Layer 6 corticothalamic neurons induce high gamma oscillations through cortico-cortical and cortico-thalamo-cortical pathways.

Layer 6 corticothalamic (L6CT) neurons project to both cortex and thalamus, inducing multiple effects including the modulation of cortical and thalamic firing, and the emergence of high gamma oscillations in the cortical local field potential (LFP). We hypothesize that the high gamma oscillations driven by L6CT neuron activation reflect the dynamic engagement of intracortical and cortico-thalamo-cortical circuits. To test this, we optogenetically activated L6CT neurons in NTSR1-cre mice (both male and female) expressing channelrhodopsin-2 in L6CT neurons. Leveraging the vibrissal pathway in awake, head-fixed mice, we presented ramp-and-hold light at different intensities while recording neural activity in the primary somatosensory barrel cortex (S1) and the ventral posteromedial nucleus (VPm) of the thalamus using silicon probes. We found that the activation of S1 L6CT neurons induces high-frequency LFP oscillations in S1 that are modulated in frequency, but not in amplitude, across light intensities and over time. To identify which neuronal classes contribute to these oscillations, we examined the temporal evolution of firing rate in S1 and VPm. While most S1 neurons were steadily suppressed, VPm and S1 Layer 4 fast spiking (L4 FS) neurons evolved from being suppressed to facilitated within 500 ms, suggesting differential recruitment of the intracortical vs cortico-thalamo-cortical pathways. Finally, we found that LFP frequency selectively correlates with VPm firing rate. Taken together, our data suggests that L6CT neurons sculpt the frequency of S1 LFP oscillations through cortico-thalamo-cortical circuits, linking the recurrent interactions mediated by L6CT neurons to the high gamma oscillations observed across physiological and pathological conditions.Significance Statement Layer 6 corticothalamic (L6CT) neurons are strategically positioned to modulate the cortex and the thalamus allowing them to engage distinct, yet interlocked, circuits. Here we show that the activation of L6CT neurons in the mouse primary somatosensory cortex induces fast cortical oscillations through the coordinated engagement of cortico-thalamo-cortical and intracortical pathways. Our work reveals that these two L6CT-mediated pathways exert competing effects: while intracortical connections suppress cortical spiking, the activity of the cortico-thalamo-cortical loop rapidly evolves, facilitating cortical spiking. We demonstrate that the cortico-thalamo-cortical pathway operates on a faster timescale than the intracortical pathway and critically shapes cortical oscillation frequency. These findings reveal how the unique position of corticothalamic neurons allows them to flexibly and dynamically modulate the thalamocortical network.