Blocking naloxone-precipitated withdrawal in rats and hamsters.

Three experiments studied the effects of putative antagonists of opiate withdrawal in hamsters and rats. In Experiment 1, the calcium channel antagonists verapamil (20 mg/kg) and nifedipine (20 mg/kg) failed to antagonize naloxone (1 mg/kg)-precipitated withdrawal in hamsters implanted with two 75-mg morphine pellets, whereas clonidine (0.4 mg/kg), the alpha 2-adrenergic agonist, blocked most withdrawal signs. In Experiment 2, clonidine (0.4 mg/kg) and verapamil (20 mg/kg) were tested against naloxone-precipitated withdrawal in hamsters made acutely dependent by a single injection of morphine (15 mg/kg). As in Experiment 1, clonidine but not verapamil was effective. In Experiment 3, the effects of verapamil on naloxone-precipitated withdrawal were studied in morphine-pelleted rats and hamsters. In rats implanted with two morphine pellets, verapamil (20 mg/kg) reversed naloxone-precipitated withdrawal. By contrast, in hamsters implanted with either one or two morphine pellets neither of two doses of verapamil (20 and 30 mg/kg) was effective. These results are discussed in terms of species' differences in sensitivity to calcium channel blockers.