominal contraction. This accelerated discharge (2- to 5-fold increase over the background activity) was maintained throughout parturition (58-93 min) and for up to 45 min after delivery of the last placenta. All 9 neurons displayed at 6-14 s periods of even higher rate of discharge (10-32 spikes s-1) after forceful abdominal contractions. The peak firing rates within these periods of accelerated discharge decreased as labor progressed. Four cells also showed a burst (5-12 s) of high-frequency activity 15-28 s before delivery of either fetuses or placentae. These 4 units were later classified as oxytocinergic on the basis of their stereotyped activation 10-12 s before reflex milk-ejection. The remaining 5 neurons which did not respond with a burst of high-frequency discharge before delivery were classed as potential vasopressin-producing cells. Four of these units displayed a phasic pattern of activity with periods of activity (5-230 s) alternating with periods of silence (4-31 s).