Thresholds and encoding of neuronal responses to mechanical stimuli in the ventro-basal thalamus during carrageenin-induced hyperalgesic inflammation in the rat.

Neuronal response thresholds and the encoding of mechanical stimulus intensity in the ventro-basal (VB) thalamus was analyzed in anaesthetized rats before and during the first two hours following induction of hyperalgesic inflammation. This inflammation was induced by the intra-plantar injection of carrageenin in the hindpaw contralateral to the recorded neurones. Only neurones exclusively driven by noxious stimuli and with a receptive field on or including the injected paw were considered. In this early phase of the inflammatory process, there was no significant modification of the response threshold to the mechanical stimulus (indentation of about 300 micron). This suggests the involvement of additional neuronal population(s) to account for the decrease in the vocalisation threshold to pressure observed in the freely moving animal at this time of the inflammation. A liner encoding of the indentation depth was observed before and after the carrageenin injection although the slope of the stimulus response-curve was steeper after the injection. The data emphasize that the carrageenin-sensitization acts differentially on the liminal and supra-liminal responses of the same neurone to a skin indentation, since in the first hour following the initiation of the inflammation the sensitization is essentially observed for responses obtained with stimulus intensity largely above the threshold value. With regard to previous observations using thermal stimulation, the results also illustrate that the carrageenin induced sensitization of responses differs depending on the stimulus intensity and modality used.