Alterations in L-type calcium channels in the brain and spinal cord of acutely treated and morphine-tolerant mice.

Opiate tolerance has been associated with changes in neuronal intracellular calcium levels. In vivo studies have indicated the involvement of dihydropyridine (DHP)-sensitive ('L-type') voltage-gated calcium channels in the morphine-tolerant state. In this study, we assessed how the morphine-tolerant state would affect the antinociception produced by intrathecal administration of two agents (Bay K 8644 and thapsigargin) that increase intracellular calcium. Morphine-tolerant mice displayed a 7-fold greater ED50 for Bay K 8644 antinociception than placebo-treated animals; no difference was found in the dose-response curve for thapsigargin. To further explore the role of the L-type channel in morphine tolerance development, we performed radiolabeled nitrendipine binding studies in the spinal cord and brain regions associated with pain modulation from acutely treated, tolerant, and control mice. Our treatment protocol produced a 4-fold shift in the ED50 for s.c. morphine antinociception, as determined by the tail-flick procedure. Binding site number and affinity were determined using Scatchard analysis for the following timepoints: 20 min and 60 min after s.c. injection of morphine or vehicle, as well as after 4 days of chronic administration, and after naloxone-precipitated withdrawal. Although some changes were observed in the affinity of nitrendipine for its receptor, a significant change in these measures was found only following naloxone administration, which produced increases in Bmax in both placebo- and morphine-treated mice.