Inhibitory effect of capsaicin evoked trigeminal pain on warmth sensation and warmth evoked potentials

• Published in: Experimental brain research Vol. 160; no. 1; pp. 29 - 37
• Main Authors: VALERIANI, Massimiliano, TINAZZI, Michele, LE PERA, Domenica, RESTUCCIA, Domenico, DE ARMAS, Liala, MAIESE, Toni, TONALI, Pietro, ARENDT-NIELSEN, Lars
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01.01.2005; Springer Nature B.V
• Subjects: Adult; Afferent Pathways - drug effects; Afferent Pathways - physiology; Biological and medical sciences; Brain - physiology; Capsaicin; Electroencephalography; Evoked Potentials, Somatosensory - drug effects; Evoked Potentials, Somatosensory - physiology; Female; Functional Laterality - drug effects; Functional Laterality - physiology; Fundamental and applied biological sciences. Psychology; Hot Temperature; Humans; Hyperalgesia; Hyperalgesia - chemically induced; Hyperalgesia - physiopathology; Lasers; Male; Nerve Fibers, Myelinated - drug effects; Nerve Fibers, Myelinated - physiology; Nerve Fibers, Unmyelinated - drug effects; Nerve Fibers, Unmyelinated - physiology; Neural Inhibition - physiology; Nociceptors - drug effects; Nociceptors - physiology; Pain; Pain - chemically induced; Pain - physiopathology; Physiology; Reaction Time - drug effects; Reaction Time - physiology; Somesthesis and somesthetic pathways (proprioception, exteroception, nociception); interoception; electrolocation. Sensory receptors; Thermosensing - physiology; Trigeminal Nerve - drug effects; Trigeminal Nerve - physiology; Vertebrates: nervous system and sense organs; Human; Nociception; Sensation; Noxious stimulus; Diffuse noxious inhibitory controls; C fiber; DNIC; Electrophysiology; Thermal stimulus; Nerve fiber C; Hyperalgesia; Lip; Pain; Thermal sensation; Somatosensory evoked potential
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-004-1983-1

The aim of the study was to evaluate the effect of tonic pain evoked by topical application of capsaicin on the somatosensory sensation of warmth. The warmth pathways were studied in ten healthy subjects by recording the scalp potentials evoked by non-painful warm laser stimuli delivered on both the right and left perioral region (warmth C-fiber related laser-evoked potentials (C-LEPs)). Tonic pain was induced by topical capsaicin application above the lateral part of the right upper lip. The area of primary and secondary hyperalgesia were mapped. C-LEPs were obtained from 31 scalp electrodes before, during, and after capsaicin application. C-LEPs from the right perioral region were evoked by laser stimuli delivered to the area of secondary hyperalgesia during capsaicin application and on both the areas of primary and secondary hyperalgesia after capsaicin removal. While the lateralized N1/P1 component (around 185 ms of latency) was not affected by the capsaicin, the amplitudes of the later vertex C-LEPs (around 260 and 410 ms of latency for the N2a and P2 potentials, respectively) evoked from the secondary hyperalgesic area on the right side and from a symmetrical non-hyperalgesic area on the left perioral region were significantly decreased during capsaicin application and after capsaicin removal, as compared with the baseline recordings. At the same times, the rating of the laser-evoked warmth sensation was reduced significantly. This inhibitory effect can occur at brainstem level and is possibly due to: 1) trigemino-cortico-trigeminal circuits, similar to those mediating the classical diffuse noxious inhibitory control, or 2) an increased background activity of the capsaicin-insensitive A-fibers, which mediate the secondary hyperalgesia. Probably due to a peripheral inhibitory mechanism, neither reliable C-LEP components nor warmth sensation were evoked by laser pulses delivered to the primary hyperalgesic area. This is the first neurophysiological evidence in humans of an inhibitory effect of pain on warmth sensation.