Role of Intracellular Calcium in Release from Nerve Terminals

Abstract

Oxytocin (OT) and vasopressin (AVP) are released from terminals in the neurohypophysis (NH). These neuropeptides (NP) are contained in large dense core vesicles (LDCV). Ryanodine receptors (RyR) in NH terminals can induce spontaneous focal Ca2+ transients (DeCrescenzo, et al., 2004 J. Neurosci.24:1226-1235), making them ideal for studying the role of intraterminal Ca2+ in NP release.Fluorescent immunolabeling and immunogold micrographs of NH terminals show that RyR are localized specifically to LDCV. Furthermore, a large conductance non-specific cation channel, previously identified in the LDCV membrane with properties similar to that of a RyR (Lee, et al., 1992 Neuron 8:335-342), is pharmacologically affected in the same characteristic manner as a RyR.We found that individual Ca2+ release events alone are not enough to drive peptide release (McNally, et al., 2009 J. Neurosci. 29:14120-14126). However, these RyR sensitive events could potentially play a role in modulating NP release. To test this hypothesis, the association of LDCV within an area 0.45 µm of the plasma membrane was assessed using immunolabeling of Neurophysins I (OT) and II (AVP) in isolated NH terminals. We found that the amount of membrane associated NP-immunoreactivity varies significantly between terminal types and that this distribution pattern can be enhanced by agonist concentrations of ryanodine, but only in OT terminals.Importantly, Ca2+-evoked NP release from permeabilized-terminals was increased by agonist concentrations of ryanodine and decreased by antagonist concentrations of this drug. Amperometric recording of spontaneous release events from artificial transmitter-loaded terminals corroborated these ryanodine effects. Agonist concentrations of ryanodine were also able to increase the asynchronous phase of low frequency electrically-stimulated capacitance increases in NH terminals. Thus, the ryanodine-sensitive mobilization of LDCV seems to have a functional role in modulating secretion of NP from NH terminals. [Support: NS29470, NS40966, P60037094900000].