Geniculo-collicular descending projections in the gerbil

Axonal projections were labeled using a combination of tract-tracing and intracellular staining to examine the connection between the auditory thalamus and tectum. We report descending projections from the medial and dorsal divisions of the medial geniculate body to the external nucleus of the ipsilateral inferior colliculus and lower brainstem of the gerbil. These were studied in quasi-parasagittal brain slice preparations containing auditory thalamus, tectum and brainstem. Slices were cut in a curved, roughly parasagittal plane from the level of the thalamus to the brainstem. When properly cut, a single thick slice included most of the ipsilateral auditory cell groups and pathways at these levels. The geniculo-collicular projections were revealed after extracellular injections of Biocytin and rhodamine-conjugated biotinylated dextran amine in the medial geniculate body were applied to these slices. Furthermore, cells in the medial geniculate body that had been retrogradely prelabeled with a fluorescent tracer, Fluoro-Gold, injected in the inferior colliculus were intracellularly labeled to confirm the presence of the descending axons. In these preparations, single cells with descending axons were traced through their entire extent to their terminals in the inferior colliculus. Our results clearly demonstrate the geniculo-collicular descending projection and suggest a thalamo-tectal feedback loop.