| Summary: | 博士 === 國立成功大學 === 基礎醫學研究所 === 93 === Previous studies have shown that neurosteroids dehydroepiandrosterone (5-androsten-3b-ol-17-one; DHEA) and pregnenolone sulfate (5-pregnen-3b-ol-20-one sulfate; PS) modulate amino acid receptor-mediated responses in brain. Compared to the extensive studies of DHEA and PS effect on amino acid receptors, relatively little is known of interaction of DHEA and PS with the capsaicin receptor. In the present study, we investigated the effects of DHEA and PS on the capsaicin receptor-mediated current in acutely dissociated rat dorsal root ganglion neurons using the whole-cell voltage-clamp recording technique. DHEA and PS rapidly and reversibly inhibited the capsaicin-induced current in a concentration-dependent manner, with EC50 values of 6.7 and 13 mM and maximal inhibition of 100 and 65%, respectively. DHEA increased the capsaicin EC50 with little effect on the capsaicin maximal response, suggesting that the blocking action of DHEA was competitive. Neither the capsaicin response nor inhibition of the capsaicin response by extracellularly applied DHEA was significantly affected by inclusion of a saturating concentration of DHEA in the electrode buffer, arguing that DHEA acted at the extracellular surface of the membrane. Moreover, DHEA did not act through protein phosphatases 1, 2A or 2B to inhibit the capsaicin-induced current. Not all steroids inhibited the capsaicin response. Progesterone did not exert any significant effect on the capsaicin-induced current, suggesting that inhibition by DHEA of the capsaicin response was a specific effect. Furthermore, the stereoisomer of DHEA, 5-androsten-3a-ol-17-one (3a-DHEA), failed to inhibit the capsaicin-induced current, producing instead a potentiating effect on the capsaicin response, demonstrating that interaction of steroids with the capsaicin receptor was stereospecific. Neither the capsaicin response nor inhibition of the capsaicin response by extracellularly applied PS was significantly affected by inclusion of a saturating concentration of PS in the electrode buffer, arguing that PS acted at the extracellular surface of the membrane. Furthermore, PS inhibited the capsaicin maximal response with little effect on the capsaicin EC50, demonstrating that the blocking action of PS was noncompetitive. The fact that antagonism of the capsaicin response by PS was neither voltage- nor agonist-dependent, indicating that PS did not act as an open-channel blocker. In addition, we investigated the effects of intradermal administration of PS on the nociceptive response evoked by intradermal injection of capsaicin into the rat hindpaw. Results revealed that PS dose-dependently inhibited the capsaicin-induced nociceptive response. Moreover, PS did not act through opioid- and cannabinoid-dependent mechanisms to reduce the capsaicin-induced nociception. Furthermore, intradermal injection of capsaicin did not cause the release of endogenous opioids and cannabinoids. Our results suggested that PS depressed the capsaicin-induced nociception via direct inhibition of the capsaicin receptor.
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