High level sensory signaling modulates input to trigeminal interpolaris (SpVi) neurons: A study in awake, head-restrained mice.
Primary vibrissal afferents terminate in the brainstem trigeminal complex, from which multiple ascending projections relay sensory information to targets in the mesencephalon and diencephalon. It is also known that the trigeminal nuclei are extensively interconnected and additionally receive descending inputs from sensory and motor cortex. These anatomical connections place the trigeminal complex within a web of closed-loop networks. It remains an unresolved issue how ascending and descending inputs combine in the trigeminal complex of awake, behaving animals. Here, we report initial steps in elucidating the role of cortical inputs in early sensory processing. We recorded the extracellular activity of putative local-circuit and projection neurons in the whisker responsive region of the trigeminal interpolaris nucleus (SpVi) of awake, head-restrained mice. Spontaneous, self-generated whisker movements (5 – 8 Hz) were monitored with high-speed video (800 frames/sec), then tracked and correlated with neural activity. Compared to baseline activity when whisker were at rest the majority of cells (90%) increased their firing rates in response to whisker movements. We found no significant difference in firing rate in response to rhythmic versus non-rhythmic movements. During rhythmic motor activity, the activity of about one-third of the movement-responsive cells in SpVi was significantly modulated by the protraction-retraction phase in a uni-modal manner (P < 0.001; KS-test). Across all recorded cells, we found no population bias of the preferred phase. The modulation of unit activity in SpVi was further explored by optical stimulation of layer 5b neurons in primary somatosensory cortex (S1) expressing the light-gated, ionotropic Channelrhodopsin-2 channel. Response latencies of layer 5b neurons to LED stimulation was 12.4 ± 1.2 ms, comparable to other studies. Consistent with results in rats (Furuta et al, 2010, J Neurosci) we found a subset of neurons in SpVi that also responded to S1 activation. Response latencies in SpVi ranged from 20 – 25 ms. Considering typical S1 response latencies to whisker deflections, we propose that S1 can modulate SpVi activity within the same or next whisking cycle following a tactile event. Our results suggest that SpVi neurons transmit information about whisker kinematics, in addition to contact related events.
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