Electrophysiological evidence for mutual excitation of oxytocin cells in the supraoptic nucleus of the rat hypothalamus.

1. Using the ventral surgical approach in vivo, extracellular recordings were made from seventy-nine cells in the supraoptic nucleus of urethane-anaesthetized male, virgin female or lactating female rats while stimulating the pituitary stalk. Cells were classed according to their spontaneous firing activity as: continuous (putative oxytocin), phasic (putative vasopressin) and silent. 2. Stimulation of the neural stalk produced an excitation (up to 25 ms poststimulus) in eleven of the seventy-nine antidromically identified magnocellular neurones, consistent with the existence of excitatory collaterals or dendritic contacts between such cells. In these recordings a second spike could frequently be seen, following the antidromic spike, with a variable latency. Such spikes consistently collided with subsequent antidromically evoked spikes. Poststimulus excitation was only seen in silent and continuously firing (putative oxytocin) cells, suggesting that oxytocin and vasopressin cells have different connections. 3. Excitatory connections were seen more frequently in lactating females (8 out of 22 cells) than in males (1 out of 15 cells) or virgin females (2 out of 10 cells), and thus may make an important contribution to the bursts of firing which precede reflex milk ejection.