Halothane sensitizes cutaneous nociceptors in monkeys

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Abstract
The effects of halothane on the responses of C-fiber (CMHs) and A-fiber (AMHs) nociceptive afferents sensitive to mechanical and heat stimuli were studied in monkeys. The response to heat stimuli was studied with use of a laser thermal stimulator that provided stepped increases in skin temperature over a 7.5-mm-diameter area with rise times to the desired temperature near 100 ms. Recordings were obtained from single fibers that innervated the glabrous skin of the hand using a teased-fiber dissection technique. In initial studies the response of 32 CMHs and 45 AMHs in monkeys anesthetized with pentobarbital sodium (3-6 mg X kg-1 X h-1) was compared with the response of 12 CMHs and 23 AMHs in monkeys anesthetized with a combination of halothane (0.8%) and N2O (67%). A standardized set of 10 3-s heat stimuli ranging from 41 to 49 degrees C delivered every 30 s were applied to the receptive field. Both AMHs and CMHs had a lower threshold and greater response to suprathreshold heat stimuli under conditions of halothane-N2O anesthesia. The threshold to mechanical stimuli, as tested by von Frey hairs, was not significantly different. Five CMHs and 5 AMHs were studied in a crossover study in which responses to the 41-49 degrees C stimuli were obtained first under halothane-N2O (0.8%-67%) anesthesia, then under an ultrashort acting barbiturate, methohexital (2-9 mg/kg over 15 min), and finally once again under halothane-N2O anesthesia. For the five CMHs, the mean cumulative response was 1.8 times greater, whereas for the five AMHs the response was 4.7 times greater under halothane as opposed to barbiturate anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)