Glucose homeostasis in mammals is achieved by the actions of counterregulatory hormones, namely insulin, glucagon and glucocorticoids. Glucose levels in the circulation are regulated by the liver, the metabolic centre which produces glucose when it is scarce in the blood. This process is catalysed by two rate-limiting enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) whose gene expression is regulated by hormones. Hormone response units (HRUs) present in the two genes integrate signals from various signalling pathways triggered by hormones. How such domains are arranged in the regulatory region of these two genes, how this complex regulation is accomplished and the latest advancements in the field are discussed in this review.

SUMMARYIn C 6 glial cells stably expressing rat -opioid receptor, opioid agonist activation is negatively coupled to adenylyl cyclase through pertussis toxin-sensitive G proteins. H]DAMGO binding in membranes with the rank order of etorphine Ͼ DAMGO ϭ ␤-endorphin Ͼ morphine Ͼ butorphanol, and the affinity of DAMGO in alkaloid-but not peptide-treated membranes was significantly lower in comparison with control. Pertussis toxin treatment of the cells before agonist treatment did not prevent the down-regulation by full agonists; DAMGO and etorphine exhibited ϳ80% internalization, whereas the ability of partial agonists was greatly impaired. In addition to establishing this cell line as a good model for further studies on the mechanisms of opioid tolerance, these results indicate important differences in the inactivation pathways of receptor triggered by full and partial agonists.Opioid receptors are activated by endogenous opioid peptides and alkaloids, which cause a multitude of important physiological functions. Recent cloning of -, ␦-, and -opioid receptors showed that these proteins contain seven transmembrane domains and belong to the family of GPCRs (1). The -opioid receptor is the molecular target for potent analgesics such as morphine and fentanyl, which are indispensable in the management of pain despite their abuse potential (2). The biochemical mechanisms of tolerance have been studied in many systems, including cell lines containing ␦-opioid receptors such as N4TG1 (3) and NG108 -15 (4) cells. Although studies conducted in the central nervous system often led to inconsistent results due to the heterogeneity of the system, experiments carried out in a single brain region, such as locus ceruleus, demonstrated the physiological relevance of the cellular model originally proposed by Sharma et al. (5) in NG108 -15 cells based on the alterations in the opioid/AC system. Subsequent studies using 7315c (6) and SH-SY5Y (7, 8) cells examined altered properties of -opioid receptor/effector components during tolerance; however, the exact mechanisms involved in this process are largely unknown. To study the molecular mechanisms of -opioid receptor selectively, we transfected C 6 glial cells that express many other receptors, but not opioid receptors (9), with the rat receptor cDNA. Transfected receptor in these cells is coupled to AC through PTX-sensitive G proteins (10). We characterized opioid agonist efficacies (11) and showed that this cell line exhibits sodium regulation of receptor in much the same fashion as SH-SY5Y cells (12).The major goal of the current study was to investigate the molecular changes involved in the development of tolerance by different agonists of varying efficacies. In the C 6 cell line stably expressing high levels of receptor (ϳ8 pmol/mg), tolerance to peptides and alkaloids was induced, and alterations were examined at every step of the signal transduction pathway (i.e., ligand/receptor interactions, G protein and effector functions). The diminished receptor activation of G protein, ...

SH-SY5Y neural cells expressing~a-and 8-opioid receptors were maintained viable in isotonic, sodiumfree buffer in vitro. Intracellular sodium levels were manipulated by various methods, and ligand binding to intact cells was studied. 50 values of 24 and 3,600 nM, respectively. Adenylyl cyclase activities measured in intact cells, at different concentrations of sodium, showed the physiological significance of this ion in signal transduction. Potency of DAMGO in inhibiting the forskolinstimulated adenylyl cyclase activity was significantly higher at lower concentrations of sodium. However, inhibition reached the maximal level only at 50 mM sodium, and typical sigmoidal dose-response curves were obtained only in the presence of 118 mM sodium. Furthermore, even at low or high intracellular sodium levels, DAMGO inhibition of cyclic AMP levels was normal. These results support a role for extracellular sodium in regulating not only the ligand interactions with the receptor, but also the signal transduction through the ,a receptor. Key Words: Sodium-Opioid receptor-Potassium-Amiloride-Monensin-Cyclic AMP.