Activity of putative oxytocin neurones during reflex milk ejection in conscious rabbits.

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Abstract
Extracellular electrical recordings were taken from 20 antidromically identified paraventricular neurons in unanesthetized, unrestrained rabbits. Neuronal activity was correlated with nursing behavior of the doe and responses of the young during suckling. Magnocellular neurons were divided into 2 groups on the basis of their activity in suckling. Group 1 (n = 14) showed several discrete bursts of high-frequency activity while neurons in group 2 (n = 6) did not. Neurons in group 1 showed 5-9 bursts of high-frequency activity in suckling. Each burst lasted 1-4 s and represented a 3- to 10-fold rise in the discharge of the cell. These units were classified as oxytocinergic, as their stereotyped activation preceded bouts of sucking behavior of the young indicative of milk ejection. All 14 cells continued to show intermittent bursts of neurosecretory activity for up to 20 min after nursing terminated. This pattern of discharge followed grooming behavior of the doe. In contrast, neurons in group 2 (n = 6) showed no high-frequency activity in suckling. They showed a significant fall in their discharge frequency compared with pre-suckling values (P < 0.05; student''s t test) and a significant (P < 0.05) lengthening of the modal interspike interval. They were classed as potential vasopressin-producing cells. Control recordings were taken from 32 neurons which could not be antidromically driven. The recording sites were shown histologically to be in the lateral hypothalamic area. These cells showed a significant fall in their discharge frequency (P < 0.05) and a significant increase (P < 0.01) in the modal interval during suckling. Cross-correlation studies of the activity, recorded from 1 electrode, of groups of neurons clustered around a single hypothalamic neuron suggest that bursting discharge from the putative oxytocin neurons in suckling is accompanied by the synchronous activation of some of the surrounding magnocellular units.