Double mutation of open syntaxin and UNC-18 P334A leads to excitatory-inhibitory imbalance and impairs multiple aspects of C. elegans behavior.

SNARE and Sec/Munc18 proteins are essential in synaptic vesicle exocytosis. Open form t-SNARE syntaxin and UNC-18 P334A are well-studied exocytosis-enhancing mutants. Here we investigate the interrelationship between the two mutations by generating double mutants in various genetic backgrounds in C. elegans . While each single mutation rescued the motility of CAPS/unc-31 and synaptotagmin/snt-1 mutants significantly, double mutations unexpectedly worsened motility or lost their rescuing effects. Electrophysiological analysis revealed that simultaneous mutations of open syntaxin and gain-of-function P334A UNC-18 induces a strong imbalance of excitatory over inhibitory transmission. We conclude that open syntaxin and P334A UNC-18 do not have additive beneficial effects on C. elegans' characteristics such as motility, growth, offspring bared, body size, and exocytosis. Our results also reveal unexpected differences in the regulation of exocytosis between excitatory versus inhibitory synapses.