The encoding of thermal stimuli applied to the tail of the rat by lowering the excitability of trigeminal convergent neurones.

Recordings were made from convergent neurones in the nucleus caudalis of the trigeminal complex. They could be excited by both innocuous and noxious stimuli applied to their excitatory receptive field located on the ipsilateral part of the muzzle. The responses to A- and C-fibre activation induced by supramaximal transcutaneous electrical stimulation of their receptive fields were conditioned by thermal stimuli applied to the tail, and the relationship between the conditioning temperatures and their ability to induce inhibitions of the evoked discharges of convergent neurones were investigated. During sequences of repetitive stimulation, the posterior two-thirds of the tail were immersed in a waterbath at various temperatures (36-52 degrees C) and the resultant inhibitions of responses were calculated. The threshold for obtaining an inhibitory effect ranged between 40 and 44 degrees C. Above this level, there was a highly significant correlation between the conditioning temperature and the degree of inhibition, which was very strong for the highest temperature (i.e. for 52 degrees C: 54.4 +/- 7.3 and 92.3 +/- 3.7% inhibitions being obtained for A- and C-fibre-evoked responses respectively); such a correlation concerned the inhibitions observed both during immersion of the tail and after the removal of the conditioning stimuli. In further experiments, identical conditioning stimuli were tested upon responses of trigeminal convergent neurones to microelectrophoretic applications of an excitatory amino acid, DL-homocysteate, and very similar results were obtained: a threshold for inhibition in the 40-44 degrees C range and a significant correlation between the conditioning temperature and the degree of inhibition in the 44-52 degrees C range (52 degrees C giving inhibitions of 98.8 +/- 0.4%.(ABSTRACT TRUNCATED AT 250 WORDS)