SUPPRESSION OF FIRST PAIN AND SLOW TEMPORAL SUMMATION OF 2ND PAIN IN RELATION TO AGE

Background. Thermal stimuli delivered to skin of the arms or legs can produce a sensation of two distinct pains. These pains have been assoc iated with activity in A-delta (first pain) and C-fiber (second pain) nociceptive fibers, respectively. Under appropriate conditions first p ain decreases in intensity (adaptation) while second pain increases in intensity (slow temporal summation). Change in first and second pain to repeated stimulation of skin has not been assessed in relation to a ge. Methods. Ten younger (M = 25 years) and ten older (M = 65 years) s ubjects participated in a study of first and second pain intensity eli cited by nociceptive range heat pulses (.7 second pulses; adapting tem perature 39 degrees C to a stimulus temperature of 51 degrees C) deliv ered to skin of arms and legs. Response times to sensations of first a nd second pain were assessed to provide evidence that subjects were re sponding to first and second pain. Results. Age groups did not differ on pain intensity ratings of initial stimuli to previously unstimulate d skin for either first or second pain. Older subjects failed to evide nce slow temporal summation of second pain at the leg. Response times to first, but not second pain from the leg were delayed in the elderly . Response times to first and second pain at the arm did not differ wi th age. Auditory response times were slower in the older group. Conclu sions. Age did not influence pain intensity to unadapted and unsensiti zed skin. Slow temporal summation of second pain was not observed at t he leg in the older group, suggesting that mechanisms subserving C-fib er mediated sensitization of second order nociceptive neurons may fail with age. Longer response times to first, but not second pain in olde r subjects may represent an age effect on myelinated (A-delta; first p ain) and not unmyelinated (C-fiber; second pain) nociceptive afferents and may represent a type of small fiber peripheral neuropathy.