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

Extracellular electrical recordings were taken from twenty antidromically identified paraventricular neurones in unanaesthetized, unrestrained rabbits. Neuronal activity was correlated with nursing behaviour of the doe and responses of the young during suckling. Magnocellular neurones were divided into two groups on the basis of their activity in suckling. Group 1 (n = 14) showed several discrete bursts of high-frequency activity whilst neurones in group 2 (n = 6) did not. Neurones in group 1 showed 5-9 bursts of high-frequency activity in suckling. Each burst lasted 1-4 s and represented a 3-10-fold rise in the discharge of the cell. These units were classified as oxytocinergic, as their stereotyped activation preceded bouts of sucking behaviour of the young indicative of milk ejection. All fourteen cells continued to show intermittent bursts of neurosecretory activity for up to 20 min after nursing terminated. This pattern of discharge followed grooming behaviour of the doe. In contrast, neurones 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 less than 0.05; Student's t test) and a significant (P less than 0.05) lengthening of the modal interspike interval. They were classed as potential vasopressin-producing cells. Control recordings were taken from thirty-two neurones 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 less than 0.05) and a significant increase (P less than 0.01) in the modal interval during suckling. Cross-correlation studies of the activity, recorded from one electrode, of groups of neurones clustered around a single hypothalamic neurone suggest that bursting discharge from the putative oxytocin neurones in suckling is accompanied by the synchronous activation of some of the surrounding magnocellular units.