[A comprehensive study of the neurochemical and immune mechanisms of morphine tolerance: the effects of naloxone].

To test the authors' hypothesis about the role of endopeptidase (enkephalinase A, in particular) in mechanisms of morphine tolerance and blocking action of small doses of naloxone, they studied nociception reactions, morphine antibodies titres and enkephalinase A activity after morphine, d-phenylalanine and naloxone injection in brain structures. It is shown that activity of enkephalinase A in structures of endogenous antinociceptive system increased simultaneously with morphine antibodies titres in tolerance condition. Injection of small dose naloxone inhibited enkephalinase activity in brain structures and decreased morphine antibodies titres to these in control morphine-sensitive rats and therefore suppressed morphine tolerance. Prolonged naloxone injection decreased morphine antibodies titres to the levels of intact animals and highly increased titers of antiidiotypic morphine antibodies. Thus, these results confirm the role of enkephalinase as a neuromodulator. A strong relationship exists between enkephalinase and immune mechanisms of development of morphine tolerance which can be blocked by small naloxone doses. It is concluded that naloxone in small doses can be used in patients to suppress morphine tolerance.