6533b7d0fe1ef96bd125a50c

RESEARCH PRODUCT

Inactivation and tachyphylaxis of heat-evoked inward currents in nociceptive primary sensory neurones of rats.

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In contrast to other sensory modalities, pain does not decrease when a noxious stimulus is applied at constant intensity (Greene & Hardy, 1962). From this lack of adaptation on the perceptive level it has traditionally been implied that primary nociceptive afferents also do not adapt upon constant stimulation. This is in contrast to the results of recordings from these afferents, which exhibit pronounced adaptation for physical as well as chemical stimuli (Meyer et al. 1994). Peripheral adaptation of nociceptive nerve endings is compensated by central summation (Mendell & Wall, 1965; Price et al. 1977); this slow summation process of small fibre input to the dorsal horn of the spinal cord is known as wind-up. Since wind-up is specific for wide dynamic range (WDR) neurones, heat responses of nociceptive-specific neurones showed adaptation whereas WDR neurones in the spinal cord did not (Coghill et al. 1993). Compensation of peripheral adaptation to yield a constant pain sensation may be the major function of wind-up. Nociceptive primary afferents act as proportional and differential sensors (PD sensors), exhibiting pronounced (up to 90 %) albeit slow adaptation (τ ≈2·5 s) when stimulated with mechanical or heat stimuli (Meyer & Campbell, 1981; Handwerker et al. 1987; Schneider et al. 1995; Treede et al. 1995). Recovery from this adapted state takes 10 min or longer (LaMotte & Campbell, 1978; Treede et al. 1998) and affects the dynamic response more than the static response (Treede, 1995). This long-lasting reduction of nociceptor discharge is called ‘suppression’. The adaptation and suppression of nociceptive afferent action potential discharges may occur at two stages of the neural encoding process: (1) transduction of physical stimuli into generator potentials and (2) transformation of generator potentials into trains of action potentials. Whereas adaptation in the transformation process is supported by slowing of conduction velocity (Thalhammer et al. 1994; Schmelz et al. 1995; Serra et al. 1999) and slow kinetics of sensory neurone-specific sodium channels (Waxman et al. 1999), there is no evidence for adaptation in the transduction process so far. The transduction process for noxious heat stimuli has been studied using dissociated neurones from dorsal root ganglia (DRG) as models of their own terminals (Cesare & McNaughton, 1996; Kirschstein et al. 1997, 1999; Nagy & Rang, 1999a,b; Vyklický et al. 1999). Brief heat stimuli (< 1 s) were found to elicit inward currents (Iheat) in DRG neurones which did not adapt (Cesare & McNaughton, 1996) and were reproducible with stimulus repetition at short intervals (Kirschstein et al. 1997, 1999; Guenther et al. 1999). Heat stimuli with slowly increasing temperatures revealed a threshold temperature of about 43°C to evoke Iheat in DRG neurones (Vyklický et al. 1999). The correlate of adaptation in the transduction process would be inactivation of Iheat upon constant stimulation; suppression would be visible as tachyphylaxis upon repeated stimulation. The transduction of heat stimuli into membrane currents has been suggested to be mediated by the heat- and capsaicin-sensitive vanilloid receptor VR1 (Caterina et al. 1997; Kirschstein et al. 1999). This receptor when transfected into human embryonic kidney (HEK293) cells shows a threshold of ∼45°C for activation (Tominaga et al. 1998). In DRG neurones, Iheat is partly antagonized by the VR antagonists Ruthenium Red and capsazepine (Kirschstein et al. 1999; Nagy & Rang, 1999b). Whereas the transduction of moderately noxious heat stimuli in DRG neurones is likely to be mediated by the capsaicin receptor VR1, a second heat transduction mechanism with a higher threshold has been described in capsaicin-insensitive DRG neurones (Nagy & Rang, 1999a). This transduction mechanism may involve the vanilloid receptor-like protein 1 (VRL1) which is activated by higher temperatures with a threshold of ∼52°C but which is not sensitive to capsaicin or capsazepine in VRL1-expressing HEK293 cells (Caterina et al. 1999). The aim of this study was to test whether inward currents elicited by moderately noxious heat stimuli show signs of inactivation and tachyphylaxis when long stimulus durations, as used previously in vivo, were applied. As inactivation and tachyphylaxis in capsaicin-induced currents are calcium dependent (Docherty et al. 1996; Liu & Simon, 1998), we further investigated the influence of extracellular and intracellular calcium on inactivation and tachyphylaxis of Iheat. Preliminary accounts of this study have appeared in abstract form (Schwarz et al. 2000).