Characteristics of early- and late-recruited oxytocin bursting cells at the beginning of suckling in rats.

1. Paired or single recordings of paraventricular and/or supraoptic oxytocin cells at the beginning of suckling in urethane-anaesthetized rats enabled us to study cell recruitment and compare the characteristics of the early- and late-recruited cells. This was done under different experimental conditions, i.e. when the reflex was triggered in less than 1 h suckling (control), and when its triggering was facilitated either by the intraventricular (i.c.v.) injection of oxytocin, of apomorphine (a dopamine agonist) or by the intravenous (i.v.) injection of propranolol (a beta-adrenoceptor antagonist) into suckled rats with no milk ejection. 2. Under control conditions, the amplitude (total number of spikes) of the successive bursts of the early-recruited cells progressively increased, generally reaching maximum by the 6th burst. This increase was more rapid and greater after oxytocin than under control conditions or after apomorphine injection, and was delayed and reduced after propranolol. The burst frequency was higher after oxytocin and apomorphine injections than under control conditions and very low after propranolol. 3. Late-recruited cells were observed under all experimental conditions, except after oxytocin injection, since all cells displayed bursts right away. Moreover, when injected during the recruitment period of a control reflex, oxytocin greatly speeded up the recruitment of the late-recruited cells. These cells generally displayed smaller amplitude bursts than the early-recruited cells. Moreover, the increase in burst amplitude was less marked for the late- than for the early-recruited cells and often was not sustained. 4. Neither the likelihood of recruitment of an oxytocin cell nor its burst amplitude could be correlated with background activity level and there was no clear relationship between the recruitment period or the bursting characteristics on one hand and the background activity on the other. 5. In conclusion, the differences between the early- and late-recruited cells in recruitment time and in burst amplitude reflected differences in cell excitability which may depend mainly on the presence of oxytocin in the magnocellular nuclei.