CHARACTERISTICS OF TEMPORAL SUMMATION OF 2ND PAIN SENSATIONS ELICITEDBY BRIEF CONTACT OF GLABROUS SKIN BY A PREHEATED THERMODE

Temporal summation of sensory intensity was investigated in normal sub jects using novel methods of thermal stimulation. A Peltier thermode w as heated and then applied in a series of brief (700 ms) contacts to d ifferent sites on the glabrous skin of either hand. Repetitive contact s on the thenar or hypothenar eminence, at interstimulus intervals (IS Is) of 3 s, progressively increased the perceived intensity of a therm al sensation that followed each contact at an onset latency >2 s. Temp oral summation of these delayed (late) sensations was proportional to thermode temperature over a range of 45 - 53 degrees C, progressing fr om a nonpainful level (warmth) to painful sensations that could be rat ed as very strong after 10 contacts. Short-latency pain sensations rar ely were evoked by such stimuli and never attained levels substantiall y above pain threshold for the sequences and temperatures presented. T emporal summation produced by brief contacts was greater in rate and a mount than increases in sensory intensity resulting from repetitive ra mping to the same temperature by a thermode in constant contact with t he skin. Variation of the interval between contacts revealed a depende nce of sensory intensity on interstimulus interval that is similar to physiological demonstrations of windup, where increasing frequencies o f spike train activity are evoked from spinal neurons by repetitive ac tivation of unmyelinated nociceptors. However, substantial summation a t repetition rates of greater than or equal to 0.33 Hz was observed fo r temperatures that produced only late sensations of warmth when prese nted at frequencies <0.16 Hz. Measurements of subepidermal skin temper ature from anesthetized monkeys revealed different time courses for st orage and dissipation of heat by the skin than for temporal summation and decay of sensory intensity for the human subjects. For example, ne gligible heat loss occurred during a 6-s interval between two trials o f 10 contacts at 0.33 Hz, but ratings of sensory magnitude decreased f rom very strong levels of pain to sensations of warmth during the same interval. Evidence that temporal summation of sensory intensity durin g series of brief contacts relies on central integration, rather than a sensitization of peripheral receptors, was obtained using two approa ches. In the first, a moderate degree of temporal summation was observ ed during alternating stimulation of adjacent but nonoverlapping skin sites at 0.33 Hz. Second, temporal summation was significantly attenua ted by prior administration of dextromethorphan, a N-methyl-D-aspartat e receptor antagonist.