Vol. 283, Issue 3, 993-999, 1997

Nifedipine, an L-type Calcium Channel Blocker, Restores the Hypnotic Response in Rats Made Tolerant to the Alpha-2 Adrenergic Agonist Dexmedetomidine¹

Kristina Reid, Tian-Zhi Guo, M. Frances Davies and Mervyn Maze

Department of Anesthesia, Stanford University and Anesthesiology Service, Veterans Affairs Palo Alto Health Care System, Palo Alto, California

Rats were made tolerant to the hypnotic effects of the alpha-2 adrenergic agonist dexmedetomidine by a 7- or 14-day continuous systemic administration of the same, and the ability of nifedipine to reverse dexmedetomidine tolerance was assessed. Acute administration of nifedipine (10 mg/kg i.p.) restored the hypnotic response to dexmedetomidine in the alpha-2 tolerant rats. Concurrent administration of nifedipine during induction of tolerance, either partially (continuous administration 10 mg/kg/day delivered by minipumps) or completely (twice daily injections, 20 mg/kg s.c.) restored hypnotic responsiveness to control levels. Induction of tolerance reduced the affinity of [³H]PN200-110 for the L-type calcium channel. Chronically administered nifedipine treatment (20 mg/kg s.c. twice daily), at doses that partially restored the behavioral response to normal, did not change ligand binding affinity of [³H]PN200-110. An increase in B_max for [³H]PN200-110 was noted in the dexmedetomidine tolerant state which did not change with chronic nifedipine. In naive rats, the phosphodiesterase inhibitor rolipram (275 µg/kg i.p.), mimicked the state of tolerance, as it resulted in a decreased hypnotic response to dexmedetomidine. Nifedipine (10 mg/kg i.p.) also reversed the rolipram-induced attenuation of the hypnotic response to dexmedetomidine. These data implicate a role for the L-type calcium channel in the mechanism of the hypnotic response in alpha-2 tolerant rats and suggest the involvement of the cAMP pathway.