Analysis of bursting responses of oxytocin neurones in the rat in late pregnancy, lactation and after weaning.

1. Electrophysiological recordings were undertaken to compare bursting characteristics of oxytocin (OT) neurones at four reproductive stages: day 20 pregnancy, day 22 of pregnancy (expected day of parturition), day 7-11 of lactation, and day 5-6 after weaning. 2. Each OT neurone was recorded for 1 h of suckling, combined with cervico-vaginal probing at 5 min intervals as an additional stimulus for bursting. Intracerebroventricular (I.C.V.) oxytocin (2.2 ng) was given after 30 min to facilitate bursting responses. Bursts observed during suckling were classified as 'spontaneous' or 'probe-evoked'. 3. The percentage of cells displaying spontaneous and/or probe-evoked bursts during the recording was low in day 20 pregnant animals, high in lactators and intermediate in day 20 pregnant and weaner groups. These differences may relate to variation in the proportion of animals with a responsive milk-ejection reflex, as well as the relative size of the population of bursting OT neurones. 4. In the period before I.C.V. OT, overall burst frequency (including both spontaneous and probe-evoked bursts) was similar across groups. After I.C.V. OT, overall burst frequency was much higher in lactators compared with other groups. Similar results were obtained when spontaneous bursts were analysed separately. 5. Burst amplitude (action potentials per burst, including both spontaneous and probe-evoked bursts) prior to I.C.V. OT was similar between the day 20 pregnant, day 22 pregnant and lactating groups, but was lower in weaners. All groups showed an increase in burst amplitude after I.C.V. OT, but values in weaners remained lower than in other groups. In a separate analysis of spontaneous bursts, burst amplitude after I.C.V. OT was higher in lactators, and lower in weaners, than in pregnant animals. 6. Background firing rates of OT cells were higher in the day 20 and day 22 pregnant groups compared with lactators, and lower in weaners. Only OT cells in lactators showed a significant increase in background firing rates following I.C.V. OT. 7. It is concluded that the bursting characteristics of OT cells change markedly between late pregnancy, mid-lactation and weaning. The factors underlying these changes, which are only loosely correlated with the sequence of morphological adaptations in OT cells surrounding lactation, remain to be established.