The α 2A adrenoceptor and the sympathetic postganglionic neuron contribute to the development of neuropathic heat hyperalgesia in mice

Abstract

We have addressed the role of the sympathetic nervous system in the development and maintenance of neuropathic pain. Using a new neuropathic mouse model, we examined the development of hyperalgesia in transgenic mice lacking functional α 2A adrenoceptors and in sympathectomized wild-type mice, to determine if sympathetic–sensory coupling generates hyperalgesia. The development of neuropathic heat hyperalgesia required the presence of both the α 2A adrenoceptor and the sympathetic postganglionic neuron (SPGN), but the development of mechanical hyperalgesia did not require either the α 2A adrenoceptor or the SPGN, indicating different mechanisms of sensitization. These results suggest that the development of neuropathic heat hyperalgesia, but not mechanical hyperalgesia, requires sympathetic–sensory coupling in the peripheral nervous system. Nerve injury enhanced the analgesic efficacy of the α 2 adrenoceptor agonist dexmedetomidine, and paradoxically also induced an analgesic response to α 2 adrenoceptor antagonists. The α 2 agonist-evoked analgesia to mechanical stimuli was mediated by activating central α 2A adrenoceptors, possibly at the spinal level. The peripherally restricted α 2 antagonist L659,066 evoked analgesia for heat, but not for mechanical stimuli, findings which support the hypothesis that the peripheral α 2 adrenoceptor plays a role in both the development and the maintenance of neuropathic heat hyperalgesia. The α 2 antagonist-evoked analgesia for heat stimuli was mediated by blocking peripheral and probably central α 2 adrenoceptors, while the analgesia for mechanical stimuli was mediated by blocking central α 2A adrenoceptors. Intradermal injections with an α 2 agonist or antagonist had no effect on nociceptive thresholds, indicating that sympathetic–sensory coupling at the level of the cutaneous nociceptor did not contribute to the maintenance of neuropathic hyperalgesia.