An index of topographic normality in rat somatosensory cortex: application to a sciatic nerve crush model

The course of recovery of locomotor function over a 10-week observation period in a rat model of femoral nerve resection and autograft repair

BACKGROUND

A great extent of knowledge on peripheral nerve regeneration has been gathered using the rat sciatic nerve model. The femoral nerve model of the rat offers an interesting alternative, as it lacks disadvantageous features such as automutilation. For the analysis of locomotor behavior in rats after sciatic nerve injury, the CatWalk^™ XT Gait Analysis System is often used. However, lesions of the femoral nerve in the rat have yet remained unstudied with this method.

MATERIAL AND METHODS

Ten male Sprague Dawley rats were evaluated with the CatWalk XT to study their gait after a 6-mm resection of the right femoral nerve and reconstruction with an autologous nerve graft. Animals were observed for 10 weeks after surgery.

RESULTS

Print Area, Print Length, Swing Speed, and Duty Cycle decreased to a minimum of 40% of baseline 2 weeks after surgery. Swing Time was elevated more than twofold at this time point. However, all these parameters recovered back to >90% of baseline values at 10 weeks after surgery. This degree of functional recovery has not been reported after sciatic nerve resection and autograft repair. Base of support varied minimally postoperatively in contrast to a strong decrement after sciatic nerve resection and repair.

CONCLUSION

We hereby provide a comprehensive in-depth analysis of how to study functional recovery after injury of the femoral nerve in the rat via the CatWalk XT. We place special emphasis on highlighting the differences between the femoral nerve and sciatic nerve injury model in this context.

Quantitative assessment of cortical somatosensory digit representations after median and ulnar nerve injury in rats

Incomplete recovery of sensory function is common after peripheral nerve injury (PNI). Despite reinnervation following injury, disorganized cortical representations persist and may contribute to functional deficits. There is a dearth of literature characterizing cortical responses after PNI in rodent models. Here we develop a quantitative electrophysiological method for mapping forepaw digit responses in primary somatosensory cortex (S1) of rats. We tested the hypothesis that PNI in the forelimb would generate significant, long lasting sensory deficits, and corresponding disorganization in S1. Rats underwent a transection of the proximal segment of the median and ulnar nerves in the forelimb followed by tubular repair. 4-12 months after nerve injury, we tested mechanosensory withdrawal thresholds and mapped S1 responses to mechanical stimulation of the digits. PNI produces persistent elevation of mechanical withdrawal thresholds, consistent with an impairment in sensory function. Assessment of cortical neurophysiology reveals a substantial disorganization of S1 somatotopy. Additionally, we document degraded timing and digit specificity of cortical responses. This quantitative measurement of long-term changes in S1 digit representations after forelimb nerve injury in rodents provides a framework for further studies focused on the development of therapeutic strategies to restore cortical and sensory function.

Functional evaluation of peripheral nerve regeneration and target reinnervation in animal models: a critical overview

Peripheral nerve injuries usually lead to severe loss of motor, sensory and autonomic functions in the patients. Due to the complex requirements for adequate axonal regeneration, functional recovery is often poorly achieved. Experimental models are useful to investigate the mechanisms related to axonal regeneration and tissue reinnervation, and to test new therapeutic strategies to improve functional recovery. Therefore, objective and reliable evaluation methods should be applied for the assessment of regeneration and function restitution after nerve injury in animal models. This review gives an overview of the most useful methods to assess nerve regeneration, target reinnervation and recovery of complex sensory and motor functions, their values and limitations. The selection of methods has to be adequate to the main objective of the research study, either enhancement of axonal regeneration, improving regeneration and reinnervation of target organs by different types of nerve fibres, or increasing recovery of complex sensory and motor functions. It is generally recommended to use more than one functional method for each purpose, and also to perform morphological studies of the injured nerve and the reinnervated targets.

AMPA and GABA(A/B) receptor subunit expression in the cortex of adult squirrel monkeys during peripheral nerve regeneration

The primate somatosensory neuroaxis provides a highly translational model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for AMPA and GABAA/B receptor subunits in the area 3b cortex of adult squirrel monkeys one and five months after median nerve compression. This method of nerve injury was selected because it allows unique insight into how receptor expression changes during the regeneration of the peripheral nerve. One month after nerve compression, the pattern of subunit staining provides evidence that the cortex enters a state of reorganization. GABA α1 receptor subunits are significantly down-regulated in layer IV, V, and VI. Glur2/3 AMPA receptor subunits and postsynaptic GABABR1b receptor subunits are up and down regulated respectively across all layers of cortex. After five months of recovery from nerve compression, the pattern of AMPA and GABAA/B receptor subunits remain significantly altered in a layer specific manner. In layer II/III, GluR1, GluR2/3, and GABA α1 subunit expression is significantly up-regulated while post synaptic GABABR1b receptor subunits are significantly down regulated. In layer VI, V, and VI the GluR2/3 and presynaptic GABABR1a receptor subunits are significantly up-regulated, while the postsynaptic GABABR1b receptor subunits remain significantly down-regulated. Taken together, these results suggest that following nerve injury the cortex enters a state of reorganization that has persistent effects on cortical plasticity even after partial or total reinnervation of the peripheral nerve.

Nerve injury-induced changes in GABA(A) and GABA(B) sub-unit expression in area 3b and cuneate nucleus of adult squirrel monkeys: further evidence of developmental recapitulation

The primate somatosensory system provides an excellent model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for the GABA(A) α1, GABA(B)R1a, and GABA(B)R1b receptor subunits in somatosensory area 3b, and cuneate nucleus one week after median nerve compression in adult squirrel monkeys. We find a significant decrease in GABA(A) α1 subunit staining across all cortical layers and within both soma and neuropil of the deprived cortical and brainstem regions. The GABA(B) staining showed an opposing shift in deprived regions, with a significant increase in presynaptic GABA(B)R1a staining, and a significant decrease in postsynaptic GABA(B)R1b staining in deprived regions of the cortex and brainstem. These changes in receptor subunit expression generate patterns that are very similar to those reported in the neonate. Furthermore, the similarities between brainstem and cortical expression suggest conserved forms of adult plasticity in these two regions. Taken together these results, along with the results from our previous paper investigating AMPA subunit expression in these same animals, support the hypothesis that deprived neurons enter a previously hidden state of developmental recapitulation that serves to prime the brain for NMDA receptor mediated receptive field reorganization.

Neurophysiological and anatomical plasticity in the adult sensorimotor cortex

Evidence supporting the plastic capacity of the adult cortex is abundant. Changes have been associated with exposure to enriched environments, learning, peripheral lesions and central lesions. The initial loss of function caused by a lesion is generally followed by a certain amount of recovery that is believed to be due, at least in part, to adaptive plasticity. In particular, the reorganization of cortical representational maps has been associated with improvement of performance. Therefore, areas undergoing such reorganization following lesions are generally assumed to participate in the recovery. We review evidence demonstrating the remodeling of representational maps of the forelimb in adult cortex and the structural plasticity that has been coupled with it. A particular emphasis is paid to non-human primate studies and stroke.

Extensive cortical rewiring after brain injury

Previously, we showed that the ventral premotor cortex (PMv) underwent neurophysiological remodeling after injury to the primary motor cortex (M1). In the present study, we examined cortical connections of PMv after such lesions. The neuroanatomical tract tracer biotinylated dextran amine was injected into the PMv hand area at least 5 months after ischemic injury to the M1 hand area. Comparison of labeling patterns between experimental and control animals demonstrated extensive proliferation of novel PMv terminal fields and the appearance of retrogradely labeled cell bodies within area 1/2 of the primary somatosensory cortex after M1 injury. Furthermore, evidence was found for alterations in the trajectory of PMv intracortical axons near the site of the lesion. The results suggest that M1 injury results in axonal sprouting near the ischemic injury and the establishment of novel connections within a distant target. These results support the hypothesis that, after a cortical injury, such as occurs after stroke, cortical areas distant from the injury undergo major neuroanatomical reorganization. Our results reveal an extraordinary anatomical rewiring capacity in the adult CNS after injury that may potentially play a role in recovery.

Limits on recovery in the corticospinal tract of the rat: partial lesions impair skilled reaching and the topographic representation of the forelimb in motor cortex

Although evidence suggests that there are impairments in skilled movements following very large lesions of the pyramidal component of the corticospinal tract, the behavioral and electrophysiological effects of partial lesion has not received equal attention. Here, rats with complete lesions or partial lesions (medial, central, or lateral third) of the pyramidal tract at the medullary pyramids were evaluated for their quantitative and qualitative postsurgical performance on a skilled reaching task, following which the topographic representation of their forelimb was mapped with intracortical microstimulation (ICMS). Complete lesions impaired reaching success, impaired the qualitative features of reaching movements, and abolished ICMS evoked movement from the forelimb region of motor cortex. Although partial lesions did not impair reaching success, they did impair qualitative aspects of limb movement including forepaw aiming, supination, and food pellet release. ICMS indicated a reduction in the size of the forelimb area, especially the distal area of the caudal forelimb area (CFA), of the motor map. The behavioral and electrophysiological impairments did not vary with lesion location within the pyramidal tract. The incomplete recovery, as measured both behaviorally and electrophysiologically, demonstrates that plasticity within the corticospinal system is limited even with lesions that permit substantial sparing of pyramidal tract fibers.

Functional and Morphological Assessment of a Standardized Rat Sciatic Nerve Crush Injury with a Non-Serrated Clamp

Peripheral nerve researchers frequently use the rat sciatic nerve crush as a model for axonotmesis. Unfortunately, studies from various research groups report results from different crush techniques and by using a variety of evaluation tools, making comparisons between studies difficult. The purpose of this investigation was to determine the sequence of functional and morphologic changes after an acute sciatic nerve crush injury with a non-serrated clamp, giving a final standardized pressure of p = 9 MPa. Functional recovery was evaluated using the sciatic functional index (SFI), the extensor postural thrust (EPT) and the withdrawal reflex latency (WRL), before injury, and then at weekly intervals until week 8 postoperatively. The rats were also evaluated preoperatively and at weeks 2, 4, and 8 by ankle kinematics, toe out angle (TOA), and gait-stance duration. In addition, the motor nerve conduction velocity (MNCV) and the gastrocnemius-soleus weight parameters were measured just before euthanasia. Finally, structural, ultrastructural and histomorphometric analyses were carried out on regenerated nerve fibers. At 8 weeks after the crush injury, a full functional recovery was predicted by SFI, EPT, TOA, and gait-stance duration, while all the other parameters were still recovering their original values. On the other hand, only two of the histomorphometric parameters of regenerated nerve fibers, namely myelin thickness/axon diameter ratio and fiber/axon diameter ratio, returned to normal values while all other parameters were significantly different from normal values. The employment of traditional methods of functional evaluation in conjunction with the modern techniques of computerized analysis of gait and histomorphometric analysis should thus be recommended for an overall assessment of recovery in the rat sciatic nerve crush model.

Training on a visual task improves the outcome of optic nerve regeneration

Optic nerve regeneration in a lizard, Ctenophorus ornatus, is dysfunctional despite survival of most retinal ganglion cells and axon regeneration to the optic tectum. The regenerated retino-tectal projection at 6 months has crude topography but by 1 year is disordered; visually-elicited behavior is absent via the experimental eye. Here, we assess the influence of training on the outcome of optic nerve regeneration. Lizards were trained to catch prey presented within the monocular field of either eye. One optic nerve was then severed and visual stimulation resumed throughout regeneration. In the trained group, presentation was restricted to the eye undergoing optic nerve regeneration; for the untrained group, the unoperated eye was stimulated. Pupil responses returned in trained but not in untrained animals. At 1 year, trained animals oriented to and captured prey; untrained animals demonstrated minimal orienting and failed to capture prey. Regenerated retino-tectal projections were topographic in the trained but not in the untrained group as assessed by in vitro electrophysiological recording and by carbocyanine dye tracing. In vitro electrophysiological recording during application of neurotransmitter antagonists to the tectum revealed that the level of GABAergic inhibition was modest in trained animals but elevated in the untrained group; responses were mainly AMPA-mediated in both groups. We conclude that training improves the behavioral outcome of regeneration, presumably by stabilizing and refining the transient retino-tectal map and preventing a build-up of tectal inhibition. The results suggest that for successful central nerve regeneration to occur in mammals, it may be necessary to introduce training to complement procedures stimulating axon regeneration.