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 ... -  《-》

Layers 3 and 4 Neurons of the Bilateral Whisker-Barrel Cortex.

In Robo3R3-5cKO mouse brain, rhombomere 3-derived trigeminal principal nucleus (PrV) neurons project bilaterally to the somatosensory thalamus. As a consequence, whisker-specific neural modules (barreloids and barrels) representing whiskers on both sides of the face develop in the sensory thalamus and the primary somatosensory cortex. We examined the morphological complexity of layer 4 barrel cells, their postsynaptic partners in layer 3, and functional specificity of layer 3 pyramidal cells. Layer 4 spiny stellate cells form much smaller barrels and their dendritic fields are more focalized and less complex compared to controls, while layer 3 pyramidal cells did not show notable differences. Using in vivo 2-photon imaging of a genetically encoded fluorescent [Ca2+] sensor, we visualized neural activity in the normal and Robo3R3-5cKO barrel cortex in response to ipsi- and contralateral single whisker stimulation. Layer 3 neurons in control animals responded only to their contralateral whiskers, while in the mutant cortex layer 3 pyramidal neurons showed both ipsi- and contralateral whisker responses. These results indicate that bilateral whisker map inputs stimulate different but neighboring groups of layer 3 neurons which normally relay contralateral whisker-specific information to other cortical areas.

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

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 ... -  《-》

Thalamic NMDA receptor function is necessary for patterning of the thalamocortical somatosensory map and for sensorimotor behaviors.

NMDARs play a major role in patterning of topographic sensory maps in the brain. Genetic knock-out of the essential subunit of NMDARs in excitatory cortical neurons prevents whisker-specific neural pattern formation in the barrel cortex. To determine the role of NMDARs en route to the cortex, we generated sensory thalamus-specific NR1 (Grin1)-null mice (ThNR1KO). A multipronged approach, using histology, electrophysiology, optical imaging, and behavioral testing revealed that, in these mice, whisker patterns develop in the trigeminal brainstem but do not develop in the somatosensory thalamus. Subsequently, there is no barrel formation in the neocortex yet a partial afferent patterning develops. Whisker stimulation evokes weak cortical activity and presynaptic neurotransmitter release probability is also affected. We found several behavioral deficits in tasks, ranging from sensorimotor to social and cognitive. Collectively, these results show that thalamic NMDARs play a critical role in the patterning of the somatosensory thalamic and cortical maps and their impairment may lead to pronounced behavioral defects.

Arakawa H ，Suzuki A ，Zhao S ，Tsytsarev V ，Lo FS ，Hayashi Y ，Itohara S ，Iwasato T ，Erzurumlu RS ... -  《-》

Optogenetic stimulation of cortex to map evoked whisker movements in awake head-restrained mice.

Whisker movements are used by rodents to touch objects in order to extract spatial and textural tactile information about their immediate surroundings. To understand the mechanisms of such active sensorimotor processing it is important to investigate whisker motor control. The activity of neurons in the neocortex affects whisker movements, but many aspects of the organization of cortical whisker motor control remain unknown. Here, we filmed whisker movements evoked by sequential optogenetic stimulation of different locations across the left dorsal sensorimotor cortex of awake head-restrained mice. Whisker movements were evoked by optogenetic stimulation of many regions in the dorsal sensorimotor cortex. Optogenetic stimulation of whisker sensory barrel cortex evoked retraction of the contralateral whisker after a short latency, and a delayed rhythmic protraction of the ipsilateral whisker. Optogenetic stimulation of frontal cortex evoked rhythmic bilateral whisker protraction with a longer latency compared to stimulation of sensory cortex. Compared to frontal cortex stimulation, larger amplitude bilateral rhythmic whisking in a less protracted position was evoked at a similar latency by stimulating a cortical region posterior to Bregma and close to the midline. These data suggest that whisker motor control might be broadly distributed across the dorsal mouse sensorimotor cortex. Future experiments must investigate the complex neuronal circuits connecting specific cell-types in various cortical regions with the whisker motor neurons located in the facial nucleus.

Auffret M ，Ravano VL ，Rossi GMC ，Hankov N ，Petersen MFA ，Petersen CCH ... -  《-》