Ipsilateral Whisker Research Articles

Induction of Fos protein-like immunoreactivity in the trigeminal spinal nucleus caudalis and upper cervical cord following noxious and non-noxious mechanical stimulation of the whisker pad of the rat with an inferior alveolar nerve transection

After transection of the inferior alveolar nerve (IAN: the third branch of the trigeminal nerve), the whisker pad area, which is innervated by the second branch of the trigeminal nerve, showed hypersensitivity to mechanical stimulation. Two days after IAN transection, the threshold intensity for escape behavior to mechanical stimulation of the ipsilateral whisker pad area was less than 1.0 g, a sign of allodynia, and returned to the preoperative level (preoperative threshold: 52.0 g) at 32 days after surgery. This decrement of escape threshold lasted for more than 3 weeks. The whisker pad area contralateral to the IAN transection also showed a decrease in escape threshold to non-noxious mechanical stimulation as compared with sham-operated rats. However, the change in threshold intensity for the side contralateral to transection was not as pronounced as that on the ipsilateral side. Fos protein-like immunoreactive (LI) cells were observed in the superficial laminae but not dominant in deeper laminae of the trigeminal spinal nucleus caudalis (Vc) and the first segment of the spinal cord (C1) after non-noxious mechanical stimulation of the whisker pad area in the rats with IAN transection. Fos protein-LI cells were expressed bilaterally in the Vc and C1, but were more numerous on the ipsilateral side to transection than on the contralateral side. The largest number of Fos protein-LI cells was observed at 2400 μm caudal from the trigeminal subnucleus interporalis (Vi)–Vc border both in ipsilateral and contralateral sides. The number of Fos protein-LI cells increased after application of 1, 4, and 16 g stimuli as compared to rats without mechanical stimulation. Furthermore, an extensively greater number of Fos protein-LI cells were expressed both in superficial and deep laminae of the bilateral Vc and C1 of the spinal cord after subcutaneous injection of mustard oil into the whisker pad. Fos protein expression after mustard oil injection was much stronger than that observed after any mechanical stimulation in the rats with IAN transection. These data suggest that the change in the numbers and spatial arrangement of nociceptive neurons in the Vc and C1 after IAN transection reflect the development of mechanical hyperalgesia in the area adjacent to the IAN innervated region.

Bilateral Integration of Whisker Information in the Primary Somatosensory Cortex of Rats

The isomorphic representation of the contralateral whisker pad in the rodent cerebral cortex has served as a canonical example in primary somatosensory areas that the contralateral body surface is spatially represented as a topographic map. By characterizing responses evoked by multiwhisker stimuli, we provide direct evidence that the whisker region of the rat primary somatosensory cortex (SI) integrates information from both contralateral and ipsilateral whisker pads. The proportions of SI neurons responsive to ipsilateral whisker stimuli, as well as their response probabilities, increased with the number of ipsilateral whiskers stimulated. Under bilateral whisker stimulation, the responses of 95% of neurons recorded were affected by stimulation of ipsilateral whiskers. Contralateral tactile responses of SI neurons were profoundly influenced by preceding ipsilateral stimuli and vice versa. This effect depended on both the spatial location and the relative timing of bilateral whisker stimuli, leading to both spatial and temporal asymmetries of interaction. Bilateral whisker stimulation resulted in only modest changes in evoked response latency. Previous ipsilateral stimulation was also shown to affect tactile responses evoked by later ipsilateral stimuli. Inactivation of the opposite SI abolished ipsilaterally evoked responses as well as their influence on subsequently evoked contralateral responses in the intact SI. Based on these results, we conclude that the rat SI integrates information from both whisker pads and propose that such interactions may underlie the ability of rats to discriminate bilateral tactile stimuli.

Neonatal whisker removal reduces the discrimination of tactile stimuli by thalamic ensembles in adult rats.

Simultaneous recordings of up to 48 single neurons per animal were used to characterize the long-term functional effects of sensory plastic modifications in the ventral posterior medial nucleus (VPM) of the thalamus following unilateral removal of facial whiskers in newborn rats. One year after this neonatal whisker deprivation, neurons in the contralateral VPM responded to cutaneous stimulation of the face at much longer minimal latencies (15.2 +/- 8.2 ms, mean +/- SD) than did normal cells (8.8 +/- 5.3 ms) in the same subregion of the VPM. In 69% of these neurons, the initial sensory responses to stimulus offset were followed for up to 700 ms by reverberant trains of bursting discharge, alternating in 100-ms cycles with inhibition. Receptive fields in the deafferented VPM were also atypical in that they extended over the entire face, shoulder, forepaw, hindpaw, and even ipsilateral whiskers. Discriminant analysis (DA) was then used to statistically evaluate how this abnormal receptive field organization might affect the ability of thalamocortical neuronal populations to "discriminate" somatosensory stimulus location. To standardize this analysis, three stimulus targets ("groups") were chosen in all animals such that they triangulated the central region of the "receptive field" of the recorded multineuronal ensemble. In the normal animals these stimulus targets were whiskers or perioral hairs; in the deprived animals the targets typically included hairy skin of the body as well as face. The measured variables consisted of each neuron's spiking response to each stimulus differentiated into three poststimulus response epochs (0-15, 15-30, and 30-45 ms). DA quantified the statistical contribution of each of these variables to its overall discrimination between the three stimulus sites. In the normal animals, the stimulus locations were correctly classified in 88.2 +/- 3.7% of trials on the basis of the spatiotemporal patterns of ensemble activity derived from up to 18 single neurons. In the deprived animals, the stimulus locations were much less consistently discriminated (reduced to 73.5 +/- 12.6%; difference from controls significant at P < 0.01) despite the fact that much more widely spaced stimulus targets were used and even when up to 20 neurons were included in the ensemble. Overall, these results suggest that neonatal damage to peripheral sense organs may produce marked changes in the physiology of individual neurons in the somatosensory thalamus. Moreover, the present demonstration that these changes can profoundly alter sensory discrimination at the level of neural populations in the thalamus provides important evidence that the well-known perceptual effects of chronic peripheral deprivation may be partially attributable to plastic reorganization at subcortical levels.

Altered expression of Growth Inhibitory Factor (GIF / MT-III) mRNA in the rat facial nucleus after facial nerve injury is closely related with facial function

Growth inhibitory factor / metallothionein III (GIF / MT-III) is reported to have the unique property of suppressing neuronal survival and neurite promotion in vitro. We investigated changes in the expression of GIF mRNA within the facial nucleus using in situ hybridization as well as changes in the function of the facial nerve after nerve injury. Following crushing injury just distal to the stylomastoid foramen, movement of the ipsilateral whiskers was eliminated but returned by the 7th day. GIF mRNA expression decreased at 3 days after injury and returned in 7 days. However, when the nerve was cut and sutured immediately, it took one month for the facial function to recover. In this case, GIF mRNA expression decreased 3 days after injury, remained at a low level for 14 days, and finally returned in 3-4 weeks. Thus, changes in the expression of GIF mRNA were found to be closely related to the facial nerve function.

Reorganization of trigeminal primary afferents following neonatal infraorbital nerve section in hamster

The infraorbital nerve was sectioned and the ipsilateral whisker follicles were cauterized in hamsters within 12 h of birth. Sixty to ninety days later application of HRP to the proximal stumps of the ipsilateral lingual, inferior alveolar, mylohyoid and auriculotemporal nerves resulted in increased numbers of labeled somata in trigeminal ganglion regions which contain primarily infraorbital cell bodies in normal animals. The labeled central processes of mandibular nerves also occupied portions of the brainstem trigeminal complex normally innervated by infraorbital axons. These findings represent the first anatomical demonstration of trigeminal primary afferent plasticity.

Responses in the rat thalamus to whisker movements produced by motor nerve stimulation.

1. The effect of electrical stimulation of the motor nerve supplying the whiskers on the activity of single cells in the vibrissal region of the ventrobasal complex of the thalamus has been studied in rats under urethane anaesthesia.2. The stimulation caused protraction of the ipsilateral whiskers. 60% of the cells which fired to mechanical movements of the whiskers were found to respond to this electrical stimulus with 1-2 impulses at short latency (average 7.7 msec), provided the stimulus was sufficient to move the whiskers.3. When the moving whiskers hit a barrier, 92% of the cells responded to the stimulus. The most effective position of the barrier was in front of the whiskers, although other positions often produced a response as well. Static displacement of the whiskers, particularly in the forward direction, could abolish the response or increase its latency.4. The following-frequencies for these cells were 5-10 stimuli/sec. Combinations of electrical stimuli with mechanical ramp movements of the whiskers showed that similar recovery times followed both types of stimuli.5. These results are compared with those reported from studies in the afferent nerve fibres after electrical stimulation of the motor nerve and also with responses in the thalamus following mechanical movements of the whiskers. The possible importance of the latency of these sensory responses is considered.

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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 (<i>Robo3^R3–5cKO</i>) 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. <b>SIGNIFICANCE STATEMENT</b> 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.