Title : Intraspinal microstimulation and serotonergic neurotransmission synergistically modulate nociceptive pathways after spinal cord injury
Abstract:
Background: Spinal cord injury (SCI) often results in motor impairments and neuropathic pain. Not only are these conditions irreversible, they disrupt a crucial pathway for transmitting signals between the brain and the rest of the body, affecting movement, sensation, and autonomic functions, such as limb function and bladder control. Intraspinal microstimulation (ISMS), or delivery of small amounts of electrical current directly to enhance motor recovery in regions below SCI, can potentially reduce neural transmission in spinal pain pathways. Additionally, serotonin, a monoamine neurotransmitter, may have the unique ability to simultaneously reduce transmission in spinal pain pathways while increasing transmission in spinal motor pathways.
Methods: In Sprague-Dawley rats under urethane anesthesia, the SCI model consisted of an incomplete impact injury at the T8 level. Microelectrode arrays were used for simultaneous extracellular recordings and sub-motor ventral horn stimulation (vISMS) via implantation in a spinal segment corresponding to the L5 dermatome, which was mechanically stimulated by applying non-painful light touch every 10 seconds or painful pinches every 30 seconds. A serotonin (5-HT) antagonist drug, WAY-100365, was pipetted onto thin tissue paper and placed to cover the exposed cord under the electrode implantation site. Nociceptive stimulation was repeated with and without ISMS. Animals were sacrificed in terminal experiments and perfused. Spinal cord sections from the lumbar enlargement were collected and analyzed via immunofluorescence of the serotonin transporter (SERT) in injured rats and controls with and without ISMS.
Results: Based on a comparison of 5-HT transporter immunostaining in dorsal horn and intermediate zone of lumbar enlargement in sham vs SCI (n=5 rats), serotonin transporter levels (5-HT transporter, SERT) were drastically decreased in the injured cord. However, it is still unclear if ISMS has a consistent ability to transiently rescue SERT expression to a significant extent. Following application of WAY-100635 in vivo, multi-unit neuron firing frequencies were depressed 96% and remained consistent within an established ~75 minute time frame in which drug potency was stable even after saline washout, minimizing drug wear-off effects on subsequent experiments within the timeframe. We established that one 30 minute session of sub-motor threshold ISMS intended to enhance recovery of movement also depresses and modulates spinal nociceptive transmission after SCI. In injured rats experiencing behavioral signs of chronic pain as well as neurologically intact rats, the percentage of electrodes with a depressed response compared to baseline pre-ISMS was significant compared to rats with SCI without neuropathic pain. However, ISMS after application of WAY-100635 in neurologically intact animals (n=3) did not significantly recover nor further depress pre-drug firing frequencies in response to nociceptive stimuli. Additionally, firing frequencies elicited by non-nociceptive stimuli (light touch) were not significantly affected by the drug or vISMS, revealing unexpected modal specificity. Conclusions: Working serotonin pathways are crucial in the ability of ISMS to mediate normal nociceptive transmission. Pairing the effects of ISMS for motor rehabilitation with monoaminergic agonists may be a particularly effective strategy for enhancing long-lasting neuroplastic changes and restoration of natural, cohesive patterns of spinal sensorimotor transmission.
Audience Takeway Notes:
- Multi-modal nociceptive mechanisms of neuropathic pain: the audience would gain a better understanding of how pain signals are elicited and processed, specific neurotransmitter
- interactions and circuits involved in pain processing, and neuron responsiveness to different stimuli. This, combined with ISMS, can link long-term effects of therapeutic stimulation on spinal cord circuits with neural plasticity.
- Innovative: suggests potential directions to develop novel therapeutic approaches for reducing pain in SCI patients, including avenues to expand existing epidural stimulation to combine electrical stimulation and pharmacological treatments targeting serotonergic pathways. This discovery could lead to development of neuroprosthetic therapies that provide multimodal (sensory and movement) rehabilitation benefits after SCI.
- Experimental: strengthens the efficacy of previous ISMS studies from an in-vivo pharmacology standpoint, introducing the role of serotonin and potential implications of antagonist or agonist manipulation on existing pathways in real time. The electrode system used, stimulation protocol, and immunostaining techniques used in experiments can be adapted, modified, and/or translated to researchers studying both sensory and motor recovery.
