ABNORMALITIES OF MECHANORECEPTORS IN A RAT MODEL OF NEUROPATHIC PAIN - POSSIBLE INVOLVEMENT IN MEDIATING MECHANICAL ALLODYNIA

1. Peripheral nerve injury sometimes leads to the development of neuro pathic pain. One of the symptoms of such neuropathic pain is mechanica l allodynia, pain in response to normally innocuous mechanical stimuli . We hypothesized that sympathetically driven dysfunction of cutaneous mechanoreceptors is responsible for signaling mechanical allodynia. T he present study was undertaken to identify the types of sensory recep tors that potentially mediate mechanical allodynia, with the use of a rat neuropathic pain model we have developed. 2. One week to 10 days a fter tight ligations of the L5 and L6 spinal nerves on one side, the r ats fully developed behavioral signs of mechanical allodynia on the af fected hindlimb. Various cutaneous mechanoreceptors originating from t he neuropathic foot were examined by single-fiber recordings from the L4 dorsal root. 3. Although no particular abnormalities were found in other types of cutaneous mechanoreceptors, an unusual type of mechanor eceptor was found to be innervating the neuropathic foot. The response characteristics of this type of receptor resemble those of rapidly ad apting mechanoreceptors (RAs), but with low and irregular static disch arges during a maintained mechanical stimulus. We termed this unusual type as a ''modified rapidly adapting'' mechanoreceptor (MRA). 4. The response characteristics of MRAs change to those of typical RAs after a systemic injection of phentolamine, an alpha-adrenergic receptor blo cker. 5. We conclude that many RAs become abnormal under the influence of sympathetic efferents in neuropathic pain, so that their response patterns change to those of MRAs. We propose that this abnormality is responsible for signaling the mechanical allodynia that can be seen in neuropathic pain states such as causalgia.