Spinal stimulation for motor rehabilitation immediately modulates nociceptive transmission

Grid-based transcutaneous spinal cord stimulation: probing neuromodulatory effect in spinal flexion reflex circuits

Objective.Non-invasive spinal stimulation has the potential to modulate spinal excitability. This study explored the modulatory capacity of sub-motor grid-based transcutaneous spinal cord stimulation (tSCS) applied to the lumbar spinal cord in neurologically intact participants. Our objective was to examine the effect of grid spinal stimulation on polysynaptic reflex pathways involving motoneurons and interneurons likely activated by Aβ/δfiber-mediated cutaneous afferents.Approach.Stimulation was delivered using two grid electrode montages, generating a net electric field in transverse or diagonal directions. We administered tSCS with the center of the grid aligned with the T10-T11 spinous process. Participants were seated for the 20 min stimulation duration. At 30 min after the cessation of spinal stimulation, we examined neuromodulatory effects on spinal circuit excitability in the tibialis anterior muscle by employing the classical flexion reflex paradigms. Additionally, we evaluated spinal motoneuron excitability using theH-reflex paradigm in the soleus muscle to explore the differential effects of tSCS on the polysynaptic versus monosynaptic reflex pathway and to test the spatial extent of the grid stimulation.Main results.Our findings indicated significant neuromodulatory effects on the flexion reflex, resulting in a net inhibitory effect, regardless of the grid electrode montages. Our data further indicated that the flexion reflex duration was significantly shortened only by the diagonal montage.Significance.Our results suggest that grid-based tSCS may specifically modulate spinal activities associated with polysynaptic flexion reflex pathways, with the potential for grid-specific targeted neuromodulation.

Targeted inactivation of spinal α2 adrenoceptors promotes paradoxical anti-nociception

Noradrenergic drive from the brainstem to the spinal cord varies in a context-dependent manner to regulate the patterns of sensory and motor transmission that govern perception and action. In sensory networks, it is traditionally assumed that activation of spinal α2 receptors is anti-nociceptive, while spinal α2 blockade is pro-nociceptive. Here, however, we demonstrate in vivo in rats that targeted blockade of spinal α2 receptors can promote anti-nociception. The anti-nociceptive effects are not contingent upon supraspinal actions, as they persist below a chronic spinal cord injury and are enhanced by direct spinal application of antagonist. They are also evident throughout sensory-dominant, sensorimotor integrative, and motor-dominant regions of the gray matter, and neither global changes in spinal neural excitability nor off-target activation of spinal α1 adrenoceptors or 5HT 1A receptors abolished the anti-nociception. Together, these findings challenge the current understanding of noradrenergic modulation of spinal nociceptive transmission.

The ins and outs of spinal cord stimulation

This scientific commentary refers to 'Intraspinal microstimulation of the ventral horn has therapeutically relevant cross-modal effects on nociception', by Bandres et al. (https://doi.org/10.1093/braincomms/fcae280).

Intraspinal microstimulation of the ventral horn has therapeutically relevant cross-modal effects on nociception

Electrical stimulation of spinal networks below a spinal cord injury is a promising approach to restore functions compromised by inadequate and/or inappropriate neural drive. The most translationally successful examples are paradigms intended to increase neural transmission in weakened yet spared descending motor pathways and spinal motoneurons rendered dormant after being severed from their inputs by lesion. Less well understood is whether spinal stimulation is also capable of reducing neural transmission in pathways made pathologically overactive by spinal cord injury. Debilitating spasms, spasticity and neuropathic pain are all common manifestations of hyperexcitable spinal responses to sensory feedback. Whereas spasms and spasticity can often be managed pharmacologically, spinal cord injury-related neuropathic pain is notoriously medically refractory. Interestingly, however, spinal stimulation is a clinically available option for ameliorating neuropathic pain arising from aetiologies other than spinal cord injury, and the limited evidence available to date suggests that it holds considerable promise for reducing spinal cord injury-related neuropathic pain, as well. Spinal stimulation for pain amelioration has traditionally been assumed to modulate sensorimotor networks overlapping with those engaged by spinal stimulation for rehabilitation of movement impairments. Thus, we hypothesize that spinal stimulation intended to increase the ability to move voluntarily may simultaneously reduce transmission in spinal pain pathways. To test this hypothesis, we coupled a rat model of incomplete thoracic spinal cord injury, which results in moderate to severe bilateral movement impairments and spinal cord injury-related neuropathic pain, with in vivo electrophysiological measures of neural transmission in networks of spinal neurons integral to the development and persistence of the neuropathic pain state. We find that when intraspinal microstimulation is delivered to the ventral horn with the intent of enhancing voluntary movement, transmission through nociceptive specific and wide dynamic range neurons is significantly depressed in response to pain-related sensory feedback. By comparison, spinal responsiveness to non-pain-related sensory feedback is largely preserved. These results suggest that spinal stimulation paradigms could be intentionally designed to afford multi-modal therapeutic benefits, directly addressing the diverse, intersectional rehabilitation goals of people living with spinal cord injury.

Sensory-Targeted Intraspinal Microstimulation for Spinal Cord Injury Rehabilitation

Spinal Cord Injury (SCI) alters the excitability of neurons below the lesion, contributing to sensorimotor impairments. Less commonly acknowledged is SCI-related neuropathic pain (SCI-NP) that develops during the chronic phase of SCI, whose neural mechanisms are not yet fully understood. Previous work has demonstrated that electrical intraspinal microstimulation (ISMS) delivered amidst the spinal motor pools inside the ventral spinal gray matter causes an unexpected depression of sensory neuron responsiveness to nociceptive transmissions [1]. Given the lack of efficacious therapeutics for SCI-NP, we explored whether ISMS delivered in the sensory-dominant dorsal horn of the spinal cord could drive more robust depressive effects on pain-related transmission. Preliminary outcomes looked at population-level and single neuron-level modulatory actions of ISMS on neural transmission throughout the gray matter of a single segment of the lumbar enlargement in vivo in rats. Our preliminary results suggest dorsal ISMS may be less efficacious than motor-targeted ISMS at reducing spinal responsiveness to nociceptive sensory feedback. Future work will be required to determine the consistency of this finding and, if warranted, to optimize the stimulation parameters for multi-modal and/or pain-related applications.

Spinal Noradrenergic Alpha-2 Receptors Mediate the Antinociceptive Effects of Therapeutic Intraspinal Microstimulation

This study investigates the role of noradrenergic signaling in the antinociceptive effects of intraspinal microstimulation intended to restore movement after spinal cord injury. Two rat cohorts, intact and with chronic spinal cord injury, underwent intraspinal microstimulation (ISMS), and the multiunit neuronal response to peripheral noxious stimulation was examined at the spinal cord level. The observed effects were compared with those after blocking alpha-2 adrenergic receptors using RX821002. SCI heightened the nociceptive response to pinch stimulation, but ISMS exhibited clear antinociceptive effects in both intact and injured groups. These effects were nullified by blocking alpha-2 receptors, highlighting their involvement in the depression of nociceptive processing induced by ISMS.

Chronic Spinal Cord Injury Increases Spontaneous Intraspinal Neural Transmission and Spike Train Variability

Spinal cord injury (SCI) often leads to increased spontaneous activity (SpAP) and responsiveness to sensory feedback. This hyperexcitable state contributes to the development and maintenance of SCI-related neuropathic pain (SCI-NP). In general, characterizations of SpAP in the context of SCI-NP focus on the dorsal horns and sensory-processing neurons. As a result, changes in SpAP in the integrative intermediate gray and the motor-dominant ventral horn after SCI are considerably less understood. Thus, it is unclear whether altered firing dynamics in SpAP throughout the dorso-ventral extent of the spinal gray matter contribute to SCI-NP. Here we characterize the firing dynamics of dorsal and ventral interneurons in vivo in neurologically intact rats and rats with chronic SCI with and without SCI-NP. We find (1) robust SpAP throughout the dorsoventral extent of the spinal gray matter after chronic SCI and chronic SCI-NP; (2) that animals with SCI-NP showed higher spontaneous discharge rates compared to the other animal groups, particularly in the ventral horns; and (3) that animals with chronic SCI exhibited higher neuronal variability compared to neurologically intact animals. Together, the results suggest that increased SpAP in dorsal and ventral neurons may further exacerbate pathological sensory transmission after SCI and be a component of SCI-NP.

Nonlinear Firing Dynamics in Spinal Interneurons May Delineate the Presence or Absence of Spinal Cord Injury-related Neuropathic Pain

Spinal cord injury (SCI) often results in hyperexcitability of sensory-dominant regions of the spinal cord, leading to muscle spasms, spasticity, and SCI-related neuropathic pain (SCI-NP). In part, these symptoms are due to injury-induced alterations in neuromodulatory drive. Altered neuromodulatory drive changes a neuron's input-output function, increasing or decreasing its sensitivity to a given excitatory or inhibitory input through actions on metabotropic receptors on the dendrites. Many of these actions result in downstream effects on the voltage-gated ion channels that mediate persistent inward currents (PICs), which can markedly exaggerate the gain of a given spinal neuron to a synaptic input(s). Here, we hypothesize that SCI-NP is associated with altered neuromodulatory drive, revealed through the firing dynamics of sensory interneurons in the spinal cord. We find evidence of altered PICs and associated self-sustained firing as well as firing state changes in rats with and rats without SCI-NP, however we find a greater prevalence of features associated with altered neuromodulation in animals with SCI-NP than those without.

Unintentionally intentional: unintended effects of spinal stimulation as a platform for multi-modal neurorehabilitation after spinal cord injury

Electrical stimulation of spinal neurons has emerged as a valuable tool to enhance rehabilitation after spinal cord injury. In separate parameterizations, it has shown promise for improving voluntary movement, reducing symptoms of autonomic dysreflexia, improving functions mediated by muscles of the pelvic floor (e.g., bowel, bladder, and sexual function), reducing spasms and spasticity, and decreasing neuropathic pain, among others. This diverse set of actions is related both to the density of sensorimotor neural networks in the spinal cord and to the intrinsic ability of electrical stimulation to modulate neural transmission in multiple spinal networks simultaneously. It also suggests that certain spinal stimulation parameterizations may be capable of providing multi-modal therapeutic benefits, which would directly address the complex, multi-faceted rehabilitation goals of people living with spinal cord injury. This review is intended to identify and characterize reports of spinal stimulation-based therapies specifically designed to provide multi-modal benefits and those that report relevant unintended effects of spinal stimulation paradigms parameterized to enhance a single consequence of spinal cord injury.

Cypin Inhibition as a Therapeutic Approach to Treat Spinal Cord Injury-Induced Mechanical Pain

Cypin (cytosolic postsynaptic density protein 95 interactor) is the primary guanine deaminase in the central nervous system (CNS), promoting the metabolism of guanine to xanthine, an important reaction in the purine salvage pathway. Activation of the purine salvage pathway leads to the production of uric acid (UA). UA has paradoxical effects, specifically in the context of CNS injury as it confers neuroprotection, but it also promotes pain. Since neuropathic pain is a comorbidity associated with spinal cord injury (SCI), we postulated that small molecule cypin inhibitor B9 treatment could attenuate SCI-induced neuropathic pain, potentially by interfering with UA production. However, we also considered that this treatment could hinder the neuroprotective effects of UA and, in doing so, exacerbate SCI outcomes. To address our hypothesis, we induced a moderate midthoracic contusion SCI in female mice and assessed whether transient intrathecal administration of B9, starting at 1 d postinjury (dpi) until 7 dpi, attenuates mechanical pain in hindlimbs at 3 weeks pi. We also evaluated the effects of B9 on the spontaneous recovery of locomotor function. We found that B9 alleviates mechanical pain but does not affect locomotor function. Importantly, B9 does not exacerbate lesion volume at the epicenter. In accordance with these findings, B9 does not aggravate glutamate-induced excitotoxic death of SC neurons in vitro. Moreover, SCI-induced increased astrocyte reactivity at the glial scar is not altered by B9 treatment. Our data suggest that B9 treatment reduces mechanical pain without exerting major detrimental effects following SCI.

Neural population dynamics reveal that motor-targeted intraspinal microstimulation preferentially depresses nociceptive transmission in spinal cord injury-related neuropathic pain

The purpose of this study is to determine whether intraspinal microstimulation (ISMS) intended to enhance voluntary motor output after spinal cord injury (SCI) modulates neural population-level spinal responsiveness to nociceptive sensory feedback. The study was conducted in vivo in three cohorts of rats: neurologically intact, chronic SCI without behavioral signs of neuropathic pain, and chronic SCI with SCI-related neuropathic pain (SCI-NP). Nociceptive sensory feedback was induced by application of graded mechanical pressure to the plantar surface of the hindpaw before, during, and after periods of sub-motor threshold ISMS delivered within the motor pools of the L5 spinal segment. Neural population-level responsiveness to nociceptive feedback was recorded throughout the dorso-ventral extent of the L5 spinal segment using dense multi-channel microelectrode arrays. Whereas motor-targeted ISMS reduced nociceptive transmission across electrodes in neurologically intact animals both during and following stimulation, it was not associated with altered nociceptive transmission in rats with SCI that lacked behavioral signs of neuropathic pain. Surprisingly, nociceptive transmission was reduced both during and following motor-targeted ISMS in rats with SCI-NP, and to an extent comparable to that of neurologically intact animals. The mechanisms underlying the differential anti-nociceptive effects of motor-targeted ISMS are unclear, although they may be related to differences in the intrinsic active membrane properties of spinal neurons across the cohorts. Nevertheless, the results of this study support the notion that it may be possible to purposefully engineer spinal stimulation-based therapies that afford multi-modal rehabilitation benefits, and specifically that it may be possible to do so for the individuals most in need - i.e., those with SCI-related movement impairments and SCI-NP.

Precision neuromodulation: Promises and challenges of spinal stimulation for multi-modal rehabilitation

Spinal cord injury results in multiple, simultaneous sensorimotor deficits. These include, but are not limited to, full or partial paralysis of muscles below the lesion, muscle spasms, spasticity, and neuropathic pain. Bowel, bladder, and sexual dysfunction are also prevalent. Yet, the majority of emerging spinal stimulation-based therapies focus on a single issue: locomotor rehabilitation. Despite the enormous potential of these translational advances to transform the lives of people living with spinal cord injury, meaningful recovery in other domains deemed critical priorities remains lacking. Here, we highlight the importance of considering the diverse patterns of neural transmission that underlie clinically similar presentations when developing spinal stimulation-based therapies. We also motivate advancement of multi-modal rehabilitation paradigms, which leverage the dense interconnectivity of sensorimotor spinal networks and the unique ability of electrical stimulation to modulate these networks to facilitate and guide simultaneous rehabilitation across domains.

Motor-targeted spinal stimulation promotes concurrent rebalancing of pathologic nociceptive transmission in chronic spinal cord injury

Electrical stimulation of spinal networks below a spinal cord injury (SCI) is a promising approach to restore functions compromised by inadequate excitatory neural drive. The most translationally successful examples are paradigms intended to increase neural transmission in weakened yet spared motor pathways and spinal motor networks rendered dormant after being severed from their inputs by lesion. Less well understood is whether spinal stimulation is also capable of reducing neural transmission in pathways made pathologically overactive by SCI. Debilitating spasms, spasticity, and neuropathic pain are all common manifestations of hyperexcitable spinal responses to sensory feedback. But whereas spasms and spasticity can often be managed pharmacologically, SCI-related neuropathic pain is notoriously medically refractory. Interestingly, however, spinal stimulation is a clinically available option for ameliorating neuropathic pain arising from etiologies other than SCI, and it has traditionally been assumed to modulate sensorimotor networks overlapping with those engaged by spinal stimulation for motor rehabilitation. Thus, we reasoned that spinal stimulation intended to increase transmission in motor pathways may simultaneously reduce transmission in spinal pain pathways. Using a well-validated pre-clinical model of SCI that results in severe bilateral motor impairments and SCI-related neuropathic pain, we show that the responsiveness of neurons integral to the development and persistence of the neuropathic pain state can be enduringly reduced by motor-targeted spinal stimulation while preserving spinal responses to non-pain-related sensory feedback. These results suggest that spinal stimulation paradigms could be intentionally designed to afford multi-modal therapeutic benefits, directly addressing the diverse, intersectional rehabilitation goals of people living with SCI.