Chronic Adaptations in the Dorsal Horn Following a Cervical Spinal Cord Injury in Primates.

Spinal cord injury (SCI) is devastating, with limited treatment options and variable outcomes. Most in vivo SCI research has focused on the acute and early post-injury periods, and the promotion of axonal growth, so little is understood about the clinically stable chronic state, axonal growth over time, and what plasticity endures. Here, we followed animals into the chronic phase following SCI, to address this gap. Male macaques received targeted deafferentation, affecting three digits of one hand, and were divided into short (4-6 months) or long-term (11-12 months) groups, based on post-injury survival times. Monkeys were assessed behaviorally, where possible, and all exhibited an initial post-injury deficit in manual dexterity, with gradual functional recovery over 2 months. We previously reported extensive sprouting of somatosensory corticospinal (S1 CST) fibers in the dorsal horn in the first five post-injury months. Here, we show that by 1 year, the S1 CST sprouting is pruned, with the terminal territory resembling control animals. This was reflected in the number of putatively "functional" synapses observed, which increased over the first 4-5 months, and then returned to baseline by 1 year. Microglia density also increased in the affected dorsal horn at 4-6 months and then decreased, but did not return to baseline by 1 year, suggesting refinement continues beyond this time. Overall, there is a long period of reorganization and consolidation of adaptive circuitry in the dorsal horn, extending well beyond the initial behavioral recovery. This provides a potential window to target therapeutic opportunities during the chronic phase.

Small sensory spinal lesions that affect hand function in monkeys greatly alter primary afferent and motor neuron connections in the cord.

Small sensory spinal injuries induce plasticity across the neuraxis, but little is understood about their effect on segmental connections or motor neuron (MN) function. Here, we begin to address this at two levels. First, we compared afferent input distributions from the skin and muscles of the digits with corresponding MN pools to determine their spatial relationship, in both the normal state and 4-6 months after a unilateral dorsal root/dorsal column lesion (DRL/DCL), affecting digits 1-3. Second, we looked at specific changes to MN inputs and membrane properties that likely impact functional recovery. Monkeys received a targeted unilateral DRL/DCL, and 4-6 months later, cholera toxin subunit B (CT-B) was injected bilaterally into either the distal pads of digits 1-3, or related intrinsic hand muscles, to label inputs to the cord, and corresponding MNs. In controls (unlesioned side), cutaneous and proprioceptive afferents from digits 1-3 showed different distribution patterns but similar rostrocaudal spread within the dorsal horn from C1 to T2. In contrast, MNs were distributed across just two segments (C7-8). Following the lesion, sensory inputs were significantly diminished across all 10 segments, though this did not alter MN distributions. Afferent and monoamine inputs, as well as KCC2 cotransporters, were also significantly altered on the cell membrane of CT-B labeled MNs postlesion. In contrast, inhibitory neurotransmission and perineuronal net integrity were not altered at this prechronic timepoint.  Our findings indicate that even a small sensory injury can significantly impact sensory and motor spinal neurons and provide new insight into the complex process of recovery.

Semi-quantitative Assessment of Alzheimer's-like Pathology in Two Aged Polar Bears (Ursus maritimus).

Age-associated neurodegenerative changes, including amyloid ß (Aß) plaques, neurofibrillary tangles (NFTs), and amyloid angiopathy comparable to those seen in the brains of human patients with Alzheimer's disease (AD), have been reported in the brains of aged bears. However, the significance of these findings in bears is unclear due to the difficulty in assessing cognitive impairment and the lack of standardized approaches for the semiquantitative evaluation of Aß plaques and NFTs. In this study, we evaluate the neuropathologic changes in archival brain tissue of 2 aged polar bears (Ursus maritimus, ages 28 and 37) using the National Institute of Aging-Alzheimer Association (NIA-AA) consensus guidelines for the neuropathologic assessment of Alzheimer's Disease (AD). Both bears had an Aß (A) score of 3 of 3, Braak stage (B score) of 2 of 3, and neuritic plaque (C) score of 3 of 3. These findings are consistent with the neurodegenerative changes observed in brains of patients with AD. The application of NIA-AA consensus guidelines, as applied to the neuropathologic assessment of the aged bears in this report, demonstrates the use of standardized semiquantitative assessment systems for comparative, translational studies of aging in a vulnerable wildlife species.

Behavioral recovery after a spinal deafferentation injury in monkeys does not correlate with extent of corticospinal sprouting.

A long held view in the spinal cord injury field is that corticospinal terminal sprouting is needed for new connections to form, that then mediate behavioral recovery. This makes sense, but tells us little about the relationship between corticospinal sprouting extent and recovery potential. The inference has been that more extensive axonal sprouting predicts greater recovery, though there is little evidence to support this. Here we addressed this by comparing behavioral data from monkeys that had received one of two established deafferentation spinal injury models in monkeys (Darian-Smith et al., 2014, Fisher et al., 2019, 2020). Both injuries cut similar afferent pools supplying the thumb, index and middle fingers of one hand but each resulted in a very different corticospinal tract (CST) sprouting response. Following a cervical dorsal root lesion, the somatosensory CST retracted significantly, while the motor CST stayed largely intact. In contrast, when a dorsal column lesion was combined with the DRL, somatosensory and motor CSTs sprouted dramatically within the cervical cord. How these two responses relate to the behavioral outcome was not clear. Here we analyzed the behavioral outcome for the two lesions, and provide a clear example that sprouting extent does not track with behavioral recovery.

Reorganization of the Primate Dorsal Horn in Response to a Deafferentation Lesion Affecting Hand Function.

The loss of sensory input following a spinal deafferentation injury can be debilitating, and this is especially true in primates when the hand is involved. Although significant recovery of function occurs, little is currently understood about the reorganization of the neuronal circuitry, particularly within the dorsal horn. This region receives primary afferent input from the periphery, and cortical input via the somatosensory subcomponent of the corticospinal tract (S1 CST), and is critically important in modulating sensory transmission, both in normal and lesioned states. To determine how dorsal horn circuitry alters to facilitate recovery post-injury, we used an established deafferentation lesion model (dorsal root/dorsal column) in male monkeys to remove sensory input from just the opposing digits (digits 1-3) of one hand. This results in a deficit in fine dexterity that recovers over several months. Electrophysiological mapping, tract tracing, and immunolabeling techniques were combined to delineate specific changes to dorsal horn input circuitry. Our main findings show that (1) there is complementary sprouting of the primary afferent and S1 CST populations into an overlapping region of the reorganizing dorsal horn; (2) S1 CST and primary afferent inputs connect in different ways within this region to facilitate sensory integration; and (3) there is a loss of larger S1 CST terminal boutons in the affected dorsal horn, but no change in the size profile of the spared/sprouted primary afferent terminal boutons post-lesion. Understanding such changes helps to inform new and targeted therapies that best promote recovery.SIGNIFICANCE STATEMENT Spinal injuries that remove sensation from the hand, can be debilitating, though functional recovery does occur. We examined changes to the neuronal circuitry of the dorsal horn in monkeys following a lesion that deafferented three digits of one hand. Little is understood about dorsal horn circuitry, despite the fact that this region loses most of its normal input after such an injury, and is clearly a major focus of reorganization. We found that both the spared primary afferents and somatosensory corticospinal efferents sprouted in an overlapping region of the dorsal horn after injury, and that larger (presumably faster) corticospinal terminals are lost, suggesting a significantly altered cortical modulation of primary afferents. Understanding this changing circuitry is important for designing targeted therapies.

Somatosensory corticospinal tract axons sprout within the cervical cord following a dorsal root/dorsal column spinal injury in the rat.

The corticospinal tract (CST) is the major descending pathway controlling voluntary hand function in primates, and though less dominant, it mediates voluntary paw movements in rats. As with primates, the CST in rats originates from multiple (albeit fewer) cortical sites, and functionally different motor and somatosensory subcomponents terminate in different regions of the spinal gray matter. We recently reported in monkeys that following a combined cervical dorsal root/dorsal column lesion (DRL/DCL), both motor and S1 CSTs sprout well beyond their normal terminal range. The S1 CST sprouting response is particularly dramatic, indicating an important, if poorly understood, somatosensory role in the recovery process. As rats are used extensively to model spinal cord injury, we asked if the S1 CST response is conserved in rodents. Rats were divided into sham controls, and two groups surviving post-lesion for ~6 and 10 weeks. A DRL/DCL was made to partially deafferent one paw. Behavioral testing showed a post-lesion deficit and recovery over several weeks. Three weeks prior to ending the experiment, S1 cortex was mapped electrophysiologically, for tracer injection placement to determine S1 CST termination patterns within the cord. Synaptogenesis was also assessed for labeled S1 CST terminals within the dorsal horn. Our findings show that the affected S1 CST sprouts well beyond its normal range in response to a DRL/DCL, much as it does in macaque monkeys. This, along with evidence for increased synaptogenesis post-lesion, indicates that CST terminal sprouting following a central sensory lesion, is a robust and conserved response.

Spinal cord injury transiently alters Meissner's corpuscle density in the digit pads of macaque monkeys.

Meissner's corpuscles (MCs) are cutaneous mechanoreceptors found in glabrous skin and are exquisitely sensitive to light touch. Along with other receptors, they provide continuous sensory feedback that informs the execution of fine manual behaviors. Following cervical spinal deafferentation injuries, hand use can be initially severely impaired, but substantial recovery occurs over many weeks, even when ~95% of the original input is permanently lost. While most SCI research focuses on central neural pathway responses, little is known about the role of peripheral receptors in facilitating recovery. We begin to address this by asking the following: (1) What is the normal pattern of MCs in the distal pads of all five digits in the macaque monkey (with hands similar to humans)? (2) What happens to these receptors 4-5 months following either a dorsal column lesion (DCL) or a combined dorsal root/dorsal column lesion (DRL/DCL), when functional recovery is largely complete? (3) What happens chronically, 12-14 months later? Our findings show that in normal monkeys, MCs are densest in the distal pads of the opposing thumb and index finger, with the greatest concentration on the thumb. This reflects a close functional relationship between receptor density and precision grip. At 4-5 months post-injury, there was a (~30%) loss of MCs on the deafferented digits of the injured hand compared with the contralateral side. However, 12-14 months after a DRL/DCL, receptor densities had returned to normal levels. Our findings indicate a complex peripheral response and highlight the importance of the periphery in shaping central changes.

Extensive somatosensory and motor corticospinal sprouting occurs following a central dorsal column lesion in monkeys.

The corticospinal tract (CST) forms the major descending pathway mediating voluntary hand movements in primates, and originates from ∼nine cortical subdivisions in the macaque. While the terminals of spared motor CST axons are known to sprout locally within the cord in response to spinal injury, little is known about the response of the other CST subcomponents. We previously reported that following a cervical dorsal root lesion (DRL), the primary somatosensory (S1) CST terminal projection retracts to 60% of its original terminal domain, while the primary motor (M1) projection remains robust (Darian-Smith et al., J. Neurosci., 2013). In contrast, when a dorsal column lesion (DCL) is added to the DRL, the S1 CST, in addition to the M1 CST, extends its terminal projections bilaterally and caudally, well beyond normal range (Darian-Smith et al., J. Neurosci., 2014). Are these dramatic responses linked entirely to the inclusion of a CNS injury (i.e., DCL), or do the two components summate or interact? We addressed this directly, by comparing data from monkeys that received a unilateral DCL alone, with those that received either a DRL or a combined DRL/DCL. Approximately 4 months post-lesion, the S1 hand region was mapped electrophysiologically, and anterograde tracers were injected bilaterally into the region deprived of normal input, to assess spinal terminal labeling. Using multifactorial analyses, we show that following a DCL alone (i.e., cuneate fasciculus lesion), the S1 and M1 CSTs also sprout significantly and bilaterally beyond normal range, with a termination pattern suggesting some interaction between the peripheral and central lesions.

Corticospinal sprouting differs according to spinal injury location and cortical origin in macaque monkeys.

The primate corticospinal tract (CST), the major descending pathway mediating voluntary hand movements, comprises nine or more functional subdivisions. The role of subcomponents other than that from primary motor cortex, however, is not well understood. We have previously shown that following a cervical dorsal rhizotomy (Darian-Smith et al., 2013), CST projections originating from primary somatosensory (S1) and motor (M1) cortex responded quite differently to injury. Terminal projections from the S1 (areas 3b/1/2) shrank to <60% of the contralateral side, while M1 CST projections remained robust or expanded (>110%). Here, we asked what happens when a central lesion is added to the equation, to better simulate clinical injury. Monkeys (n = 6) received either a unilateral (1) dorsal root lesion (DRL), (2) or a combined DRL/dorsal column lesion (DRL/DCL), or (3) a DRL/DCL where the DCL was made 4 months following the initial DRL. Electrophysiological recordings were made in S1 4 months postlesion in the first two groups, and 6 weeks after the DCL in the third lesion group, to identify the reorganized region of D1-D3 (thumb, index finger, and middle finger) representation. Anterograde tracers were then injected bilaterally to assess spinal terminal labeling. Remarkably, in all DRL/DCL animals, terminal projections from the S1 and M1 extended bilaterally and caudally well beyond terminal territories in normal animals or following a DRL. These data were highly significant. Extensive sprouting from the S1 CST has not been reported previously, and these data raise important questions about S1 CST involvement in recovery following spinal injury.

Corticospinal sprouting occurs selectively following dorsal rhizotomy in the macaque monkey.

The corticospinal tract in the macaque and human forms the major descending pathway involved in volitional hand movements. Following a unilateral cervical dorsal root lesion, by which sensory input to the first three digits (D1-D3) is removed, monkeys are initially unable to perform a grasp retrieval task requiring sensory feedback. Over several months, however, they recover much of this capability. Past studies in our laboratory have identified a number of changes in the afferent circuitry that occur as function returns, but do changes to the efferent pathways also contribute to compensatory recovery? In this study we examined the role of the corticospinal tract in pathway reorganization following a unilateral cervical dorsal rhizotomy. Several months after animals received a lesion, the corticospinal pathways originating in the primary somatosensory and motor cortex were labeled, and terminal distribution patterns on the two sides of the cervical cord were compared. Tracers were injected only into the region of D1-D3 representation (identified electrophysiologically). We observed a strikingly different terminal labeling pattern post lesion for projections originating in the somatosensory versus motor cortex. The terminal territory from the somatosensory cortex was significantly smaller compared with the contralateral side (area mean = 0.30 vs. 0.55 mm2), indicating retraction or atrophy of terminals. In contrast, the terminal territory from the motor cortex did not shrink, and in three of four animals, aberrant terminal label was observed in the dorsal horn ipsilateral to the lesion, indicating sprouting. These differences suggest that cortical regions play a different role in post-injury recovery

Adult neurogenesis occurs in primate sensorimotor cortex following cervical dorsal rhizotomy.

Adult neurogenesis remains controversial in the cerebral cortex. We have previously shown in monkeys and rats that reactive neurogenesis occurs in the spinal dorsal horn 6-8 weeks after a cervical dorsal rhizotomy. Here, in three monkeys with the same lesion, we asked whether it also occurs coincidentally in the corresponding primary somatosensory and motor cortex, where significant topographic and neuronal reorganization is known to occur. Monkeys (male Macaca fascicularis) were given 5-bromo-2-deoxyuridine (BrdU) injections 2-3 weeks after the rhizotomy, and were perfused 4-6 weeks later. Cells colabeled for BrdU and five different neuronal markers were observed within the primary somatosensory and motor cortex, and their distributions were compared bilaterally. Cells colabeled with BrdU and the astrocytic marker glial fibrillary acidic protein (GFAP) were also quantified for comparison. A significant number of BrdU/NeuN- and BrdU/calbindin-colabeled cells were observed in topographically reorganized cortex. Small numbers of BrdU/GFAP-colabeled cells were also consistently observed bilaterally, but these cells were never colabeled with any of the neuronal markers. Of the cells colabeled with BrdU and a neuronal marker, at least half had an inhibitory phenotype. However, excitatory pyramidal neurons were also identified with classic pyramidal morphology. Cortical neurogenesis was not observed in other cortical regions. It was also not observed in the primary sensorimotor, prefrontal, or posterior parietal cortex in an additional control monkey (male Macaca fascicularis) that had no surgical intervention. Our findings provide evidence for reactive endogenous cortical neurogenesis after a dorsal rhizotomy, which may play a role in functional recovery.

Changes in synaptic populations in the spinal dorsal horn following a dorsal rhizotomy in the monkey.

Studies in monkeys have shown substantial neuronal reorganization and behavioral recovery during the months following a cervical dorsal root lesion (DRL; Darian-Smith [2004] J. Comp. Neurol. 470:134-150; Darian-Smith and Ciferri [2005] J. Comp. Neurol. 491:27-45, [2006] J. Comp. Neurol. 498:552-565). The goal of the present study was to identify ultrastructural synaptic changes post-DRL within the dorsal horn (DH). Two monkeys received a unilateral DRL, as described previously (Darian-Smith and Brown [2000] Nat. Neurosci. 3:476-481), which removed cutaneous and proprioceptive input from the thumb, index finger, and middle finger. Six weeks before terminating the experiment at 4 post-DRL months, hand representation was mapped electrophysiologically within the somatosensory cortex, and anterograde tracers were injected into reactivated cortex to label corticospinal terminals. Sections were collected through the spinal lesion zone. Corticospinal terminals and inhibitory profiles were visualized by using preembedding immunohistochemistry and postembedding gamma-aminobutyric acid (GABA) immunostaining, respectively. Synaptic elements were systematically counted through the superficial DH and included synaptic profiles with round vesicles (R), pleomorphic flattened vesicles (F; presumed inhibitory synapses), similar synapses immunolabeled for GABA (F-GABA), primary afferent synapses (C-type), synapses with dense-cored vesicles (D, mostly primary afferents), and presynaptic dendrites of interneurons (PSD). Synapse types were compared bilaterally via ANOVAs. As expected, we found a significant drop in C-type profiles on the lesioned side ( approximately 16% of contralateral), and R profiles did not differ bilaterally. More surprising was a significant increase in the number of F profiles ( approximately 170% of contralateral) and F-GABA profiles ( approximately 315% of contralateral) on the side of the lesion. Our results demonstrate a striking increase in the inhibitory circuitry within the deafferented DH.

Synaptic plasticity, neurogenesis, and functional recovery after spinal cord injury.

Spinal cord injury research has greatly expanded in recent years, but our understanding of the mechanisms that underlie the functional recovery that can occur over the weeks and months following the initial injury, is far from complete. To grasp the scope of the problem, it is important to begin by defining the sensorimotor pathways that might be involved by a spinal injury. This is done in the rodent and nonhuman primate, which are two of the most commonly used animal models in basic and translational spinal injury research. Many of the better known experimentally induced models are then reviewed in terms of the pathways they involve and the reorganization and recovery that have been shown to follow. The better understood neuronal mechanisms mediating such post-injury plasticity, including dendritic spine growth and axonal sprouting, are then examined.

Adult neurogenesis in primate and rodent spinal cord: comparing a cervical dorsal rhizotomy with a dorsal column transection.

Neurogenesis has not been shown in the primate spinal cord and the conditions for its induction following spinal injury are not known. In the first part of this study, we report neurogenesis in the cervical spinal dorsal horn in adult monkeys 6-8 weeks after receiving a well-defined cervical dorsal rhizotomy (DRL). 5-bromo-2-deoxyuridine (BrdU) was administered 2-4 weeks following the lesion. Cells colabeled with BrdU and five different neuronal markers were observed in the peri-lesion dorsal horn 4-5 weeks after BrdU injection. Those colabeled with BrdU and neuron-specific nuclear protein, and BrdU and glial fibrillary acidic protein were quantified in the dorsal horn peri-lesion region, and the ipsi- and contralateral sides were compared. A significantly greater number of BrdU/neuron-specific nuclear protein- and BrdU/glial fibrillary acidic protein-colabeled cells were found on the lesion side (P<0.01). These findings led us to hypothesize that neurogenesis can occur within the spinal cord following injury, when the injury does not involve direct trauma to the cord and glial scar formation. This was tested in rats. Neurogenesis and astrocytic proliferation were compared between animals receiving a DRL and those receiving a dorsal column lesion. In DRL rats, neurogenesis was observed in the peri-lesion dorsal horn. In dorsal column lesion rats, no neurogenesis was observed but astrocytic activation was intense. The rat data support our hypothesis and findings in the monkey, and show that the response is not primate specific. The possibility that new neurons contribute to recovery following DRL now needs further investigation.

Monkey models of recovery of voluntary hand movement after spinal cord and dorsal root injury.

The hand is unique to the primate and manual dexterity is at its finest in the human (Napier 1980), so it is not surprising that cervical spinal injuries that even partially block sensorimotor innervation of the hand are frequently debilitating (Anderson 2004). Despite the clinical need to understand the neuronal bases of hand function recovery after spinal and/or nerve injuries, relatively few groups have systematically related subtle changes in voluntary hand use following injury to neuronal mechanisms in the monkey. Human and macaque hand anatomy and function are strikingly similar, which makes the macaque the favored nonhuman primate model for the study of postinjury dexterity. In this review of monkey models of cervical spinal injury that have successfully related voluntary hand use to neuronal responses during the early postinjury months, the focus is on the dorsal rhizotomy (or dorsal rootlet lesion) model developed and used in our laboratory over the last several years. The review also describes macaque monkey models of injuries to the more central cervical spine (e.g., hemisection, dorsal column) that illustrate methods to assess postlesion hand function and that relate it to neurophysiological and neuroanatomical changes. Such models are particularly important for understanding what the sensorimotor pathways are capable of, and for assessing the outcome of therapeutic interventions as they are developed.

Synaptic protein dynamics in hibernation.

Neurons in hibernating mammals exhibit a dramatic form of plasticity during torpor, with dendritic arbors retracting as body temperature cools and then regrowing rapidly as body temperature rises. In this study, we used immunohistochemical imaging and Western blotting of several presynaptic and postsynaptic proteins to determine the synaptic changes that accompany torpor and to investigate the mechanisms behind these changes. We show torpor-related alterations in synaptic protein localization that occur rapidly and uniformly across several brain regions in a temperature-dependent manner. Entry into torpor is associated with a 50-65% loss of synapses, as indicated by changes in the extent of colocalization of presynaptic and postsynaptic markers. We also show that the loss of synaptic protein clustering occurring during entry into torpor is not attributable to protein loss. These findings suggest that torpor-related changes in synapses stem from dissociation of proteins from the cytoskeletal active zone and postsynaptic density, creating a reservoir of proteins that can be quickly mobilized for rapid rebuilding of dendritic spines and synapses during the return to euthermia. A mechanism of neural plasticity based on protein dissociation rather than protein breakdown could explain the hibernator's capacity for large, rapid, and repeated microstructural changes, providing a fascinating contrast to neuropathologies that are dominated by protein breakdown and cell death.

Ubiquitous and temperature-dependent neural plasticity in hibernators.

Hibernating mammals are remarkable for surviving near-freezing brain temperatures and near cessation of neural activity for a week or more at a time. This extreme physiological state is associated with dendritic and synaptic changes in hippocampal neurons. Here, we investigate whether these changes are a ubiquitous phenomenon throughout the brain that is driven by temperature. We iontophoretically injected Lucifer yellow into several types of neurons in fixed slices from hibernating ground squirrels. We analyzed neuronal microstructure from animals at several stages of torpor at two different ambient temperatures, and during the summer. We show that neuronal cell bodies, dendrites, and spines from several cell types in hibernating ground squirrels retract on entry into torpor, change little over the course of several days, and then regrow during the 2 h return to euthermia. Similar structural changes take place in neurons from the hippocampus, cortex, and thalamus, suggesting a global phenomenon. Investigation of neural microstructure from groups of animals hibernating at different ambient temperatures revealed that there is a linear relationship between neural retraction and minimum body temperature. Despite significant temperature-dependent differences in extent of retraction during torpor, recovery reaches the same final values of cell body area, dendritic arbor complexity, and spine density. This study demonstrates large-scale and seemingly ubiquitous neural plasticity in the ground squirrel brain during torpor. It also defines a temperature-driven model of dramatic neural plasticity, which provides a unique opportunity to explore mechanisms of large-scale regrowth in adult mammals, and the effects of remodeling on learning and memory.

Cuneate nucleus reorganization following cervical dorsal rhizotomy in the macaque monkey: its role in the recovery of manual dexterity.

Immediately following a dorsal rhizotomy that removes input from the thumb, index, and middle fingers, the macaque is unable to execute movements that require controlled apposition of these digits. We have previously shown that within the early weeks and months following one of these lesions, there is 1) a re-emergence of part or all of the cortical hand map; 2) central axonal sprouting of spared primary afferents into the dorsal horn and cuneate nucleus; and 3) substantial although incomplete recovery of hand function (Darian-Smith [204] J. Comp. Neurol. 470:134-150; Darian-Smith and Ciferri [2005] J. Comp. Neurol. 491:27-45). In this study we asked: What neuronal reorganization occurs in the cuneate nucleus during this "recovery" period? And, does it contribute to the recovery of manual dexterity? To address these questions, the representation of the hand was electrophysiologically mapped (by unitary receptive field [RF] recordings) in the pars rotunda of the cuneate nucleus at either 1-2 weeks (short term) or 16-32 weeks (long term) post-rhizotomy. In short-term monkeys, the region deprived of input from the thumb, index, and middle finger was found to be unresponsive to cutaneous stimulation. However, at 16-32 weeks later, when dexterity had largely recovered, RFs of cuneate neurons could again be mapped within the cuneate nucleus, primarily in a region bordering the deprived zone. We conclude that the cuneate pre- and postsynaptic reorganization that occurs following dorsal rhizotomy plays a key role in the recovery of hand function.

Loss and recovery of voluntary hand movements in the macaque following a cervical dorsal rhizotomy.

The recovery of manual dexterity was analyzed in the macaque following a cervical dorsal root section that abolished cutaneous feedback from selected digits of one hand. Monkeys were trained to retrieve a target object from a clamp using thumb and index finger opposition. Dorsal rootlets containing electrophysiologically identified axons projecting from the thumb and index finger were then cut in two monkeys (Group 1). In four others (Group 2), additional rootlets shown to innervate the middle finger and thenar eminence were also transected. Three performance parameters were analyzed before and following the rhizotomy: 1) percentage of successful retrievals; 2) digital stratagem (the pattern of digit opposition); and 3) contact time (duration of digit contact with the object before its retrieval). During the first postoperative week, hand function was severely impaired in all monkeys. Over the following weeks, Group 1 monkeys recovered the ability to retrieve the object by opposing the index finger and thumb in >80% of trials. Group 2 monkeys also regained some function in the impaired hand: each monkey adopted a stratagem for grasping the target, using digits that were incompletely deafferented. In the terminal experiment, hand representation in the contralateral somatosensory cortex was electrophysiologically mapped to define hand deafferentation and cortical reactivation further. There was a close correspondence between the cortical map and digit use. Our data imply that the recovery of precision grip using the thumb and index finger depends on the survival of afferents innervating these digits, as well as the proliferation of their central terminals.

Primary afferent terminal sprouting after a cervical dorsal rootlet section in the macaque monkey.

We examined the role of primary afferent neurons in the somatosensory cortical "reactivation" that occurs after a localized cervical dorsal root lesion (Darian-Smith and Brown [2000] Nat. Neurosci. 3:476-481). After section of the dorsal rootlets that enervate the macaque's thumb and index finger (segments C6-C8), the cortical representation of these digits was initially silenced but then re-emerged for these same digits over 2-4 postlesion months. Cortical reactivation was accompanied by the emergence of physiologically detectable input from these same digits within dorsal rootlets bordering the lesion site. We investigated whether central axonal sprouting of primary afferents spared by the rhizotomy could mediate this cortical reactivation. The cortical representation of the hand was mapped electrophysiologically 15-25 weeks after the dorsal rootlet section to define this reactivation. Cholera toxin subunit B conjugated to horseradish peroxidase was then injected into the thumb and index finger pads bilaterally to label the central terminals of any neurons that innervated these digits. Primary afferent terminal proliferation was assessed in the spinal dorsal horn and cuneate nucleus at 7 days and 15-25 postlesion weeks. Labeled terminal bouton distributions were reconstructed and the "lesion" and control sides compared within each monkey. Distributions were significantly larger on the side of the lesion in the dorsal horn and cuneate nucleus at 15-25 weeks after the dorsal rootlet section, than those mapped only 7 days postlesion. Our results provide direct evidence for localized sprouting of spared (uninjured) primary afferent terminals in the dorsal horn and cuneate nucleus after a restricted dorsal root injury.