Onset of bursting in oxytocin cells in suckled rats.

1. We tested whether firing characteristics are associated with the onset of bursting in oxytocin cells during suckling. Extracellular electrical activity of paraventricular and supraoptic oxytocin cells was recorded in lactating rats from the beginning of suckling up to the first milk-ejection burst, which occurred either within less than 1 h of suckling (bursting cells) or after injecting facilitatory drugs (non-bursting cells). 2. Significant differences in the distributions of firing rate (using low, intermediate and high categories, < or = 1, 1-3 and > 3 spikes s-1, respectively) of bursting and non-bursting cells were observed during suckling. Thirty minutes after pup application, most bursting cells (56%) had an intermediate firing rate, whereas non-bursting cells had either a low (36%) or high (40%) firing rate. 3. Thirty minutes after applying the pups, probability of bursting was highest for cells firing at 1-4 spikes s-1, and lowest for those firing above 5 spikes s-1. 4. Bursting cells with high initial firing rate decreased their firing rate substantially during suckling while most of those with low initial firing rate showed increases. For non-bursting cells, activity was maintained. 5. There were marked differences in firing rate and its evolution between paired bursting cells. The firing rates for non-bursting cell pairs were similar (mostly either low or high), and remained stable during suckling. 6. In conclusion, we suggest that, as suckling proceeds, probability of bursting is related to the firing rate of oxytocin cells within the whole population, more specifically to the proportion of cells within the animal initially or subsequently displaying a critical firing rate (between 1 and 3 spikes s-1). The firing rates of cells which eventually burst and which are firing outside this range change in a direction which brings them into the preferred range for bursting. 7. We suggest that when bursting occurs unaided, a majority of oxytocin cells fire in the preferred bursting range and facilitate the bursting of cells outside this range. Since such cooperativity does not develop between non-bursting cells, it might be due to centrally released oxytocin.