Axonal projections of single auditory neurons in the thalamic reticular nucleus: implications for tonotopy-related gating function and cross-modal modulation.

Tonotopically comparable subfields of the primary auditory area (AI) and nonprimary auditory areas (non-AI), i.e. posterodorsal area (PD) and ventral auditory area (VA), in the rat cortex have similar topographies in the projection to the ventral division of the medial geniculate nucleus (MGV), but reverse topographies in the projection to the thalamic reticular nucleus (TRN). In this study, we examined axonal projections of single auditory TRN cells, using juxtacellular recording and labeling techniques, to determine features of TRN projections and estimate how the TRN mediates corticofugal inhibition along with the reverse topographies of cortical projections to the TRN. Auditory TRN cells sent topographic projections to limited parts of the MGV in a manner that relays cortical inputs from tonotopically comparable subfields of the AI and non-AI (PD and VA) to different parts of the MGV. The results suggest that corticofugal excitations from the AI and non-AI modulate thalamic cell activity in the same part of the MGV, whereas corticofugal inhibitions via the TRN modulate cell activity in different parts of the MGV with regard to tonotopic organization. The AI and non-AI could serve distinctive gating functions for auditory attention through the differential topography of inhibitory modulation. In addition, we obtained an intriguing finding that a subset of auditory TRN cells projected to the somatosensory but not to the auditory thalamic nuclei. There was also a cell projecting to the MGV and somatosensory nuclei. These findings extend the previously suggested possibility that TRN has a cross-modal as well as an intramodal gating function in the thalamus.