Corticofugal control of brainstem sensory gating in the rodent whisker system

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Rodents exhibit a wide repertoire of behaviors whilst exploring their immediate environment through the use of their large vibrissae or whiskers. In a specific behavior, known as active touch, they explore objects with their whiskers and manipulate the position of these tactile sensors to extract maximal information about the object. During such contacts, sensory processing is altered as compared to periods of quiescence. Specifically, the sensory responses in the tactile responsive neurons of the trigeminal complex are reduced. By contrast, passive contacts with an object in the absence of whisker movements evoke large responses in these neurons. Such sensory gating apparently maximizes detection of unexpected objects in the vicinity. Recent studies suggest that gating through brainstem trigeminal nuclei is mediated by corticofugal inputs which differentially modulate trigeminal circuitry. Neurons in specific compartments of primary somatosensory cortex (S1 septa) increase sensory transmission through the trigeminal brainstem whereas secondary somatosensory cortex (S2) inhibits it. However, several key factors in such a scheme remain to be elucidated. First, the projections from septa to the trigeminal complex have never been anatomically determined. Second, the responses of the various trigeminal nuclei in an awake behaving animal during active touch/quiescence have not been characterized. Third, the effect of modulatory influence of cortical projections on trigeminal nuclei on perception has never been tested. In this proposal we aim to clarify these points via a combined approach utilizing anatomical tract tracing, neurophysiological recordings, optogenetics and behavioral methods. First, we will determine and quantify the projections from S1 septa to the trigeminal nuclei. Second, we will record from these trigeminal nuclei and cortex in the same awake animal and characterize trigeminal responses in the different behavioral states. Cross correlation analysis will be applied to study their dependence on cortical drive. Finally, we will optogenetically manipulate S1 and S2 recipient structures in the trigeminal nuclei and quantify ensuing changes in perception. This will be the first attempt to characterize top down, cortical influences on whisker-related tactile gating in the brainstem in an awake animal and has important implications in the study of cortical effects on perception.