Nifedipine-morphine interaction: a further investigation on nociception and locomotor activity in mice

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Abstract
Nociception and locomotor activity were tested in mice (C57BL/6 and DBA/2 strains), receiving the dihydropyridine calcium-channel blocker nifedipine, alone or combined with morphine. The calcium antagonist did not change the reaction time to thermal stimulation (tail-flick test), when administered alone, but combinations of nifedipine and morphine prolonged tail-flick latencies less than did the opiate alone. Nifedipine decreased locomotion in both strains, reduced the hypermotility induced by morphine in C57 mice, and enhanced the locomotor depression induced by the opiate in DBA mice. A comparison of the effects of nifedipine with those of the non-calcium antagonist vasodilator, hydralazine, suggests that the interactions with morphine were not exclusively related to neuronal changes produced by calcium channel blockade, but also to haemodynamic factors. In fact, except for the lack of interference with morphine-induced hypermotility in C57 mice, hydralazine, given alone or in combination with morphine, produced effects similar to those of nifedipine.