Abstract
A GABAergic component has been identified in the projection from the inferior colliculus (IC) to the medial geniculate body (MG) in cats and rats. We sought to determine if this GABAergic pathway exists in guinea pig, a species widely used in auditory research. The guinea pig IC contains GABAergic cells, but their relative abundance in the IC and their relative contributions to tectothalamic projections are unknown. We identified GABAergic cells with immunochemistry for glutamic acid decarboxylase (GAD) and determined that ~21% of IC neurons are GABAergic. We then combined retrograde tracing with GAD immunohistochemistry to identify the GABAergic tectothalamic projection. Large injections of Fast Blue, red fluorescent beads or FluoroGold were deposited to include all subdivisions of the MG. The results demonstrate a GABAergic pathway from each IC subdivision to the ipsilateral MG. GABAergic cells constitute ~22% of this ipsilateral pathway. In addition, each subdivision of the IC had a GABAergic projection to the contralateral MG. Measured by number of tectothalamic cells, the contralateral projection is about 10% of the size of the ipsilateral projection. GABAergic cells constitute about 20% of the contralateral projection. In summary, the results demonstrate a tectothalamic projection in guinea pigs that originates in part from GABAergic cells that project ipsilaterally or contralaterally to the MG. The results show similarities to both rats and cats, and carry implications for the role of GABAergic tectothalamic projections vis-à-vis the presence (in cats) or near absence (in rats and guinea pigs) of GABAergic interneurons in the MG.
Highlights
The inferior colliculus (IC) is a large midbrain structure that is a target of both ascending and descending auditory pathways as well as a source of projections to numerous areas
We used immunolabeling for glutamic acid decarboxylase (GAD) to identify GABAergic IC cells and combined retrograde tracing with the immunostain to identify GABAergic IC cells that project to the medial geniculate body (MG)
We noted a wide range of sizes in all IC subdivisions (∼8 μm to ∼40 μm in major diameter), suggesting broad similarity with GABAergic IC cells described in other species
Summary
The inferior colliculus (IC) is a large midbrain structure that is a target of both ascending and descending auditory pathways as well as a source of projections to numerous areas. The IC contains a mixture of excitatory and inhibitory neurons. Unlike many other brain areas, the GABAergic cells provide a significant contribution to the output pathways from the IC (Appell and Behan, 1990; Wenstrup, 2005). The largest and best studied pathway is the tectothalamic projection from the IC to the medial geniculate nucleus (MG), the auditory center of the thalamus. Quantitative analyses have been completed in two species, where the GABAergic cells constitute about 40% of the tectothalamic
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