Complexin that regulates SNARE protein complex-mediated synaptic vesicle fusion (PubMed:19386896). Required for the maintenance of synaptic ultrastructure in the adult retina (PubMed:19386896). Positively regulates synaptic transmission through synaptic vesicle availability and exocytosis of neurotransmitters at photoreceptor ribbon synapses in the retina (PubMed:15911881, PubMed:19386896, PubMed:27335398). Suppresses tonic photoreceptor activity and baseline 'noise' by suppression of Ca(2+) vesicle tonic release and the facilitation of evoked synchronous and asynchronous Ca(2+) vesicle release (PubMed:22694764, PubMed:27335398). Weakly binds to the SNARE core complex containing SNAP25, VAMP2 and STX1A. Enriched at the synaptic terminal. Present specifically in the retina (at protein level) (PubMed:15911881, PubMed:19386896, PubMed:22694764, PubMed:27335398). Expressed in the outer nuclear layer of the retina (at protein level) (PubMed:22694764). Strongly expressed at rod photoreceptor ribbon synapses (at protein level) (PubMed:15911881, PubMed:22694764). Not expressed at conventional amacrine cell synapses, nor at cone photoreceptor ribbon synapses (at protein level) (PubMed:15911881). Weakly expressed at cone photoreceptor synaptic terminals (at protein level) (PubMed:22694764). Not expressed in the brain (at protein level) (PubMed:19386896). Farnesylation mediates presynaptic targeting and is important for function in neurotransmitter release. Knockout mice are generally phenotypically normal, viable, and fertile (PubMed:19386896). Normal overall retina structure and morphology of the outer plexiform layer (OPL) and inner plexiform layer (IPL) (PubMed:19386896). Abundance and distribution of synaptic proteins remain consistent (PubMed:19386896). Reduced inner retinal processing and retinal synaptic transmission at low light intensities (PubMed:19386896). Cplx3 and Cplx4 double knockout mice are generally phenotypically normal, viable, and fertile, however show disordered morphology of the OPL and vision perturbation when compared to single knockout mice (PubMed:19386896). Cplx3 and Cplx4 double knockout mice show evidence of mild vision perturbation, with a reduction in the number of morphologically normal anchored presynaptic ribbon synapses and a decrease in controlled neurotransmitter release at photoreceptor ribbon synapses (PubMed:19386896). Cplx3 and Cplx4 double knockout mice show a reduced response and sensitivity of ON and OFF ganglion cell response as a result of disrupted synaptic transmission (PubMed:22694764). Cplx3 and Cplx4 double knockout mice show a greater variance in photoreceptor activity response and a decrease in sustained response, this is caused by an increase in release and fusion of synaptic vesicles in an asynchronous manner, this is particularly evident following multiple stimuli (PubMed:27335398). Belongs to the complexin/synaphin family.