Encoding of noxious heat messages in neurons of the ventrobasal thalamic complex of the rat

Ventrobasal thalamic neurons responsive to noxious mechanical stimuli were tested with noxious heat stimuli graded in temperature, surface-area and duration. Experiments were performed by plunging the tail of intact, lightly anesthetized rats into a temperature-controlled water bath. Seventeen of 24 neurons encoded stimulus temperature by frequency of discharge although the responses of 6 of these 17 reached a plateau at the highest temperatures. Alternatively, the cell population might code stimulus temperature by a recruiting mechanism since response thresholds were distributed between 40 and 50 degrees C. Some units also increased their discharge in parallel with an increase in stimulus area and/or duration. Analyses of discharge patterns were performed. A decrease of discharge frequency during stimulation was not observed before several tens of seconds had elapsed. Thus, the average response of the cell population to 15, 30 and 60 sec stimuli showed no clear 'adaptation'. In our conditions, i.e. with most of the stimulations limited to a duration of 15 sec each, sensitization to heat was observed after 55 and 60 degrees C, but not after 50 degrees C. These data indicate that noxious heat stimulus parameters are coded at the thalamic level in the rat by both an increase in discharge and a progressive recruitment of units.