<i>C</i>. <i>elegans</i> strains used in this study.

<div> The balance between synaptic excitation and inhibition (E/I) is essential for coordinating motor behavior, yet the differential roles of exocytosis regulators in this balance are less understood. In this study, we investigated the roles of 2 conserved exocytosis regulators, complexin/CPX-1 and CAPS/UNC-31, in excitatory versus inhibitory synapses at <i>Caenorhabditis elegans</i> neuromuscular junctions. <i>cpx-1</i> null mutants exhibited a marked increase in spontaneous release specifically at excitatory synapses, alongside an unequal reduction in excitatory and inhibitory evoked release. A clamping-specific knockin mutant, <i>cpx-1(Δ12)</i>, which preserved evoked release, also showed a biased enhancement in excitatory spontaneous release. Conversely, the <i>unc-31</i> null mutation, while maintaining normal spontaneous release, displayed a more pronounced reduction in evoked release at excitatory synapses. Notably, we found that CPX-1’s clamping function is dependent on UNC-31 and is sensitive to external Ca²⁺. Pull-down experiments confirmed that CAPS/UNC-31 does not directly interact with complexin, implying an indirect regulatory mechanism. Moreover, complexin regulates activity-dependent synaptic plasticity, which is also UNC-31 dependent. The unexpected role of CAPS/UNC-31 in the absence of CPX-1 clamping function may underpin the synaptic E/I balance and coordinated behavioral outputs in different species. </div>