Differences among mouse strains in the regulation by mu, delta 1 and delta 2 opioid receptors of striatal adenylyl cyclases activated by dopamine D1 or adenosine A2a receptors

Characterization of the discriminative stimulus effect of quinpirole: Further evidence for functional interaction between central dopamine D1/D2-receptors

Dysfunction of the central dopamine D₂-receptor-related network has been proposed to play a critical role in dopamine-related diseases, such as schizophrenia and drug dependence. Generally, the stimulation of dopamine D₂-receptors on medium spiny neurons (MSN) induces several behavioral effects, such as sedation, hallucination, aversion and motivation. Furthermore, such physiological responses through dopamine D₂-receptor-containing MSN (D₂-MSN) may be synchronized with the activity of dopamine D₁-receptor-containing MSN (D₁-MSN), or both may exhibit dual agonistic/antagonistic innervation. In the present study, we characterized the discriminative stimulus effect of the selective dopamine D₂-receptor agonist quinpirole to further investigate the "D_1/D₂-MSN" interaction using dopamine-related agents, hallucinogens and sedatives in rats. Among dopamine receptor agonists, only selective dopamine D₂-receptor agonists substituted for the discriminative stimulus effects of quinpirole. Neither the δ-opioid receptor agonist SNC80 nor the adenosine A_2A-receptor antagonist istradefylline, both of which may act on D₂-MSNs, substituted for the discriminative stimulus effects of quinpirole. Interestingly, the dopamine D₁-receptor antagonist SCH23390 and the GABA_B-receptor agonist baclofen, but not hallucinogens or sedatives, substituted for the discriminative stimulus effects of quinpirole. These results suggest that stimulation of central dopamine D₂-receptors exerts a distinct discriminative stimulus effect, and blockade of dopamine D₁-receptors and agonistic modulation of GABA_B-receptors may share the discriminative stimulus effect via the activation of central dopamine D₂-receptors.

New concepts in opioid analgesia

INTRODUCTION

Opioids are the oldest and most potent drugs for the treatment of severe pain, but they are burdened by detrimental side effects such as respiratory depression, addiction, sedation, nausea, and constipation. Their clinical application is undisputed in acute (e.g. perioperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny and has contributed to the current 'opioid crisis.'

AREAS COVERED

This article reviews pharmacological principles and research strategies aiming at novel opioids with reduced side effects. Basic mechanisms underlying pain, opioid analgesia, and other opioid actions are outlined. To illustrate the clinical situation and medical needs, plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic, and side effects are discussed.

EXPERT OPINION

The epidemic of opioid misuse has taught us that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be considered in the context of drug development, and that novel analgesics with less abuse liability are badly needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and selectively targeting pathological conformations of peripheral opioid receptors.

Ligand stabilization of CXCR4/delta-opioid receptor heterodimers reveals a mechanism for immune response regulation

The CXCR4 chemokine receptor and the delta opioid receptor (DOR) are pertussis toxin-sensitive G protein-coupled receptors (GPCR). Both are widely distributed in brain tissues and immune cells, and have key roles in inflammation processes and in pain sensation on proximal nerve endings. We show that in immune cells expressing CXCR4 and DOR, simultaneous addition of their ligands CXCL12 and [D-Pen2, D-Pen5]enkephalin does not trigger receptor function. This treatment does not affect ligand binding or receptor expression, nor does it promote heterologous desensitization. Our data indicate that CXCR4 and DOR form heterodimeric complexes that are dynamically regulated by the ligands. This is compatible with a model in which GPCR oligomerization leads to suppression of signaling, promoting a dominant negative effect. Knockdown of CXCR4 and DOR signaling by heterodimerization might have repercussions on physiological and pathological processes such as inflammation, pain sensation and HIV-1 infection.

Theophylline inhibits tolerance and sensitization induced by morphine: a conditioned place preference paradigm study in female mice

The effect of theophylline on reward properties of morphine was examined in the present study. A biased conditioned place preference paradigm was used to study the effects of theophylline on the development of conditioned place preference by morphine in sensitized and tolerant female mice. Subcutaneous injection of morphine (0.5-10 mg/kg) induced conditioned place preference in mice, while intraperitoneal administration of theophylline (2.5-100 mg/kg) did not induce conditioned place preference or conditioned place aversion. Theophylline (2.5-100 mg/kg) in combination with morphine (5 mg/kg), during conditioning sessions, decreased the acquisition of morphine conditioned place preference dose independently. Administration of theophylline (2.5-100 mg/kg) before testing also caused a significant reduction of the expression of morphine-induced conditioned place preference in a dose-independent manner. Administration of morphine (12.5, 25 or 50 mg/kg) daily, for 3 days, produced tolerance to conditioned place preference induced by the drug (5 mg/kg). Administration of theophylline (2.5 and 10 mg/kg) 1 h before morphine (12.5, 25 mg/kg), during development of tolerance, abolished morphine tolerance. A higher dose of theophylline (100 mg/kg), however, did not alter morphine tolerance. In addition, theophylline (2.5, 10 and 100 mg/kg) failed to reduce tolerance to a higher dose of morphine (50 mg/kg). Daily administration of morphine (5 mg/kg) for 3 days followed by a 5-day interval caused sensitization to morphine place conditioning. When theophylline was administered (2.5, 10 and 100 mg/kg) 1 h before morphine (5 mg/kg), during development of sensitization, inhibition of morphine-induced sensitization was demonstrated. The effect of theophylline was dose independent. It is concluded that while theophylline has no effect by itself, it reduced both the acquisition and expression of morphine conditioned place preference. In addition, theophylline reduced the acquisition of morphine conditioned place preference in morphine-sensitized and morphine-tolerant mice.

Up-regulation of adenosine transporter-binding sites in striatum and hypothalamus of opiate tolerant mice

Opioid-adenosine interactions have been demonstrated at both cellular and behavioral levels. Short-term morphine treatment has been shown to enhance adenosine release in brain and spinal tissues. Since adenosine uptake and release is regulated by a nitrobenzylthioinosine-sensitive adenosine transporter, we examined the effects of morphine treatment on this transporter-binding site. Adenosine transporter-binding sites were examined using equilibrium binding studies with [3H]nitrobenzylthioinosine in brain regions of morphine-treated mice. A 72-hour morphine pellet implantation procedure, which previously produced up-regulation of central adenosine A1 receptors and created a state of opiate dependence [G.B. Kaplan, K.A. Leite-Morris and M.T. Sears, Alterations in adenosine A receptors in morphine dependence, Brain Res., 657 (1994) 347-350], was used in this current study. This chronic morphine treatment significantly increased adenosine transporter-binding site concentrations in striatum and hypothalamus by 12 and 37%, respectively, compared to vehicle pellet implantation. No effects of morphine treatment were demonstrated in cortex, hippocampus, brainstem or cerebellum. In behavioral studies, mice receiving this same chronic morphine or vehicle treatment were given saline or morphine injections (40 or 50 mg/kg i.p.) followed by ambulatory activity monitoring. In the chronic vehicle treatment group, morphine injections significantly stimulated ambulatory activity while in the chronic morphine treatment group there was no such stimulation by acute morphine, suggestive of opiate tolerance. Morphine-induced up-regulation of striatal and hypothalamic adenosine transporter sites could potentially alter extracellular adenosine release and adenosine receptor activation and mediate aspects of opiate tolerance and dependence.