Metabotropic glutamate receptors drive global persistent inhibition in the visual thalamus

Within the dorsal lateral geniculate nucleus (dLGN) of the thalamus, retinal ganglion cell (RGC) projections excite thalamocortical (TC) cells that in turn relay visual information to the cortex. Local interneurons in the dLGN regulate the output of TC cells by releasing GABA from their axonal boutons and specialized dendritic spines. Here we examine the functional role of these highly specialized interneurons and how they inhibit TC cells in mouse brain slices. It was widely thought that activation of metabotropic glutamate receptor type 5 (mGluR5) on interneuron spines leads to local GABA release restricted to sites receiving active RGC inputs. We reexamined experiments that supported this view, and found that in the presence of TTX, mGluR5 agonists evoked GABA release that could instead be explained by interneuron depolarization and widespread intracellular calcium increases. We also examined GABA release evoked by RGC activation and found that high-frequency stimulation induces a long-lasting subthreshold afterdepolarization, persistent firing, or prolonged plateau potentials in interneurons and evokes sustained GABA release. mGluR5 antagonists virtually eliminated sustained spiking and the resulting widespread calcium-signals, and reduced inhibition by >50%. The remaining inhibition appeared to be mediated by a fraction of interneurons in which plateau potentials produced large and widespread calcium increases. Local calcium signals required for local GABA release were not observed. These findings indicate that, contrary to the previous view, RGC activation does not simply evoke localized GABA release by activating mGluR5, rather, synaptic activation of mGluR5 acts primarily by depolarizing interneurons and evoking widespread dendritic GABA release.