The effect of cyclosporin A on morphine tolerance and dependence: involvement of L-arginine/nitric oxide pathway.

Cyclosporin A is known to decrease nitric oxide (NO) production in nervous tissues. The effects of systemic cyclosporine A on the induction and expression of morphine tolerance and dependence, acute morphine-induced antinociception, and the probable involvement of the L-arginine/nitric oxide pathway in these effects were assessed in mice. Cyclosporin A (20 mg/kg), N(G)-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg) and a combination of the two at lower and per se non-effective doses (5 and 3 mg/kg, respectively) showed a similar pattern of action, inhibiting the induction of tolerance to morphine-induced antinociception and increasing the antinociception threshold in the expression phase of morphine tolerance. These agents also inhibited the expression of morphine dependence as assessed by naloxone-precipitated withdrawal signs, while having no effect on the induction of morphine dependence. L-Arginine, at a per se non-effective dose (60 mg/kg), inhibited the effects of Cyclosporin A. Moreover, acute administration of Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) enhanced the antinociception induced by acute administration of morphine (5 mg/kg), while chronic pretreatment with Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) for 2 days (twice daily) did not affect morphine-induced antinociception. The inducible nitric oxide synthase inhibitor, aminoguanidine (100 mg/kg), did not alter morphine antinociception, tolerance or dependence. In conclusion, decreasing NO production through constitutive nitric oxide synthase may be a mechanism through which cyclosporin A differentially modulates morphine tolerance, dependence and antinociception.