Naloxone potentiates the depressant effect of chlordiazepoxide on spontaneous activity in mice

The role of delta opioid receptors in the anxiolytic actions of benzodiazepines

The anxiolytic effects of benzodiazepines appear to involve opioid processes in the amygdala. In previous experiments, overexpression of enkephalin in the amygdala enhanced the anxiolytic actions of the benzodiazepine agonist diazepam in the elevated plus maze. The effects of systemically administered diazepam are also blocked by injections of naltrexone into the central nucleus of the amygdala. The current studies investigated the role of delta opioid receptors in the anxiety-related effects of diazepam. Three days following bilateral stereotaxic injections of viral vectors containing cDNA encoding proenkephalin or beta-galactosidase (control vector), the delta opioid receptor antagonist naltrindole (10 mg/kg, s.c.) attenuated the enhanced anxiolytic effects of 1-2 mg/kg diazepam in rats overexpressing preproenkephalin in the amygdala. Despite this effect, naltrindole failed to attenuate the anxiolytic action of higher diazepam doses (3 mg/kg) in animals with normal amygdalar enkephalin expression. Similarly, the mu opioid receptor antagonist, beta-funaltrexamine (20 mg/kg, s.c.), had no effect on the anxiolytic effect of diazepam alone. These data support a role for delta opioid receptors in the opioid-enhanced anxiolytic effects of diazepam.

Naltrexone treatment of tardive dyskinesia in patients with schizophrenia

OBJECTIVE

Treatment of dyskinetic disorders, in general, and of tardive dyskinesia (TD), in particular, is difficult. The opiate peptide enkephalin coexits with gamma aminobutyric acid in the projection neurons of striatum forming the "indirect" pathway. Several lines of preclinical evidence implicate this enkephalin-comediated pathway in the pathophysiology and therapeutics of dyskinesia. However, previous studies, most using relatively low doses of opioid antagonists, showed mixed results. The goal of the current study was to test whether moderately high doses of naltrexone, alone or in combination with a subtherapeutic dose of a gamma aminobutyric acid agonist, improve TD.

METHODS

In 2 double-blind, placebo-controlled, randomized, crossover trials, effects of naltrexone alone (n = 9) and in combination with clonazepam (n = 14) were tested on TD. In both trials, patients' antipsychotic medication and dose remained unchanged through the trial. Naltrexone dose was increased over a period of 3 weeks to 200 mg/d and maintained at that dose for another week. In the second study, patients were first stabilized on low dose (0.25 to 0.5 mg) of clonazepam for 4 weeks or longer. In addition to the TD scores, saccadic peak velocity and latency, as measures of vigilance, and antisaccade error rate were obtained during the fourth week of placebo and naltrexone in a subgroup of patients.

RESULTS

There were no significant effects of naltrexone alone on TD (mean +/- SD decrease in TD score = 0.1 +/- 4.8), psychosis scores, or eye movement measures. Low dose of clonazepam had no effect on TD. However, addition of naltrexone significantly improved TD (mean, SD decrease in TD score 4.0 +/- 3.6). There was no clinical or laboratory evidence of increased sedation during treatment with naltrexone compared to placebo. There were no significant effects on the antisaccade error rate or psychosis scores.

CONCLUSION

These findings suggest effectiveness of a strategy of combining a GABA(gamma aminobutyric acid)mimetic drug with an enkephalin antagonist to treat dyskinesia.

Flunitrazepam variably alters morphine, buprenorphine, and methadone lethality in the rat

Opiates and substitution products are frequently abused, alone and in association with benzodiazepines. While this combination may result in severe respiratory depression and death, the quantitative relationship remains uncertain. We performed randomized, blinded intravenous median lethal dose (MLD) studies in Sprague-Dawley rats of morphine, buprenorphine, and methadone, alone and in combination with intraperitoneal flunitrazepam pretreatment. We employed the up-and-down method, performed in quadruplicate, comparing time to death following opioid injection. Results are expressed as median of four series (extremes). The MLDs of morphine, buprenorphine, and methadone alone were 64.0 (33.6:79.5), 234.6 (168.6:284.4), and 22.5 (19.3:24.1) mg/kg, respectively, and 60.6 (35.2:88.2), 38.4 (30.6:54.0), and 13.0 (9.7:13.8) mg/kg, respectively, after pretreatment with 40 mg/kg flunitrazepam. Times to death for morphine, buprenorphine, and methadone alone were 2.5 (0.8:24), 0.02 (0.0:24), and 2.0 (0.0:24) hours, respectively, and 13.5 (0.0:144), 24.0 (0.0:120), and 0.0 (0.0:24) hours, respectively, after pretreatment with flunitrazepam 40 mg/kg, ip. Flunitrazepam significantly altered methadone (P=0.02) and buprenorphine (P=0.02) but not morphine lethality (P=0.77). Flunitrazepam significantly prolonged time to death only for buprenorphine (P<0.01). Flunitrazepam-opioid drug-drug interactions are more complex than is generally believed. Mechanistic studies of flunitrazepam-opioid lethal interactions are needed.

ACTH: a single pretreatment enhances the analgesic efficacy of and prevents the development of tolerance to morphine

Evidence suggests a close relationship between ACTH and opioids. They are derived from the same precursor molecule, released concomitantly in response to stress and are known to interact at receptor level. Acute exposure to endogenous opioids induces analgesia in the short-term yet shifts the morphine dose-response curve to the right for a period of several weeks. Therefore, the possibility exists that ACTH might also exert an influence extending beyond that suggested by its biological half-life. To investigate this further, DBA/2 mice were pretreated with porcine ACTH left undisturbed for 3 days, then challenged with morphine. Dose-response studies indicated that pretreatment with 10 but not 0, 0.1 or 1.0 IU ACTH influenced responsivity to morphine, rendering a sub-analgesic dose (1.0 mg/kg) effective and significantly enhancing the degree of analgesia observed following treatment with 5 mg/kg morphine, suggesting a shift in the dose-response curve to the left. Time-course analysis revealed 5 mg/kg morphine to induce an analgesic reaction with an onset of between 15-30 and lasting between 60 and 120 min post opiate administration. ACTH pretreatment did not influence this time course, however a significantly greater degree of analgesia was observed at 60 min post morphine injection in 10 IU ACTH pretreated animals than in saline pretreated controls. ACTH pretreatment further influenced subsequent responding to chronic morphine administration. Whilst saline pretreated animals demonstrated significant analgesia in response to the first administration, tolerance to this effect had developed following four days of repeated exposure to morphine.(ABSTRACT TRUNCATED AT 250 WORDS)