The kappa-opioid receptor: evidence for the different subtypes

Synthesis and biological activity of small peptides as NOP and opioid receptors' ligands: view on current developments

The heptadecapeptide nociceptin, also called orphanin FQ (N/OFQ), is the endogenous agonist of the N/OFQ peptide receptor (NOP receptor) and is involved in several central nervous system pathways, such as nociception, reward, tolerance, and feeding. The discovery of small molecule ligands for NOP is being actively pursued for several therapeutic applications. This review presents overview of the several recently reported NOP ligands (agonists and antagonists), with an emphasis of the structural features that may be important for modulating the intrinsic activity of these ligands. In addition, a brief account on the characterization of newly synthesized ligands of NOP receptor with aminophosphonate moiety and β-tryptophan analogues will be presented.

Bioinformatics and evolution of vertebrate nociceptin and opioid receptors

G protein-coupled receptors (GPCRs) are ancestrally related membrane proteins on cells that mediate the pharmacological effect of most drugs and neurotransmitters. GPCRs are the largest group of membrane receptor proteins encoded in the human genome. One of the most famous types of GPCRs is the opioid receptors. Opioid family receptors consist of four closely related proteins expressed in all vertebrate brains and spinal cords examined to date. The three classical types of opioid receptors shown unequivocally to mediate analgesia in animal models and in humans are the mu- (MOR), delta- (DOR), and kappa-(KOR) opioid receptor proteins. The fourth and most recent member of the opioid receptor family discovered is the nociceptin or orphanin FQ receptor (ORL). The role of ORL and its ligands in producing analgesia is not as clear, with both analgesic and hyperalgesic effects reported. All four opioid family receptor genes were cloned from expressed mRNA in a number of vertebrate species, and there are enough sequences presently available to carry out bioinformatic analysis. This chapter presents the results of a comparative analysis of vertebrate opioid receptors using pharmacological studies, bioinformatics, and the latest data from human whole-genome studies. Results confirm our initial hypotheses that the four opioid receptor genes most likely arose by whole-genome duplication, that there is an evolutionary vector of opioid receptor type divergence in sequence and function, and that the hMOR gene shows evidence of positive selection or adaptive evolution in Homo sapiens.

Direct inhibition of hypothalamic proopiomelanocortin neurons by dynorphin A is mediated by the μ-opioid receptor

It has recently been shown that dynorphin A (Dyn A), an endogenous agonist of the κ-opioid receptor (KOR), directly inhibits proopiomelanocortin (POMC) neurons in the hypothalamus through activation of G-protein-coupled inwardly rectifying K(+) channels (GIRKs). This effect has been proposed to be mediated by the putative κ2-opioid receptor (KOR-2), and has been suggested as a possible mechanism for the orexigenic actions of KOR agonists. Using whole-cell voltage clamp recordings in brain slice preparations, the present study demonstrates that Dyn A (1 or 5 μm) induces an outward current in POMC neurons that is reversed by the highly selective μ-opioid receptor (MOR) antagonist CTAP and is absent in mice lacking MORs. Additionally, the KOR-2-selective agonist GR89696 binds MORs on POMC neurons but fails to induce an outward current. Similar to Dyn A, the KOR-selective antagonist nor-binaltorphimine (nor-BNI) lacked specificity when used at sufficiently high concentrations. Maximal concentrations of the MOR-selective agonist DAMGO induced outward currents in POMC neurons that were completely reversed by a relatively high concentration of nor-BNI. Experiments using a half-maximal concentration of DAMGO demonstrate that nor-BNI must be used at concentrations <100 nm to avoid non-specific actions of the antagonist at MORs located on POMC neurons. These data suggest that direct inhibition of POMC neurons by Dyn A is mediated through the MOR, not the KOR-2, which is consistent with previous studies demonstrating that Dyn A can act at the μ-opioid receptor (MOR) when present in high concentrations.

Opioid receptor subtypes: fact or artifact?

There is a vast amount of pharmacological evidence favouring the existence of multiple subtypes of opioid receptors. In addition to the primary classification of µ (mu: MOP), δ (delta: DOP), κ (kappa: KOP) receptors, and the nociceptin/orphanin FQ peptide receptor (NOP), various groups have further classified the pharmacological µ into µ(1-3), the δ into δ(1-2)/δ(complexed/non-complexed), and the κ into κ(1-3). From an anaesthetic perspective, the suggestions that µ(1) produced analgesia and µ(2) produced respiratory depression are particularly important. However, subsequent to the formal identification of the primary opioid receptors (MOP/DOP/KOP/NOP) by cloning and the use of this information to produce knockout animals, evidence for these additional subtypes is lacking. Indeed, knockout of a single gene (and hence receptor) results in a loss of all function associated with that receptor. In the case of MOP knockout, analgesia and respiratory depression is lost. This suggests that further sub-classification of the primary types is unwise. So how can the wealth of pharmacological data be reconciled with new molecular information? In addition to some simple misclassification (κ(3) is probably NOP), there are several possibilities which include: (i) alternate splicing of a common gene product, (ii) receptor dimerization, (iii) interaction of a common gene product with other receptors/signalling molecules, or (iv) a combination of (i)-(iii). Assigning variations in ligand activity (pharmacological subtypes) to one or more of these molecular suggestions represents an interesting challenge for future opioid research.

Binding of synthetic peptide TPLVTLFK to nonopioid beta-endorphin receptor on rat brain membranes

The synthetic peptide TPLVTLFK corresponding to the sequence 12-19 of beta-endorphin (referred to as octarphin) was found to bind to high-affinity naloxone-insensitive binding sites on membranes isolated from the rat brain cortex (K(d) = 2.6 +/- 0.2 nM). The binding specificity study revealed that these binding sites were insensitive not only to naloxone but also to alpha-endorphin, gamma-endorphin, [Met(5)]enkephalin, and [Leu(5)]enkephalin, as well. The [(3)H]octarphin specific binding with brain membranes was inhibited by unlabeled beta-endorphin (K(i) = 2.4 +/- 0.2 nM) and a selective agonist of nonopioid beta-endorphin receptor decapeptide immunorphin SLTCLVKGFY (K(i) = 2.9 +/- 0.2 nM). At the same time, unlabeled octarphin completely (by 100%) inhibited the specific binding of [(3)H]immunorphin with membranes (K(i) = 2.8 +/- 0.2 nM). Thus, octarphin binds with a high affinity and specificity to nonopioid receptor of beta-endorphin on rat brain cortex membranes.

Identification, structure-activity relationships and molecular modeling of potent triamine and piperazine opioid ligands

Opioid receptors are important targets for pain management. Here, we report the synthesis and biological evaluation of three positional scanning combinatorial libraries, consisting of linear triamines and piperazines. A highly potent (14 nM) and selective (IC(50(mu))/IC(50(kappa))=71; IC(50(delta))/IC(50(kappa))=714) triamine for the kappa-opioid receptor was found. In addition, non-selective mu-kappa binders were obtained, with binding affinities of 54 nM and 22 nM for mu- and kappa-opioid receptors, respectively. Structure-activity relationships of each subset are described. 3D molecular alignments based on shape similarity to internal and external query molecules were carried out. For the combinatorial chemistry dataset studied here a 1.3 similarity cut-off value was observed to be efficient in the rocs-based alignment method. Interactions from the overlays analyzed in the binding sites of homology models of the receptors revealed specific substitution patterns for enhancing binding affinity in the piperazine series. Pharmacophore modeling of the compounds found from the three combinatorial libraries was also performed. The pharmacophore model indicated that the important feature for receptor binding activity with the mu-receptor was the presence of at least one hydrogen bond acceptor and one aromatic hydrophobic group. Whereas for the kappa-receptor two binding modes emerged with one set of compounds employing the hydrogen bond acceptor and aromatic hydrophobic group, and a second set possibly via interactions with the receptor by hydrophobic and ionic salt-bridges.

The evolution of vertebrate opioid receptors

The proteins that mediate the analgesic and other effects of opioid drugs and endogenous opioid peptides are known as opioid receptors. Opioid receptors consist of a family of four closely-related proteins belonging to the large superfamily of G-protein coupled receptors. The three types of opioid receptors shown unequivocally to mediate analgesia in animal models are the mu (MOR), delta (DOR), and kappa (KOR) opioid receptor proteins. The role of the fourth member of the opioid receptor family, the nociceptin or orphanin FQ receptor (ORL), is not as clear as hyperalgesia, analgesia, and no effect was reported after administration of ORL agonists. There are now cDNA sequences for all four types of opioid receptors that are expressed in the brain of six species from three different classes of vertebrates. This review presents a comparative analysis of vertebrate opioid receptors using bioinformatics and data from recent human genome studies. Results indicate that opioid receptors arose by gene duplication, that there is a vector of opioid receptor divergence, and that MOR shows evidence of rapid evolution.

Effects of kappa opioid agonists alone and in combination with cocaine on heart rate and blood pressure in conscious squirrel monkeys

As kappa agonists have been proposed as treatments for cocaine abuse, the cardiovascular effects of the kappa opioid receptor agonists ethylketocyclazocine (EKC) and enadoline were investigated in conscious squirrel monkeys. Both EKC and enadoline increased heart rate with little effect on blood pressure. This effect appeared to be specific for kappa receptors as the mu opioid agonist morphine did not mimic the effects of the kappa agonists. The opioid antagonist naltrexone, at a dose of 1.0 mg/kg, blocked the effect of EKC. An action at both central and peripheral receptors may be responsible for the heart rate increase following kappa agonist treatment. The ganglionic blocker chlorisondamine partially antagonized the effect of EKC on heart rate, suggesting central involvement, while the peripherally-acting agonist ICI 204,448 ((+/-)-1-[2,3- (Dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride) also increased heart rate, supporting a peripheral site of action. When given in combination with cocaine, EKC produced effects that were sub-additive, suggesting that the kappa agonists may be used safely as cocaine abuse treatments.

Effects of opioid subtypes on detrusor overactivity in rats with cerebral infarction

AIM

In order to determine the influence of different opioid receptor subtypes on detrusor overactivity after left middle cerebral artery (MCA) occlusion, cystometric recordings were obtained in conscious rats.

METHODS

Female Sprague-Dawley rats were used in this study. Control cystometrography was followed by left MCA occlusion. The sham-operated (SO) rats underwent the same procedures except for MCA occlusion. [D-Ala(2), Phe(4), Gly(5)]-enkephalin (DAGO; mu-opioid agonist), [D-Pen(2,5)]-enkephalin (DPDPE; delta1-opioid agonist), deltorpin II (delta2-opioid agonist), and U-50488 (kappa-opioid agonist) were administered intracerebroventricularly at graded doses. The bladder capacity, residual volume, micturition threshold pressure, and bladder contraction pressure were determined. Finally, the volume of the infarction was measured.

RESULTS

The intracerebroventricular administration of DAGO and DPDPE significantly increased the bladder capacity in the cerebrally infarcted (CI) and SO rats, but differences in the changes in bladder capacity between the CI and SO rats were not significant. Deltorpin II did not produce any changes in the bladder capacity in the CI or SO rats at any dose examined. However, the intracerebroventricular administration of U-50488 significantly increased the bladder capacity in the CI rats but not in the SO rats. None of the drugs affected the residual volume, micturition threshold pressure or bladder contraction pressure at any dosage examined. The mean infarcted volumes were not significantly different from those in the vehicle-treated rats.

CONCLUSION

These results suggest that the opioid receptor subtypes, mu and delta1 in the brain, are related to the micturition reflex. Furthermore, the kappa opioid agonist might be useful for the suppression of detrusor overactivity caused by cerebral infarction.

Modulation of systemic and renal haemodynamics by κ-opioids in conscious lambs

The purpose of the present study was to determine the cardiovascular effects of the kappa-opioid receptor agonist U-50488H at two stages of postnatal maturation under physiological conditions. Experiments were carried out firstly to define systemic and renal haemodynamic responses to kappa-opioid receptor activation and, secondly, to determine whether these effects are altered during postnatal maturation. To investigate whether the responses to U-50488H resulted from receptor-dependent effects, responses to U-50488H were also tested in the presence of the specific kappa-opioid receptor antagonist 5'-guanidinonaltrindole (GNTI). Experiments were carried out in two groups of conscious, chronically instrumented lambs aged approximately 1 and approximately 6 weeks. Mean arterial pressure, mean venous pressure and renal blood flow (RBF) were measured for 30 min before and 90 min after i.v. injection of U-50488H or vehicle. Heart rate increased in both age groups of lambs within 10 min of U-50488H administration. Mean arterial pressure decreased for 50 min following U-50488H administration at 1 week but, in contrast, increased transiently at 10 min in 6-week-old lambs, returning to control levels by 20 min. In both age groups, there was a sustained decrease in RBF following U-50488H. The aforementioned responses to U-50488H were abolished by pretreatment with GNTI. These data provide the first measurements of systemic and renal haemodynamic responses to kappa-opioid receptor activation during postnatal maturation.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

Kappa opioids as potential treatments for stimulant dependence

Stimulant abuse is a major problem in the United States and the development of pharmacological treatments for stimulant abuse remains an important therapeutic goal. Classically, the "dopamine hypothesis" has been used to explain the development of addiction and dependence of stimulants. This hypothesis involves the direct increase of dopamine as the major factor in mediating the abuse effects. Therefore, most treatments have focused on directly influencing the dopamine system. Another approach, which has been explored for potential treatments of stimulant abuse, is the use of kappa opioid agonists. The kappa receptor is known to be involved, via indirect effects, in synaptic dopamine levels. This review covers several classes of kappa opioid ligands that have been explored for this purpose.

Butorphanol: effects of a prototypical agonist-antagonist analgesic on kappa-opioid receptors

The opioid analgesic, butorphanol (17-cyclobutylmethyl-3,14-dihydroxymorphinan) tartrate is a prototypical agonist-antagonist opioid analgesic agent whose potential for abuse has been the cause of litigation in the United States. With a published affinity for opioid receptors in vitro of 1:4:25 (mu:delta:kappa), the relative contribution of actions at each of these receptors to the in vivo actions of the drug are an issue of active investigation. A body of evidence has been developed which indicates that a substantial selective action of butorphanol on the kappa-opioid receptor mediates the development of tolerance to butorphanol and cross-tolerance to other opioid agonists; to the production of dependence upon butorphanol, particularly in the rodent; and to compensatory alterations in brain opioid receptor-effector systems. This perspective will identify the current state of understanding of the effects produced by butorphanol on brain opioid receptors, particularly on the kappa-opioid receptor subtype, and on the expression of phosphotyrosyl proteins following chronic treatment with butorphanol.

Kappa opioid receptor is expressed in the rat cerebellar cortex

Classically, the cerebellum has been shown to be involved in motor and visual functions, although recent evidence point to new roles of this organ. Pain processing is one of the recently described functions of the cerebellum. According to the importance of the opioid system in nociception, a detailed characterization of the expression pattern of opioid peptides in the cerebellum is the first step towards understanding the precise involvement of this organ in pain management. By using two different approaches (reverse transcription/polymerase chain reaction and in situ hybridization), we have detected, for the first time, expression of the kappa opioid receptor (KOR) gene in the cerebellar cortex of the rat. Expression is found in the molecular and granular layers in all the lobules of the cerebellum. Approximately 34% of the cells present in the molecular layer express KOR mRNA. This work contributes to the deeper knowledge of the mechanisms that are involved in cerebellar function and may lead to a better understanding of the relationships between nociceptive activity and drug abuse potential.

Kappa-opioid receptors are differentially labeled by arylacetamides and benzomorphans

Using Chinese Hamster Ovary cell membranes that stably expressed the human kappa-opioid receptor, we investigated the hypothesis that kappa(1)- and kappa(2)-opioid receptors, historically defined by their pharmacological selectivity for either arylacetamides or benzomorphans are, in fact, different affinity states or binding sites on the same kappa-opioid receptors. Receptor binding studies showed that GTP gamma S potently inhibited [3H](5 alpha,7 alpha,8 beta)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro [4.5]dec-8-yl)-benzeneacetamide (U69,593) binding, compared to virtually no inhibition of [3H]bremazocine binding. Saturation binding experiments showed a three-fold decrease in [3H]U69,593 affinity in the presence of GTP gamma S, but GTP gamma S had no effect on [3H]bremazocine affinity. The kappa-opioid receptor antagonist nor-binaltorphimine had a four-fold higher affinity for [3H]U69,593-labeled receptors than for [3H]bremazocine-labeled receptors. Functional selectivity studies, measuring the stimulation of [35S]GTP gamma S agonist-induced binding, showed a significantly higher U69,593-induced G protein-receptor activation in comparison to the stimulation observed with bremazocine. These results suggest that pharmacologically defined 1 kappa-opioid receptor subtypes may be different affinity states of the same receptor.

Opioid research in amphibians: an alternative pain model yielding insights on the evolution of opioid receptors

This review summarizes the work from our laboratory investigating mechanisms of opioid analgesia using the Northern grass frog, Rana pipiens. Over the last dozen years, we have accumulated data on the characterization of behavioral effects after opioid administration on radioligand binding by using opioid agonist and antagonist ligands in amphibian brain and spinal cord homogenates, and by cloning and sequencing opioid-like receptor cDNA from amphibian central nervous system (CNS) tissues. The relative analgesic potency of mu, delta, and kappa opioids is highly correlated between frogs and other mammals, including humans. Radioligand binding studies using selective opioid agonists show a similar selectivity profile in amphibians and mammals. In contrast, opioid antagonists that are highly selective for mammalian mu, delta, and kappa opioid receptors were not selective in behavioral and binding studies in amphibians. Three opioid-like receptor cDNAs were cloned and sequenced from amphibian brain tissues and are orthologs to mammalian mu, delta, and kappa opioid receptors. Bioinformatics analysis of the three types of opioid receptor cDNAs from all vertebrate species with full datasets gave a pattern of the molecular evolution of opioid receptors marked by the divergence of mu, delta, and kappa opioid receptor sequences during vertebrate evolution. This divergence in receptor amino acid sequence in later-evolved vertebrates underlies the hypothesis that opioid receptors are more type-selective in mammals than in nonmammalian vertebrates. The apparent order of receptor type evolution is kappa, then delta, and, most recently, the mu opioid receptor. Finally, novel bioinformatics analyses suggest that conserved extracellular receptor domains determine the type selectivity of vertebrate opioid receptors.

Non-opioid actions of opioid peptides

Beside the well known actions of opioid peptides on mu-, delta- and kappa-opioid receptors, increasing amount of pharmacological and biochemical evidence has recently been published about non-opioid actions of various opioid peptides. These effects are not abolished by naloxone treatments. Such non-opioid effects are observed both in nervous tissues and in the cellular elements of the immune system. Peptides exhibiting non-opioid effects include beta-endorphin, dynorphin A, nociceptin/OFQ, endomorphins, hemorphins and a number of Proenkephalin A derived peptides, such as Met-enkephalin, Met-enkephalin-Arg-Phe (MERF) and bovine adrenal medullary peptide (BAM22). Non-opioid actions are exerted through different neuronal receptors, e.g., dynorphin hyperalgesia through NMDA receptor, Met-enkephalin induced regulation of cell growth through zeta receptors, pain modulation by nociceptin through ORL-1 or NOP receptors, while BAM22 acts through sensory neuron specific G protein-coupled receptors (SNSR). We have investigated Met-enkephalin-Arg-Phe (MERF) and its analogues by the means of direct and indirect radioligand binding assays. It has been found that in addition to kappa(2) and delta-opioid receptors, MERF can act also through sigma(2)- or probably via FMRF-NH(2) receptors in rat cerebellum. A role of functionally assembling heterodimer receptors in mediating the non-conventional actions of these peptide ligands can not be excluded as well.

Synthesis and binding characteristics of a novel enkephalin analogue, [3H]Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe

The endogenous opioid heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) has been shown to interact with multiple opioid as well as non-opioid sites in mammalian brain membranes. To increase the stability and bioavailability of MERF, new synthetic derivatives with D-amino acid substitutions were prepared and studied. One of the new compounds in this series, Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN), had only moderate affinity in competing with [3H]MERF, whereas it displayed the highest potency in producing antinociception following intrathecal administration. DADN was radiolabeled with 41Ci/mmol specific activity. Specific binding of [3H]DADN was saturable, stereoselective and of high affinity. Chemical stability, increased micro-receptor selectivity, and hydrophobicity of the peptide all contribute to the effectiveness observed in biochemical and pharmacological studies.

Kappa-receptor selective binding of opioid ligands with a heterocyclic bicyclo[3.3.1]nonan-9-one structure

Previous pharmacological results have suggested that members of the heterocyclic bicyclo[3.3.1]nonan-9-one-like compounds are potent kappa-opioid receptor specific agonists. One lead molecule of this series. called compound 1 (dimethyl 7-methyl-2,4-di-2-pyridyl-3.7-diazabicyclo[3.3.1]nonan-9-one-1,5-dicarboxylate) exhibited high affinity for [3H]ethylketocyclazocine and [3H]U-69.593 binding sites in guinea pig cerebellar membranes which known to be a good source for kappa1 receptors. It was shown by molecular modelling that heterocyclic bicyclo[3.3.1]nonan-9-ones fit very well with the structure of ketazocine, a prototypic kappa-selective benzomorphan compound; when compared to the arylacetamide structure of U-69.593, a specific kappa1-receptor agonist, a similar geometry was found with a slightly different distribution of the charges. It is postulated, that the essential structural skeleton involved in the opioid activity is an aryl-propyl-amine element distributed along the N7-C6-C5-C4-aryl bonds.

Josef Rudinger Memorial Lecture 2002. The chemistry of the opioid receptor binding sites

Since the discovery of the opioid receptors and their endogenous ligands an immense research work has been devoted to the exploration of their specificity, the mechanism of ligand binding and ligand-receptor interactions. One of the main goals has been the location and characterization of the binding sites. The present review compiles the results achieved in this field in the last quarter of a century, and puts some questions concerning the success of these efforts.

Synthesis and antinociceptive activity of chimonanthines and pyrrolidinoindoline-type alkaloids

Hodgkinsine, a trimeric pyrrolidinoindoline type alkaloid, present as a major constituent of Psychotria spp. (Rubiaceae), has shown to produce dose-dependent, naloxone reversible, analgesic effect in thermal models of nociception and in the capsaicin-induced pain. SAR studies have been initiated by synthesizing the three diastereomeric dimers (chimonanthines) (11-13) which were evaluated in vitro and in vivo along with the synthetic intermediates. Strong binding affinities for mu opioid receptors were found for (-)- and (+)-chimonanthine monourethanes (9 and 10), whereas (-)-, (+)- and (meso)-chimonanthine (11-13) and hodgkinsine displayed low affinity. In vivo data have shown that only (+)-chimonanthine (12) and calycosidine resemble the analgesic profile found for hodgkinsine.

Synthetic peptide SLTCLVKGFY competes with beta-endorphin for naloxone-insensitive binding sites on rat brain membranes

The synthetic decapeptide Ser-Leu-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr (termed immunorphin) corresponding to the sequence 364-373 of the CH3 domain of human immunoglobulin G heavy chain and its synthetic fragment VKGFY were found to compete with 125I-labeled beta-endorphin for high-affinity naloxone-insensitive binding sites on membranes isolated from the rat brain cortex (K(i)=1.18+/-0.09 and 1.58+/-0.11 nM, respectively). The binding specificity study revealed that these binding sites were insensitive not only to naloxone but to [Met(5)]enkephalin and [Leu(5)]enkephalin as well. The K(d) values characterizing the specific binding of 125I-labeled immunorphin and its fragment Val-Lys-Gly-Phe-Tyr to these binding sites were determined to be 2.93+/-0.27 nM and 3.17+/-0.29 nM, respectively.

[(11)C]-GR89696, a potent kappa opiate receptor radioligand; in vivo binding of the R and S enantiomers

The R and S enantiomers of [(11)C]GR89696, [(11)C]-methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl)methyl]-1-piperazinecarboxylate, were synthesized from their appropriate chiral precursors and [(11)C]methyl chloroformate. The [(11)C]-labeled R enantiomer of GR89696, also known as GR103545, demonstrated high affinity in mouse brain with region to cerebellar ratios at 90 minutes of 11.4 and 8.7 for the hypothalamus and olfactory tubercle, respectively. The [(11)C]-labeled S enantiomer showed low affinity and region to cerebellar ratios of 1 for all brain regions. The [(11)C]-labeled GR103545 exhibited a selective and saturable binding for the kappa opioid receptor.

Alterations in cortical and basal ganglia levels of opioid receptor binding in a rat model of l-DOPA-induced dyskinesia

Opioid receptor-binding autoradiography was used as a way to map sites of altered opioid transmission in a rat model of l-DOPA-induced dyskinesia. Rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathways sustained a 3-week treatment with l-DOPA (6 mg/kg/day, combined with 12 mg/kg/day benserazide), causing about half of them to develop dyskinetic-like movements on the side of the body contralateral to the lesion. Autoradiographic analysis of mu-, delta-, and kappa-opioid binding sites was carried out in the caudate-putamen (CPu), the globus pallidus (GP), the substantia nigra (SN), the primary motor area, and the premotor-cingulate cortex. The dopamine-denervating lesion alone caused an ipsilateral reduction in opioid radioligand binding in the CPu, GP, and SN, but not in the cerebral cortex. Chronic l-DOPA treatment affected opioid receptor binding in both the basal ganglia and the cerebral cortex, producing changes that were both structure- and receptor-type specific, and closely related to the motor response elicited by the treatment. In the basal ganglia, the most clear-cut differences between dyskinetic and nondyskinetic rats pertained to kappa opioid sites. On the lesioned side, both striatal and nigral levels of kappa binding densities were significantly lower in the dyskinetic group, showing a negative correlation with the rats' dyskinesia scores on one hand and with the striatal expression of opioid precursor mRNAs on the other hand. In the cerebral cortex, levels of mu and delta binding site densities were bilaterally elevated in the dyskinetic group, whereas kappa radioligand binding was specifically increased in the nondyskinetic cases and showed a negative correlation with the rats' dyskinesia scores. These data demonstrate that bilateral changes in cortical opioid transmission are closely associated with l-DOPA-induced dyskinesia in the rat. Moreover, the fact that dyskinetic and nondyskinetic animals often show opposite changes in opioid radioligand binding suggests that the motor response to l-DOPA is determined, at least in part, by compensatory adjustments of brain opioid receptors.

Analysis of [3H]bremazocine binding in single and combinatorial opioid receptor knockout mice

Despite ample pharmacological evidence for the existence of multiple mu-, delta- and kappa-opioid receptor subtypes, only three genes encoding mu-(MOR), delta-(DOR) and kappa-(KOR) opioid receptor have been cloned. The KOR gene encodes kappa(1)-sites, which specifically bind arylacetamide compounds, and the possible existence of kappa-opioid receptor subtypes derived from another kappa-opioid-receptor gene, yet to be characterized, remains a very contentious issue. kappa(2)-Opioid receptors are described as binding sites typically labelled by the non-selective benzomorphan ligand [3H]bremazocine in the presence of mu-, delta- and kappa(1)-opioid receptor blocking ligands. To investigate the genetic origin of kappa(2)-opioid receptors, we have carried out homogenate binding experiments with [3H]bremazocine in brains of single MOR-, DOR-, KOR- and double MOR/DOR-deficient mice. Scatchard analysis showed that 68+/-12% of the binding sites arise from the MOR gene, 27+/-1% from the DOR gene and 14.5+/-0.2% from the KOR gene, indicating that the three known genes account for total [3H]bremazocine binding. Experiments in the presence of mu-, delta- and kappa(1)-opioid receptor suppressor ligands further showed that non-kappa(1)-opioid receptor labelling can be accounted for by binding to both the mu- and delta-opioid receptors. Finally, [3H]bremazocine binding experiments performed on brain membranes from the triple MOR/DOR/KOR-deficient mice revealed a complete absence of binding sites, confirming definitively that no additional gene is required to explain the total population of [3H]bremazocine binding sites. Altogether the data show that the putative kappa(2)-opioid receptors are in fact a mixed population of KOR, DOR and predominantly MOR gene products.

Morphine inhibits resting respiration, but it attenuates reflexive respiratory suppression in anesthetized cat through kappa-receptor

Noxious stimulation of thin-fiber muscular afferents induces a reflexive respiratory suppression that we call "poststimulus respiratory suppression." In anesthetized, vagotomized, paralyzed, and artificially ventilated cats, morphine depressed the level of resting respiration (inhibitory effect on resting respiration) and attenuated the magnitude of the poststimulus respiratory suppression (excitatory effect on the reflexively modified respiration). These two kinds of morphine effects were antagonized by naloxone, suggesting the participation of opioid receptors. To clarify the opioid receptor subtypes responsible for these effects of morphine, three type-selective opioid antagonists-naltrindole (delta antagonist), gamma-funaltrexamine (mu antagonist), and Mr2266 (kappa antagonist)-were tested. The morphine-induced depression in the resting respiration was antagonized by pretreatment with the kappa antagonist, not with the mu or delta antagonist. Furthermore, the morphine-induced attenuation in the magnitude of the poststimulus suppression was also blocked by the kappa antagonist, but not by the mu or delta antagonist. In conclusion, (1) morphine inhibits resting respiration, but it attenuates the magnitude of the poststimulus respiratory suppression; (2) both these morphine effects are mediated by kappa opioid receptors. The possibility that the kappa(3) receptor, one of the kappa receptors subtypes, mediates the two kinds of morphine effects has been discussed.

Kappa opioid receptors are expressed by interneurons in the CA1 area of the rat hippocampus: a correlated light and electron microscopic immunocytochemical study

A local GABA-system is known to have a mediatory function between several afferents and the principal cells of the hippocampus. This study examines the distribution and fine structure of kappa opioid receptor-immunoreactive elements in the CA1 subfield and reveals some new aspects concerning the structural basis of opioid-GABA interaction in the rat hippocampal formation. Kappa receptors were visualized immunocytochemically with a previously produced and characterized monoclonal antibody, the mAb KA8 (Maderspach, K., Németh, K., Simon, J., Benyhe, S., Szûcs, M., Wollemann, M., 1991. A monoclonal antibody recognizing kappa-, but not mu- and delta-opioid receptors. J. Neurochem. 56, 1897-1904). The antibody selectively recognizes the kappa opioid receptor with preference to the kappa(2) subtype. Neuronal cell bodies, proximal dendrites and occasionally glial processes surrounding neuronal perikarya were labelled in the CA1 area. The immunopositive cells were present mainly in the stratum oriens, followed by the stratum pyramidale in a rostrocaudally increasing number. Their shape was fusiform, or multipolar. Occasionally kappa receptor-immunoreactive boutons surrounding weakly immunopositive somata were also observed. Electron microscopy of immunopositive neurons showed that the DAB labelling was intensive in the perinuclear cytoplasm. The widths and electron densities of the postsynaptic densities of some axosomatic synapses were remarkably increased. Similar increase of postsynaptic densities were observable at some axodendritic and axospinous synapses. On the basis of their location and fine structural properties the labelled cells are suggested to be GABAergic inhibitory interneurons, probably belonging to the somatostatinergic sub-population. The axons of these inhibitory interneurons are known to arborize in the stratum lacunosum-moleculare where the entorhinal afferents terminate. A modulatory effect of opioids on the entorhinal input, mediated by somatostatinergic interneurons is suggested

Tritiated kappa receptor antagonist norbinal torphimine: synthesis and in vitro binding in three different tissues

Recently a new antagonist with high selectivity for the kappa receptors (norbinaltorphimine) was developed and tested in various systems. This compound was radiolabelled with tritium resulting in high specific radioactivity (47.2 Ci/mmol). [3H]Norbinaltorphimine was characterized by in vitro radioligand binding assays. The radioligand binds to kappa-opioid receptors with a high potency and selectivity in guinea pig, frog and rat brain membranes. Our results suggest the kappa1 specificity of the radioligand.

Receptor binding and G-protein activation by new Met5-enkephalin-Arg6-Phe7 derived peptides

Met5-enkephalin-Arg6-Phe7 (Tyr-Gly-Gly-Phe-Met-Arg-Phe, MERF) is a naturally occurring heptapeptide that binds to opioid and non-opioid recognition sites in the central nervous system. Four synthetic analogs with single or double amino acid substitutions were prepared by solid phase peptide synthesis to achieve proteolytically more stable structures: Tyr-D-Ala-Gly-Phe-Met-Arg-Phe (I), Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (II), Tyr-D-Ala-Gly-Phe-L-Nle-Arg-Phe (III) and Tyr-Gly-Gly-Phe-L-Nle-Arg-Phe (IV). In this study receptor binding characteristics and G-protein activation of MERF and its derivatives were compared in crude membrane fractions of frog and rat brain. Synthetic MERF-derived peptides were potent competitors for [3H]MERF and [3H]naloxone binding sites with the exception of analog (II) which turned to be substantially less active. The presence of 100 mM NaCl or 100 microM 5'-guanylylimidodiphosphate, Gpp(NH)p, decreased the affinity of the peptides in [3H]naloxone binding assays, suggesting that these ligands might act as agonists at the opioid receptors. Some of the compounds were also used to stimulate guanosine-5'-O-(3-[gamma-[35S]thio)triphosphate ([35S]GTPgammaS) binding in rat and frog brain membranes at concentrations of 10(-9)-10(-5) M. The EC50 values of analog (II) were the highest in both tissues. Analog (I) was as effective as MERF in rat brain membranes, but showed lower maximal stimulation in frog brain preparation. Again, analog (II) seemed to be the least efficacious peptide that stimulated [35S]GTPgammaS binding only by 59%. Specificity of the peptides was further investigated by the inhibition of agonist-stimulated [35S]GTPgammaS binding in the presence of selective antagonists for the opioid receptor types. The mu-selective antagonist cyprodime displayed the lowest potency in inhibiting the effects of the peptides, whereas norbinaltorphimine (kappa-selective antagonist) and naltrindole (delta-selective antagonist) were quite potent in both tissues. We concluded that MERF and its derivatives are able to activate G-proteins mainly via kappa- and delta-opioid receptors.

Characterization of the antinociceptive effects of TRK-820 in the rat

We have already reported that TRK-820, (-)-17-cyclopropylmethyl-3, 14b-dihydroxy-4, 5a-epoxy-6b-[N-methyl-trans-3-(3-furyl)acrylamido]morphinan hydrochloride, a new selective kappa-opioid receptor agonist, has affinity for kappa-subtype opioid receptors other than the kappa(1)-opioid receptor. It would be of interest to examine whether the different kappa-opioid receptor subtype properties of TRK-820 participate in its antinociceptive action in the inflamed paw test and the formalin test. TRK-820 produced a potent antinociceptive effect, which was inhibited by the selective kappa-opioid receptor antagonist nor-binaltorphimine, but not by the mu-opioid receptor antagonist naloxone in the mechanical paw pressure test. TRK-820 also produced a potent antinociceptive effect in rats with adjuvant-induced arthritis. TRK-820 and morphine, a prototype mu-opioid receptor agonist, were equally effective in inhibiting the nociceptive responses in the arthritic rats and in the normal rats, while ICI-199441, 2-(3, 4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-(1-pyrrolidinyl)ethyl]- acetamide, a kappa-opioid receptor agonist, was about 5-fold less potent in the arthritic rats than in the normal rats. In the formalin test TRK-820 had a very similar antinociceptive potency to that of ICI-199441, unlike in the arthritic rats in which TRK-820 was 2.5 times more potent than ICI-199441. It is concluded that TRK-820 produced a potent antinociceptive action via the stimulation of kappa-opioid receptors in rats. TRK-820 has a unique antinociceptive profile different from that of the other kappa-opioid receptor agonists such as ICI-199441 in arthritic rats.

Opioid peptide receptor studies. 10. Nor-BNI differentially inhibits kappa receptor agonist-induced G-protein activation in the guinea pig caudate: further evidence of kappa receptor heterogeneity

There is strong evidence supporting the existence of multiple kappa receptors. Previous studies proposed that U69,593 and (+)-tifluadom act on different kappa receptor subtypes, kappa(1) (kappa(1)) and kappa(2) (kappa(2)), respectively. In this study, we investigated the effects of the kappa selective antagonist nor-binaltorphimine (Nor-BNI) on U69,593- and (+)-tifluadom-induced receptor-mediated stimulation of [(35)S]-GTP-gamma-S binding in the guinea pig caudate. The IC(50) value of Nor-BNI in the presence of a stimulating concentration of U69,593 (1 microM) was 0.19+/-0.02; while the IC(50) for Nor-BNI in the presence of (+)-tifluadom (1 microM) was 13.9+/- 1.62 nM. The mu-opioid receptor antagonist CTAP (10,000 nM) significantly reduced (+)-tifluadom-stimulated [(35)S]-GTP-gamma-S binding in rat brain sections and guinea pig brain membranes, indicating that (+)-tifluadom has mu agonist activity. Under conditions in which the mu agonist activity of (+)-tifluadom was blocked by 1000 nM CTAP the Ki value for Nor-BNI for inhibition of U69,593-stimulated [(35)S]-GTP-gamma-S binding was 0.036+/-.004 nM, whereas, its Ki value for the (+)-tifluadom-stimulated [(35)S]-GTP-gamma-S binding was 0.27+/-.015 nM. These results suggest that (+)-tifluadom and U69,593 activate pharmacologically different receptors. This study provides functional evidence in support of kappa receptor heterogeneity.

Dynorphin selectively augments the M-current in hippocampal CA1 neurons by an opiate receptor mechanism

Most electrophysiological studies of opioids on hippocampal principal neurons have found indirect actions, usually through interneurons. However, our laboratory recently found reciprocal alteration of the voltage-dependent K(+) current, known as the M-current (I(M)), by kappa and delta opioid agonists in CA3 pyramidal neurons. Recent ultrastructural studies have revealed postsynaptic delta opiate receptors on dendrites and cell bodies of CA1 and CA3 hippocampal pyramidal neurons (HPNs). Reasoning that previous electrophysiological studies may have overlooked voltage-dependent postsynaptic effects of the opioids in CA1, we reevaluated their role in CA1 HPNs using the rat hippocampal slice preparation for intracellular current- and voltage-clamp recording. None of the delta and mu; receptor-selective opioids tested, including [D-Pen(2,5)]-enkephalin (DPDPE), [D-Ala(2)]-deltorphin II (deltorphin), [D-Ala(2), NMe-Phe(4), Gly-ol]-enkephalin (DAMGO), and [D-Ala(2), D-Leu(5)] enkephalin (DADLE), altered membrane properties such as I(M) or Ca(2+)-dependent spikes in CA1 HPNs. The nonopioid, Des-Tyr-dynorphin (D-T-dyn), also had no effect. By contrast, dynorphin A (1-17) markedly increased I(M) at low concentrations and caused an outward current at depolarized membrane potentials. The opioid antagonist naloxone and the kappa receptor antagonist nor-binaltorphimine (nBNI) blocked the I(M) effect. However, the kappa-selective agonists U69,593 and U50,488h did not significantly alter I(M) amplitudes when averaged over all cells tested, although occasional cells showed an I(M) increase with U50,488h. Our results suggest that dynorphin A postsynaptically modulates the excitability of CA1 HPNs through opiate receptors linked to voltage-dependent K(+) channels. These findings also provide pharmacological evidence for a functional kappa opiate receptor subtype in rat CA1 HPNs but leave unanswered questions on the role of delta receptors in CA1 HPNs.

Characterization of kappa1-opioid receptor binding in human insular cortex

Mesolimbic dopaminergic neurotransmission is modulated by dynorphin peptides binding to kappa-opioid receptors. The interaction between dynorphin and dopamine systems makes the kappa-opioid receptor a potential drug discovery target for the development of therapeutic agents for schizophrenia and drug abuse. This study reports the specificity and parameters of [3H]U69593 binding in the insular cortex, a representative corticolimbic area of the human brain. The results demonstrate that the radioligand [3H]U69593 labels a single population of receptors in human insular cortex with an affinity in the low nanomolar range. The pharmacological profile for inhibition of [3H]U69593 binding was determined in this brain region using drugs known to bind to mu, kappa and delta opioid receptors. The results show that kappa-opioid selective agonists and antagonists inhibit binding of this ligand in human brain with comparable affinities and rank order as previously described for rat and guinea pig brain and the cloned kappa1-opioid receptor subtype.

Irreversible labelling of the opioid receptors by a melphalan-substituted [Met5]enkephalin-Arg-Phe derivative

[Met5]enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe) was modified with the methyl esther of melphalan (Mel; 4-bis(2-chloroethyl)amino-L-phenylalanine) and the resulting compounds were studied for their opioid binding properties in guinea pig and rat brain membranes. Three new peptides, with a substitution of a single amino acid, were synthesized (Mel-Gly-Gly-Phe-Met-Arg-Phe, Tyr-Gly-Gly-Mel-Met-Arg-Phe and Tyr-Gly-Gly-Phe-Met-Arg-Mel). In the rat brain, none of these ligands displayed any type specificity, whereas in guinea pig brain membranes the C-terminally modified peptide, Tyr-Gly-Gly-Phe-Met-Arg-Mel ([Mel7]peptide), displayed a kappa-binding profile and was a weak kappa-opioid-receptor agonist in isolated guinea pig ileum. The effect of sodium ions on [Mel7]peptide competition against [3H]naloxone binding indicated a weak agonist nature of the compound. When guinea pig brain membranes were preincubated with 1-10 microM of [Mel7]peptide, an apparently irreversible inhibition of [3H]naloxone ligand binding was observed. These results suggest that the heptapeptide containing melphalan at the C-terminus can be used as a relatively high-affinity irreversible label for the kappa-opioid receptor.

Evaluation of hypothermia-induced analgesia and influence of opioid antagonists in leopard frogs (Rana pipiens)

Hypothermia results in diminished voluntary muscle activity, and is frequently used as a means of providing deep anesthesia to ectotherms and some mammals. In ectotherms, however, it is unclear if hypothermia produces true pain insensation. A needle-probe thermometer was used to demonstrate in frogs (Rana pipiens) that local hypothermia (9 degrees C) could be induced by placement of a tourniqueted leg into ice water (6 degrees C) for 10 min in contrast to the contralateral nontourniqueted leg (21.8 degrees C) kept out of ice water. Analgesia was tested by placement of dilutions of acetic acid on the rear leg. Further tests using groups of 10 frogs demonstrated that frogs with local hypothermia tolerated greater concentrations of acetic acid (mean acetic acid test score = 11) than morphine (10 mg/kg)-treated (9.6) or nontreated (5.8) frogs. Additional studies showed that morphine analgesia was blocked with naloxone doses as low as 0.01 mg/kg and hypothermia-induced analgesia at 10 mg/kg. Naltrexone blocked morphine analgesia at dosages as low as 0.01 mg/kg and hypothermia-induced analgesia at 0.10 mg/kg. In summary, this study demonstrates that hypothermia induces significant analgesia in an amphibian, and that this analgesia is partially blocked by naloxone and naltrexone, suggesting that the effect is mediated at least partially by opioid receptors.

Characterization of [3H]Met-enkephalin-Arg6-Phe7 binding to multiple sites in rat and guinea pig cerebellum

[3H]Met-enkephalin-Arg6-Phe7 (MERF) has been shown to label opioid (kappa2 and delta) and sigma2 sites in rat and frog brain membrane preparations, and no specific binding to kappa1 opioid receptors could be established (refs. 6 and 8). In this study the binding was examined in rat cerebellar membranes which are relatively rich in kappa2-sites, and in guinea pig cerebellar preparations where kappa1 opioid receptors are almost exclusively present. In accordance with our previous results, [3H]MERF binding could not be displaced in guinea pig cerebellar membranes neither with U-69,593 nor with naloxone or levorphanol suggesting no interaction with opioid sites, nevertheless a Kd of 2.8 nM was calculated in cold saturation experiments. In rat cerebellar membrane fractions about the half of the specific [3H]MERF binding sites was inhibited by opiate alkaloids such as naloxone, ethylketocyclazocine, or bremazocine. This portion of the heptapeptide binding sites was stereoselective as demonstrated by the difference in the affinities of the enantiomeric compounds levorphanol and dextrorphan, therefore it would represent an opioid site. In both tissues (-)N-allyl-normetazocine (SKF-10,047), which is also considered as sigma2 ligand, displayed the highest affinities. Among opioid peptides beta-endorphin and dynorphin(1-13) showed the highest potencies, displacing [3H]MERF also from its non-opioid sites. It was concluded therefore that [3H]MERF does not bind to kappa1 sites, and besides kappa2-opioid sites substantial binding to peptide preferring non-opioid sites, and/or sigma2 receptors also occurs.

Spinal administration of selective opioid antagonists in amphibians: evidence for an opioid unireceptor

In mammals, opioids act by interactions with three distinct types of receptors: mu, delta, or kappa opioid receptors. Using a novel assay of antinociception in the Northern grass frog, Rana pipiens, previous work demonstrated that selective mu, delta, or kappa opioids produced a potent antinociception when administered by the spinal route. The relative potency of this effect was highly correlated to that found in mammals. Present studies employing selective opioid antagonists, beta-FNA, NTI, or nor-BNI demonstrated that, in general, these antagonists were not selective in the amphibian model. These data have implications for the functional evolution of opioid receptors in vertebrates and suggest that the tested mu, delta, and kappa opioids mediate antinociception via a single type of opioid receptor in amphibians, termed the unireceptor.

Opioids: first lessons from knockout mice

Opioid receptors of the mu-, delta- and kappa-subtypes mediate the potent analgesic and addictive actions of opioid drugs. They also regulate responses to pain, stress and emotions when activated by endogenous opioid peptides. Recently, mice lacking opioid receptors or opioid peptides have been produced by gene targeting, providing molecular tools to study opioid function in vivo. Observations on mutant mice have shed new light on the mode of action of opioids, opioid receptor heterogeneity and interactions, and the involvement of each component of the opioid system in mouse physiology. In this article, Brigitte L. Kieffer reviews the first reported studies and discusses their therapeutic implications.

Weaning-induced development of delta-opioid receptors in rat brain: differential effects of guanine nucleotides and sodium upon ligand-receptor recognition

1. We have previously shown that weaning at day 21 increases delta-opioid receptor binding in the brain at day 25, which might be due to stimulation of the development of a delta-opioid receptor subtype or activation of G-protein coupling processes. 2. We have addressed the possibility that weaning stimulates coupling of the delta-receptor by homogenate binding studies with four agonist and one antagonist radioligand in the presence of a GTP analogue and Na+ in brain tissue from weaned and non-weaned animals. 3. Saturation studies with three agonist ligands ([3H]-deltorphin I, [3H]-S-Atc-Ile(5,6)deltorphin I and [3H]-R-Atc-Ile(5,6)deltorphin II) showed higher levels of maximal binding in brains from 25-day weaned than in brains from non-weaned rats. The magnitude of the effects of GMPPNP and Na+ in decreasing this binding was ligand dependent and in each case was significantly more marked in brains from weaned animals. GMPPNP and Na+ were completely without effect on Bmax for, [3H]-S-Atc-Ile(5,6)deltorphin I and [3H]-R-Atc-Ile(5,6)deltorphin II in brains from non-weaned rats. 4. [3H]-Ile(5,6)deltorphin II and [3H]-naltrindole showed no differences in labelling between weaned and non-weaned groups and both groups responded similarly to the effects of GMPPNP and Na+ treatment. 5. GMPPNP and Na+ had small effects on binding affinity (K(D)) for some of the agonist radioligands which were similar in both weaned and non-weaned groups. 6. Weaning induced increases in binding of delta-receptors in 25-day rats can be explained in terms of the way delta-agonist radioligands recognize the receptor environment.

Glucose stimulation of pancreatic beta-cell lines induces expression and secretion of dynorphin

To investigate adaptive responses of pancreatic beta-cells to hyperglycemia, genes induced by glucose stimulation were identified by subtraction cloning. Among 53 clones representing differentially expressed genes, 20 encoded the endogenous opioid precursor, prodynorphin. The amino acid sequence of murine prodynorphin is identical to the rat protein in sequences comprising the opioid peptides and 86% identical in the remainder of the molecule. Stimulation of MIN6 cells increased prodynorphin RNA levels to more than 20-fold in proportion to physiological glucose concentrations. Similar induction levels were observed in murine betaTC3 and rat Rinm5F beta-cell lines. Prodynorphin RNA expression increased within 1 h of glucose stimulation, achieved maximal levels by 4 h, and remained elevated for at least 24 h. By using RIA, MIN6 cells were shown to contain and secrete increased amounts of dynorphin-A following glucose stimulation. Treatment of MIN6 cells with KCl, forskolin, or isobutyl-methyl-xanthine strongly induced prodynorphin RNA expression, suggesting that induction may be related to secretion-coupled signaling pathways. The induction of prodynorphin in several beta-cell lines is consistent with previous demonstrations of beta-cell synthesis of other endogenous opioids, including beta-endorphin, and suggests that opioids may have a potentially significant role in regulating beta-cell secretion.

Lack of evidence of kappa2-selective activation of G-proteins: kappa opioid receptor stimulation of [35S] GTPgammaS binding in guinea pig brain

Although only one gene for kappa opioid receptors has been cloned to date, kappa1 and kappa2 receptors have been defined pharmacologically, with drugs such as bremazocine binding to both putative kappa receptor subtypes. To examine whether kappa receptor subtypes can be distinguished at the level of the G-protein, the ability of the kappa1 agonist (trans-(dl)-3,4-dichloro-N- methyl-N-[2-(1 -pyrrolidinyl)cyclohexyl]-benzeneacetamide) methane sulfonate (U-50488H) to stimulate [35S]guanosine-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding in guinea pig brain was compared with that of bremazocine and dynorphin. In membranes prepared from guinea pig striatum, both bremazocine and U-50488H stimulated [35S]GTPgammaS binding with the same relative efficacy, while dynorphin produced at least two-fold greater efficacy than the other two agonists. In vitro autoradiography of agonist-stimulated [35S]GTPgammaS binding revealed similar regional distributions of bremazocine- and U-50488H-activated G-proteins. In striatal membranes, the kappa antagonist nor-binaltorphimine (nor-BNI) blocked both bremazocine- and U-50488H-stimulated [35S]GTPgammaS binding with similar Ke values. In agonist additivity experiments, the stimulation of [35S]GTPgammaS binding by the delta agonist [D-pen2'5, p-Cl-Phe4]enkephalin (p-Cl-DPDPE) was approximately additive with the two kappa agonists. Stimulation of [35S]GTPgammaS binding by the mu agonist [D-Ala2, N-Me4, Gly5-ol]-enkephalin (DAMGO) was additive with U-50488H, but not with bremazocine, reflecting the mu antagonist properties of this compound. The combination of bremazocine and U-50488H together produced no greater stimulation of binding than either agonist alone, indicating that they were binding to the same site. These results demonstrate that bremazocine and U-50488H activate G-proteins in guinea pig brain through the same receptor, and suggest that kappa2 receptors are not coupled through the same signal transduction mechanisms as kappa1 receptors.

Effect of the peripherally selective kappa-opioid agonist, asimadoline, on adjuvant arthritis

1. Opioids, though widely used as analgesics, have not been seriously considered as therapy for rheumatoid arthritis. The present study evaluated the dose-effect and time-dependence relationships of a new peripherally selective kappa agonist, asimadoline, in rats with adjuvant arthritis. 2. The arthritis was assessed by a pooled severity index combining the comprehensive criteria of oedema, radiography and histological changes, in the hind limbs. Asimadoline was extremely effective in attenuating joint damage (by up to 80%) when administered parenterally (0.5 to 10 mg kg(-1) day(-1), i.p.) throughout the disease or during its early phase; treatment was less successful if confined to the latter stages. Ten fold higher doses were effective orally. 3. Equimolar doses of a peripherally-selective antagonist, naloxone methiodide, and the kappa-selective antagonist, MR2266, fully reversed the peripheral anti-arthritic effects of asimadoline (5 mg kg(-1) day(-1)), indicating that asimadoline acts through peripheral kappa-opioid receptors. However, an equivalent dose of MR2266 did not fully reverse the anti-arthritic effects of the highest dose of asimadoline (40 mg kg(-1) day(-1)), suggesting a loss of kappa-selectivity at this dose. 4. Asimadoline also exhibited analgesic effects (mechanical nociceptive thresholds) in arthritic but not non-arthritic rats, indicating that inflammation is necessary for asimadoline-induced analgesia. 5. These data confirm our previous findings that kappa-opioids possess anti-arthritic properties and that these effects are mediated via peripheral kappa-receptors. The present results are new in showing that the peripherally acting kappa-opioid agonist, asimadoline, is a potent anti-arthritic agent. Such novel drugs, essentially lacking central side effects, herald new treatments for rheumatoid arthritis.

Receptor-mediated activation of G-proteins by kappa opioid agonists in frog (Rana esculenta) brain membranes

This study delineates the heterotrimeric guanine nucleotide binding regulatory protein (G-protein) types in frog (Rana esculenta) brain membranes and their activation by kappa opioid agonists. Ethylketocyclazocine (EKC), trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl)b enzeneacetamide (U-50,488) and bremazocine displayed dose-dependent, norbinaltorphimine-reversible stimulation of guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding in crude membrane preparations. G-proteins were identified by Western-blotting using previously characterized specific antisera that were generated against mammalian G-protein alpha-subunits and beta-subunits. A photoreactive guanosine 5'-triphosphate (GTP) analog, [alpha-32P]GTP azidoanilide ([alpha-32P]AA-GTP) irreversibly labeled four proteins in the molecular weight range of 39-43 kDa. Ethylketocyclazocine and U-50,488 stimulated photolabelling of these proteins among which the 39 kDa band comigrated with the protein specifically labelled with the alpha(i2) antibody and the 40 kDa band was identified as alpha(o1). The other two bands were also stained with the alpha(common) antibody, but were not further identified. These results suggest that the endogenously expressed kappa opioid receptors that are present in frog brain interact with multiple G-proteins in situ. Furthermore, the structure of most G-proteins seems to be well preserved during phylogenesis.

Effects of niravoline (RU 51599), a selective kappa-opioid receptor agonist on intracranial pressure in gradually expanding extradural mass lesion

It has recently been reported that kappa-opioid receptor agonists inhibit antidiuretic hormone secretion and promote water excretion in humans and animals. We investigated the effect of niravoline (RU 51599), a selective kappa-opioid receptor agonist in the treatment of intracranial hypertension. Acute intracranial hypertension was induced in cats by continuous inflation of an extradural balloon with physiological saline at the constant rate of 0.5 ml/h for 3 h. At this point, inflation was discontinued and the balloon remained expanded for an additional hour after which it was deflated. In the post-deflation period, monitoring continued for 1 h. The control group (n = 8) received ringer's lactate solution only, while the treatment group (n = 8) received an intravenous (IV) injection of 1.0 mg/kg of niravoline, every hour at the beginning of balloon inflation, during balloon inflation, in post-inflation, and at deflation time (5 doses). Changes in intracranial pressure (ICP), mean arterial blood pressure (MAP), cerebral perfusion pressure (CPP), electroencephalogram (EEG), blood gases, pupil size, serum electrolytes, and osmolality were measured in both groups. Brain water content was determined in a separate group of cats at the end of a 3-h extradural brain compression. Compared to the untreated group, the niravoline-treated group had a significantly lower ICP and higher CPP at the 2 and 3 h during balloon inflation, in post-inflation, and in post-deflation periods. Brain water content was significantly reduced in niravoline-treated animals. No significant change was observed in serum osmolality throughout the experiment. Our results indicate that the mechanism by which niravoline reduces ICP is partly via a reduction in brain water content. Also, the current findings suggest that in clinical situations in which ICP is elevated due to the pressure of an extradural mass, niravoline may effectively reduce ICP while maintaining adequate CPP until the mass is removed.

Quantitative autoradiographic mapping of mu-, delta- and kappa-opioid receptors in knockout mice lacking the mu-opioid receptor gene

Mice lacking the mu-opioid receptor (MOR) gene have been successfully developed by homologous recombination and these animals show complete loss of analgesic responses to morphine as well as loss of place-preference activity and physical dependence on this opioid. We report here quantitative autoradiographic mapping of opioid receptor subtypes in the brains of wild-type, heterozygous and homozygous mutant mice to demonstrate the deletion of the MOR gene, to investigate the possible existence of any mu-receptor subtypes derived from a different gene and to determine any modification in the expression of other opioid receptors. Mu-, delta-, kappa1- and total kappa-receptors, in adjacent coronal sections in fore- and midbrain and in sagittal sections, were labelled with [3H]DAMGO (D-Ala2-MePhe4-Gly-ol5 enkephalin), [3H]DELTI (D-Ala2 deltorphinI), [3H]CI-977 and [3H]bremazocine (in the presence of DAMGO and DPDPE) respectively. In heterozygous mice, deficient in one copy of the MOR gene, mu-receptors were detectable throughout the brain at about 50% compared to wild-type. In brains from mu-knockout mice there were no detectable mu-receptors in any brain regions and no evidence for mu-receptors derived from another gene. Delta-, kappa1- and total kappa-receptor binding was present in all brain regions in mutant mice where binding was detected in wild-type animals. There were no major quantitative differences in kappa- or delta-binding in mutant mice although there were some small regional decreases. The results indicate only subtle changes in delta- and kappa-receptors throughout the brains of animals deficient in mu-receptors.

Kappa2 opioid receptors in limbic areas of the human brain are upregulated by cocaine in fatal overdose victims

Cocaine is thought to be addictive because chronic use leads to molecular adaptations within the mesolimbic dopamine (DA) circuitry that affect motivated behavior and emotion. Although the reinforcing effects of cocaine are mediated primarily by blocking DA reuptake into the presynaptic nerve terminal, reciprocal signaling between DA and endogenous opioids has important implications for cocaine dependence. The present study used the opioid antagonist 6 beta-[125iodo]-3,14-dihydroxy-17-cyclopropylmethyl-4,5 alpha-epoxymorphinan ([125I]IOXY) after pretreatment with the site-directed acylating agents 2-(p-ethoxybenzyl)-1-diethylaminoethyl-5-isothiocyanatobenzimid iazole -HCl (mu-selective) and N-phenyl-N-[1-(2-(4-isothiocyanato)-phenethyl)-4-piperidinyl]-p ropana mide-HCl (delta-selective) to examine the effect of cocaine exposure on the distribution and density of kappa2 receptors in autopsy studies of human cocaine fatalities. The selective labeling of the kappa2 receptor subtype was demonstrated by competition binding studies, which gave a pharmacological signature (IOXY >/= (+)-bremazocine >> U50,488 >/= U69,593) distinct from either the kappa1 or kappa3 receptor subtypes. Visualization of [125I]IOXY labeling revealed that kappa2 receptors localize to mesocortical and subcortical limbic areas, including the cingulate, entorhinal, insular, and orbitofrontal cortices and the nucleus accumbens and amygdala. The number of kappa2 receptors in the nucleus accumbens and other limbic brain regions from cocaine fatalities was increased twofold as compared with age-matched and drug-free control subjects. Cocaine overdose victims, who experienced paranoia and marked agitation before death, also had elevated densities of kappa2 receptors in the amygdala. These findings demonstrate for the first time that kappa2 receptor numbers are upregulated by cocaine exposure. The molecular adaptation of kappa2 receptor numbers may play a role in the motivational incentive associated with episodes of binge cocaine use and in the dysphoria that follows abrupt cocaine withdrawal.