Electrophysiological characteristics of dorsal horn cells in rats with cutaneous inflammation resulting from chronic arthritis

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Abstract
Electrophysiological properties of dorsal horn neurons were investigated in decerebrate, immobilized spinal rats rendered polyarthritic by intradermal injection of Freund''s adjuvant. Since arthritis is associated with pronounced erythema and edema of the foot sole and ankle areas, this study was devoted to induced modifications of responses of units driven by cutaneous inputs. It allowed comparison with previous studies performed in healthy animals. Superficial dorsal horn cells could be separated in those driven from non-edematous skin and which had properties similar to those observed in healthy animals; and those driven from edematous skin and which characterized by unclassical electrophysiological properties such as a relatively high level of background activity, frequently with bursting pattern and sometimes exhibiting dramatic increases (these neurons are normally silent in healthy animals), or a high degree of responsivity to light mechanical stimuli associated with a clear decrease in threshold. Units which initially only responded to high intensity or frankly noxious stimuli probably shifted to become highly or maximally responsive to gentle stimuli. Similar modifications were also observed for some of the neurons located in deeper areas of the spinal cord where it is suggested that some units initially responding to both non-noxious and noxious stimuli (class 2 cells) in healthy rats shifted their characteristics to resemble units maximally driven by non-noxious stimuli (class 1 cells) in arthritic animals. The numerous structural alterations reported in cutaneous tissues compared to the relatively normal structure of peripheral nerves and CNS suggest that these observations can be related to peripheral sensitization of the sensory endings of thin peripheral fibers induced by the release of chemalgogens in inflammatory exudates. For both superficial and deeper dorsal horn neurons, arthritis also affected the late responses of neurons to transcutaneous electrical stimulation and their responses to radiant heat. Responses to radiant heat showed an increase in threshold, sometimes associated with a very high degree of adaptation. Another group of neurons displayed fading responses to repetitive mechanical stimuli. All were located in the deeper 2/3 of the dorsal horn. Evidently, chronic pathological conditions related to articular and/or cutaneous diseases strongly modify the responses of dorsal horn neurons, some being at the origin of ascending tracts. This model seems to be a promising approach for studying the fundamental mechanisms involved in chronic pain in order to address these problems in humans.