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The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 2853-2860
Copyright ©1999 by the American Physiological Society
1Institute of Physiology and Pathophysiology, Johannes Gutenberg University, D-55099 Mainz; and 2Center of Physiology and Pathophysiology, Georg August University, D-37073 Göttingen, Germany
Kirschstein, Timo,
Wolfgang Greffrath,
Dietrich Büsselberg, and
Rolf-Detlef Treede.
Inhibition of Rapid Heat Responses in Nociceptive Primary Sensory
Neurons of Rats by Vanilloid Receptor Antagonists. J. Neurophysiol. 82: 2853-2860, 1999. Recent
studies demonstrated that heat-sensitive nociceptive primary sensory
neurons respond to the vanilloid receptor (VR) agonist capsaicin, and
the first cloned VR is a heat-sensitive ion channel. Therefore we
studied to what extent heat-evoked currents in nociceptive dorsal root
ganglion (DRG) neurons can be attributed to the activation of native
vanilloid receptors. Heat-evoked currents were investigated in 89 neurons acutely dissociated from adult rat DRGs as models for their own
terminals using the whole cell patch-clamp technique. Locally applied
heated extracellular solution (effective temperature ~53°C) rapidly
activated reversible and reproducible inward currents in 80% (62/80)
of small neurons (
32.5 µm), but in none of nine large neurons
(P < 0.001,
2 test). Heat and
capsaicin sensitivity were significantly coexpressed in this
subpopulation of small DRG neurons (P < 0.001,
2 test). Heat-evoked currents were accompanied by an
increase of membrane conductance (320 ± 115%; mean ± SE, n = 7), had a reversal potential of
5 ± 2 mV (n = 5), which did not differ from
that of capsaicin-induced currents in the same neurons (4 ± 3 mV), and were carried at least by Na+ and Ca2+
(pCa2+ > pNa+). These observations are
consistent with the opening of temperature-operated nonselective cation
channels. The duration of action potentials was significantly higher in
heat-sensitive (10-90% decay time: 4.45 ± 0.39 ms,
n = 12) compared with heat-insensitive neurons (2.18 ± 0.19 ms, n = 6; P < 0.005, Student's t-test), due to an inflection in
the repolarizing phase. This property as well as capsaicin sensitivity
and small cell size are characteristics of nociceptive DRG neurons.
When coadministered with heat stimuli, the competitive VR antagonist
capsazepine (1 µM to 1 mM) significantly reduced heat-evoked currents
in a dose-dependent manner (IC50 13 µM, Hill slope
0.58, maximum effect 75%). Preincubation for 12-15 s shifted the
IC50 by ~0.5 log10 units to an estimated
IC50 of ~4 µM. The noncompetitive VR antagonist
ruthenium red (5 µM) significantly reduced heat-evoked currents by
33 ± 6%. The effects of both VR antagonists were rapidly
reversible. Our results provide evidence for a specific activation of
native VRs in nociceptive primary sensory neurons by noxious heat. The
major proportion of the rapid heat-evoked currents can be attributed to
the activation of these temperature-operated channels, and noxious heat
may be the signal detected by VRs under physiological conditions.
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