The nature of opiate receptors in toad brain

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.

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.

Delta and mu opioid receptors from the brain of a urodele amphibian, the rough-skinned newt Taricha granulosa: cloning, heterologous expression, and pharmacological characterization

Two full-length cDNAs, encoding delta (delta) and mu (mu) opioid receptors, were cloned from the brain of the rough-skinned newt Taricha granulosa, complementing previous work from our laboratory describing the cloning of newt brain kappa (kappa) and ORL1 opioid receptors. The newt delta receptor shares 82% amino acid sequence identity with a frog delta receptor and lower (68-70%) identity with orthologous receptors cloned from mammals and zebrafish. The newt mu receptor shares 79% sequence identity with a frog mu receptor, 72% identity with mammalian mu receptors, and 66-69% identity with mu receptors cloned from teleost fishes. Membranes isolated from COS-7 cells transiently expressing the newt delta receptor possessed a single, high-affinity (Kd = 2.4 nM) binding site for the nonselective opioid antagonist [3H]naloxone. In competition binding assays, the newt delta receptor displayed highest affinity for Met-enkephalin, relatively low affinity for Leu-enkephalin, beta-endorphin, and [D-penicillamine, D-penicillamine] enkephalin (DPDPE) (a delta-selective agonist in mammals), and very low affinity for mu-, kappa-, or ORL1-selective agonists. COS-7 cells expressing the newt mu receptor also possessed a high-affinity (Kd = 0.44 nM) naloxone-binding site that showed highest affinity for beta-endorphin, moderate-to-low affinity for Met-enkephalin and Leu-enkephalin and DAMGO (a mu-selective agonist in mammals), and very low affinity for DPDPE and kappa- or ORL1-selective agonists. COS-7 cells expressing either receptor type (delta or mu) showed very high affinity (Kd = 0.1-0.3 nM) for the nonselective opioid antagonist diprenorphine. Taricha granulosa expresses the same four subtypes (delta, mu, kappa, and ORL1) of opioid receptors found in other vertebrate classes, but ligand selectivity appears less stringent in the newt than has been documented in mammals.

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.

Characterization of mu, kappa, and delta opioid binding in amphibian whole brain tissue homogenates

Opioid agonists produce analgesia in mammals through the activation of mu, kappa, or delta opioid receptors. Previous behavioral and binding studies from our laboratory using an amphibian model suggested that mu, kappa, or delta opioid agonists may activate a single type of opioid receptor in the grass frog, Rana pipiens. In the present study, kinetic, saturation, and competitive binding profiles for three opioid radioligands, [(3)H]DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin) (mu-selective), [(3)H]U65953 [(5 alpha, 7 alpha,8 beta)-(+)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide] (kappa-selective), and [(3)H]DPDPE ([D-Pen(2),D-Pen(5)]-enkephalin) (delta-selective) were determined using frog whole brain homogenates. Kinetic analyses and experimentally derived values from saturation experiments gave affinity constants (K(D)) in the low nanomolar range. The density of opioid binding sites (B(max)) was 224.4, 118.6, and 268.9 fmol/mg for mu, kappa, and delta opioid radioligands, respectively. The affinity values did not significantly differ among the three opioid radioligands, but the kappa radioligand bound to significantly fewer sites than did the mu or delta radioligands. K(i) values for unlabeled mu, kappa, and delta competitors, including highly selective opioid antagonists, were consistent with each radioligand selectivity profile. The present data suggest that mu, kappa, and delta opioid radioligands bind to distinct opioid receptors in amphibians that are surprisingly similar to those found in mammalian brain.

Fish, amphibian, and reptile analgesia

Pain perception and appropriate behavioral responses are important for survival. The conservation of the opioid ligand and receptor suggests evolution of opioid receptors mediating antinociception throughout vertebrate phylogeny. Fish, amphibians, and reptiles have appropriate neurologic components, display the appropriate behavior in response to a painful stimulus, and possess antinociceptive mechanisms to modulate pain. Because pain perception in these species is therefore likely to be analogous to that of mammals, invasive and painful procedures should always be accompanied by appropriate analgesia and anesthesia. Although specific doses have not been established in clinical trials, clinicians should attempt to provide lower vertebrates with appropriate analgesia during painful procedures. Further experimental and clinical investigations are necessary to expand the current veterinary literature in the area of pain and analgesia in lower vertebrates such as fish, amphibians, and reptiles.

Selective opioid agonist and antagonist competition for [3H]-naloxone binding in amphibian spinal cord

Opioids elicit antinociception in mammals through three distinct types of receptors designated as mu, kappa and delta. However, it is not clear what type of opioid receptor mediates antinociception in non-mammalian vertebrates. Radioligand binding techniques were employed to characterize the site(s) of opioid action in the amphibian, Rana pipiens. Naloxone is a general opioid antagonist that has not been characterized in Rana pipiens. Using the non-selective opioid antagonist, [3H]-naloxone, opioid binding sites were characterized in amphibian spinal cord. Competitive binding assays were done using selective opioid agonists and highly-selective opioid antagonists. Naloxone bound to a single-site with an affinity of 11.3 nM and 18.7 nM for kinetic and saturation studies, respectively. A B(max) value of 2725 fmol/mg protein in spinal cord was observed. The competition constants (K(i)) of unlabeled mu, kappa and delta ranged from 2.58 nM to 84 microM. The highly-selective opioid antagonists yielded similar K(i) values ranging from 5.37 to 31.1 nM. These studies are the first to examine opioid binding in amphibian spinal cord. In conjunction with previous behavioral data, these results suggest that non-mammalian vertebrates express a unique opioid receptor which mediates the action of selective mu, kappa and delta opioid agonists.

Selective opioid receptor agonist and antagonist displacement of [3H]naloxone binding in amphibian brain

Opioid receptor ligands have been shown to elicit antinociception in mammals through three distinct types of receptors designated as mu, delta and kappa. These opioid receptors have been characterized and cloned in several mammalian species. Radioligand binding techniques were employed to characterize the sites of opioid action in the amphibian, Rana pipiens. Naloxone is a general opioid receptor antagonist which has not been characterized in R. pipiens. Kinetic analyses of [3H]naloxone in the amphibian yielded a K(D) of 6.84 nM while the experimentally derived K(D) value from saturation experiments was found to be 7.11 nM. Density data were also determined from saturation analyses which yielded a B(max) of 2170 fmol/mg. Additionally, K(i) values were calculated in competition studies for various unlabelled mu-, delta- and kappa-opioid receptor ligands to isolate their site of action. Highly selective antagonists for mu-, delta- and kappa-opioid receptors yielded nearly identical K(i) values against [3H]naloxone.

Nociceptin binding sites in frog (Rana esculenta) brain membranes

The recently discovered natural heptadecapeptide nociceptin (orphanin FQ) shares some homology with the opioid peptides but it binds to a distinct receptor type, termed nociceptin receptor. This study demonstrates the presence of specific nociceptin recognition sites in brain membrane fractions of an amphibian, Rana esculenta. Para-iodo-Phe(1)-nociceptin-amide was radiolabelled by catalytic dehalotritiation, resulting in p[(3)H]Phe(1)-nociceptin-amide of 25 Ci/mmol specific radioactivity. Specific binding of [(3)H]nociceptin-amide to frog brain membranes was found to be saturable and of high affinity with equilibrium K(d) values in the low nanomolar range. A single set of binding sites with about 180 fmol/mg protein maximal binding capacity was obtained in saturation and competition experiments. [(3)H]Nociceptin-amide binding could easily be inhibited by synthetic nociceptin compounds but not by opioid ligands. Both sodium ions and 5'-guanylylimidodiphosphate decreased the binding of the radioligand by transferring the receptor to a lower affinity state. Nociceptin dose-dependently stimulated the binding of the nonhydrolysable, radiolabeled GTP-analogue guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS) to G-proteins in frog brain membranes. Addition of 1 microM naloxone caused no significant change in the curves, indicating that nociceptin-mediated activation of G-proteins occurred through nonopioid mechanism.

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.

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.

Supraspinal administration of opioids with selectivity for mu-, delta- and kappa-opioid receptors produces analgesia in amphibians

Previous results using an amphibian model showed that systemic and spinal administration of opioids selective for mu, delta and kappa-opioid receptors produce analgesia. It is not known whether non-mammalian vertebrates also contain supraspinal sites mediating opioid analgesia. Thus, opioid agonists selective for mu (morphine; fentanyl), delta (DADLE, [D-Ala2, D-Leu5]-enkephalin; DPDPE, [D-Pen2, D-Pen5]-enkephalin) and kappa (U50488, trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide methanesulfonate; CI977, (5R)-(544alpha,744alpha,845beta)-N-methyl-N-[7-(1-p yrr olidinyl)-1-oxaspiro[4,5]dec-8yl]-4-benzofuranaceta mide++ + monohydrochloride) opioid receptors were tested for analgesia following i.c.v. administration in the Northern grass frog, Rana pipiens. Morphine, administered at 0.3, 1, 3 and 10 nmol/frog, produced a dose-dependent and long-lasting analgesic effect. Concurrent naltrexone (10 nmol) significantly blocked analgesia produced by i.c.v. morphine (10 nmol). ED50 values for the six opioids ranged from 2.0 for morphine to 63.9 nmol for U50488. The rank order of analgesic potency was morphine > DADLE > DPDPE > CI977 > fentanyl > U50488. These results show that supraspinal sites mediate opioid analgesia in amphibians and suggest that mechanisms of supraspinal opioid analgesia may be common to all vertebrates.

[3H]dynorphin1-8 binding sites in frog (Rana esculenta) brain membranes

Opioid binding sites specific for [3H]dynorphin1-8 were characterized in the particulate membrane fraction of frog (Rana esculenta) brain. The degradation of the radioligand during the assay was prevented by the use of a broad spectrum of peptidase inhibitors. The binding of [3H]dynorphin1-8 to frog brain membranes was stereoselective, reversible, saturable, and displaceable by a series of opioid ligands including dynorphin1-13, bremazocine, levorphanol and naloxone. The specific binding of [3H]dynorphin1-8 can be significantly inhibited by Na+ ions and/or guanine nucleotides confirming the agonist property of the ligand in vitro. A single set of high affinity opioid binding sites with a Kd approximately 7.5 nM is present in the membranes. The maximum density of binding sites (Bmax approximately 1.1 pmol [3H]dynorphin1-8 per mg protein) was considerably higher than such sites in guinea-pig brain. In addition, comparison with binding of tritiated opioid peptides selective for the mu- and delta-types of opioid receptor showed that in the frog brain most of the sites labelled by [3H]dynorphin1-8 are kappa-sites and that this is a rich source of such sites.

Species differences in the stereoselectivity of kappa opioid binding sites for [3H]U-69593 and [3H]ethylketocyclazocine

Stereoselectivity of the binding sites for the specific kappa-opioid agonist [3H]U-69593, a benzeneacetamido based ligand was investigated in membrane suspension prepared from frog and rat brain, as well as guinea pig cerebellum, using the pure chiral forms of different unlabelled opiates. The ligand binding sites showed stereospecificity with at least three orders of magnitude differences in the affinities (measured as Ki values) of the opioid stereoisomer pairs both in rat and guinea pig membrane fractions. However, in frog brain membranes there was no substantial difference in potencies of the (-) and (+) isomers competing for the [3H]U-69593 binding sites. Another type of the kappa-site preferring opioid ligand, [3H]ethylketocyclazocine, a benzomorphan derivative was able to discriminate between (-) and (+) forms of the same compounds even in frog brain membrane preparation. Our data concerning binding profile of [3H]U-69593 in frog brain membranes are consistent with the observation that kappa opioid binding sites in frog (Rana esculenta) brain differ from those kappa-sites found in mammalian brains.

Characterization of kappa 1 and kappa 2 opioid binding sites in frog (Rana esculenta) brain membrane preparation

The distribution and properties of frog brain kappa-opioid receptor subtypes differ not only from those of the guinea pig brain, but also from that of the rat brain. In guinea pig cerebellum the kappa 1 is the dominant receptor subtype, frog brain contains mainly the kappa 2 subtype, and the distribution of the rat brain subtypes is intermediate between the two others. In competition experiments it has been established that ethylketocyclazocine and N-cyclopropylmethyl-norazidomorphine, which are nonselective kappa-ligands, have relatively high affinities to frog brain membranes. The kappa 2 ligands (Met5)enkephalin-Arg6-Phe7 and etorphine also show high affinities to the frog brain. Kappa 1 binding sites measured in the presence of 5 microM/D-Ala2-Leu5/enkephalin represent 25-30% of [3H]ethylketocyclazocine binding in frog brain membranes. The kappa 2 subtype in frog brain resembles more to the mu subtype than the delta subtype of opioid receptors, but it differs from the mu subtype in displaying low affinity toward beta-endorphin and /D-Ala2-(Me)Phe4-Gly5-ol/enkephalin (DAGO). From our data it is evident that the opioid receptor subtypes are already present in the amphibian brain but the differences among them are less pronounced than in mammalian brain.

Apparent precoupling of kappa- but not mu-opioid receptors with a G protein in the absence of agonist

Rabbit and guinea-pig cerebellum membranes contain a very high (greater than 80%) proportion of mu- and kappa-opioid receptors, respectively. Rabbit (mu) and guinea-pig (kappa) cerebellum membranes were (i) labeled either with the opiate agonist, [3H]etorphine (Kd = 0.1-0.2 nM), or with the opiate antagonist, [3H]diprenorphine (Kd = 0.1 nM), in the absence or presence of Na+ and/or 5'-guanylylimidodiphosphate (GppNHp), (ii) solubilized with digitonin (1%, w:v) and (iii) the radioactivity in the soluble extracts analyzed by ultracentrifugation in sucrose gradients. In the soluble extracts from rabbit cerebellum (mu) membranes, bound [3H]etorphine sedimented faster (S20,w congruent to 12S) than bound [3H]diprenorphine (10S), while in those from guinea-pig cerebellum (kappa) membranes, bound [3H]etorphine and bound [3H]diprenorphine sedimented at the same position (12S). Na+ selectively decreased recovery of the bound tritiated agonist in the two soluble preparations. When they had been generated in the presence of GppNHp but in the absence of Na+, the [3H]etorphine complexes of the mu- and kappa-opioid receptors as well as the [3H]diprenorphine complex of the kappa-opioid receptor were all recovered at position 10S, indicating that GppNHp had induced a decrease of the apparent molecular size of the two types of opioid receptors. These data are interpreted in terms of mu- and kappa-opioid receptors being capable of physically interacting with a G protein (GTP binding regulatory protein) yet, unlike the mu-opioid receptor which does so only in the presence of an agonist, the kappa-opioid receptor appears to be precoupled with a G protein.

Effects of corticotropin-releasing factor (CRF) and opiates on amphibian locomotion

Male rough-skinned newts (Taricha granulosa) were used as a model for the study of the neuroendocrine regulation of locomotion. Intracerebroventricular (i.c.v.) injections of nanogram quantities of corticotropin-releasing factor (CRF) dose-dependently increased locomotion as measured in a circular open-field test arena. In other studies animals received intraperitoneal (i.p.) injections of saline or naloxone, a synthetic opioid antagonist, followed by i.c.v. injections of saline or CRF. With 1-min intervals between injections, neither i.p. saline nor naloxone injections modified the stimulatory effects of CRF injections on locomotor activity. In contrast, with 20-min intervals between injections, the naloxone-plus-CRF injected newts displayed more locomotor activity than the saline-plus-CRF injected newts, suggesting that the opioid system modulated the behavioral effects of CRF. An i.p. injection of bremazocine, an opiate kappa-receptor agonist, suppressed spontaneous locomotion but not CRF-induced locomotion. In contrast, an i.p. injection of morphine, an opiate mu-receptor agonist, did not affect spontaneous locomotion but reduced CRF-induced locomotion, indicating further that the opioid system may modulate the behavioral effects of CRF in this amphibian. The present study provides the first evidence that both CRF and opioids may be involved in the regulation of amphibian locomotor activity.

Effects of sodium and temperature on naloxone binding in brain tissues of a urodele amphibian

1. Partially purified brain membranes obtained from male rough-skinned newts (Taricha granulosa) were used to determine the effects of NaCl and temperature on the specific binding of the opioid receptor antagonist [3H]naloxone. 2. The addition of NaCl to the incubation medium at concentrations up to 400 mM produced a dose-related increase of the specific binding of [3H]naloxone. 3. The addition of other salts to the incubation medium had less pronounced effects: KCl and MgCl2 slightly increased and decreased, respectively, the specific binding of naloxone, and CaCl2 had no effect. 4. Results of an equilibrium saturation experiment showed that the addition of 200 mM NaCl resulted in over a 10-fold increase in the number of high affinity (KD = 0.61 nM) binding sites for naloxone, with no changes in the number of low affinity (KD = 21.8 nM) binding sites. 5. Changes in NaCl concentrations did not significantly affect either dissociation constant. 6. The binding of [3H]naloxone was temperature-dependent; it increased when the incubation temperatures were elevated from 0 degree C to 37 degrees C. 7. Results obtained for this urodele amphibian are compared with those available for other vertebrate species.

Neuropeptides induce directional asymmetry in brain and spinal cord: facts and hypotheses

Directional behavioral and functional asymmetries (i.e., left-biased or right-biased in all or most animals of the population) induced by certain chemical substances are new types of brain and spinal cord asymmetry. The revealed asymmetry comprises: (1) left- or right-biased circle rotation in rat, (2) hind limb postural asymmetry resulting from alteration of the left or right flexion reflex in rat and cat, and (3) asymmetric alterations of the evoked potentials (EP) in the turtle visual cortex. Circle rotation of animals is induced by hypothalamic neurohormones (somatostatin, LH-RH, substance P, and TRH). Postural asymmetry develops under the effect produced by enkephalins and opioid kappa- and delta-agonists, sigma-agonist SKF 10.047, Arg-vasopressin. Endogenous peptide factors, the activity (or content) of which increased under brain and spinal cord unilateral injury, as well as the ones localized in the left or right hemisphere, also induced postural asymmetry. EP of the left and right turtle visual cortex were inhibited by enkephalins and opioid kappa-, and delta- and mu-agonists, and factors predominantly localized in the left or right turtle visual cortex in a different manner. The data reported here suggest the existence of a side-specific mechanism for a selective neurohormonal regulation of the neuronal activity and other processes in the left and right halves of brain and spinal cord which involves lateralized neuropeptides and their receptors. This mechanism might serve to maintain a certain balance between the activity of the left and right-side neurons, and other contralateral processes in the paired and bilateral structures in brain and spinal cord. Significant deviations from the balance occur most likely due to powerful unilateral stimuli, e.g., unilateral trauma. Many neuropeptides (opioid ones, somatostatin, MSH, ACTH) are, presumably, involved in the regeneration processes in the central and peripheral nervous system. In the case of brain lesions, some lateralized endogenous peptides may participate in the regulation of regeneration process on the left, whereas the other ones, on the right side of the midline, which depends on the side of the lesion. Some lateralized receptors and ligands may serve as positional markers of the left, whereas the other ones may serve as those of the right brain hemisphere. In ontogenesis, these markers are probably necessary to perform the function of the mechanism responsible for symmetrical brain formation.

Effect of sodium on [3H]ethylketocyclazocine binding to opioid receptors in frog brain membranes

The specific binding of [3H]ethylketocyclazocine to frog brain membrane preparation was enhanced in the presence of sodium ions administered as NaCl, both at 0 degree C and at room temperature. The optimal NaCl concentration was 25 mM at 0 degree C and 50 mM at 24 degrees C. MgCl2 inhibited the [3H]ethylketocyclazocine binding. Two binding sites (high and low affinity) were established with [3H]ethylketocyclazocine as ligand by equilibrium binding studies. Addition of NaCl increased the Bmax of the low-affinity site more than that of the high-affinity site at both temperatures. Affinities were higher at 0 degree C than at 24 degrees C. The KD values were not significantly influenced by sodium ions. The dissimilarities between the rat and frog brain opioid receptors in [3H]ethylketocyclazocine binding are attributed to the different lipid composition of the two membranes.

Distribution of mu, delta, and kappa opiate receptor types in the forebrain and midbrain of pigeons

Ligands that are highly specific for the mu, delta, and kappa opiate receptor binding sites in mammalian brains have been identified and used to map the distribution of these receptor types in the brains of various mammalian species. In the present study, the selectivity and binding characteristics in the pigeon brain of three such ligands were examined by in vitro receptor binding techniques and found to be similar to those reported in previous studies on mammalian species. These ligands were then used in conjunction with autoradiographic receptor binding techniques to study the distribution of mu, delta, and kappa opiate receptor binding sites in the forebrain and midbrain of pigeons. The autoradiographic results indicated that the three opiate receptor types showed similar but not identical distributions. For example, mu, delta, and kappa receptors were all abundant within several parts of the cortical-equivalent region of the telencephalon, particularly the hyperstriatum ventrale and the medial neostriatum. In contrast, in other parts of the cortical-equivalent region of the avian telencephalon, such as the dorsal archistriatum and caudal neostriatum, only kappa receptors appeared to be abundant. Within the basal ganglia, all three types of opiate receptors were abundant in the striatum and low in the pallidum. Within the diencephalon, kappa and delta binding was high in the dorsal and dorsomedial thalamic nuclei, but the levels of all three receptor types were generally low in the specific sensory relay nuclei of the thalamus. Kappa binding and delta binding were high, but mu was low in the hypothalamus. Within the midbrain, all three receptor types were abundant in both the superficial and deep tectal layers, in periventricular areas, and in the tegmental dopaminergic cell groups. In many cases, the distribution of opiate receptors in the pigeon forebrain generally showed considerable overlap with the distribution of opioid peptide-containing fiber systems (for example, in the striatal portion of the basal ganglia), but there were some clear examples of receptor-ligand mismatch. For example, although all three receptor types are very abundant in the hyperstriatum ventrale, opioid peptide-containing fibers are sparse in this region. Conversely, within the pallidal portion of the basal ganglia, opioid peptide-containing fibers are abundant, but the levels of opiate receptors appear to be considerably lower than would be expected. Thus, receptor-ligand mismatches are not restricted to the mammalian brain, since they are a prominent feature of the organization of the brain opiate systems in pigeons.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of oxymorphazone in frogs: long lasting antinociception in vivo, and apparently irreversible binding in vitro

Oxymorphazone (at doses of 50-200 mg/kg) was found to be a relatively weak antinociceptive drug in intact frog (Rana esculenta) when acetic acid was used as pain stimulus. Frogs remained analgesic for at least 48 hrs following oxymorphazone (200 mg/kg) administration. The ligand increased the latency of wiping reflex in spinal frogs too. These effects were blocked by naloxone. In equilibrium binding studies (3H)oxymorphazone had high affinity to the opioid receptors of frog brain and spinal cord as well (apparent Kd values were 8.9 and 10.6 nM, respectively). Kinetic experiments show that only 25% of the bound (3H)oxymorphazone is readily dissociable. Preincubation of the membranes with labeled oxymorphazone results in a washing resistant inhibition of the opioid binding sites. At least 70% of the (3H)oxymorphazone specific binding is apparently irreversible after reaction at 5 nM ligand concentration, and this can be enhanced by a higher concentration of tritiated ligand.

Opioid antinociception in amphibians

Systemic and spinal administration of opioids produces a behaviorally defined antinociception in a variety of mammalian models. Although endogenous opioid peptides and opioid binding sites are ubiquitous throughout phylogeny, little attention has been paid to the function of endogenous opioid system(s) or development of nociceptive models in nonmammalian species. Recent work has shown that the amphibian, Rana pipiens, provides an appropriate model for assessment of opioid antinociception and that endogenous opioid systems may likewise modulate the central processing of noxious information in amphibians as well as mammals.

Evidence for a new type of opioid binding site in the brain of the frog Rana ridibunda

The crude membrane fraction from the brain of the frog Rana ridibunda was shown to contain 0.7-0.8 pmol/mg protein for a site with high (KD = 0.1 nM) and about 3.2 pmol/mg protein for a site with lower (KD = 10-15 nM) affinity for the opiate agonist [3H]etorphine and for the opiate antagonist [3H]diprenorphine. In addition to its very high affinity for the two tritiated oripavine derivatives, the high affinity site displayed (i) a considerably reduced ability to bind the agonist but not the antagonist in the presence of Na+ ions and (ii) pronounced stereospecificity. These properties are all typical of an opioid receptor site. The lower affinity site, which was about four times as abundant as the other exhibited none of the aforementioned characteristics and is therefore probably not opioid in nature. Detailed testing of the potency of various unlabelled opioid ligands to inhibit the binding of [3H]etorphine at the high affinity site showed that the latter consists of a mixture of several types of opioid sites, including a major type with an apparent binding profile clearly different from those of mammalian brain mu, delta- and kappa-opioid sites. In particular, this major type of site, which accounted for about 70% of the opioid binding in frog brain membranes, bound mu ([D-Ala2,MePhe4,Glyol5]enkephalin), delta ([D-Thr2,Leu5]enkephalyl-Thr) and kappa (U50,488) selective ligands with much lower affinity than did mu-, delta- and kappa-opioid receptor sites, respectively.

Selective effects of ethanol on opiate receptor subtypes in brain

Large concentrations of ethanol in vitro decreased ligand binding to mu and delta opiate receptors in the frontal cortex of the C57BL mouse, but did not alter binding to kappa opiate receptors. Mu and delta receptors were equally sensitive to the inhibitory effect of ethanol. Since the effects of ethanol were significant only in large concentrations, ethanol may alter opiate binding through its membrane lipid-perturbing actions, and the selectivity of the effects of ethanol may reflect differences in the microenvironments of the opiate receptor subtypes in membranes. After chronic ingestion of ethanol by mice, in vivo, there was a selective decrease in the number of mu receptors in the frontal cortex. This change may result from indirect effects of ethanol on the opiate receptor and may contribute to specific central effects of ethanol.

Reptilian enkephalins: implications for the evolution of proenkephalin

To shed light on the evolution of the opioid peptide precursor proenkephalin, we have investigated the profile of immunoreactive enkephalins in reptilian brain using six antisera directed toward different portions of the proenkephalin molecule. Three different orders of reptiles--turtles, alligators, and lizards--were studied; these orders represent lineages diverging more than 250 million years ago. Reptilian brain was found to contain large quantities of Met5-enkephalin (Met5-enk), Leu5-enkephalin (Leu5-enk), and Met5-enk-Arg6-Phe7 (MERF); gel filtration and reverse-phase chromatography of acid extracts prepared from turtle brain indicated that nearly all of the immunoreactivity corresponding to these peptides could be ascribed to the authentic penta- to heptapeptides. Immunoreactive metorphamide, but not amidorphin, was present in all three species. Unlike mammalian brain, reptilian brain does not contain immunoreactive Met5-enk-Arg6-Gly7-Leu8 (MERGL). These results indicate that reptilian proenkephalin, while similar to mammalian enkephalin in containing Met5-enk, Leu5-enk, MERF, and metorphamide, nevertheless differs from mammalian proenkephalin. Reptilian proenkephalin either may contain the MERGL sequence in a form not recognized by the MERGL antibody, or may lack the MERGL sequence entirely.

Solubilization and characterization of kappa opioid binding sites from guinea pig cerebellum

Solubilization of opioid binding sites from guinea pig cerebellum by digitonin, in the absence and presence of NaCl, resulted in very similar yields (25-30%) of [3H]bremazocine binding. Saturation curves of [3H]bremazocine binding give linear Scatchard plots for both soluble and membrane-bound binding sites yielding similar Kd's and Bmax's. Soluble kappa sites seem to resemble closely their membrane-bound counterparts and retain high affinity and selectivity for various kappa opioid ligands. The apparent molecular weight of soluble kappa sites is ca. 4 X 10(5). Results from this study, along with our previous findings with toad and guinea pig brain, indicate that kappa sites (unlike mu and delta) can be solubilized in good yield by digitonin even in the absence of NaCl. This supports the hypothesis that kappa sites may represent molecular species different from those of mu and delta sites.

Solubilization and characterization of opioid binding sites from frog (Rana esculenta) brain

Active opioid receptors were solubilized from frog (Rana esculenta) brain membrane fractions by the use of 1% digitonin. It was found by kinetic as well as by equilibrium measurements that both the membrane and the solubilized fractions contain two binding sites. For the membrane preparations, KD values were 0.9 and 3.6 nM, and Bmax values were 293 and 734 fmol/mg protein. For the solubilized preparations, KD values were 0.4 and 2.6 nM, an Bmax values were 35 and 266 fmol/mg protein. The stereospecificity of the binding did not change during solubilization. Both the membrane-bound and the solubilized receptors showed weak binding of enkephalin and mu-specific drugs, suggesting that they are predominantly of the kappa-type. The membrane-bound and the soluble receptors showed the same distribution of subtypes, i.e., 70% kappa, 13% mu, and 17% delta for the membrane-bound and 71% kappa, 17% mu, and 12% delta for the soluble receptors.

Analgesic effects of ethylketocyclazocine and morphine in rat and toad

We have previously found rat and toad (Bufo marinus) brain to contain inverse ratios of benzomorphan-preferring (kappa/sigma) and morphine-preferring (mu) opioid receptor types. The aim of the present study was to compare in vivo pharmacologic activity of a benzomorphan, ethylketocyclazocine (EKC) and morphine sulfate (MS) in rat and toad. Footshock intensity thresholds for eliciting locomotion were determined and dose-response curves for EKC and MS analgesia were obtained. Drugs were injected subcutaneously. In rats (high mu, low kappa in brain), both compounds produced analgesia and displayed similar sensitivity to naloxone antagonism. The analgesic effects of EKC and MS may, therefore, be mediated by a common receptor type (mu) in this pain test in rats. In toads (high kappa, low mu in brain), MS produced naloxone-reversible analgesia at doses 20-fold higher than were effective in rats. Toads did not display EKC analgesia at doses below those producing motor impairment. Moreover, 50-fold higher doses were required to produce such impairment in toads. Thirty minutes following subcutaneous injection of 3H-EKC, similar concentrations were found in rat and toad brain. Uptake into brain is probably not a factor in the behavioral resistance of toads to EKC.

Opiate binding sites and endogenous opioids in Bufo viridis oocytes

Binding sites with high affinity for [3H]naloxone, but not for [3H]morphine and [3H] (D-Ala2, D-Leu5) enkephalin, have been found in membranes of Bufo viridis oocytes. The binding is reversible and saturable. Bound [3H]naloxone is easily displaced both by unlabeled naloxone and bremazocine, much worse by morphine and SKF 10,047; (D-Ala2, D-Leu5) enkephalin and beta-endorphin practically fail to displace [3H]naloxone. Scatchard analysis is consistent with the existence of two classes of binding sites with Kd 15 nM and 10(3) nM. The number of binding sites with high affinity for naloxone is 16 pmol/mg protein of homogenized oocytes which is 20-50-fold higher than in, toad or rat brain. Oocyte extract displaces [3H]naloxone bound with oocytes' membranes and inhibits electrically evoked contractions of the rabbit vas deferens. This inhibition is reversed by naloxone. It is suggested that compounds similar to opiate kappa-agonists exist in oocytes. It cannot be ruled out that they participate via specific receptors in the regulation of oocyte maturation and egg development.

A spinal site mediates opiate analgesia in frogs

The application of acetic acid to the hind leg of a frog will induce a spinally mediated wiping reflex only if the acetic acid concentration is above a certain threshold. By using this reflex as the basis of a test for nociception, we show that morphine sulfate is a potent analgesic in the frog when injected into the lumbar area of the spinal cord. Significant analgesia is induced within 5 min after injection of as little as 0.0316 microgram of morphine sulfate. Low doses of morphine sulfate (0.0316 or 0.1 microgram) induce analgesia which dissipates within 1 h while for higher doses (0.316, 1.0 or 3.16 micrograms) the analgesia persists for at least 3 h. The analgesic effect of 0.316 micrograms of morphine sulfate is completely blocked by naloxone HCl at either 0.158 or 0.316 micrograms. Animals receiving naloxone alone (0.316 micrograms) appear to be slightly hyperalgesic compared to saline injected controls but this effect is not significant.