Paired recordings from supraoptic and paraventricular oxytocin cells in suckled rats: recruitment and synchronization.

Oxytocin cells in the paraventricular (p.v.) and contralateral supraoptic (s.o.) nuclei were pair‐recorded (with two micro‐electrodes) in suckled rats after being anaesthetized with urethane (1.2 g/kg), to study the synchronization of their neurosecretory bursts, the importance of cell recruitment and their firing characteristics. The synchronization of paired bursts was determined by measuring the onset time‐lag (time in milliseconds between the onset of two corresponding bursts) and the maximum firing time‐lag (time in milliseconds between the two shortest interspike intervals for the corresponding bursts). For each cell, the characteristics studied were: the background activity and the frequency and amplitude (total number of spikes) of the neurosecretory bursts. All paired p.v.‐s.o. cells recorded were activated simultaneously 12‐18 s before each milk ejection. The onset of a burst could vary either way, up to 680 ms, in relation to the other (mean onset time‐lag was 206 +/‐ 18 ms; n = 85) but the maximum activation periods fitted more closely, the mean maximum firing time‐lag being 122 +/‐ 14 ms (n = 64). Both parameters varied randomly, in duration and order from one pair of cells to another, from one pair of bursts to another for successive bursts of a given pair of cells and independently, whether the cells were in the p.v. or the s.o. nucleus. However, in most cases, the neurosecretory burst with the highest amplitude began and reached its peak firing rate before the corresponding burst from the other cell. Cell recruitment was observed when the milk ejection reflex began, for both the p.v. and the s.o. cells. The bursts of the non‐responsive cells developed progressively with the reflex, but, as soon as a cell was recruited, all its successive bursts were simultaneous with those of the first‐recruited oxytocin cells. During a regular pattern of milk ejections, the mean background activity of sixty p.v. cells (3.1 +/‐ 0.2 spikes/s) was significantly higher than that of their s.o. counterparts (1.9 +/‐ 0.2 spikes/s). Nevertheless, the mean amplitude of the neurosecretory bursts of the sixty p.v. cells (49 +/‐ 3 spikes) did not differ significantly from that of their s.o. counterparts (55 +/‐ 4 spikes).(ABSTRACT TRUNCATED AT 400 WORDS)