Modulation of adenylate cyclase activity by a cytosolic factor following chronic opiate exposure in neuroblastoma x glioma NG108-15 hybrid cells

Inhibition of the morphine withdrawal syndrome and the development of physical dependence by lithium in mice

Due to the claim that lithium (Li+) reduces morphine self-administration in dependent rats, the effects of acute and chronic Li+ treatments on naloxone-precipitated withdrawal syndrome and physical dependence development to morphine in mice chronically treated with morphine, were evaluated. Morphine dependency was induced by the ingestion of morphine through drinking water in increasing doses for 10 days. Physical dependence to morphine was observed by precipitating an abstinence syndrome with naloxone (2 mg/kg, i.p.). In the acute experiments, Li+ (1 and 10 mg/kg, i.p.) was administered 1 hr prior to challenge with naloxone to morphine-dependent mice whereas for chronic studies, mice received morphine concomitant with Li+ (1200 mg/l) as drinking fluid for 10 days. Results obtained indicate that acute Li+ administration significantly reduced the withdrawal signs, and we were unable to induce some degree of morphine dependency in co-administration of Li+ to mice receiving chronic morphine treatment as compared to chronic morphine administration alone. The present study revealed that even in mice with very much lower serum Li+ levels than the commonly accepted therapeutic range there was a significant reduction in the withdrawal signs. It has been shown that Li+ and morphine have diverse effects on the transmembrane signal control systems. The interaction of Li+ and morphine might be through these systems.

Role of receptor regulation in opioid tolerance mechanisms

The molecular basis of opioid tolerance/dependence has long eluded researchers, but recent advances in receptor regulation have suggested a useful conceptual approach to the problem. In NG108-15 neuroblastoma x glioma hybrid (NG) cells, opioid agonists inhibit adenylate cyclase in a dose-dependent, naloxone-antagonizable fashion. Chronic treatment with opioid agonists results in a series of molecular processes that, in a tolerance-like fashion, counteract this inhibition. These processes include desensitization and down-regulation of receptors and an increase in adenylate cyclase activity. Opioid inhibition of adenylate cyclase and opioid receptor down-regulation also have been observed in the brain. However, most studies have found that the receptors coupled to adenylate cyclase are not of the mu type, which are thought to be the primary mediators of opioid analgesia. Down-regulation has been observed for both mu and delta opioid receptors in the brain. However, in most cases, the time course of down-regulation is not correlated with that for tolerance development, and chronic morphine treatment does not result in down-regulation. Thus, opioid receptors in the brain, like those in NG cells, are subject to dynamic regulation by agonists, which probably has an important role in their function. However, it remains to be established that opioid receptor regulation is the basis of opioid tolerance and dependence.

The adenylate cyclase rebound response to naloxone in the NG108-15 cells. Effects of etorphine and other opiates

The adenylate cyclase (AC) of the neuroblastoma-glioma hybrid cells (NG108-15), is generally considered to be a model for the study of the biochemical correlates of opiate tolerance and dependence. However, the naloxone-induced rebound response of adenylate cyclase, described in some recent reports, is much smaller than that originally described by Sharma, Klee and Nirenberg (1975). Possible explanations for these discrepancies are: (1) a marked down-regulation of opioid receptors and tolerance produced by the use of delta agonists or (2) the use of etorphine, a relatively hydrophobic drug which has slower dissociation rates than morphine. To test these possibilities, neuroblastoma-glioma hybrid cells were treated cells with morphine, etorphine, [D-Ala2,D-Leu5]enkephalin (DADLE), [D-Ala2]Leu-enkephalinamide (DALAMID) or vehicle. In addition, some of the cells treated with etorphine were washed with DADLE to replace the etorphine without producing the rebound response of adenylate cyclase prior to the addition of naloxone. The cells treated with morphine, DADLE and DALAMID, and incubated with prostaglandin E1 (PGE1) and naloxone showed a significant rebound of adenylate cyclase when compared with control groups and opiate-treated cells, incubated only with PGE1. In contrast, naloxone did not induce any significant rebound response in cells treated with etorphine unless they were previously washed with DADLE. These results demonstrate that the lack of a rebound response in cells treated with etorphine was due to the slow dissociation rates of the opiate and not to tolerance or to down-regulation of opioid receptors produced by agonists of high intrinsic activity.

The effect of tolerance on opiate dependence as measured by the adenylate cyclase rebound response to naloxone in the NG108-15 model system

The naloxone-induced adenylate cyclase rebound response produced by opiates in the NG108-15 cells is quite apparent after 48 h treatment with M but it is undetectable after incubation with the high intrinsic activity agonist etorphine. Replacement of agonists by antagonists in receptors that have been rendered inactive by tolerance does not elicit the AC rebound response, which is considered a biochemical correlate of dependence. This indicates that marked tolerance and receptor down-regulation induced by prolonged treatment with high efficacy agonists may reduce the level of dependence.