Metabotropic glutamate receptor 8 (mGlu8) is a heterogeneously expressed and poorly understood glutamate receptor with potential pharmacological significance. The thalamic reticular nucleus (TRN) is a critical inhibitory modulator of the thalamocortical-corticothalamic (TC-CT) network and plays a crucial role in information processing throughout the brain, is implicated in a variety of psychiatric conditions, and is also a site of significant mGlu8 expression. Using both male and female mice, we determined via fluorescent in situ hybridization that parvalbumin-expressing cells in the TRN core and shell matrices (identified by spp1+ and ecel1+ expression, respectively) as well as the cortical layers involved in corticothalamic signaling, express grm8 mRNA. We then assayed the physiological and behavioral impacts of perturbing grm8 signaling in the TC circuit through conditional (AAV-CRE mediated) and cell type-specific constitutive deletion strategies. We show that constitutive parvalbumin grm8 knockout (PV grm8 KO) mice exhibited 1) increased spontaneous excitatory drive onto dorsal thalamus relay cells and 2) impaired sensorimotor gating, measured via paired-pulse inhibition, but observed no differences in locomotion and thigmotaxis in repeated bouts of open field testing. Conversely, we observed hyperlocomotive phenotypes and anxiolytic effects of AAV-mediated conditional knockdown of grm8 in the TRN (TRN grm8 KD) in repeated open field testing. Our findings underscore a role for mGlu8 in regulating excitatory neurotransmission as well as anxiety-related locomotor behavior and sensorimotor gating, revealing potential therapeutic applications for various neuropsychiatric disorders and guiding future research endeavors into mGlu8 signaling and TRN function.Significance statement Group III mGlu receptors and the Thalamic Reticular Nucleus (TRN) are critical modulators of reciprocal cortico-thalamic neurotransmission and are implicated in anxiety and locomotor behaviors. The present study demonstrates a specific enrichment of grm8 mRNA within the TRN and thalamus-projecting cortical layers and characterizes the role of mGlu8 receptors in controlling spontaneous excitatory neurotransmission onto cells located within the dorsal thalamus and regulating sensorimotor behaviors from open field and PPI testing. These findings add to growing bodies of literature regarding both TRN and grm8 regulation of thalamocortical activity and related behaviors implicated in neurological and neuropsychiatric disorders.