Susceptibility to amphetamine-elicited dyskinesias following chronic methadone treatment in monkeys

Rats sensitized to morphine are resistant to the behavioral effects of an unavoidable stress

In agreement with the results of other authors, rats sensitized to morphine and challenged with 5 mg/kg of morphine after 7 days of wash-out showed intense stereotyped movements, the expression of which was selectively antagonized by SCH 23390. Sensitized rats were exposed to an unavoidable stress (which consistently produces an escape deficit in control animals) after 3, 7 and 21 days of morphine wash-out. Twenty-four hours after the unavoidable stress, animals were tested for their capacity to escape and their performance was compared to that of control-stressed and naive rats. Morphine sensitization completely prevented the development of escape deficit. This protective effect was similar to that induced by a chronic imipramine treatment and, like the effect of imipramine, it was antagonized by the administration of SCH 23390 before the unavoidable stress. However, it was not affected by the administration of naloxone. Moreover, when rats presenting a clear-cut escape deficit, induced by a 10-day treatment with SKF 38393, were exposed to the morphine sensitization protocol, a complete recovery of their capacity to avoid a noxious stimulus was observed. Finally, the down-regulation of both the number of D(1)-dopamine receptors and of the coupled adenylyl cyclase activity in the pre-frontal cortex (PFC) produced by long-term SKF 38393 administration was reverted by the superimposed morphine sensitization. Thus, the condition of morphine sensitization appears to share several common effects with chronic imipramine treatment.

Levodopa and receptor sensitivity modification in tardive dyskinesia

Tardive dyskinesia (TD), a syndrome of involuntary hyperkinetic movements, purportedly involves the development of dopamine (DA) receptor hypersensitivity following long-term receptor blockade with neuroleptic drugs. It has been proposed that through a process of receptor hypersensitivity modification, TD can be treated by reversing the receptor hypersensitivity with DA agonists. Thirteen patients with TD were treated for 4--8 weeks with levodopa plus benserazide, a peripheral decarboxylase inhibitor (Madopar, Roche, Basel, Switzerland) over a wide dose range corresponding to 3.0--9.0 g/day levodopa. Drug effects was assessed by blind evaluations of randomly sequenced videotapes made before, during, and for 6 weeks following treatment. TD scores moderately increased during levodopa. After the drug was discontinued, TD scores returned to pretreatment baseline levels without further improvement in those patients receiving concurrent neuroleptic medications (N = 9), but in the neuroleptic-free patient TD scores decreased 25% in three patients and were resolved in one younger patient. Psychological effects of depression or increased psychotic symptoms occurred at higher drug doses. These results do not support the proposal that receptor sensitivity modification with levodopa is an effective therapeutic approach to TD, though selected patient and drug treatment variables, including other DA agonists, are considerations for further investigation.

Time course of dopaminergic hypersensitivity following chronic narcotic treatment

Guinea pigs were injected SC for 3 weeks with 3 different dosage schedules of morphine or methadone, or with saline. For 8 weeks thereafter they were challenged weekly with the dopamine agonist apomorphine. Hypersensitivity was manifested in more intense stereotypies, as compared to the saline group, by all morphine and methadone groups. Hypersensitivity persisted longer after the termination of methadone treatment (maximum of 8 weeks) than after morphine administration (maximum of 3 weeks). The degree of hypersensitivity, and its duration after treatment, was positively related to methadone dosage. In some groups a period of hyposensitivity was seen following hypersensitivity. These data are interpreted with reference to the hypothesized mechanism underlying the development of hypersensitivity, the different durations of action of morphine and methadone, and the retention of methadone in brain following treatment.

Behavioral supersensitivity to apomorphine following chronic narcotic treatment in the guinea pig

Male albino guinea pigs were treated for 3 weeks with methadone, morphine, haloperidol, or saline. One week and 5 weeks following termination of treatment they were challenged with the directly acting dopaminergic agonist apomorphine. At the week 1 test the haloperidol and saline groups did not differ, but behavioral supersensitivity was apparent in significantly elevated mean stereotypy scores of the methadone and morphine groups relative to the saline group. The source of differences in mean scores was a higher peak score rather than increased duration of action. At the week 5 test the scores of the methadone group were even higher, the morphine group's scores were equivalent to the saline group's, and the haloperidol group's scores were significant depressed. This study indicates that a 3-week treatment period with methadone or morphine is sufficient to induce dopaminergic supersensitivity and suggests that there may be different time courses for the retention or expression of supersensitivity following these narcotics.