Identification of oxytoxin cells in the rat supraoptic nucleus by their response to cholecystokinin injection.

Identification of specific cell types is fundamental to interpreting single cell recording studies. The hypothalamic supraoptic nucleus (SON) contains phasic (putative vasopressin) cells and a mixed population of continuously firing vasopressin and oxytocin cells. We injected cholecystokinin (CCK-8; i.v.), which is known to release oxytocin but not vasopressin, to see if such injections might differentiate oxytocin from vasopressin cells. Recordings made, using the ventral surgical approach to the SON in female rats under urethane anaesthesia (1.3 g/kg) from 49 non-phasic cells showed that CCK-8 (20 micrograms/kg) excited 41. Twenty of these 41 cells were tested for their response to i.v. injections of phenylephrine (10 micrograms), which interrupts the firing of putative vasopressin cells, and none were inhibited. Only one out of 8 cells recorded from suckled, lactating rats which showed a burst of spikes just before reflex milk-ejection was inhibited by phenylephrine. Injections of CCK-8 also excited 4 out of four such 'milk ejection' cells. Cells activated by CCK, cells unaffected by phenylephrine, cells activated just before reflex milk-ejection and cells which fire continuously appear to be a single population, thus continuous firing gives a good indication that an SON cell secretes oxytocin.