Cross-dependence to opioid and alpha 2-adrenergic receptor agonists in NG108-15 cells

Yohimbine does not affect opioid receptor activation but prevents adenylate cyclase regulation by morphine in NG108-15 cells

AIMS

Yohimbine has been shown to modulate the pharmacological actions of opioid drugs in a way that could be of potential therapeutic interest. This work tries to study if this interaction involves the impairment of opioid receptor activation at the cellular level by studying the effects of morphine and yohimbine on NG108-15 neuroblastoma x glioma hybrid cells.

MAIN METHODS

[(35)S]GTPγS binding assays were performed to study δ-opioid and α(2B)-adrenoceptor activation by opioid and adrenoceptor agonists in the presence and absence of yohimbine. The effect of morphine was also studied after 6 h pre-incubations with morphine, yohimbine and combinations of these drugs taking into account previous results showing an interaction between both drugs in these conditions. Forskolin-induced cAMP accumulation was also studied by immunoassay in cells incubated with morphine for 6 h in the presence and absence of naloxone and yohimbine.

KEY FINDINGS

Yohimbine behaved as a competitive antagonist/inverse agonist on α(2B)-adrenoceptors but did not modify G-protein activation by morphine, either in acute conditions or after 6 h of incubation. However, morphine-induced inhibition of cAMP accumulation was prevented both by naloxone and yohimbine when these drugs were present in the incubation medium.

SIGNIFICANCE

Yohimbine seems to desensitise adenylate cyclase to the inhibitory effect of opioid-activated G proteins. This cellular effect could underlie the antagonistic actions of yohimbine on many pharmacological effects of the opioid both in vitro and in vivo.

Somatostatin and opioid receptors in mammary tissue. Role in cancer cell growth

Somatostatin and opioid systems, are the two main inhibitory systems in mammals. Both classes of substances have been identified in normal and malignant mammary gland, as well as their cognitive receptors. They have been implied in the inhibition of cell growth of cancer cells and cell lines, in a dose-dependent and reversible manner. Somatostatin acts through homologous receptors (SSTRs), belonging to five distinct classes (SSTR1-5). We, and others have identified SSTR2 and 3 as been the only SSTRs present in the breast. Furthermore, opioids act through the three classes of opioid receptors (mu, delta,kappa). In the breast, kappa opioid receptor subtypes (kappa 1-kappa 3) are the most widely expressed. We further have shown that opioids, in addition to their binding to opioid receptors, compete for binding to SSTRs. This functional interaction, together with other identified modes of opioid action in the breast (modulation of steroid receptors, proteases' secretion, interaction with cytoskeletal elements), will be discussed, taking into consideration also the possible local production of casomorphins (casein-derived opioids), which are very potent antiproliferative agents.

Chronic exposure of NG 108-15 cells to inhibitory acting drugs reduces stimulatory prostaglandin E1 receptor number

Prolonged exposure of neuroblastoma x glioma (NG 108-15) hybrid cells to inhibitory acting drugs results in sensitization of adenylate cyclase. We now report that chronic activation (3 days) of either inhibitory delta-opioid receptors, alpha 2B-adrenoceptors, or muscarinic M4 receptors significantly decreases the number of stimulatory, adenylate cyclase-coupled prostaglandin E1 receptors. Pharmacological characterization further revealed that the loss of [3H]prostaglandin E1-binding sites most likely corresponds to a reduction of the number of high-affinity, G protein-coupled prostaglandin E1 receptors. The decline in functionally active prostaglandin E1 receptors developed in a time- and dose-dependent manner and could be prevented by pretreatment of the cells with pertussis toxin. Heterologous prostaglandin E1 receptor regulation was blocked by concomitant exposure of the cells to antagonists for inhibitory receptors and was rapidly reversed (t 1/2 < 30 min) upon termination of chronic inhibitory drug treatment. The decrease in high-affinity prostaglandin E1 receptors developed regardless of whether full or partial agonists were used for pretreatment. In addition, the concentrations of inhibitory drugs required to maximally affect prostaglandin E1 receptor number closely resembled those mediating maximal adenylate cyclase inhibition. The data demonstrate that chronic inhibitory drug treatment of NG 108-15 hybrid cells reduces the number of functionally active, excitatory prostaglandin E1 receptors. Thus, it is proposed that adaptations at the level of stimulatory receptor systems contribute to the regulatory mechanisms associated with drug dependence.

A general role for adaptations in G-proteins and the cyclic AMP system in mediating the chronic actions of morphine and cocaine on neuronal function

Previous studies have shown that chronic morphine increases levels of the G-protein subunits Gia and Goa, adenylate cyclase, cyclic AMP-dependent protein kinase, and certain phosphoproteins in the rat locus coeruleus, but not in several other brain regions studied, and that chronic morphine decreases levels of Gia and increases levels of adenylate cyclase in dorsal root ganglion/spinal cord (DRG-SC) co-cultures. These findings led us to survey the effects of chronic morphine on the G-protein/cyclic AMP system in a large number of brain regions to determine how widespread such regulation might be. We found that while most regions showed no regulation in response to chronic morphine, nucleus accumbens (NAc) and amygdala did show increases in adenylate cyclase and cyclic AMP-dependent protein kinase activity, and thalamus showed an increase in cyclic AMP-dependent protein kinase activity only. An increase in cyclic AMP-dependent protein kinase activity was also observed in DRG-SC co-cultures. Morphine regulation of G-proteins was variable, with decreased levels of Gia seen in the NAc, increased levels of Gia and Goa in amygdala, and no change in thalamus or the other brain regions studied. Interestingly, chronic treatment of rats with cocaine, but not with several non-abused drugs, produced similar changes compared to morphine in G-proteins, adenylate cyclase, and cyclic AMP-dependent protein kinase in the NAc, but not in the other brain regions studied. These results indicate that regulation of the G-protein/cyclic AMP system represents a mechanism by which a number of opiate-sensitive neurons adapt to chronic morphine and thereby develop aspects of opiate tolerance and/or dependence. The findings that chronic morphine and cocaine produce similar adaptations in the NAc, a brain region important for the reinforcing actions of many types of abused substances, suggest further that common mechanisms may underlie psychological aspects of drug addiction mediated by this brain region.

Drugs and receptors. An overview of the current state of knowledge

This paper reviews the theoretical concepts and methods utilised with isolated tissues to characterise drugs and drug receptors. Specifically the impact, on the in vitro measurement of agonist affinity and relative efficacy, of the idea that receptors bind to transduction proteins in the lipid bilayer of the cell membrane is discussed. The effects of ternary complex formation of agonist-receptor equilibria raise theoretical objections to the measurement of agonist receptor equilibrium dissociation constants. Possible 'promiscuity' of receptors with respect to the G-proteins with which they can interact makes classification of receptors by agonists suspect. The use of Schild analysis for the measurement of antagonist affinity and subsequent classification of receptors is considered in the light of recent data showing that estimates calculated with this method are heterogeneous. Resultant analysis for the detection of allosteric effects is also discussed. Lastly, the impact of molecular biology on the drug and drug receptor classification process is considered, as well as the effects of pathological processes on drug action at the receptor level.

Dilator actions of endothelin in coronary resistance vessels and the abdominal aorta of the guinea pig

Endothelin has been characterized as a potent constricting factor. The purpose of this study was to investigate possible dilator effects of this peptide and to examine whether dilator responses occur through an endothelium-mediated mechanism in guinea pig coronary resistance vessels and isolated aortic rings. Changes in perfusion pressure after bolus injections of endothelin were measured using a constant-flow modified Langendorff preparation with a transducer between the flow pump and the heart. An immediate fall in perfusion pressure, averaging 6 mmHg, was observed after injection of endothelin (10(-14)-10(-12) moles). This effect was maximal at 1 minute and tended to return toward baseline levels within 4 minutes. In response to endothelin (10(-9) M), isolated aortic rings relaxed 35% after being contracted with prostaglandin F2 alpha (10(-7) M). In both preparations, dilation was converted to constriction after endothelium damage by oxygen radicals or endothelium removal (mechanical rubbing). Dilator responses to endothelin were blocked by pretreatment for 30 minutes with indomethacin (14 microM) in the presence of an intact endothelium in coronary resistance vessels, whereas in the abdominal aorta they were not. We conclude that endothelin has significant dilator properties and that this effect is opposed by its constrictor action at higher doses. In addition, dilator responses to endothelin require an intact endothelium in both coronary vessels and abdominal aorta. Finally, endothelin-induced dilation in coronary resistance vessels appears to occur through a cyclooxygenase product-mediated mechanism.