QUANTITATIVE-ANALYSIS OF OROFACIAL THERMORECEPTIVE NEURONS IN THE SUPERFICIAL MEDULLARY DORSAL HORN OF THE RAT

Surprisingly little is known concerning the central processing of inno cuous thermal somatosensory information. The aim of the present study was to obtain quantitative data on the characteristics of neurons in t he rat superficial medullary dorsal horn (sMDH) that responded to inno cuous thermal stimulation of the rat's face and tongue. Single-unit ex tracellular recordings were obtained in choralose-urethane anesthetize d rats. A total of 153 thermoreceptive neurons was studied. Of these, 146 were excited by cooling and inhibited by warming and were classifi ed as COLD cells. The remaining seven cells were excited by innocuous warming of the skin or tongue. Of 123 COLD cells tested, 33% were exci ted by touch and 22% by pinch stimuli delivered to the thermoreceptive field. Of the 50 COLD cells tested, 46% were excited also by noxious healing (greater than or equal to 50 degrees C for 5 s). Most (82/121) of the receptive fields were located on the upper lip, 25 on the tong ue, and most of the remaining on the lower Lip. Receptive fields were generally small (1-5 mm(2)). In some experiments, electrical stimulati on in the thalamus was performed, and nine COLD cells could be activat ed antidromically. The responses of 38 COLD cells to incremental 5 deg rees C cooling steps were examined quantitatively. Thermal stimuli wer e applied to facial or Lingual receptive fields of sMDH neurons with a computer-controlled Peltier thermode starting from 33 degrees C, decr easing to 8 or 3 degrees C, and returning to 33 degrees C. Most COLD c ells (26/38) had both static and dynamic responses; 7 had mainly dynam ic and 5 mainly static responses to step decreases in temperature. Rat sMDH COLD cells could be classified into three groups depending on th eir stimulus-response functions. The first group (Type 1, n = 19) had a bell-shaped static stimulus response function. The second group (Typ e 2) had a high maintained or increasing static firing rate as the tem perature decreased < 18 degrees C (n = 10). Type 3 COLD cells had main ly dynamic properties (n = 7). Many of the cells in all groups were ex cited by noxious mechanical stimulation. Type 2 cells differed from th e other two groups in that most did not respond to noxious thermal sti muli (hot) and many responded to innocuous tactile stimuli. Neurons fr om each of the three groups of COLD cells could be activated antidromi cally from contralateral thalamus. These data suggest that there is li ttle central processing of thermal information at the first central sy napse for Type 1 neurons, however, the responses of the other two type s may be due to central processing and convergence. The demonstration of rat sMDH COLD cells with distinctive stimulus-response functions to thermal shifts suggests separate functional roles of these neurons in the ascending thermal sensory pathway.