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Abstract

Pain is a perceptually salient association of a distressing, unpleasant emotional experience with actual or potential tissue damage. The combination of sensory perception with an emotional valence that gives rise to a singular painful experience is thought to rely on cortical communication with thalamic contributions. As a modulator of both thalamocortical signalling and signalling in cortical layer 5 (L5), the numerous corticothalamic cells in cortical layer 6 (L6CT) can potentially influence both what and how sensory information reaches and leaves the cortex. Alteration in sensory L6CT activity has been implicated in altering sensory perception but has not been directly investigated in the context of pain perception in a targeted, cell-specific manner. Here, I used in vivo cell-type specific optogenetic manipulation of L6CT and L5 during extracellular electrophysiological recordings in primary somatosensory cortex (S1) to show that L6CT excitation inhibits cortical output in L5 whilst increasing thalamocortical throughput. In collaboration with colleagues, we demonstrate that this L6CT inhibition of L5 can account for the increased mechanical sensitivity and aversive effect of L6CT excitation through its stifling of anti-nociceptive signalling from L5. My research subsequently reveals that L6CT can in fact bidirectionally modulate cortical activity as a function of its own activity levels, causing facilitation in cortical subpopulations and entraining these subpopulations to its firing patterns. The firing patterns of L6CT also bidirectionally influence the degree of thalamic burst-firing in a manner that is somewhat dissociable from total L6CT activity. This research further demonstrates that S1 L6CT can be a flexible and potent modulator of signalling to and from the cortex, providing a partial mechanistic basis for its dramatic modulation of multiple qualia underlying pain perception. My observations that S1 L6CT can bidirectionally modulate cortical firing and thalamic firing mode reveal a more complex role for L6CT than previously recognised.