Unique agonist-bound cannabinoid CB1 receptor conformations indicate agonist specificity in signaling.

Cannabinoid drugs differ in their rank order of potency to produce analgesia versus other central nervous system effects. We propose that these differences are due to unique agonist-bound cannabinoid CB1 receptor conformations that exhibit different affinities for individual subsets of intracellular signal transduction pathways. In order to test this hypothesis, we have used plasmon-waveguide resonance (PWR) spectroscopy, a sensitive method that can provide direct information about ligand-protein and protein-protein interactions, and can detect conformational changes in lipid-embedded proteins.

Quantitative evaluation of human delta opioid receptor desensitization using the operational model of drug action.

Agonist-mediated desensitization of the opioid receptors is thought to function as a protective mechanism against sustained opioid signaling and therefore may prevent the development of opioid tolerance. However, the exact molecular mechanism of opioid receptor desensitization remains unresolved because of difficulties in measuring and interpreting receptor desensitization. In the present study, we investigated deltorphin II-mediated rapid desensitization of the human delta opioid receptors (hDOR) by measuring guanosine 5'-O-(3-[(35)S]thio)-triphosphate binding and inhibition of cAMP accumulation.

Chronic morphine-mediated adenylyl cyclase superactivation is attenuated by the Raf-1 inhibitor, GW5074.

The utility of morphine for the treatment of chronic pain is limited by the development of analgesic tolerance. Adenylyl cyclase (AC) superactivation, induced by chronic opioid agonist administration, is regarded as one of the molecular mechanisms leading to tolerance. In the present work, we tested the role of Raf-1 in morphine-mediated AC superactivation in CHO cells stably expressing the human micro-opioid receptor.

Morphine promotes phosphorylation of the human delta-opioid receptor at serine 363.

After prolonged stimulation, the delta-opioid receptor becomes desensitized by regulatory mechanisms such as receptor phosphorylation, internalization and down-regulation. In this study, we demonstrate that morphine treatment causes phosphorylation of S363 in the C-terminus of the human delta-opioid receptor. Morphine-mediated phosphorylation reached 53+/-8% of maximum deltorphin II-mediated phosphorylation.

Agonist-specific regulation of the delta-opioid receptor.

Delta opioid receptor (DOR) agonists are attractive potential analgesics, since these compounds exhibit strong antinociceptive activity with relatively few side effects. In the past decade, several novel classes of delta-opioid agonists have been synthesized. Recent experimental data indicate that structurally distinct opioid agonists interact differently with the delta-opioid receptor.

Mutation S363A in the human delta-opioid receptor selectively reduces down-regulation by a peptide agonist.

Chemically distinct opioid agonists have different abilities to down-regulate opioid receptors. The present study investigated the role of Ser(363) in human delta-opioid receptor down-regulation by a delta-selective peptide- and non-peptide agonist. Cyclic[D-Pen(2),D-Pen(5)]enkephalin (DPDPE)-mediated down-regulation was significantly attenuated by a S363A mutation.

Molecular mechanisms of excitatory signaling upon chronic opioid agonist treatment.

Opioid receptor agonists mediate their analgesic effects by interacting with Gi/o protein-coupled opioid receptors. Acute treatment with opioid agonists is thought to mediate analgesia by hyperpolarization of presynatic neurons, leading to the inhibition of excitatory (pain) neurotransmitters release. After chronic treatment however, the opioid receptors gradually become less responsive to agonists, and increased drug doses become necessary to maintain the therapeutic effect (tolerance).

Converging protein kinase pathways mediate adenylyl cyclase superactivation upon chronic delta-opioid agonist treatment.

Adenylyl cyclase (AC) superactivation is thought to play an important role in opioid tolerance, dependence, and withdrawal. In the present study, we investigated the involvement of protein kinases in chronic delta-opioid agonist-mediated AC superactivation in Chinese hamster ovary (CHO) cells stably expressing the human delta-opioid receptor (hDOR/CHO). Maximal forskolin-stimulated cAMP formation in hDOR/CHO cells increased by 472 +/- 91, 399 +/- 2, and 433 +/- 73% after chronic treatment with the delta-opioid agonists (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N,N-diethyl benzamide (SNC 80), [d-Pen2,d-Pen5]-enkephalin, and deltorphin II, respectively.

Involvement of Raf-1 in chronic delta-opioid receptor agonist-mediated adenylyl cyclase superactivation.

Chronic delta-opioid receptor agonist treatment of Chinese hamster ovary (CHO) cells stably expressing the human delta-opioid receptor (hDOR/CHO) leads to increased cAMP formation after the removal of the agonist (adenylyl cyclase superactivation). We have previously found that at the same time, chronic delta-opioid receptor agonist treatment augments phosphorylation of the adenylyl cyclase VI isoenzyme. Since phosphorylation of adenylyl cyclase VI by Raf-1 protein kinase was recently shown, we tested the role of Raf-1 in adenylyl cyclase superactivation in hDOR/CHO cells.