Behavioral Consequences of a Bifacial Map in the Mouse Somatosensory Cortex

Abstract

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 (Robo3^R3-5cKO) 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.

Keywords: Robo3; barrel cortex; barreloids; midline crossing defects; optical imaging; whisker-sensitive behaviors.

Figure 1.

Patterning of the whisker-barrel pathway in Robo3^R3–5cKO mice. Glutamate transporter immunohistochemistry reveals afferent terminal patterns that correspond to barrelettes in the PrV, barreloids in the VPM nucleus, and barrels in the cortex (BCx). Left column photomicrographs represent normal patterning in control animals at successive levels of the trigeminal neuraxis. Right column photomicrographs represent photomicrographs in Robo3^R3–5cKO mice at successive levels of the trigeminal neuraxis. Although barrelette patterns in the principal sensory nucleus are normal, there is aberrant patterning in the VPM and in the barrel cortex (BCx). In the immunostained mutant barrel cortex, asterisks indicate the boundary of the ipsilateral map. Whisker barrel rows are indicated in the control cortex. CO-stained sections shown in the bottom micrographs further illustrate the smaller size of the barrels in the Robo3^R3–5cKO cortex. C2 whisker is indicated and the approximate locations of the ipsilateral (i) and contralateral (c) C2 whisker barrels in the Robo3^R3–5cKO cortex section are marked. Bottom diagrams illustrate the ipsilateral and contralateral whisker representations in the barrel cortex of normal (left) and mutant (right) mice.

Figure 2.

VSD imaging in barrel cortex after C2 whisker stimulation. A, Pseudocolor map of the responses in the barrel cortex to contralateral C2 stimulation. B, Pseudocolor map of the ipsilateral (blue) and contralateral (red) responses in the left hemisphere in a Robo3^R3–5cKO mouse. The time after stimulus onset is at the top left corner of each plate. C, D, The time course of the integrated fluorescence signals (blue represents ipsilateral; red represents contralateral) calculated in the squares (7 × 7 pixels) in insets at 40 ms image in control (C) and Robo3^R3–5cKO case (D). To visualize the color-coded squares, pseudocolor-coded responses are filtered, the unfiltered images are seen in 40 ms response images in A and B.

Figure 3.

Object and texture recognition and whisking laterality tests. In object recognition test, two identical objects (an Easter egg) were presented in opposite corners of the cage and mice were allowed to freely investigate them during Trial 1 (T1). In Trial 2 (T2), one of the objects was removed and replaced with a novel object (small flower pot). In Trial 3 (T3), one object from the first trial (Easter egg) and yet another novel object (toothbrush) were presented. Normally, mice spend more time sniffing and whisking at the new object. In this experiment, mice were allowed to use visual cues. The texture discrimination test was conducted the same way but under infrared lighting, and the object shapes were the same but they all had different textures. Significant differences between the control and Robo3^R3–5cKO mice were found on T1 and T2 only in the texture discrimination test. In the whisking laterality test, there was no significant difference in rod detection between the control and cKO mice (t₍₇₎ = 0.09827, p = 0.9245). When mice oriented their snout and left or right set of whiskers to the rod, significant differences in laterality were observed. “Ipsilateral” indicates that the mouse whisked the same side of whiskers at a rod located on the same side (e.g., left side rod, left whiskers). “Medial” indicates that the mouse oriented its snout to the rod. “Contralateral” indicates that the mouse oriented the opposite side of whiskers to a rod located at one side (e.g., left side rod, right whiskers). Control mice tended to use the medial approach or ipsilateral whiskers to investigate a rod, whereas Robo3^R3–5cKO mice tended to use the contralateral set of whiskers to explore the rod (lateral: t₍₁₄₎ = 0.701158, p = 0.4947; medial: t₍₁₄₎ = 9.0911, p < 0.0001; and contralateral: t₍₇₎ = 24.677, *p < 0.0001). Data are mean ± SEM.