Nerve injury induces plasticity that results in spinal inhibitory effects of galanin

Galanin is a 29-amino-acid neuropeptide that has been implicated in the processes of nociception. This study examines the effect of exogenous galanin on dorsal horn neurone activity in vivo in the spinal nerve ligation (SNL) model of neuropathic pain. SNL rats but not naive or sham-operated rats exhibited behaviour indicative of allodynia. In anaesthetized rats, extracellular recordings were made from individual convergent dorsal horn neurones following stimulation of peripheral receptive fields electrically or with natural (innocuous mechanical, noxious mechanical and noxious thermal) stimuli. Spinal administration of galanin (0.5-50 microg) caused a slight facilitation of the neuronal responses to natural and electrical stimuli in naive rats and up to a 65% inhibition of neuronal responses in sham-operated rats following 50 microg galanin. In contrast, there was a marked inhibition of up to 80% of responses to both natural and electrical stimuli in SNL rats following spinal galanin administration. These results suggest that following peripheral nerve injury, there is plasticity in the levels of galanin and/or its receptors at spinal cord level so that the effect of exogenous galanin favours inhibitory function.