TY - JOUR T1 - Inhibitory projections from the inferior colliculus to the medial geniculate body originate from four subtypes of GABAergic cells JF - eneuro JO - eNeuro DO - 10.1523/ENEURO.0406-18.2018 SP - ENEURO.0406-18.2018 AU - N. L. Beebe AU - J. G. Mellott AU - B. R. Schofield Y1 - 2018/10/29 UR - http://www.eneuro.org/content/early/2018/10/29/ENEURO.0406-18.2018.abstract N2 - GABAergic cells constitute 20-40% of the cells that project from the inferior colliculus (IC, a midbrain auditory hub) to the medial geniculate body (MG, the main auditory nucleus of the thalamus). Four subtypes of GABAergic IC cells have been identified based on their association with perineuronal nets (PNs) and dense rings of axosomatic terminals expressing vesicular glutamate transporter 2 (VGLUT2 rings). These subtypes differ in their soma size and distribution within the IC. Based on previous work emphasizing large GABAergic cells as the origin of GABAergic IC-MG projections, we hypothesized that GABAergic IC cells surrounded by PNs and VGLUT2 rings, which tend to have larger somas, were more likely to project to the MG than smaller cells lacking these extracellular markers. Here, we injected retrograde tract tracers into the MG of guinea pigs of either sex and analyzed retrogradely labeled GABAergic cells in the ipsilateral IC for soma size and association with PNs and/or VGLUT2 rings. We found a range of GABAergic soma sizes present within the IC-MG pathway, which were reflective of the full range of GABAergic soma sizes present within the IC. Further, we found that all four subtypes of GABAergic IC cells participate in the IC-MG pathway, and that GABAergic cells lacking PNs and VGLUT2 rings were more prevalent within the pathway than would be expected based on their overall prevalence in the IC. These results may provide an anatomical substrate for the multiple roles of inhibition in the IC-MG pathway which have emerged in electrophysiological studies.Significance Statement GABAergic cells constitute a substantial proportion (20-40%) of the cells that project from the inferior colliculus to the auditory thalamus. Previous studies highlight a single subtype of GABAergic colliculothalamic cell characterized by a large soma and dense perisomatic input from glutamatergic boutons. In response to a sound, these cells could provide an inhibitory signal to the thalamus that precedes excitation. Here, we demonstrate that colliculothalamic projections arise from four GABAergic cell subtypes. The predominant subtype has a small soma, suggesting slower inhibition that presumably overlaps the arrival of excitatory inputs to the thalamus. The results suggest extensive opportunity for temporal integration of inhibitory and excitatory inputs, and a wider diversity of functions than previously suggested for inhibitory colliculothalamic projections. ER -