Alteration of lymphocyte opioid receptors in methadone maintenance subjects

Methadone maintenance therapy is the most widely used treatment in patients with heroin addiction. Multiple studies have suggested that both current and former heroin addicts entering a methadone maintenance treatment program have altered immune function. Our previous study indicated that heroin addicts have depressed mitogen-stimulated lymphocyte proliferation and a decrease in the modulation of lymphocyte surface markers. This immunosuppression may be mediated via the direct interaction of opiates with lymphocyte opioid receptors. In order to test this hypothesis, the levels of opioid receptors on immune cells obtained from heroin users were determined using saturation binding, and it was found that former heroin addicts on methadone maintenance treatment had a significantly reduced maximum number (Bmax) of [3H]naloxone binding. The Bmax values were 51.3 ± 7.6 fmol/mg protein for the non-addicted group and 25.3 ± 3.1 fmol/mg protein for the methadone maintenance group. Opioid receptor gene expression on the immune cell was determined using a semi-quantitative reverse-transcription polymerase chain reaction technique with specific pairs of primers to amplify mu- and delta-opioid receptor mRNAs. Both types of mRNAs were significantly decreased in lymphocytes obtained from the former heroin addicts on methadone maintenance subjects. Similarly, in an in vitro study, 100 μM methadone significantly down-regulated both mu- and delta-opioid receptor mRNA expressions in cultured lymphocytes obtained from naïve subjects. This effect was prevented by including 100 μM naloxone or pretreating with 50 ng/ml pertussis toxin. The data presented indicate that chronic opiate exposure was associated with down-regulation of G-protein-coupled opioid receptor gene expression in human lymphocytes.