Journal of neurophysiology
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1. Nocigenic inhibition is the inhibition of neural, behavioral, or reflex responses to a nociceptive test stimulus produced by another, conditioning, nociceptive stimulus. The present study examines whether a natural noxious visceral stimulus, colorectal distension, used as a conditioning stimulus would inhibit neuronal or reflex responses to noxious cutaneous stimuli. ⋯ All neurons inhibited by colorectal distension (51 class 2 and 8 class 3 neurons) were also inhibited by noxious pinch of the nose or forepaw. The magnitude of the nocigenic inhibition of responses during heating of the hindpaw was graded with the intensity and duration of the noxious conditioning colorectal distension, was a function of the number of preceding distensions given to the rat, and outlasted the distending stimulus. Conditioning colorectal distension also produced a parallel shift to the right in stimulus-response functions relating responses of neurons to the intensity of the noxious test stimulus (42-50 degrees C).(ABSTRACT TRUNCATED AT 400 WORDS)
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1. Psychophysical studies were made, in humans, of the sensory characteristics and underlying mechanisms of the hyperalgesia (often termed "secondary hyperalgesia") that occurs in uninjured skin surrounding a local cutaneous injury. The hyperalgesia was characterized by lowered pain thresholds and enhanced magnitude of pain to normally painful stimuli. ⋯ Heat stimulation of the skin that produced pain that was equivalent in magnitude and time course to that produced by an injection of capsaicin (10 micrograms) resulted in much smaller areas of mechanical hyperalgesia. It was postulated that there exist special chemosensitive primary afferent nerve fibers that are more effective in producing mechanical hyperalgesia than are the known thermo- and mechanosensitive nociceptive nerve fibers. 6. Once developed, the mechanical hyperalgesia became only partially dependent on peripheral neural activity originating at the site of injury.(ABSTRACT TRUNCATED AT 400 WORDS)
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Comparative Study
Neurogenic hyperalgesia: central neural correlates in responses of spinothalamic tract neurons.
1. The contribution of activity in spinothalamic tract (STT) neurons to the pain and neurogenic hyperalgesia produced by an intradermal injection of 100 micrograms of capsaicin was investigated. Electrophysiological responses of identified STT neurons recorded in anesthetized monkeys were compared with psychophysical measurements of pain and hyperalgesia obtained in humans using identical stimuli. 2. ⋯ Capsaicin significantly increased the responses of HT neurons (9-fold) and the responses of WDR neurons (2-fold) to stroking the skin within the receptive field. Similar increases in responses to a standard punctate stimulus were observed at test sites, 1, 2, and 3 cm away from the injection site. After injection of vehicle, the responses to punctate stimulation increased by a mean of only 1.2- and 1.4-fold for HT and WDR neurons, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)
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1. A local cutaneous injury can produce primary hyperalgesia within the injured area and secondary hyperalgesia in the normal surrounding skin. An intradermal injection of capsaicin in humans causes intense pain and hyperalgesia to heat and to mechanical stimuli in the surrounding skin. ⋯ In some cases, similar results were produced by the vehicle alone. However, capsaicin and not the vehicle lowered the thresholds of some CMHs to heat. Thus the sensitization of CMHs contributes to the primary hyperalgesia known to occur within the area of skin directly exposed to topically applied capsaicin.(ABSTRACT TRUNCATED AT 400 WORDS)