# Thalamic reticular neurons provide cell type-specific modulation of sound processing in the auditory thalamus

## TL;DR

This study reveals how two types of inhibitory neurons in the thalamic reticular nucleus modulate sound processing in the auditory thalamus in mice.

## Contribution

The study identifies cell type-specific modulation of auditory thalamus activity by PVTRN and SSTTRN neurons.

## Key findings

- PVTRN neurons project to ventral MGB, while SSTTRN neurons project to dorso-medial MGB.
- Optogenetic inactivation of PVTRN neurons bidirectionally modulates sound-evoked activity in MGB.
- Inactivating SSTTRN neurons largely suppresses tone-evoked activity in MGB neurons.

## Abstract

Inhibition plays an important role in controlling the flow and processing of auditory information throughout the central auditory pathway, yet how inhibitory circuits shape auditory processing in the medial geniculate body (MGB), the key region in the auditory thalamus, is poorly understood. The MGB gates the flow of auditory information to the auditory cortex, and it is inhibited largely by the thalamic reticular nucleus (TRN). The TRN contains two major classes of inhibitory neurons: parvalbumin (PVTRN)-positive and somatostatin (SSTTRN)-positive neurons. PV and SST neurons have been shown to play differential roles in controlling sound responses in auditory cortex. In the somatosensory and visual subregions of the TRN, PVTRN and SSTTRN neurons exhibit anatomical and functional differences. However, it remains unknown whether and how PVTRN and SSTTRN neurons differ in their anatomical projections from the TRN to the auditory thalamus, and whether and how they differentially modulate activity in the MGB. Here, we investigated virally labeled projections of PVTRN or SSTTRN neurons, and recorded neuronal responses in the MGB of awake, head-fixed mice while presenting sound stimuli and selectivity suppressing PVTRN or SSTTRN neurons on a subset of trials. We find that PVTRN and SSTTRN neurons exhibit differential projection patterns within the auditory thalamus: PVTRN neurons predominantly project to ventral MGB, whereas SSTTRN neurons project to the dorso-medial regions of MGB. Optogenetic inactivation of PVTRN neurons bidirectionally modulated sound-evoked activity in MGB, increasing firing in 29% of MGB neurons, while suppressing firing in 41%. In contrast, inactivating SSTTRN neurons largely suppressed tone-evoked activity in MGB neurons. Cell type-specific computational models identified candidate circuit mechanisms for generating the differential effects of TRN inactivation on MGB sound responses. These distinct inhibitory pathways within the auditory thalamus reveal cell type-specific organization of thalamic inhibition in auditory computation.

The thalamic reticular nucleus is an important regulator of sensory processing and computation, but the functional roles of distinct neuron types in this brain region are unknown. By examining responses to tones in mice, this study shows that the two key inhibitory neuron types of the thalamic reticular nucleus exert unexpected, bidirectional effects on sound processing in the auditory thalamus.