Exocytosis in peptidergic nerve terminals exhibits two calcium-sensitive phases during pulsatile calcium entry

The link between electrical activity, Ca2+ entry through voltage-gated channels, and transmitter or hormone secretion is a central issue in neurobiology. In peptidergic nerve terminals of the mammalian neurohypophysis (NHP), secretion is elicited by patterned bursts of action potentials (APs). All parameters of the bursts are important to elicit efficient secretion, including AP frequency, AP broadening, burst duration, and interburst interval (Leng, 1988). We have studied Ca(2+)-secretion coupling of peptide-containing large dense-core vesicles (LDCV) in isolated rat NHP terminals. Ca2+ influx through voltage-gated Ca2+ channels was elicited and recorded by the whole-cell patch-clamp technique. Exocytosis was monitored on line with high temporal resolution by the capacitance detection technique (Neher and Marty, 1982). AP bursts were simulated by depolarizing pulse trains that mimic pulsatile submembrane Ca2+ elevations predicted for physiological stimuli. The characteristic capacitance response (delta Cm) to a train of depolarizing pulses was triphasic. It consisted of a threshold phase during which early pulses did not elicit secretion, a subsequent secretory phase during which Cm increases were coupled to depolarizing pulses, and a fatigued or inactivated state during which additional Ca2+ entry was ineffective. Both the threshold phase and secretory phase were correlated with the integrals of Ca2+ current. Ca2+ chelators affect both the threshold and secretory phase at submillimolar concentrations. Thus, a "shell" rather than "microdomain" model of Ca2+ elevation is appropriate for analyzing Ca(2+)-secretion coupling in NHP terminals (Nowycky and Pinter, 1993). We propose a two-step model, with a ca(2+)-dependent preparatory step followed by a final exocytotic step that is coupled to active Ca2+ influx. The results suggest that under physiological conditions, APs early in a burst prepare an NHP terminal for secretion, but later APs actually trigger exocytosis. Since NHP terminals do not possess a readily releasable pool of vesicles that require only a single Ca2+ step for exocytosis as seen in chromaffin cells (Neher and Zucker, 1993) and melanotrophs (Thomas et al, 1993a), Ca(2+)-secretion coupling mechanisms may be heterologous even within a single class of vesicles.