Paradoxical and subtype-specific effects of opiate antagonists on the expression of opioid receptors in rat brain cultures

✍ Scribed by J. Barg; R. Levy; Dr. R. Simantov

• Publisher: John Wiley and Sons
• Year: 1989
• Tongue: English
• Weight: 828 KB
• Volume: 22
• Category: Article
• ISSN: 0360-4012
• DOI: 10.1002/jnr.490220312

✦ Synopsis

Cultures of aggregating fetal rat brain cells express p, 6, and K opioid receptors. The potent and longlasting opioid antagonist naltrexone was used to investigate whether different regulatory mechanisms are involved in the expression of the three receptor subtypes. In cultures treated for seven days, naltrexone increased dose-dependently the binding of 3H-diprenorphine to the three receptor subtypes, with the p sites being affected at a lower concentration than the other two; A Scatchard analysis indicated that this receptor up-regulation was obtained by an increase in the BMax, with no significant change in the affinity of the ligand to the receptors. In contrast to these effects in cultures treated for 7 days, it was surprising to find that a 48 hr treatment with naltrexone had an apparent converse and subtypespecific influence; the antagonist decreased significantly the binding of 3H-diprenorphine to both p and 6 receptors but had no effect on K sites. Two other opioid antagonists, naloxone and levallorphan, had a similar effect. Further analysis of naltrexone's mode of action was obtained by studying its effect on the adenylate cyclase activity. Of several inducers of this enzyme, the P-adrenergic inducer isoproterenol gave the highest increase in cyclic AMP. Naltrexone had no significant effect on the basal adenylate cyclase activity but it altered the pattern of cyclic AMP formation in isoproterenol-stimulated cultures. Overall, the results indicate that in addition to its classic antagonistic activity, naltrexone exhibits in fetal brain aggregates some properties associated with opiate agonists.