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.
© 2020 British Society for Neuroendocrinology.