Enigmatic action of ciclosporine A on the naloxone-precipitated morphine withdrawal syndrome in mice.

Various alterations of the immune system have recently been reported to attenuate the severity of morphine withdrawal. The effect of the immunosuppressive agent ciclosporine A on the naloxone-induced morphine withdrawal syndrome in the chronically dependent mouse was investigated. Ciclosporine A significantly suppressed stereotyped behaviour such as jumping and forepaw treading while wet shakes were potentiated. Withdrawal diarrhoea was diminished as a consequence of a promotive action of ciclosporine A on the intestine. A ciclosporine derivative, which is devoid of immunosuppressive activity, had no influence on withdrawal signs. The attenuating effect of ciclosporine A was observed at a dose of 20 mg/kg i.p., which is not regarded as immunosuppressive in the mouse. It was also effective in animals lacking an intact immune system as a result of a genetic T-cell defect (nude mouse) or after selective ablation by whole body irradiation. Nude mice and irradiated normal mice developed dependence on morphine to the same extent as normal animals, as could be derived from the severity of their withdrawal signs. These results suggest that an intact immune system is not a necessary prerequisite for ciclosporine A to attenuate morphine withdrawal and that its action may be attributable to mechanisms other than immunosuppression. It is possibly a result of a direct effect of ciclosporine A on the central nervous system structures involved in the behavioural expression of the opiate withdrawal syndrome.