Dorsal horn (convergent) neurones in the intact anaesthetized arthritic rat. I. Segmental excitatory influences

Abstract

Recordings were made from dorsal horn neurones in intact anaesthetized rats rendered polyarthritic by s.c. injection into the base of the tail, of Mycobacterium butyricum suspended in oil; the experiments were carried out during the acute phase of the illness (3-4 weeks post inoculation) during which hyperaesthesia occurred. The majority (60.8%) of the neurones studied had properties close to those of corresponding groups of units in healthy rats. These ''typical'' neurones could be subdivided into convergent (13.2%), non-noxious (34.4%) and proprioceptive (13.2%) units. By contrast, and in agreement with a previous study in the unanaesthetized spinal arthritic rat [37] the segmental electrophysiological characteristics of the remaining large proportion of neurones were changed both in terms of the size and distribution of their excitatory receptive fields and their responsiveness to peripheral stimuli; these were designated as ''atypical'' neurones. According to their electrophysiological properties, these neurones were differentiated as atypical convergent (27.8%) and atypical non-noxious (11.4%) units. The main qualitative difference between the typical and atypical neurones was that the atypical had an additional receptive field on the oedematous ipsilateral ankle and, in several cases, showed high levels of background activity with sometimes dramatic increases. By comparison with neurones recorded in healthy rats, quantitative data revealed other modifications: typical and atypical convergent neurones and atypical non-noxious neurones had larger classical excitatory receptive fields; while C-fibre responses evoked by transcutaneous electrical stimulation were facilitated in the case of typical convergent neurones, 47% of the atypical convergent neurones had no C-fibre responses, and when present (53%) the threshold for obtaining these C-fibre responses was higher with suprathreshold stimuli producing a minimal number of spikes; in these cells, gentle mechanical stimuli gave rise to high rates of firing which sometimes resulted in dramatic, long lasting after-discharges. The possibility that typical convergent, atypical convergent and atypical non-noxious neurones were derived from the homogenous population of convergent neurones in the healthy rat is discussed; the atypical properties could be the result of a change in the characteristics of convergent neurones, resulting from arthritis. In conclusion, from a general point of view, increases in segmental excitatory processes mechanically triggered from lesioned hind paws can be observed in the dorsal horn of arthritic rats: these findings could account for experimentally observed hyperaesthesia in human patients suffering from chronic pain due to rheumatoid arthritis and for the sudden flashes of pain reported by such patients even in the absence of stimulation of the part of the body concerned with the disease.