Sensation of pain is maybe the most important sense
quoad vitam since it protects against injury and tissue damage by harmful stimuli. Molecular sensors detecting thermal or mechanical tissue damaging stimuli, however, still remain elusive; particularly those detecting threat and not actual damage—the first line of defense. Translation of findings from one system level (in vitro) to another (in vivo) and finally to humans is one of the great challenges in medical science in general [
15], as direct translatability of methods and tools, applicable at more than one system level, is often difficult [
61]. We now investigated intensity coding of the same adequate stimulus at different system levels, facilitating translation of peripheral encoding of noxious heat pain from cellular models to human experiments using a specific non-damaging noxious heat stimulus.
The capsaicin receptor TRPV1 has been described as a cation channel gated by noxious heat [
10]. Phosphorylation by different kinases (PKA, PKC, MAPK), mirrors peripheral sensitization to heat by inflammatory mediators [
25] and the resulting primary hyperalgesia following injury [
9,
10,
13]. The competitive TRPV1 antagonist capsazepine (CPZ) inhibits heat-induced inward currents in TRPV1-expressing cells [
10] and nociceptive neurons [
31,
34]. While mice with depleted TRPV1 carrying nerve fibers selectively lose heat sensitivity [
11], TRPV1 knockout mice still have behavioral sensitivity to ramped heat stimuli [
9,
13]. Two additional TRP channels, TRPM3 and TRPA1, seem to contribute to the perception of heat pain [
60]. The lack of an acute heat phenotype when knocking out TRPV1 may be explained by absence of functional relevance for acute pain or by mismatch of heat stimulus characteristics and TRPV1 thermal gating properties. Until now, there is a mismatch of stimulation paradigms between recent molecular, electrophysiological and behavioural studies of heat pain. Contact heat stimuli are applied by thermodes in human and animal studies [
12,
16,
60], while at the cellular and molecular level superfusion with heated solutions is used [
52,
60]. Radiant heat stimuli that allow precise control of stimulus timing, are considered gold standard for clinical electrophysiological studies of heat pain pathways [
23] but are rarely used in vitro [
19,
27,
64]. Infrared lasers have been used in a few studies on animal behaviour [
1,
8,
39,
63], electrophysiology [
14,
56,
58], dorsal root ganglion (DRG) neurons [
19,
40] and heterologously expressed TRPV1 [
27,
64]. Wavelengths of these lasers, however, range between 980 and 10,600 nm [
19,
59], spot sizes between 0.1 and 10 mm [
27,
41] and pulse durations between 3 and 400 ms [
19,
53], making comparisons across studies difficult.
The aim of this study was to facilitate the direct translation of peripheral encoding of noxious heat from cellular models to human experiments using the same near-infrared laser stimuli in three system levels. Encoding properties of transient, non-damaging laser heat stimuli will be characterized in human psychophysics and rat DRG neurons and compared to heterologously transfected HEK293 cells. Involvement of TRPV1 in signal transduction will be compared between native neurons and a heterologous expression system regarding thresholds, suprathreshold encoding, strength-duration curves and tachyphylaxis.