Use of a near-infrared diode laser to activate mouse cutaneous nociceptors in vitro

A skin-nerve preparation is useful for study of heat transduction mechanisms of A- and C-high threshold primary afferents (nociceptors), but the small dimension and liquid environment of the skin organ bath do not readily accommodate conventional noxious heat delivery systems. For these reasons, a 980 nm (near-infrared) diode laser was tested for activation and differentiation of cutaneous afferents. Current to the laser driver was varied. Exposure time and area, angle of approach, and stand-off distance from the bath solution surface were held constant. Seventy-eight fibers were classified by: conduction velocity, mechanical threshold, and responsiveness to laser radiation. A subset of the sampled fibers was also tested for sensitivity to convective heat. Most C (30/43) and a few A (6/25) nociceptors responded to laser irradiation. All low mechanical threshold primary afferents (10/10) were unresponsive to laser irradiation. Laser-sensitive fibers responded to convective heat, whereas laser-insensitive fibers did not. Laser-induced responses were consistent with literature reports of responses to traditional heat stimulation. Laser stimulation proved to be a rapid, unobtrusive method for reproducible heat stimulation of primary afferents of the mouse skin-nerve preparation. It is effective for defining subpopulations of primary afferent fibers and holds promise as a tool for gauging modification of C-fiber activity.