Modulatory effects of activation of metabotropic glutamate receptors on GABAergic circuits in the mouse thalamus

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