Central changes in processing of mechanoreceptive input in capsaicin‐induced secondary hyperalgesia in humans.

TLDR

The study used intradermal capsaicin injections (100 µg in 10 µl) to induce localized pain and hyperalgesia, and employed selective nerve compression to block A‑ or C‑fiber conduction, revealing differential contributions of these fibers to pain and mechanical hyperalgesia. The experiments showed that C‑fiber activity drives capsaicin‑induced pain and heat hyperalgesia, while A‑fiber conduction is required for mechanical hyperalgesia, and that intraneural microstimulation becomes painful with lowered thresholds in the hyperalgesic zone, indicating reversible central changes in mechanoreceptive processing. The abstract is organized into numbered sections 1–5.

Abstract

1. Capsaicin, the algesic substance in chilli peppers, was injected intradermally in healthy human subjects. A dose of 100 micrograms given in a volume of 10 microliters caused intense pain lasting for a few minutes after injection and resulted in a narrow area of hyperalgesia to heat and a wide surrounding area of hyperalgesia to mechanical stimuli (stroking) lasting for 1‐2 h. 2. Nerve compression experiments with selective block of impulse conduction in myelinated (A) but not in unmyelinated (C) fibres indicated that afferent signals in C fibres contributed to pain from capsaicin injection and to heat hyperalgesia, whereas conduction in afferent A fibres was necessary for the perception of mechanical hyperalgesia. 3. Electrical intraneural microstimulation normally eliciting non‐painful tactile sensations was accompanied by pain when the sensation was projected to skin areas within the region of mechanical hyperalgesia induced by capsaicin injection. 4. The threshold for pain evoked by intraneural microstimulation was reversibly lowered and pain from suprathreshold stimulation was exaggerated during the period of mechanical hyperalgesia, regardless of lidocaine anaesthesia of the cutaneous innervation territory of the stimulated fibres. 5. The results indicate that hyperalgesia to stroking on a skin area surrounding a painful intradermal injection of capsaicin is due to reversible changes in the central processing of mechanoreceptive input from myelinated fibres which normally evoke non‐painful tactile sensations.