Serotonergic neural precursor cell grafts attenuate bilateral hyperexcitability of dorsal horn neurons after spinal hemisection in rat

Abstract

Hemisection of the rat spinal cord at thoracic level 13 provides a model of spinal cord injury that is characterized by chronic pain attributable to hyperexcitability of dorsal horn neurons. Presuming that this hyperexcitability can be explained in part by interruption of descending inhibitory modulation by serotonin, we hypothesized that intrathecal transplantation of RN46A-B14 serotonergic precursor cells, which secrete serotonin and brain-derived neurotrophic factor, would reduce this hyperexcitability by normalizing the responses of low-threshold mechanoreceptive, nociceptive-specific, and multireceptive dorsal horn neurons. Three groups ( n=45 total) of 30-day-old male Sprague–Dawley rats underwent thoracic level 13 spinal hemisection, after which four weeks were allowed for development of allodynia and hyperalgesia. The three groups of animals received transplants of no cells, 10 6 RN46A-V1 (vector-only) or 10 6 RN46A-B14 cells at lumbar segments 2–3. Electrophysiological experiments were done two weeks later. Low-threshold mechanoreceptive, nociceptive-specific, and multireceptive cells ( n=394 total) were isolated at depths of 1–300 and 301–1000 μm in the lumbar enlargement. Responses to innocuous and noxious peripheral stimuli were characterized, and analyses of population responses were performed. Compared with normal animals, dorsal horn neurons of all types in hemisected animals showed increased responsiveness to peripheral stimuli. This was true for neurons on both sides of the spinal cord. After hemisection, the proportion of neurons classified as multireceptive cells increased, and interspike intervals of spontaneous discharges became less uniform after hemisection. Transplantation of RN46A-B14 cells restored evoked responses to near-control levels, normalized background activity, and returned the proportion of multireceptive cells to the control level. Restoration of normal activity was reversed with methysergide. These electrophysiological results corroborate anatomical and behavioral studies showing the effectiveness of serotonergic neural precursors in correcting phenomena associated with chronic central pain following spinal cord injury, and provide mechanistic insights regarding mode of action.