Negative regulation of neurotransmitter release by calpain: a possible involvement of specific SNAP‐25 cleavage

Abstract Synaptic transmission is conducted by neurotransmitters released from presynaptic nerve terminals by means of Ca 2+ ‐dependent exocytosis of synaptic vesicles. Formation of a complex of soluble N ‐ethylmaleimide‐sensitive fusion protein receptor (SNARE) proteins, including vesicle‐associated membrane protein‐2 (VAMP‐2) in the synaptic vesicle membrane, and syntaxin 1 and synaptosomal‐associated protein of 25 kDa (SNAP‐25) in the plasma membrane, is essential for exocytosis. Ionomycin treatment of cultured rat cerebellar granule cells led to cleavage of SNAP‐25, but not syntaxin 1 and VAMP‐2, that was dependent on extracellular Ca 2+ . Cleavage was also induced by N ‐methyl‐ d ‐aspartate (NMDA) treatment, but not by depolarization. The use of various site‐specific antibodies to SNAP‐25, suggested that the cleavage site was in the N‐terminal domain of SNAP‐25. Calpain inhibitors abolished the Ca 2+ ‐dependent cleavage of SNAP‐25 and markedly facilitated Ca 2+ ‐dependent glutamate (Glu) release from cerebellar granule cells. These results suggest that calpain may play an important role in the long‐lasting regulation of synaptic transmission by suppressing neurotransmitter release, possibly through the proteolytic cleavage of SNAP‐25.