Inhibitory Projections from the Inferior Colliculus to the Medial Geniculate body Originate from Four Subtypes of GABAergic Cells

Nov 21, 2018eNeuro

Four types of inhibitory brain cells send signals from the sound processing center to the hearing relay area

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Abstract

GABAergic cells make up 20-40% of the projections from the inferior colliculus to the medial geniculate body.

Four subtypes of GABAergic cells in the inferior colliculus are identified based on their association with specific extracellular markers.
These subtypes vary in size and distribution within the inferior colliculus.
Larger GABAergic cells surrounded by perineuronal nets and VGLUT2 rings are hypothesized to project to the medial geniculate body.
Analysis revealed a range of soma sizes for GABAergic cells in the IC-MG pathway, reflecting the diversity found in the inferior colliculus.
All four subtypes of GABAergic cells are involved in the IC-MG pathway, with smaller cells lacking PNs and VGLUT2 rings being more common than expected.

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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.

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Inhibitory Projections from the Inferior Colliculus to the Medial Geniculate body Originate from Four Subtypes of GABAergic Cells

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