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

1. Extracellular electrical recordings were taken from nine antidromically identified paraventricular units in unanaesthetized, unrestrained rats. Neuronal activity was correlated with the observed events of parturition, i.e. abdominal contractions and delivery of young or placentae. 2. The level of spontaneous activity (0.15--3.2 spikes s-1) of all nine units began to increase 15 min before the first signs of abdominal contraction. This accelerated discharge (2--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. 3. All nine neurones 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 labour progressed. 4. Four cells also showed a burst (5--12 s) of high-frequency activity 15--28 s before delivery of either fetuses or placentae. These four units were later classified as oxytocinergic on the basis of their stereotyped activation 10--12 s before reflex milk-ejection. 5. The remaining five neurones 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).