Cellular/Molecular A Presynaptic Group III mGluR Recruits G/SNARE Interactions to Inhibit Synaptic Transmission by Cone Photoreceptors in the Vertebrate Retina X Matthew J. Van Hook, 1 X Norbert Babai, 2 Zack Zurawski, 3 X Yun Young Yim, 3 X Heidi E. Hamm, 3 and X Wallace B. Thoreson 1,4 1 Truhlsen Eye Institute, Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska 68198, 2 Department of Biology, Animal Physiology, University of Erlangen-Nuremberg, 91058 Erlangen, Germany, 3 Department of Pharmacology, Vanderbilt University Medical School, Nashville, Tennessee 37232, and 4 Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, Nebraska 68198 G-protein  subunits (G) interact with presynaptic proteins and regulate neurotransmitter release downstream of Ca 2 influx. To accomplish their roles in sensory signaling, photoreceptor synapses use specialized presynaptic proteins that support neurotransmission at active zone structures known as ribbons. While several G-protein coupled receptors (GPCRs) influence synaptic transmission at ribbon synapses of cones and other retinal neurons, it is unknown whether G contributes to these effects. We tested whether activation of one particular GPCR, a metabotropic glutamate receptor (mGluR), can reduce cone synaptic transmission via G in tiger salamander retinas. In recordings from horizontal cells, we found that an mGluR agonist (L-AP4) reduced cone-driven light responses and mEPSC frequency. In paired recordings of cones and horizontal cells, L-AP4 slightly reduced cone I Ca (10%) and caused a larger reduction in cone-driven EPSCs (30%). Proximity ligation assay revealed direct interactions between SNAP-25 and G subunits in retinal synaptic layers. Pretreatment with the SNAP-25 cleaving protease BoNT/A inhibited L-AP4 effects on synaptic transmission, as did introduction of a peptide derived from the SNAP-25 C terminus. Introducing G subunits directly into cones reduced EPSC amplitude. This effect was inhibited by BoNT/A, supporting a role for G/SNAP-25 interactions. However, the mGluR-dependent reduction in I Ca was not mim- icked by G, indicating that this effect was independent of G. The finding that synaptic transmission at cone ribbon synapses is regulated by G/SNAP-25 interactions indicates that these mechanisms are shared by conventional and ribbon-type synapses. G liberated from other photoreceptor GPCRs is also likely to regulate synaptic transmission. Key words: G-protein coupled receptor; mGluR; retina; ribbon; SNARE complex; synapse Introduction Early-stage visual processing in the vertebrate retina relies on the unique synaptic signaling capabilities of rod and cone photore- ceptors. Changes in membrane voltage due to photon absorption in photoreceptor outer segments regulate tonic glutamate release at the synaptic terminal. Photoreceptor synapses differ from their conventional counterparts in a number of ways. For instance, exocytosis depends on large planar protein structures known as Received Sept. 20, 2016; revised March 27, 2017; accepted March 28, 2017. Author contributions: M.J.V.H., N.B., Z.Z., Y.Y.Y., H.E.H., and W.B.T. designed research; M.J.V.H., N.B., and W.B.T. performed research; Z.Z., Y.Y.Y., and H.E.H. contributed unpublished reagents/analytic tools; M.J.V.H., N.B., and W.B.T. analyzed data; M.J.V.H. and W.B.T. wrote the paper. This work was supported by National Institutes of Health Grants EY10542 to W.B.T., F32EY023864 to M.J.V.H., EY010291 to H.E.H., MH101679 to H.E.H., and DK109204 to H.E.H., and Research to Prevent Blindness Senior Scien- tific Investigator Award to W.B.T. The authors declare no competing financial interests. Significance Statement Dynamic regulation of synaptic transmission by presynaptic G-protein coupled receptors shapes information flow through neural circuits. At the first synapse in the visual system, presynaptic metabotropic glutamate receptors (mGluRs) regulate cone photo- receptor synaptic transmission, although the mechanisms and functional impact of this are unclear. We show that mGluRs regulate light response encoding across the cone synapse, accomplished in part by triggering G-protein  subunits (G) interactions with SNAP-25, a core component of the synaptic vesicle fusion machinery. In addition to revealing a role in visual processing, this provides the first demonstration that G/SNAP-25 interactions regulate synaptic function at a ribbon-type synapse, contributing to an emerging picture of the ubiquity of G/SNARE interactions in regulating synaptic transmission throughout the nervous system. 4618 • The Journal of Neuroscience, April 26, 2017 • 37(17):4618 – 4634