Role of Inhibitory Neuron Types of the TRN in Auditory Processing
Sensory perception requires a set of complex feedforward and feedback processes between the periphery, subcortical pathways, and the sensory cortices. For most sensory systems synapses within the thalamic sensory nuclei play a key role in fine-tuning sensory information. The thalamic reticular nucleus (TRN), a thin sheet of GABAergic neurons encapsulating the thalamus, is the primary source of inhibition to the thalamic nuclei. Through its inhibitory projections to thalamic relay cells, it filters relevant sensory information between the thalamus and cortices. Two dominant subclasses of inhibitory neurons are found in the TRN: parvalbumin (PV) and somatostatin (SST) neurons, neuron types that play differential roles in auditory processing. Although the TRN is necessary for the transfer of important acoustic information to higher-order structures, little is known of the function of the TRN and its inhibitory neuronal subtypes in the auditory pathway. In chapter 2 we tested how arising auditory information is relayed from the basolateral amygdala (BLA) to the auditory cortex; two functionally important areas for fear related behaviors. We found that activation of direct projections from the BLA to the TRN decreases spontaneous activity but amplifies auditory responses to pure tones in the auditory cortex and thalamus. This suggests that the BLA-TRN circuit is crucial for relaying behaviorally relevant signals to auditory cortex. To further understand the TRNs contributions to auditory processing, in chapter 3 we explored the circuit mechanisms between the TRN and the auditory thalamus, the medial geniculate body (MGB). Via anatomical tracing we found that PV neurons of the TRN project to the ventral MGB, which relays sensory information to primary auditory areas, and that SST cells of the TRN project to dorsal and medial MGB, which relay information to secondary auditory association areas. Furthermore, through optogenetic inhibition of PV and SST neurons of the TRN we find that responses to pure tones in excitatory relay cells of the MGB are both facilitated and suppressed. Together, these results establish a nuanced yet crucial role of the TRN in auditory processing.