Synchronized Periodic Ca Pulses Define Neurosecretory Activities in Magnocellular Vasotocin and Isotocin Neurons

The electrical activity of magnocellular neurosecretory cells (NSCs) is correlated with the release rates of neurohypophysial hormones. NSCs may control their secretory activity in a cooperative manner by changing their electrical activity in response to changes in the internal milieu. In the present study, we applied confocal Ca imaging to a sagittally hemisected rainbow trout brain to simultaneously monitor the neuronal activity of a number of NSCs. We found that NSCs in vitro showed synchronized pulsatile elevations of intracellular Ca levels at regular intervals. Double immunostaining of vasotocin (VT) and isotocin (IT) after the confocal imaging clarified that each of the VT and IT neuronal populations showed a distinct pattern of periodic Ca pulses. Simultaneous cell-attached patch recordings ensured that individual Ca pulses were associated with a phasic burst firing. Depolarizing stimuli by increasing the extracellular K concentration from 5 to 7–9 mM reversibly shortened the interpulse intervals in both VT and IT neurons. Interpulse intervals but not durations of pulses were shortened by hypo-osmotic stimuli and prolonged by hyperosmotic stimuli, consistent with the osmoregulatory function of teleost NSCs. We therefore hypothesize that NSCs use intervals of synchronized periodic burst discharges to fit the levels of secretory activity to physiological requirements.