Synchronized periodic Ca2+ pulses define neurosecretory activities in magnocellular vasotocin and isotocin neurons

The electrical activity of magnocellular neurosecretory cells (NSCs) is cor related with the release rates of neurohypophysial hormones. NSCs may contr ol their secretory activity in a cooperative manner by changing their elect rical activity in response to changes in the internal milieu. In the presen t study, we applied confocal Ca2+ imaging to a sagittally hemisected rainbo w trout brain to simultaneously monitor the neuronal activity of a number o f NSCs. We found that NSCs in vitro showed synchronized pulsatile elevation s of intracellular Ca2+ levels at regular intervals. Double immunostaining of vasotocin (VT) and isotocin (IT) after the confocal imaging clarified th at each of the VT and IT neuronal populations showed a distinct pattern of periodic Ca2+ pulses. Simultaneous cell-attached patch recordings ensured t hat individual Ca2+ pulses were associated with a phasic burst firing. Depo larizing stimuli by increasing the extracellular K+ concentration from 5 to 7-9 mM reversibly shortened the interpulse intervals in both VT and IT neu rons. Interpulse intervals but not durations of pulses were shortened by hy po-osmotic stimuli and prolonged by hyperosmotic stimuli, consistent with t he osmoregulatory function of teleost NSCs. We therefore hypothesize that N SCs use intervals of synchronized periodic burst discharges to fit the leve ls of secretory activity to physiological requirements.