Inward currents in primary nociceptive neurons of the rat and pain sensations in humans elicited by infrared diode laser pulses

Abstract

Radiant heat is often used to study nociception in vivo. We now used infrared radiation generated by a diode laser stimulator (wavelength 980 nm) to investigate transduction mechanisms for noxious heat stimuli in acutely dissociated dorsal root ganglion (DRG) neurons of rats in vitro. The laser stimulator offered the unique opportunity to test whether the same stimuli also elicit pain sensations in humans. A specific heat-induced current (I(heat)) was elicited in six of 13 small DRG neurons (diameter < or =30 microm) tested in the whole-cell configuration of the patch-clamp mode. Current responses in the seven heat-insensitive neurons were within the range explainable by the temperature dependence of the recording setup. I(heat) was characterized by: (1) non-linearity of its amplitude during a suprathreshold heat ramp as well as with stimuli of increasing intensity with an estimated threshold of 42 +/- 1 degrees C; (2) fast rise time and even faster decay time (t(1/2) = 96.5 +/- 5.9 and 27.7 +/- 1.5 ms, respectively); and (3) rate dependence of its induction. All three heat-sensitive neurons tested were also sensitive to capsaicin. The mean threshold for the induction of I(heat) was 2.8 +/- 0.3 J mm(-2). The threshold for the induction of action potentials by depolarizing current pulses was significantly reduced after laser stimulation, suggesting a sensitization at the transformation stage. No such change was seen in heat-insensitive neurons that underwent the same heat stimuli. The same diode laser elicited pain sensations and laser-evoked potentials in human subjects. No significant differences were seen between the pain thresholds in hairy and in glabrous skin, probably due to the deep penetration of this laser radiation. The mean pain threshold for stimuli > or =200 ms in humans was 2.5 +/- 0.2 J mm(-2) (n = 11), and did not differ from the thresholds for the induction of I(heat) in vitro. Our results indicate that I(heat) in primary sensory neurons can be activated by infrared laser pulses that are painful in humans.

Heat-Induced Action Potential Discharges in Nociceptive Primary Sensory Neurons of Rats

“…This response pattern resembles that of heat-evoked currents (Greffrath et al 2002;Schwarz et al 2000;Vyklický et al 1999). Decreasing depolarizing generator current I heat leads in current-clamp mode to a reduction of membrane depolarization.…”

“…In native DRG neurons, peak values of heat-evoked depolarizing currents vary greatly between 50 and 10,000 pA (Cesare and McNaughton 1996;Greffrath et al 2002;Kirschstein et al 1999;Liu and Simon 2000;Nagy and Rang 1999;Rau et al 2007;Reichling and Levine 1997;Vyklický et al 1999). These currents may or may not be sufficient to reach the threshold of APs (Scholz et al 1998a).…”

Laser acupuncture - innovative basic research: visual and laser-induced evoked potentials

“…8) Other experiments have indicated that brief high heating rate diode laser pulses can selectively activate myelinated Aδ fiber nociceptors in rats and produce pricking pain in humans, whereas low heating rate, longer pulses can preferentially activate unmyelinated C fibers in rats and produce burning pain in humans. [39][40][41][42][43] There is no evidence whether these different pulse parameters will differentially activate fibers in humans.…”

Optical Stimulation of Neurons

“…The authors noted that the temperature effects of irradiation were responsible for the outcomes of the experiments. A recent study showed that irradiation with a pulsed, near-infrared diode laser (l ¼ 980 nm and 4 ms to 400 ms pulse length) induced inward currents in dissociated cell cultures of nocioceptive rat dorsal root ganglion cells [46]. At the stimulation threshold fluence of 2.8 J cm À2 , they calculated a temperature of 42 C at the neurons.…”