Mechanical activation of dorsal root ganglion cells in vitro:: comparison with capsaicin and modulation by κ-opioids

The aim of this study was to characterize plasma membrane pathways involved in the intracellular calcium ([Ca2+](i)) response of small DRG neurons to mechanical stimulation and the modulation of these pathways by kappa-opioids. [Ca2+](i) responses were measured by fluorescence video microscopy of Fura-2 labeled lumbosacral DRG neurons obtained from adult rats in short-term primary culture. Transient focal mechanical stimulation of the soma, or brief superfusion with 300 nM capsaicin, resulted to [Ca2+](i) increases which were abolished in Ca2+-free solution, but unaffected by lanthanum (25 mu M) or tetrodotoxin (10(-6) M). 156 out of 465 neurons tested (34%) showed mechanosensitivity while 55 out of 118 neurons (47%) were capsaicin-sensitive. Ninety percent of capsaicin-sensitive neurons were mechanosensitive. Gadolinium (Gd3+; 250 mu M) and amiloride (100 mu M) abolished the [Ca2+](i) transient in response to mechanical stimulation, but had no effect on capsaicin-induced [Ca2+](i) transients. The kappa-opioid agonists U50,488 and fedotozine showed a dose-dependent inhibition of mechanically stimulated [Ca2+](i) transients but had little effect on capsaicin-induced [Ca2+](i) transients. The inhibitory effect of U50,488 was abolished by the kappa-opioid antagonist nor-Binaltorphimine dihydrochloride (nor-BNI; 100 nM), and by high concentrations of naloxone (30-100 nM), but not by low concentrations of naloxone (3 nM). We conclude that mechanically induced [Ca2+](i) transients in small diameter DRG somas are mediated by influx of Ca2+ through a Gd3+- and amiloride-sensitive plasma membrane pathway that is co-expressed with capsaicin-sensitive channels. Mechanical-, but not capsaicin-mediated, Ca2+ transients are sensitive to kappa-opioid agonists. (C) 2000 Elsevier Science B.V. All rights reserved.