Opioid agonists are the therapeutic choice in the management of severe chronic pain (1). Despite effective analgesic benefit, extended periods of opioid treatment can result in a range of undesirable side effects, not the least of which is the development of tolerance. Opioids such as morphine demonstrate potent and stereoselective effects that are reversed by selective antagonists, suggesting an interaction with a specific membrane receptor (1). Selective, saturable, and high affinity membrane binding sites were subsequently demonstrated by radioligand binding assays on native tissue preparations (1). et al. (2) proposed the hypothesis of multiple opioid classes and defined receptors as the site of morphine action. The classification of opioid receptors into , , and ␦ was subsequently reinforced by the development of highly selective ligands for each class (3). Recently, the genetic basis for the three opioid classes was obtained, with the molecular cloning of the ␦, , and opioid receptors (for review, see Ref. 4). MartinOf the three opioid receptor classes, the receptor is thought to be the principal site of analgesic interaction, since most of the clinically relevant opioids used in pain management bind to this receptor with high affinity (5-7). Studies on the receptor offer potential molecular insights into the cellular basis for tolerance and dependence, serious side effects of prolonged opioid usage which may result from effects on receptor regulation (8).Whereas morphine induces dependence, methadone is used in the treatment of opioid addiction, despite being a full agonist at the receptor (9). The therapeutic efficacy of methadone has been linked to a lower abuse potential, although it is not established why methadone has lower addictive properties than morphine. Buprenorphine is the other opioid agent used in the treatment of addiction and has not been found to precipitate withdrawal in animal and human studies (9, 10). Buprenorphine has been reported to be a partial agonist or mixed agonist/antagonist at the receptor, and these properties have been implicated as the basis for addiction therapy (10).Despite the clinical importance of tolerance development and dependence, relatively little is known about the molecular and cellular events induced by morphine (8). Also, the cellular events that accompany the therapeutic actions of methadone and buprenorphine in treating addiction are poorly understood.The recent cloning and heterologous expression of the receptor have facilitated attempts to determine the biochemical mechanisms of clinically used opioids. In this study we investigated the molecular consequences of prolonged opioid agonist exposure on the cloned murine receptor stably expressed in HEK 1 293 cells. Continuous treatment of the receptor with morphine or DAMGO does not appear to desensitize the receptor, but it does result in an adaptive cellular response that is manifested by an increase in forskolin-stimulated intracellular cAMP levels. In contrast, methadone and buprenorphine treatments res...

1 Imidacloprid is a new insecticide with selective toxicity for insects over vertebrates. Recombinant (a4b2) chicken neuronal nicotinic acetylcholine receptors (AChRs) and a hybrid nicotinic AChR formed by co-expression of a Drosophila melanogaster neuronal a subunit (SAD) with the chicken b2 subunit were heterologously expressed in Xenopus oocytes by nuclear injection of cDNAs. The agonist actions of imidacloprid and other nicotinic AChR ligands ((+)-epibatidine, (7)-nicotine and acetylcholine) were compared on both recombinant nicotinic AChRs by use of two-electrode, voltage-clamp electrophysiology. 2 Imidacloprid alone of the 4 agonists behaved as a partial agonist on the a4b2 receptor; (+)-epibatidine, (7)-nicotine and acetylcholine were all full, or near full, agonists. Imidacloprid was also a partial agonist of the hybrid Drosophila SAD chicken b2 receptor, as was (7)-nicotine, whereas (+)-epibatidine and acetylcholine were full agonists. 3 The EC 50 of imidacloprid was decreased by replacing the chicken a4 subunit with the Drosophila SAD a subunit. This a subunit substitution also resulted in an increase in the EC 50 for (+)-epibatidine, (7)-nicotine and acetylcholine. Thus, the Drosophila (SAD) a subunit contributes to the greater apparent anity of imidacloprid for recombinant insect/vertebrate nicotinic AChRs. 4 Imidacloprid acted as a weak antagonist of ACh-mediated responses mediated by SADb2 hybrid receptors and as a weak potentiator of ACh responses mediated by a4b2 receptors. This suggests that imidacloprid has complex eects upon these recombinant receptors, determined at least in part by the a subunit.

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Molecular modelling and energy minimisation calculations that incorporate solvent effects have been used to investigate the complexation of delta and lambda-[Ru(1,10-phenanthroline]2+ to DNA. The most stable binding geometry for both enantiomers is one in which a phenanthroline chelate is positioned in the major groove. The chelate is partially inserted between neighbouring base pairs, but is not intercalated. For delta, though not for lambda, a geometry with two chelates in the major groove is only slightly less favourable. Minor groove binding is shown to be no more favourable than external electrostatic binding. The optimised geometries of the DNA/[Ru(1,10-phenanthroline]2+ complexes enable published linear dichroism spectra to be used to determine the percentage of each enantiomer in the two most favourable major groove sites. For delta 57 +/- 15% and for lambda 82 +/- 7% of bound molecules are in the partially inserted site.