Descending facilitation from the brainstem determines behavioural and neuronal hypersensitivity following nerve injury and efficacy of pregabalin

Abstract

Various mechanisms at peripheral, spinal and/or supraspinal levels may underlie neuropathic pain. The nervous system's capacity for long-term reorganisation and chronic pain may result from abnormalities in RVM facilitatory On cells. Hence, via brainstem injections of the toxic conjugate dermorphin-saporin, which specifically lesions facilitatory cells expressing the mu-opioid receptor (MOR), we sought to determine the influence of these cells in normal and spinal nerve-ligated (SNL) rats. We combined behavioural, electrophysiological and pharmacological techniques to show that the supraspinal facilitatory drive is essential for neuronal processing of noxious stimuli in normal and neuropathic states, and that descending facilitatory neurones maintain behavioural hypersensitivities to mechanical stimuli during the late stages of nerve injury. Furthermore, we showed that these neurones are essential for the state-dependent inhibitory actions of pregabalin (PGB), a drug used in the treatment of neuropathic pain. During the early stages of nerve injury, or following medullary MOR cell ablation, PGB is ineffective at inhibiting spinal neuronal responses possibly due to quiescent spinal 5HT(3) receptors. This can however be overcome, and PGB's efficacy restored, by pharmacologically mimicking the descending drive at the spinal level with a 5HT(3) receptor agonist. Since RVM facilitatory neurones are integral to a spino-bulbo-spinal loop that reaches brain areas co-ordinating the sensory and affective components of pain, we propose that activity therein may influence painful outcome following nerve injury, and responsiveness to treatment.