Activation of group II metabotropic glutamate receptors inhibits glutamate release from salamander retinal photoreceptors

We investigated the effects of group II metabotropic glutamate receptor (mGluR) activation on excitatory synaptic transmission in the salamander retinal slice preparation. The group II selective agonists DCG-IV and LY354740 reduced light-evoked excitatory postsynaptic currents (EPSCs) in ganglion cells. To determine the synaptic basis of this effect, we also recorded from bipolar cells and horizontal cells. In ON bipolar cells, DCG-IV increased the inward current in darkness but did not affect the peak current at light onset. In OFF bipolar cells and horizontal cells, DCG-IV had the opposite effect, reducing the inward current in darkness. Given the opposite polarities of these two classes of synapses, our results suggest that group II mGluRs act presynaptically to reduce glutamate release from photoreceptors. To determine whether DCG-IV affected rods or cones, we applied light stimuli that selectively activate each type of photoreceptor. In horizontal cells, most of which receive mixed synaptic input from rods and cones, DCG-IV reduced rod-driven EPSCs evoked by 470-nm stimuli and cone-driven EPSCs elicited by 700-nm stimuli in the presence of a rod-saturating background. Thus, activation of group II mGluRs reduced rod- and cone-mediated glutamate release. Our results suggest that group II mGluRs could mediate feedback by which extracellular glutamate inhibits glutamate release from photoreceptor terminals.