Chronically implanted electrodes for repeated stimulation and recording of spinal cord potentials

Novel Evoked Synaptic Activity Potentials (ESAPs) Elicited by Spinal Cord Stimulation

Spinal cord stimulation (SCS) evokes fast epidural evoked compound action potential (ECAP) that represent activity of dorsal column axons, but not necessarily a spinal circuit response. Using a multimodal approach, we identified and characterized a delayed and slower potential evoked by SCS that reflects synaptic activity within the spinal cord. Anesthetized female Sprague Dawley rats were implanted with an epidural SCS lead, epidural motor cortex stimulation electrodes, an epidural spinal cord recording lead, an intraspinal penetrating recording electrode array, and intramuscular electromyography (EMG) electrodes in the hindlimb and trunk. We stimulated the motor cortex or the epidural spinal cord and recorded epidural, intraspinal, and EMG responses. SCS pulses produced characteristic propagating ECAPs (composed of P1, N1, and P2 waves with latencies <2 ms) and an additional wave ("S1") starting after the N2. We verified the S1-wave was not a stimulation artifact and was not a reflection of hindlimb/trunk EMG. The S1-wave has a distinct stimulation-intensity dose response and spatial profile compared with ECAPs. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX; a selective competitive antagonist of AMPA receptors (AMPARs)] significantly diminished the S1-wave, but not ECAPs. Furthermore, cortical stimulation, which did not evoke ECAPs, produced epidurally detectable and CNQX-sensitive responses at the same spinal sites, confirming epidural recording of an evoked synaptic response. Finally, applying 50-Hz SCS resulted in dampening of S1-wave but not ECAPs. Therefore, we hypothesize that the S1-wave is synaptic in origin, and we term the S1-wave type responses: evoked synaptic activity potentials (ESAPs). The identification and characterization of epidurally recorded ESAPs from the dorsal horn may elucidate SCS mechanisms.

Spinal cord bioelectronic interfaces: opportunities in neural recording and clinical challenges

Bioelectronic stimulation of the spinal cord has demonstrated significant progress in restoration of motor function in spinal cord injury (SCI). The proximal, uninjured spinal cord presents a viable target for the recording and generation of control signals to drive targeted stimulation. Signals have been directly recorded from the spinal cord in behaving animals and correlated with limb kinematics. Advances in flexible materials, electrode impedance and signal analysis will allow SCR to be used in next-generation neuroprosthetics. In this review, we summarize the technological advances enabling progress in SCR and describe systematically the clinical challenges facing spinal cord bioelectronic interfaces and potential solutions, from device manufacture, surgical implantation to chronic effects of foreign body reaction and stress-strain mismatches between electrodes and neural tissue. Finally, we establish our vision of bi-directional closed-loop spinal cord bioelectronic bypass interfaces that enable the communication of disrupted sensory signals and restoration of motor function in SCI.

Chronic softening spinal cord stimulation arrays

OBJECTIVE

We sought to develop a cervical spinal cord stimulator for the rat that is durable, stable, and does not perturb the underlying spinal cord.

APPROACH

We created a softening spinal cord stimulation (SCS) array made from shape memory polymer (SMP)-based flexible electronics. We developed a new photolithographic process to pattern high surface area titanium nitride (TiN) electrodes onto gold (Au) interconnects. The thiol-ene acrylate polymers are stiff at room temperature and soften following implantation into the body. Durability was measured by the duration the devices produced effective stimulation and by accelerated aging in vitro. Stability was measured by the threshold to provoke an electromyogram (EMG) muscle response and by measuring impedance using electrochemical impedance spectroscopy (EIS). In addition, spinal cord modulation of motor cortex potentials was measured. The spinal column and implanted arrays were imaged with MRI ex vivo, and histology for astrogliosis and immune reaction was performed.

MAIN RESULTS

For durability, the design of the arrays was modified over three generations to create an array that demonstrated activity up to 29 weeks. SCS arrays showed no significant degradation over a simulated 29 week period of accelerated aging. For stability, the threshold for provoking an EMG rose in the first few weeks and then remained stable out to 16 weeks; the impedance showed a similar rise early with stability thereafter. Spinal cord stimulation strongly enhanced motor cortex potentials throughout. Upon explantation, device performance returned to pre-implant levels, indicating that biotic rather than abiotic processes were the cause of changing metrics. MRI and histology showed that softening SCS produced less tissue deformation than Parylene-C arrays. There was no significant astrogliosis or immune reaction to either type of array.

SIGNIFICANCE

Softening SCS arrays meet the needs for research-grade devices in rats and could be developed into human devices in the future.

A continuous spinal cord stimulation model attenuates pain-related behavior in vivo following induction of a peripheral nerve injury

OBJECTIVES

Models that simulate clinical conditions are needed to gain an understanding of the mechanism involved during spinal cord stimulation (SCS) treatment of chronic neuropathic pain. An animal model has been developed for continuous SCS in which animals that have been injured to develop neuropathic pain behavior were allowed to carry on with regular daily activities while being stimulated for 72 hours.

MATERIAL AND METHODS

Sprague-Dawley rats were randomized into each of six different groups (N = 10-13). Three groups included animals in which the spared nerve injury (SNI) was induced. Animals in two of these groups were implanted with a four-contact electrode in the epidural space. Animals in one of these groups received stimulation for 72 hours continuously. Three corresponding sham groups (no SNI) were included. Mechanical and cold-thermal allodynia were evaluated using von Frey filaments and acetone drops, respectively. Mean withdrawal thresholds were compared. Statistical significance was established using one-way ANOVAs followed by Holm-Sidak post hoc analysis.

RESULTS

Continuous SCS attenuates mechanical allodynia in animals with neuropathic pain behavior. Mechanical withdrawal threshold increases significantly in SNI animals after 24 and 72 hours stimulation vs. SNI no stimulation (p = 0.007 and p < 0.001, respectively). SCS for 24 and 72 hours provides significant increase in mechanical withdrawal thresholds relative to values before stimulation (p = 0.001 and p < 0.001, respectively). Stimulation did not provide recovery to baseline values. SCS did not seem to attenuate cold-thermal allodynia.

CONCLUSION

A continuous SCS model has been developed. Animals with neuropathic pain behavior that were continuously stimulated showed significant increase in withdrawal thresholds proportional to stimulation time.

Novel aspects of spinal cord evoked potentials (SCEPs) in the evaluation of dorso-ventral and lateral mechanical impacts on the spinal cord

OBJECTIVES

The aim of the current study was to mimic mechanical impacts on the spinal cord by manifesting the effects of dorsoventral (DVMP) and lateral (LMP) mechanical pressure on neural activity to address points to be considered during surgery for different purposes, including spinal cord decompression.

APPROACHES

Spinal cords of anesthetized rats were compressed at T13. Different characteristics of axons, including vulnerability, excitability, and conduction velocity (CV), in response to promptness, severity, and duration of pressure were assessed by spinal cord evoked potentials (SCEPs). Real-time SCEPs recorded at L4-5 revealed N1, N2, and N3 peaks that were used to represent the activity of injured sensory afferents, interneurons, and MN fibers. The averaged SCEP recordings were fitted by trust-region algorithm to find the equivalent Gaussian and polynomial equations.

MAIN RESULTS

The pyramidal and extrapyramidal pathways possessed CVs of 3-11 and 16-80 m s(-1), respectively. DVMP decreased the excitability of myelinated neural fibers in antidromic and orthodromic pathways. The excitability of fibers in extrapyramidal and pyramidal pathways of lateral corticospinal (LCS) and anterior corticospinal (ACS) tracts decreased following LMP. A significant drop in the amplitude of N3 and its conduction velocity (CV) revealed higher susceptibility of less-myelinated fibers to both DVMP and LMP. The best parametric fitting model for triplet healthy spinal cord CAP was a six-term Gaussian equation (G6) that fell into a five-term equation (G5) at the complete compression stage.

SIGNIFICANCE

The spinal cord is more susceptible to dorsoventral than lateral mechanical pressures, and this should be considered in spinal cord operations. SCEPs have shown promising capabilities for evaluating the severity of SCI and thus can be applied for diagnostic or prognostic intraoperative monitoring (IOM).

Co-transplantation of olfactory ensheathing glia and mesenchymal stromal cells does not have synergistic effects after spinal cord injury in the rat

BACKGROUND AIMS

Olfactory ensheathing glia (OEG) and mesenchymal stromal cells (MSC) are suitable candidates for transplantation therapy of spinal cord injury (SCI). Both facilitate functional improvement after SCI by producing trophic factors and cytokines. In this study, the co-transplantation of both types of cells was studied to clarify their additive and/ or synergistic effects on SCI.

METHODS

A balloon-induced compression lesion was used to produce SCI in rats. OEG, MSC or both OEG and MSC (3 x 10(5) cells of each cell type) were implanted by intraspinal injection 1 week after SCI. The effect of transplantation was assessed using behavioral, electrophysiologic and histologic methods.

RESULTS

Hindlimb function was examined with Basso, Beattie and Bresnahan (BBB) and Plantar tests. Improvement was found in all three groups of transplanted rats with different time-courses, but there was no significant difference among the groups at the end of the experiment. Motor-evoked potentials after SCI decreased in amplitude from 7 mV to 10 microV. Linear regression analysis showed a modest recovery in amplitude following transplantation, but no change in the control rats. Histologic findings showed that the white and gray matter were significantly spared by transplantation after SCI.

CONCLUSIONS

Functional improvement was achieved with transplantation of OEG and/or MSC, but the co-transplantation of OEG and MSC did not show synergistic effects. The poor migration of OEG and MSC might prevent their concerted action. Pre-treatment with a Rho antagonist and a combination of intraspinal and intravenous injection of the cells might be beneficial for SCI therapy.

Long-Term Changes in Spinal Cord Evoked Potentials After Compression Spinal Cord Injury in the Rat

1. After traumatic spinal cord injury (SCI), histological and neurological consequences are developing for several days and even weeks. However, little is known about the dynamics of changes in spinal axonal conductivity. The aim of this study was to record and compare repeated spinal cord evoked potentials (SCEP) after SCI in the rat during a 4 weeks' interval. These recordings were used: (i) for studying the dynamics of functional changes in spinal axons after SCI, and (ii) to define the value of SCEP as an independent outcome parameter in SCI studies. 2. We have used two pairs of chronically implanted epidural electrodes for stimulation/recording. The electrodes were placed below and above the site of injury, respectively. Animals with implanted electrodes underwent spinal cord compression injury induced by epidural balloon inflation at Th8-Th9 level. There were five experimental groups of animals, including one control group (sham-operated, no injury), and four injury groups (different degrees of SCI). 3. After SCI, SCEP waveform was either significantly reduced or completely lost. Partial recovery of SCEPs was observed in all groups. The onset and extent of recovery clearly correlated with the severity of injury. There was good correlation between quantitated SCEP variables and the volumes of the compressing balloon. However, sensitivity of electropohysiological parameters was inferior compared to neurological and morphometric outcomes. 4. Our study shows for the first time, that the dynamics of axonal recovery depends on the degree of injury. After mild injury, recovery of signal is rapid. However, after severe injury, axonal conductivity can re-appear after as long as 2 weeks postinjury. In conclusion, SCEPs can be used as an independent parameter of outcome after SCI, but in general, the sensitivity of electrophysiological data were worse than standard morphological and neurological evaluations.