Mechanism of α-latrotoxin action at nerve endings of neurohypophysis

Abstract

The neurotoxin α-latrotoxin elicits spontaneous exocytosis of neurotransmitter from neurons and peptide hormones from endocrine cells. While the mechanism of action is not fully understood, both Ca 2+-dependent and Ca 2+-independent pathways participate in the facilitation of release, with the relative contribution of the pathways differing among neuronal and endocrine cell types. Here, we investigate the actions of α-latrotoxin on neuroendocrine nerve endings that emanate from central nervous system neurons and, therefore, are unique in that they possess properties of central nerve endings and endocrine cells. Using intracellular [Ca 2+] measurements both calcium-independent receptors for latrotoxin (CIRL or latrophilin) and neurexin 1α receptors were found to be functionally present. Interaction of α-latrotoxin with these receptors stimulated secretion of vasopressin and oxytocin neuropeptide. The secretory response was entirely dependent upon toxin-mediated extracellular Ca 2+ influx, although α-latrotoxin also consistently triggered mobilization of Ca 2+ from an intracellular store. The mobilization of intracellular Ca 2+ relied on α-latrotoxin-mediated Na + influx and was blocked by the protonophore FCCP, thereby implicating mitochondria as the Ca 2+ store being mobilized. Using the whole cell recording configuration of the patch clamp, we report that α-latrotoxin interaction with the CIRL receptor on these nerve endings resulted in ionic pore formation, generating unitary inward current steps of 20 pA and a channel conductance of approximately 220 pS in Ca 2+-free saline. Thus, α-latrotoxin stimulates Ca 2+-dependent exocytosis in neurohypophysial nerve endings through receptor interaction and insertion of Ca 2+ permeable membrane pores. While α-latrotoxin mobilizes intracellular Ca 2+ stores the elevation in [Ca 2+] reached is insufficient to trigger measurable exocytosis.