Electrophysiological evidence that olfactory cell transplants improve function after spinal cord injury.

Transplants of cells obtained from the olfactory system are a potential treatment for spinal cord injury and a number of clinical trials are in progress. However, the extent to which transplants improve recovery of function remains unclear and there are contradictory reports on the extent to which they support axonal regeneration. Here, we have used anatomical and electrophysiological techniques to investigate the repair promoted by olfactory cell transplants after a dorsal column lesion. Since the use of olfactory cells of varying type and origin may contribute to the differing outcomes of previous studies, regeneration of dorsal column axons was compared following transplants of pure olfactory ensheathing cells from neonatal animals and mixed olfactory cells from both neonatal and adult rats. Two to three months after lesioning, numerous regenerating fibres could be seen in each type of transplant. However, tracing of ascending dorsal column fibres showed that few regenerated beyond the lesion, even when transplanted with mixed olfactory cells from the adult olfactory bulb which have previously been reported to support regeneration which bridges a lesion. Despite the absence of axonal regeneration across the injury site, olfactory cell transplants led to improved spinal cord function in sensory pathways investigated electrophysiologically. When cord dorsum potentials (CDPs), evoked by electrical stimulation of the L4/L5 dorsal roots, were recorded from the spinal cord above and below a lesion at the lumbar 3/4 level, CDPs recorded from transplanted animals were significantly larger than those recorded from lesioned controls. In addition, sensory evoked potentials recorded over the sensorimotor cortex were larger and detectable over a more extensive area in transplanted animals. These results provide direct evidence that transplants of olfactory cells preserve the function of circuitry in the region of the lesion site and of ascending pathways originating near the injury. These actions, rather than axonal regeneration, may help ameliorate the effects of spinal cord injury.

Upregulation of substance P in low-threshold myelinated afferents is not required for tactile allodynia in the chronic constriction injury and spinal nerve ligation models.

It has been proposed that substance P and calcitonin gene-related peptide (CGRP) are upregulated in low-threshold myelinated primary afferents after certain types of nerve injury, and that release of substance P from these afferents contributes to the resulting tactile allodynia. To test this hypothesis, we looked for neuropeptides in Abeta primary afferent terminals in the ipsilateral gracile nucleus and spinal dorsal horn in three nerve injury models: sciatic nerve transection (SNT), spinal nerve ligation (SNL), and chronic constriction injury (CCI). We also looked for evidence of neurokinin 1 (NK1) receptor internalization in the dorsal horn after electrical stimulation of Abeta afferents. We found no evidence of either substance P or CGRP expression in injured Abeta terminals in the spinal cord in any of the models. Although substance P was not detected in terminals of injured afferents in the gracile nucleus, CGRP was expressed in between 32 and 68% of these terminals, with a significantly higher proportion in the SNL and CCI models, compared with SNT. In addition, we did not detect any Abeta-evoked NK1 receptor internalization in neurons from laminas I, III, or IV of the dorsal horn in the CCI or SNL models. These results do not support the proposal that substance P is present at significant levels in the terminals of injured Abeta primary afferents in neuropathic models. They also suggest that any release of substance P from injured Abeta afferents is unlikely to activate NK1 receptors in the dorsal horn or contribute to neuropathic pain.

Lack of evidence for sprouting of Abeta afferents into the superficial laminas of the spinal cord dorsal horn after nerve section.

The central arborizations of large myelinated cutaneous afferents normally extend as far dorsally as the ventral part of lamina II in rat spinal cord. Woolf et al. (1992) reported that after nerve injury some of these afferents sprouted into lamina I and the dorsal part of lamina II, and it has been suggested that this could contribute to allodynia associated with neuropathic pain. Part of the evidence for sprouting was on the basis of the use of cholera toxin B subunit as a selective tracer for A-fibers, and the validity of this approach has recently been questioned; however, sprouting was also reported in experiments involving intra-axonal labeling of chronically axotomized afferents. We have used intra-axonal labeling in the rat to examine central terminals of 58 intact sciatic afferents of presumed cutaneous origin and 38 such afferents axotomized 7-10 weeks previously. Both normal and axotomized populations included axons with hair follicle afferent-like morphology and arbors that entered the ventral half of lamina II; however, none of these extended farther dorsally. We also performed bulk labeling of myelinated afferents by injecting biotinylated dextran into the lumbar dorsal columns bilaterally 8-11 weeks after unilateral sciatic nerve section. We observed that both ipsilateral and contralateral to the sectioned nerve, arbors of axons with hair follicle afferent-like morphology in the sciatic territory extended only as far as the ventral half of lamina II. Therefore these results do not support the hypothesis that Abeta afferents sprout into the superficial laminas after nerve section.