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

Abstract

The aim of this study was to characterize plasma membrane pathways involved in the intracellular calcium ([Ca 2+] i) response of small DRG neurons to mechanical stimulation and the modulation of these pathways by κ-opioids. [Ca 2+] 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 [Ca 2+] i increases which were abolished in Ca 2+-free solution, but unaffected by lanthanum (25 μM) or tetrodotoxin (10 −6 M). 156 out of 465 neurons tested (34%) showed mechanosensitivity while 55 out of 118 neurons (47%) were capsaicin-sensitive. Ninty percent of capsaicin-sensitive neurons were mechanosensitive. Gadolinium (Gd 3+; 250 μM) and amiloride (100 μM) abolished the [Ca 2+] i transient in response to mechanical stimulation, but had no effect on capsaicin-induced [Ca 2+] i transients. The κ-opioid agonists U50,488 and fedotozine showed a dose-dependent inhibition of mechanically stimulated [Ca 2+] i transients but had little effect on capsaicin-induced [Ca 2+] i transients. The inhibitory effect of U50,488 was abolished by the κ-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 [Ca 2+] i transients in small diameter DRG somas are mediated by influx of Ca 2+ through a Gd 3+- and amiloride-sensitive plasma membrane pathway that is co-expressed with capsaicin-sensitive channels. Mechanical-, but not capsaicin-mediated, Ca 2+ transients are sensitive to κ-opioid agonists.