Somatic responses of ventrobasal thalamic neurones in polyarthritic rats

Rats rendered polyarthritic by injection of Mycobacterium butyricum into the tail were used as a model for the study of 'chronic pain'. In such rats unitary responses of ventrobasal thalamic neurons to somatic stimulations were dramatically modified by comparison to those described in normal rats investigated in the same anaesthetic conditions. (1) Only the neurons with receptive fields located on inflamed areas (168/194 in 33 rats) have been considered in this study. 27/168 activated only by brushing displayed the classical properties of lemniscal responses; only 20/168 were activated exclusively by intense cutaneous stimuli and 13/168 already activated by light cutaneous stimuli had enhanced discharges when the stimulus intensity was increased. By contrast numerous units (108/168) were excited by mild stimulations applied to the joints or to adjacent cutaneous areas (82 were driven by joint movement and/or mild lateral pressure on the articulation, 26 by brushing the overlapping skin); these responses presented atypical characteristics and displayed unusual patterns with very long afterdischarges of duration several times that of the stimulus. (2) In 20 additional arthritic rats, responses to transcutaneous electrical stimulation (TES) and/or to noxious heat, were obtained for 34 neurones responding to joint stimuli. (a) 16 of 18 neurones tested with transcutaneous electrical stimulation had latencies of 25-100 ms, and thresholds of 1-4 mA (width of shock 2 ms). (b) Neurones activated by joint stimuli frequently responded to noxious heat (radiant or waterbath). Initially, their response thresholds tested in 16 neurones were higher by about 4 degrees C than those of 'noxious' VB neurones in normal rats; however, following sensitization to heat, thresholds were decreased by 4 degrees C. For 8 neurones there was a linear relation between stimulus intensity and responses. (3) Several different factors which could explain the important modification of neuronal responses in VB complex of arthritic rats by comparison with normal are proposed in the discussion.