Modulatory Effects of Activation of Metabotropic Glutamate Receptors on

17 Metabotropic glutamate receptors (mGluRs) are widely distributed in the central nervous 18 system and modulate the release of neurotransmitters in different ways. We have previously 19 shown that activation of presynaptic group II mGluRs reduces the gain of GABAergic inputs in 20 both primary visual and auditory cortices (V1 and A1). In the present study, we sought to 21 determine whether activation of mGluRs can also affect the inhibitory inputs in thalamus. Using 22 whole cell recordings in a mouse slice preparation, we studied two GABAergic inputs to 23 thalamic relay cells: that of the thalamic reticular nucleus (TRN) to cells of the ventral 24 posteromedial nucleus (VPM), and that of interneurons to cells of lateral geniculate nucleus 25 (LGN). We found that activation of mGluRs significantly reduced the amplitudes of inhibitory 26 postsynaptic currents (IPSCs) evoked from TRN inputs to VPM cells, and further experiments 27 indicated that this was due to activation of presynaptic group I and group II mGluRs. Similar 28 results were found in the interneuronal inputs to LGN cells. Activation of presynaptic group I 29 (type 1 but not type 5) and group II mGluRs significantly reduced the amplitudes of evoked 30 IPSCs of the axonal inputs to relay cells, and additional experiments were consistent with 31 previous observations that activation of type 5 mGluRs on the dendritic terminals of interneurons 32 enhanced postsynaptic IPSCs. We concluded that group I and II mGluRs may generally reduce 33 the amplitude of evoked GABAergic IPSCs of axonal inputs to thalamic relay cells, operating 34 through presynaptic mechanisms, and this extends our previous findings in cortex. 35