Regulation of dihydropyridine-sensitive Ca 2+ channels during naloxone-induced opioid supersensitivity in rats

Abstract

The Ca 2+ channel blocker, nimodipine, increases the chronic naltrexone-induced supersensitivity to morphine analgesia in mice without affecting the density of up-regulated μ-opioid receptors. In the present study, the change in dihydropyridine-sensitive Ca 2+ channels associated with chronic naloxone-induced supersensitivity to morphine analgesia was studied in rat whole-brain membranes using a radiolabeled Ca 2+ channel blocker, [ 3H]PN200-110 {isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-methoxycarbonylpyridine-3-carboxylate} (0.02–1.0 nmol/l). Rats were chronically treated with naloxone (1 mg/kg, i.p.), nimodipine (1 mg/kg, i.p.) or their respective vehicles twice daily for 10 days. On the 11th day, 16 h after the last injection of either nimodipine or naloxone, morphine (2 mg/kg, i.p.)-induced tail-flick analgesia was determined or rats were killed for the binding study. Chronic naloxone significantly potentiated (+84%) the morphine-induced analgesia. Chronic nimodipine also potentiated (+76%) morphine analgesia. In rats treated with nimodipine and naloxone, there was an additive increase (206%) in morphine analgesia. In binding studies, chronic naloxone resulted in a significant decrease (−39%) in the density ( B max) of [ 3H]PN200-110 binding with no change in K d value when compared to the effect of chronic vehicle. Chronic nimodipine resulted in a slight but significant (+14.5%) increase in the B max of [ 3H]PN200-110. However, when nimodipine was co-administered with naloxone, it not only reversed the down-regulation but actually up-regulated (+25%) [ 3H]PN200-110 binding with no change in K d value. Our results show significant down-regulation of [ 3H]PN200-110 binding in association with naloxone-induced analgesic supersensitivity to morphine. The supersensitivity was also observed following chronic blockade of up-regulated Ca 2+ channels by nimodipine. These results indicate an important role of L-type Ca 2+ channel regulation in naloxone-induced analgesic supersensitivity to morphine.