Tachykinin NK₁ receptor antagonist co-administration attenuates opioid withdrawal-mediated spinal microglia and astrocyte activation.

Prolonged morphine treatment increases pain sensitivity in many patients. Enhanced spinal Substance P release is one of the adaptive changes associated with sustained opioid exposure. In addition to pain transmitting second order neurons, spinal microglia and astrocytes also express functionally active Tachykinin NK₁ (Substance P) receptors.

Repeated morphine treatment-mediated hyperalgesia, allodynia and spinal glial activation are blocked by co-administration of a selective cannabinoid receptor type-2 agonist.

Spinal glial activation has been implicated in sustained morphine-mediated paradoxical pain sensitization. Since activation of glial CB2 cannabinoid receptors attenuates spinal glial activation in neuropathies, we hypothesized that CB2 agonists may also attenuate sustained morphine-mediated spinal glial activation and pain sensitization. Our data indicate that co-administration of a CB2-selective agonist (AM 1241) attenuates morphine (intraperitoneal; twice daily; 6 days)-mediated thermal hyperalgesia and tactile allodynia in rats.

Sustained morphine treatment augments prostaglandin E2-evoked calcitonin gene-related peptide release from primary sensory neurons in a PKA-dependent manner.

Tissue damage leads to pain sensitization due to peripheral and central release of excitatory mediators such as prostaglandin E₂ (PGE₂). PGE₂ sensitizes spinal pain neurotransmitter such as calcitonin gene-related peptide (CGRP) release via activation of cyclic AMP (cAMP)/protein kinase A (PKA)-dependent signaling mechanisms. Our previous data demonstrate that sustained morphine pretreatment sensitizes adenylyl cyclase(s) (AC) toward the direct stimulator, forskolin, in cultured primary sensory neurons (AC superactivation).

Sustained morphine treatment augments capsaicin-evoked calcitonin gene-related peptide release from primary sensory neurons in a protein kinase A- and Raf-1-dependent manner.

Studies have shown that long-term (5alpha,6alpha)-7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol (morphine) treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia). The cellular adaptations contributing to sustained morphine-mediated pain sensitization are not fully understood. It was shown previously (J Neurosci 22:6747-6755, 2002) that sustained morphine exposure augments pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] release in the dorsal horn of the spinal cord in response to the heat-sensing transient receptor potential vanilloid 1 receptor agonist 8-methyl-N-vanillyl-6-nonenamide (capsaicin).

Sustained cannabinoid agonist treatment augments CGRP release in a PKA-dependent manner.

Studies have shown that sustained cannabinoid treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia) and innocuous mechanical stimuli (tactile allodynia). It has been suggested that augmented release of pain neurotransmitters (such as calcitonin gene-related peptide, CGRP) might be responsible for this abnormal pain sensitization. We hypothesize that intracellular adaptations upon sustained cannabinoid treatment causes augmented release of CGRP from primary nociceptors leading to increased pain sensitivity.

Intrathecal Raf-1-selective siRNA attenuates sustained morphine-mediated thermal hyperalgesia.

Studies have demonstrated that long-term opioid treatment leads to an increased sensitivity to painful (hyperalgesia) or normally innocuous (allodynia) stimuli. The molecular mechanisms that lead to paradoxical pain sensitization upon chronic opioid treatment are not completely understood. Enhanced excitatory pain neurotransmitter (such as calcitonin gene-related peptide (CGRP)) release in the dorsal horn of the spinal cord may play a role in sustained morphine-mediated paradoxical pain.

Sustained morphine treatment augments basal CGRP release from cultured primary sensory neurons in a Raf-1 dependent manner.

Recent studies suggest that sustained morphine-mediated paradoxical pain may play an important role in the development of analgesic tolerance. The intracellular signal transduction pathways involved in sustained opioid mediated augmentation of spinal pain neurotransmitter (such as calcitonin gene-related peptide (CGRP)) release are not fully clarified. Cyclic AMP (cAMP)-dependent protein kinase (PKA) plays an important role in the modulation of presynaptic neurotransmitter release.

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.

Mediating delta-opioid-initiated heart protection via the beta2-adrenergic receptor: role of the intrinsic cardiac adrenergic cell.

Stimulation of cardiac beta(2)-adrenergic receptor (beta(2)-AR) or delta-opioid receptor (DOR) exerts a similar degree of cardioprotection against myocardial ischemia in experimental models. We hypothesized that delta-opioid-initiated cardioprotection is mediated by the intrinsic cardiac adrenergic (ICA) cell via enhanced epinephrine release. Using immunohistochemical and in situ hybridization methods, we detected in situ tyrosine hydroxylase (TH) mRNA and TH immunoreactivity that was colocalized with DOR immunoreactivity in ICA cells in human and rat hearts.

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.

New paradigms and tools in drug design for pain and addiction.

New modalities providing safe and effective treatment of pain, especially prolonged pathological pain, have not appeared despite much effort. In this mini-review/overview we suggest that new paradigms of drug design are required to counter the underlying changes that occur in the nervous system that may elicit chronic pain states. We illustrate this approach with the example of designing, in a single ligand, molecules that have agonist activity at mu and delta opioid receptors and antagonist activities at cholecystokinin (CCK) receptors.

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.

Antinociception depends on the presence of G protein gamma2-subunits in brain.

We have shown previously [Hosohata, K., Logan, J.K.

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.

Postural position and neurocardiogenic syncope in late pregnancy.

A 23-year-old woman at 34 weeks' gestation developed recurrent syncope due to profound sinus arrest captured on electrocardiography. Syncopal events occurred in the same sitting position. An echocardiogram revealed severe collapse of the inferior vena cava each time the patient changed her posture from a supine to a sitting position, which was related to the syncope.

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.

(2S,3R) beta-methyl-2',6'-dimethyltyrosine-L-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-L-Tic-OH] is a potent, selective delta-opioid receptor antagonist in mouse brain.

The constrained opioid peptide (2S,3R)beta-methyl-2',6'-dimethyltyrosine-L-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-L-Tic-OH] exhibits high affinity and selectivity for the delta-opioid receptors (). In the present study, we examined the pharmacological properties of (2S,3R)TMT-L-Tic-OH in mouse brain. A 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTP gamma S) binding assay was used to determine the effect of (2S,3R)TMT-L-Tic-OH on G protein activity in vitro, in mouse brain membranes.

Agonist-specific down-regulation of the human delta-opioid receptor.

Down-regulation of the delta-opioid receptor contributes to the development of tolerance to delta-opioid receptor agonists. The involvement of the carboxy terminus of the mouse delta-opioid receptor in peptide agonist-mediated down-regulation has been established. In the present study, we examined the down-regulation of the truncated human delta-opioid receptor by structurally distinct delta-opioid receptor agonists.

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.