Voltage-induced Ca 2+ release by ryanodine receptors causes neuropeptide secretion from nerve terminals.

Depolarisation-secretion coupling is assumed to be dependent only on extracellular calcium ([Ca2+](o)). Ryanodine receptor (RyR)-sensitive stores in hypothalamic neurohypophysial system (HNS) terminals produce sparks of intracellular calcium ([Ca2+](i)) that are voltage-dependent. We hypothesised that voltage-elicited increases in intraterminal calcium are crucial for neuropeptide secretion from presynaptic terminals, whether from influx through voltage-gated calcium channels and/or from such voltage-sensitive ryanodine-mediated calcium stores. Increases in [Ca2+](i) upon depolarisation in the presence of voltage-gated calcium channel blockers, or in the absence of [Ca2+](o), still give rise to neuropeptide secretion from HNS terminals. Even in 0 [Ca2+](o), there was nonetheless an increase in capacitance suggesting exocytosis upon depolarisation. This was blocked by antagonist concentrations of ryanodine, as was peptide secretion elicited by high K+ in 0 [Ca2+](o). Furthermore, such depolarisations lead to increases in [Ca2+](i). Pre-incubation with BAPTA-AM resulted in > 50% inhibition of peptide secretion elicited by high K+ in 0 [Ca2+](o). Nifedipine but not nicardipine inhibited both the high K+ response for neuropeptide secretion and intraterminal calcium, suggesting the involvement of CaV1.1 type channels as sensors in voltage-induced calcium release. Importantly, RyR antagonists also modulate neuropeptide release under normal physiological conditions. In conclusion, our results indicate that depolarisation-induced neuropeptide secretion is present in the absence of external calcium, and calcium release from ryanodine-sensitive internal stores is a significant physiological contributor to neuropeptide secretion from HNS terminals.