The role of galanin and neuropeptide Y in a rat model of neuropathic pain

Abstract

Galanin and neuropeptide Y (NPY) are not normally produced to any significant extent by primary afferent neurones. After peripheral nerve injury however, there is a marked increase in their synthesis - galanin mainly in small to medium sized neurones, and NPY mainly in large neurones. There is evidence of transport ofthese newly produced peptides to the dorsal horn, but it is not known what stimuli result in their release in the spinal cord. The antibody microprobe technique was therefore used to study the factors that might result in release of these peptides. The model that has been studied involved placement of four loose ligatures around one sciatic nerve in the rat, reliably resulting in the development of behavioural evidence of neuropathic pain.In addition to observing for the characteristic signs associated with this model, the development of neuropathic pain was assessed quantitatively by the paw withdrawal response to von Frey hairs and pin prick. Thus, all the animals used in the study of peptide release or functional changes in the spinal cord, 10 to 14 days later, did indeed have evidence of neuropathic pain.A general assessment of altered spinal cord responses, found significant changes in the neuropathic rats. There was found to be a bilateral increase in the cord dorsum potentials evoked by A fibre stimulation of the injured nerve, as compared tot he normal nerve, despite a decrease in the afferent volley from the injured nerve. As the synthesis of both galanin and NPY is stimulated by peripheral nerve injury, alterations in their spinal release was studied, including the effects of A fibre stimulation on this central release.Microprobes bearing immobilised antibodies to galanin were inserted into the spinal cord of neuropathic rats. An increased area of immunoreactive (ir)-galanin release was detected in the superficial dorsal horn ipsilateral to nerve injury, as compared to that normally found. This occurred in the absence of any active peripheral stimulation, in an area of the dorsal horn where the small primary afferent fibres that have started to synthesise galanin terminate. Ir-galanin release was not increased by electrical stimulation of the injured nerve at a strength sufficient to activate A fibres, but there was an increase in release as C fibres were stimulated. Conduction block proximal to the dorsal root ganglia did not reduce the increased basal ir-galanin release found ipsilateral to the nerve injury. An increase in ir-galanin release was seen, however, on the contralateral side of the cord, after all neuronal input had been blocked from the injured nerve.Previous studies in this laboratory have found that, on the side ipsilateral to nerve injury, ir-NPY was released spontaneously in the dorsal horn and this release was increased by electrical stimulation of the injured nerve sufficient to activate only A fibres. The large diameter primary afferent neurones that synthesise NPY after nerve injury terminate in the deep dorsal horn, where ir-NPY was released, making it probable that these fibres are the source of this ir-NPY. Using antibody coated microprobes, the present studies found that the spontaneous release of ir-NPY was not altered by conduction block ofthe injured nerve proximal to the dorsal root ganglia. It appears therefore, that release ofNPY in the spinal cord from large diameter primary afferent neurones does not require any peripheral neuronal impulses.The large diameter neurones that start to produce NPY express the neurotrophin-3 (NT-3) receptor, trkC. It has also been shown that exogenous NT-3 can attenuate some of the increases in spinal levels ofNPY, after bilateral axotomy. The effect of exogenous NT-3 on NPY release was therefore studied using a chronic infusion of NT-3 delivered to the nerve injury site and antibody microprobes to measure spontaneous NPY release. It was found that NT-3 infusion had no effect on spontaneous NPY release, but a minor alleviation of mechanical hypersensitivity was noted 10 to 14 days after nerve injury, in those rats receiving exogenous NT-3. These studies have determined some of the stimuli responsible for spinal release of galanin and NPY after peripheral nerve injury, and excluded others. Further work is required to correlate these results with functional effects and possible role in the modulation of nociceptive transmission.