The journal of pain : official journal of the American Pain Society
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Temporal summation of second pain (TSSP) is a psychophysical indication of a central pain encoding mechanism, potentially enhanced in pathological pain conditions. Low-frequency repetitive stimulation of unmyelinated (C) nociceptors results in a progressive increase of pain intensity when thermal stimulation intensity remains constant. However, when using different methods of nociceptive delivery to the skin, regularity as well as rate of pain enhancement with repetition varies between experiments. ⋯ In the present study, TSSP by the intermittent contact with a preheated thermode and constant contact, ramp and hold methods were compared during 10 iterations of stimulation of glabrous skin of the hand or hairy forearm skin, with an onset to onset interval of 3.3 seconds and stimulus interval of .8 seconds. Significantly greater TSSP was observed for intermittent contact stimulation at both sites (P < .001). Differential activation of myelinated and unmyelinated nociceptors by ramping and tapping may account for different rates of temporal summation of heat pain.
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The Acquisition and Extinction of Fear of Painful Touch: a Novel Tactile Fear Conditioning Paradigm.
Fear of touch, due to allodynia and spontaneous pain, is not well understood. Experimental methods to advance this topic are lacking, and therefore we propose a novel tactile conditioning paradigm. Seventy-six pain-free participants underwent acquisition in a predictable as well as an unpredictable pain context. ⋯ Cue exposure reduced fear of touch, whereas context exposure reduced contextual fear. Thus, painful touch leads to increased fear, as does touch in the same context as unpredictable pain, and extinction protocols can reduce this fear. We conclude that tactile conditioning is valuable for investigating fear of touch and can advance our understanding of chronic pain.
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Little is known about endogenous descending control of itch. In chronic pain, descending pain inhibition is reduced as signified by lowered conditioned pain modulation. There are indications that patients with chronic itch may also exhibit reduced endogenous descending inhibition of itch and pain. ⋯ Itch was significantly reduced (conditioned itch modulation-effect) by contra- as well as ipsilateral applied conditioning pain (both P < .001), whereas conditioning itch stimulation only marginally reduced itch. Endogenous descending itch inhibition through mechanisms that are independent of segmental gating can be readily evoked by heterotopic conditioning pain stimulation. However, robust descending inhibition of itch cannot be evoked with conditioning itch stimulation.