Bilateral Whisker Representations in the Primary Somatosensory Cortex in Robo3cKO Mice Are Reflected in the Primary Motor Cortex.

In Robo3cKO mice, midline crossing defects of the trigeminothalamic projections from the trigeminal principal sensory nucleus result in bilateral whisker maps in the somatosensory thalamus and consequently in the face representation area of the primary somatosensory (S1) cortex (Renier et al., 2017; Tsytsarev et al., 2017). We investigated whether this bilateral sensory representation in the whisker-barrel cortex is also reflected in the downstream projections from the S1 to the primary motor (M1) cortex. To label these projections, we injected anterograde viral axonal tracer in S1 cortex. Corticocortical projections from the S1 distribute to similar areas across the ipsilateral hemisphere in control and Robo3cKO mice. Namely, in both genotypes they extend to the M1, premotor/prefrontal cortex (PMPF), secondary somatosensory (S2) cortex. Next, we performed voltage-sensitive dye imaging (VSDi) in the left hemisphere following ipsilateral and contralateral single whisker stimulation. While controls showed only activation in the contralateral whisker barrel cortex and M1 cortex, the Robo3cKO mouse left hemisphere was activated bilaterally in both the barrel cortex and the M1 cortex. We conclude that the midline crossing defect of the trigeminothalamic projections leads to bilateral whisker representations not only in the thalamus and the S1 cortex but also downstream from the S1, in the M1 cortex.

Tsytsarev V ，Plachez C ，Zhao S ，O'Connor DH ，Erzurumlu RS ... -  《-》

Behavioral Consequences of a Bifacial Map in the Mouse Somatosensory Cortex.

The whisker system is an important sensory organ with extensive neural representations in the brain of the mouse. Patterned neural modules (barrelettes) in the ipsilateral principal sensory nucleus of the trigeminal nerve (PrV) correspond to the whiskers. Axons of the PrV barrelette neurons cross the midline and confer the whisker-related patterning to the contralateral ventroposteromedial nucleus of the thalamus, and subsequently to the cortex. In this way, specific neural modules called barreloids and barrels in the contralateral thalamus and cortex represent each whisker. Partial midline crossing of the PrV axons, in a conditional Robo3 mutant (Robo3R3-5cKO) mouse line, leads to the formation of bilateral whisker maps in the ventroposteromedial, as well as the barrel cortex. We used voltage-sensitive dye optical imaging and somatosensory and motor behavioral tests to characterize the consequences of bifacial maps in the thalamocortical system. Voltage-sensitive dye optical imaging verified functional, bilateral whisker representation in the barrel cortex and activation of distinct cortical loci following ipsilateral and contralateral stimulation of the specific whiskers. The mutant animals were comparable with the control animals in sensorimotor tests. However, they showed noticeable deficits in all of the whisker-dependent or -related tests, including Y-maze exploration, horizontal surface approach, bridge crossing, gap crossing, texture discrimination, floating in water, and whisking laterality. Our results indicate that bifacial maps along the thalamocortical system do not offer a functional advantage. Instead, they lead to impairments, possibly due to the smaller size of the whisker-related modules and interference between the ipsilateral and contralateral whisker representations in the same thalamus and cortex.SIGNIFICANCE STATEMENT The whisker sensory system plays a quintessentially important role in exploratory behavior of mice and other nocturnal rodents. Here, we studied a novel mutant mouse line, in which the projections from the brainstem to the thalamus are disrupted. This led to formation of bilateral whisker maps in both the thalamus and the cortex. The two whisker maps crowd in a space normally devoted to the contralateral map alone and in a nonoverlapping fashion. Stimulation of the whiskers on either side activates the corresponding region of the map. Mice with bilateral whisker maps perform well in general sensorimotor tasks but show poor performance in specific tests that require whisker-dependent tactile discrimination. These observations indicate that contralateral, instead of bilateral, representation of the sensory space plays a critical role in acuity and fine discrimination during somesthesis.

Tsytsarev V ，Arakawa H ，Zhao S ，Chédotal A ，Erzurumlu RS ... -  《-》

Corticofugal projection patterns of whisker sensorimotor cortex to the sensory trigeminal nuclei.

Smith JB ，Watson GD ，Alloway KD ，Schwarz C ，Chakrabarti S ... -  《Frontiers in Neural Circuits》

Cortical control of whisker movement.

Petersen CC 《-》

Dual projections of tuberomammillary neurons to whisker-related, sensory and motor regions of the rat.

The primary goal of this study was to examine in the rat whether neurons in the tuberomammillary nucleus (TMN) provide axon collaterals to whisker-related, sensorimotor regions at cortical and brainstem levels, using two different retrograde tracers. When injections were made at primary sensory (S1) barrel field/primary whisker motor (M1) cortices, dual-projecting TMN neurons were observed mainly in the ventrolateral subdivision; the projection was almost exclusively ipsilateral. On the other hand, following injections of tracers into whisker-related, principal sensory trigeminal (Pr5)/lateral facial motor (Mo7) nuclei, dual-projecting cells were observed mainly in the dorsomedial subdivision; the projection was bilateral with a slight contralateral dominance. Taken together, the present observation demonstrated that each subdivision of the TMN possessed a differential functional organization with respect to its collateral projection to whisker-related sensorimotor targets, suggesting that the histaminergic projection might play a modulatory role in vibrissal sensorimotor integration, which allows the guidance of behavioral action essential for the survival of the animal.

Hong EY ，Beak SK ，Lee HS 《-》