The case for multiple opiate receptors

Bi-phasic intensity-dependent opioid-mediated neural amplitude changes in the chinchilla cochlea: partial blockade by an N-Methyl-D-Aspartate (NMDA)-receptor antagonist

Dynorphins, glutamate, and glutamate-sensitive N-Methyl-D-Aspartate (NMDA) receptors exist in the mammalian cochlea. Dynorphins produce neural excitation and excitotoxic effects in the spinal cord through a kappa-opioid facilitation of NMDA receptor-sensitivity to glutamate. The kappa-opioid receptor drug agonists N-dimethylallyl-normetazocine [(-)-pentazocine (50 mmol)] and trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide [U-50488H (100 mmol)] were administered across the cochlear round window membrane in the chinchilla. Each drug produced significant post-baseline amplitude changes in the click-evoked auditory nerve compound action potential. Amplitude changes at threshold amounted to increases in sensitivity that ranged from 4-8 decibels, measured in sound pressure level (dB SPL). The large neural amplitude increases at threshold were accompanied by progressively smaller amplitude changes at 5 and 10 dB above threshold (dB SL). However, at stimulus intensities > or =20 dB SL, post-baseline neural amplitudes were suppressed to levels below baseline and control values. These bi-phasic intensity-dependent neural amplitude changes have never before been observed following i.v. administered (-)-pentazocine in this species. Finally, the bi-phasic neural amplitude changes in U-50488H-treated (100 mmol) animals were partially blocked (except at 20 dB SL), following a round window pre-treatment with the NMDA receptor drug antagonist, dizocilpine hydrogen maleate [(+)-MK-801 (8 mmol)]. Our data suggests that endogenous dynorphins within lateral efferent olivocochlear neurons differentially modulate auditory neural excitation, possibly through cochlear NMDA receptors and glutamate. The role played by lateral efferent opioid neuromodulation at cochlear NMDA receptors, is discussed.

Cloning and characterization of Xen-dorphin prohormone from Xenopus laevis: a new opioid-like prohormone distinct from proenkephalin and prodynorphin

Opioid-like peptides mediate analgesia and induce behavioral effects such as tolerance and dependence by ligand-receptor-mediated mechanisms. The classical opioid prohormones can generate several bioactive peptides, and these divergent families of prohormones share a common well conserved ancestral opioid motif (Tyr-Gly-Gly-Phe). Evidence from pharmacological and molecular cloning studies indicates the presence of multiple isoforms of opioid ligands and receptors that are as yet uncharacterized. To identify potential new members we used the opioid motif as an anchor sequence and isolated two distinct isoforms (Xen-dorphins A and B) of an opioid prohormone from Xenopus laevis brain cDNA library. Xen-dorphin prohormones can generate multiple novel opioid ligands distinct from the known members of this family. Both isoforms are present in a wide variety of tissues including the brain. Two potential bioactive peptides, Xen-dorphin-1A and -1B, that were chemically synthesized showed opioid agonist activity in frog and rat brain membranes using a [35S]GTPgammaS assay. Initial radioligand binding experiments demonstrated that Xen-dorphin-1B binds with high affinity to opioid receptor(s) and with potential preference to the kappa-opioid receptor subtype. Cloning of the Xen-dorphin prohormone provides new evidence for the potential presence of other members in the opioid peptide superfamily.

A novel kappa-opioid receptor agonist, TRK-820, blocks the development of physical dependence on morphine in mice

The effects of a novel kappa-opioid receptor agonist, TRK-820, on the development of physical dependence on morphine were investigated in mice in comparison with those of U-50,488H. A marked body weight loss and several withdrawal signs were observed following naloxone challenge in morphine-dependent mice. Co-injection of TRK-820 (0.003-0.03 mg/kg, s.c.) but not U-50,488H (1-10 mg/kg, s.c.) during chronic morphine treatment dose-dependently suppressed the naloxone-precipitated body weight loss, jumping, wet dog shakes and diarrhea. These results suggest that TRK-820-sensitive kappa-opioid receptor subtypes may play a significant role in modulating the development of physical dependence on morphine.

Food cravings, endogenous opioid peptides, and food intake: a review

Extensive research indicates a strong relationship between endogenous opioid peptides (EOPs) and food intake. In the present paper, we propose that food cravings act as an intervening variable in this opioid-ingestion link. Specifically, we argue that altered EOP activity may elicit food cravings which in turn may influence food consumption. Correlational support for this opioidergic theory of food cravings is provided by examining various clinical conditions (e.g. pregnancy, menstruation, bulimia, stress, depression) which are associated with altered EOP levels, intensified food cravings, and increased food intake.

Conformational and electrostatic similarity between polyprotonated and Ca(2+)-bound mu-opioid peptides

In a previous paper (Zhorov and Ananthanarayanan, J. Biomol, Struct. Dynam. 1995, 13:1-13) we had calculated the minimum-energy conformations of monoprotonated and zwitterionic mu-opioid peptides and demonstrated the remarkable similarity between Ca(2+)-bound morphine on the one hand and the Ca(2+)-bound forms of these peptides on the other. We postulated that the Ca(2+)-bound forms of mu-opioids would activate the mu-receptor. To assess further the involvement of multiple positive charges on some of the mu-opioid ligands in their interaction with the receptor, we have, in this work, studied the geometry of five mu-opioid peptides containing two to four protonated groups and having chemical structures essentially different from the endogenous mu-opioid peptide Met-enkephalin (EK). Conformational space was searched using the Monte Carlo-with-energy-minimization method. Ca(2+)-bound forms of the selected peptides were found to be energetically unfavourable implying that one of the protonated groups plays a role similar to that Ca2+ plays in EK-Ca2+ complex. Bioactive conformations of the polyprotonated peptides were then selected using the criteria formulated earlier for Ca(2+)-bound ligands as well as additional criteria requiring ligands to have an elongated conical overall shape complementary to the interface between the transmembrane segments of mu-receptor. Low-energy conformations meeting these criteria were found in all the peptides considered, the protonated groups being separated from each other by about 8 and 16 A. The possible role of the ligands' cationic groups in mu-receptor activation is discussed.

Role of delta-opioid receptors in mediating the aversive stimulus effects of morphine withdrawal in the rat

An unbiased place preference conditioning procedure was used to examine the role of delta-opioid receptors in mediating the aversive effects of opioid withdrawal. Rats were implanted s.c. with two pellets each containing placebo or 75 mg morphine. Single-trial conditioning sessions with saline and the opioid receptor antagonists naloxone (0.001-1.0 mg/kg, s.c.), naltrindole (0.01-3.0 mg/kg, s.c.) or naltriben (0.01-3.0 mg/kg, s.c.) commenced 4 days later. During these conditioning sessions, physical signs of withdrawal were also quantified. Tests of conditioning were conducted on day 5. Naloxone in doses of 0.01-1.0 mg/kg produced significant conditioned place aversions in morphine-implanted animals. A dose of 0.01 mg/kg produced few physical withdrawal signs whereas higher doses resulted in marked wet dog shakes, body weight loss ptosis and diarrhea. No such effects were observed in control (placebo-implanted) animals. Administration of the selective delta-opioid receptor antagonists naltrindole and naltriben produced dose-related place aversions in morphine-implanted animals. The magnitude of these effects did not differ from that observed with naloxone. The minimum effective doses of naltrindole and naltriben were 0.1 mg/kg. Doses of 0.1-1.0 mg/kg produced few, if any, somatic signs of withdrawal whereas higher doses of these antagonists only produced diarrhea and wet-dog shakes. Other withdrawal signs were absent. In contrast to the opioid receptor antagonists tested, the dopamine D1 receptor antagonist SCH23390 failed to produced conditioned place aversions or physical signs of withdrawal in morphine-pelleted animals. These data demonstrate that the selective blockade of either delta- or mu-opioid receptors is sufficient to induce conditioned aversive effects in morphine-dependent animals. They also indicate that physical symptoms associated with precipitated morphine withdrawal differ depending upon the opioid receptor antagonist employed.

Postsynaptic and extrasynaptic localization of kappa-opioid receptor in selected brain areas of young rat and chick using an anti-receptor monoclonal antibody

kappa-opioid receptors were visualized by light and electron microscopical immunohistochemistry in young rat and chick brains, using a monoclonal antibody KA8 (IgG1, kappa) raised against a kappa-opioid receptor preparation from frog brain, which recognizes selectively the kappa-type receptor with preference for the kappa-2 subtype. The most pronounced kappa-opioid receptor-like immunoreactivity was observed in the hypothalamic nuclei of the rat brain and in the chick optic tectum, in regions where the functional significance of kappa-opioid receptors is well documented. Both neurons and glia were stained, the former on both somata and dendrites. At the ultrastructural level, the receptor-like immunoreactivity was similar in both species. Immunoprecipitate decorated the inner surface of the plasma membrane of glial cells, neuronal somata and dendrites, in a discontinuous arrangement. In the cytoplasm, labelling was associated with ribosomes, polyribosomes and rough endoplasmic reticulum membranes but not with Golgi cisternae. In the neuropil, the immunoprecipitate was observed along the dendritic microtubules and was also associated with postsynaptic sites. Nuclei and axons were devoid of label and immunoreactivity was never visible presynaptically. Our findings indicate that the antibody used in the present study marks various forms of the kappa-opioid receptor protein including those synthesised in ribosomes, transported along dendritic microtubules and incorporated into postsynaptic and non-synaptic membranes. The antibody also recognizes glial opioid receptors. The observed subcellular distribution appears to be conserved in phylogenetically distant species.

Opioidergic inhibition of capsaicin-evoked release of glutamate from rat spinal dorsal horn slices

We investigated the effects of opioid agonists on the capsaicin-evoked release of glutamate from nociceptive primary afferent fibers of the rat (6-8 weeks) using a fluorometric on-line continuous monitoring system for glutamate. In the presence of 0.3 microM tetrodotoxin, the application of 3 microM capsaicin to spinal dorsal horn slices produced an evoked glutamate release (55.9 +/- 4.02 pmol.mg-1 protein, n = 15). DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin; 0.3-10 microM) and morphine (1-30 microM), mu-opioid agonists, produced a concentration-dependent reduction (approximately 85 and approximately 77% reduction, respectively) in the capsaicin (3 microM)-evoked release of glutamate. These inhibitory effects were significantly antagonized by naloxone (1 microM). DPDPE ([D-Pen2,5]enkephalin; 1-10 microM), a delta-opioid agonist, also reduced the capsaicin-evoked release in a concentration-dependent manner (approximately 59% reduction). Naltrindole (1 microM), a selective delta-antagonist, significantly antagonized the inhibitory effect of DPDPE (10 microM). In contrast, neither U-50,488H (1-10 microM) nor U-69,593 (10 microM), kappa-opioid agonists, had any effects on the evoked release of glutamate. These results suggest that mu-, and delta-opioid agonists modulate pain transmission in the spinal dorsal horn, at least in part, by inhibiting the release of glutamate from capsaicin-sensitive primary afferents.

Mu, delta, and kappa opioid receptor mRNA expression in the rat CNS: an in situ hybridization study

The mu, delta, and kappa opioid receptors are the three main types of opioid receptors found in the central nervous system (CNS) and periphery. These receptors and the peptides with which they interact are important in a number of physiological functions, including analgesia, respiration, and hormonal regulation. This study examines the expression of mu, delta, and kappa receptor mRNAs in the rat brain and spinal cord using in situ hybridization techniques. Tissue sections were hybridized with 35S-labeled cRNA probes to the rat mu (744-1,064 b), delta (304-1,287 b), and kappa (1,351-2,124 b) receptors. Each mRNA demonstrates a distinct anatomical distribution that corresponds well to known receptor binding distributions. Cells expressing mu receptor mRNA are localized in such regions as the olfactory bulb, caudate-putamen, nucleus accumbens, lateral and medial septum, diagonal band of Broca, bed nucleus of the stria terminalis, most thalamic nuclei, hippocampus, amygdala, medial preoptic area, superior and inferior colliculi, central gray, dorsal and median raphe, raphe magnus, locus coeruleus, parabrachial nucleus, pontine and medullary reticular nuclei, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis and cuneatus, dorsal motor nucleus of vagus, spinal cord, and dorsal root ganglia. Cellular localization of delta receptor mRNA varied from mu or kappa, with expression in such regions as the olfactory bulb, allo- and neocortex, caudate-putamen, nucleus accumbens, olfactory tubercle, ventromedial hypothalamus, hippocampus, amygdala, red nucleus, pontine nuclei, reticulotegmental nucleus, motor and spinal trigeminal, linear nucleus of the medulla, lateral reticular nucleus, spinal cord, and dorsal root ganglia. Cells expressing kappa receptor mRNA demonstrate a third pattern of expression, with cells localized in regions such as the claustrum, endopiriform nucleus, nucleus accumbens, olfactory tubercle, medial preoptic area, bed nucleus of the stria terminalis, amygdala, most hypothalamic nuclei, median eminence, infundibulum, substantia nigra, ventral tegmental area, raphe nuclei, paratrigeminal and spinal trigeminal, nucleus of the solitary tract, spinal cord, and dorsal root ganglia. These findings are discussed in relation to the physiological functions associated with the opioid receptors.

Cloning, characterization, and distribution of a mu-opioid receptor in rat brain

We report the isolation and characterization of a rat cDNA clone encoding a mu-opioid receptor. This receptor, a 398 amino acid protein, shares 59% overall identity with the mouse delta- and kappa-opioid receptors. Transient expression of the receptor in COS cells revealed high-affinity binding of mu-selective opioid antagonists and agonists, with a KD for naloxone approximately 1.5 nM, and for [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and morphine at the high-affinity site of 2-4 nM, confirming a mu-opioid pharmacological profile. Northern blotting and in situ hybridization histochemistry revealed that the mu-opioid receptor mRNA was expressed in many brain regions, including cerebral cortex, caudate putamen, nucleus accumbens, olfactory tubercle, septal nuclei, thalamus, hippocampus, and medial habenular nucleus, in keeping with the known distribution of the mu-opioid receptor.

Biphasic effects of intraaccumbens mu-opioid peptide agonist DAMGO on locomotor activities

The effects of bilateral microinjections of mu-opioid receptor agonist DAMGO (0.00, 0.01, 0.1, or 1.0 microgram/side) were tested in rats for 120 min in activity monitors. The horizontal movement, rearing, and stereotypy times in seconds were measured during 12 consecutive 10-min time blocks. DAMGO (0.01, 0.1, and 1.0 microgram) resulted in biphasic effects, inhibition followed by activation for each of the three measures. These data replicate the behavioral effects of ICV DAMGO except that the duration of the behavioral effects were longer with Acb injections.

Blockade of morphine reward through the activation of kappa-opioid receptors in mice

The effects of systemic (s.c.) treatment with the kappa-agonists U-50,488H and E-2078 (a stable dynorphin analog) on the morphine-induced place preference were examined in mice. Morphine (s.c.) caused a dose-related preference for the drug-associated place; the effects at doses of 3 and 5 mg/kg were significant. On the other hand, U-50,488H or E-2078 produced a dose-related conditioned place aversion. Both U-50,488H (1 mg/kg, s.c.) and E-2078 (0.1 mg/kg, s.c.) induced a slight, nonsignificant place aversion. Pretreatment with U-50,488H (1 mg/kg) abolished the morphine (3 mg/kg)-induced place preference. The morphine-induced place preference was also significantly decreased by pretreatment with E-2078 (0.1 mg/kg). The inhibitory effects of the kappa-agonists were antagonized by the kappa-antagonist nor-binaltorphimine (nor-BNI; 3 mg/kg, s.c.). In contrast, pretreatment with U-50,488H did not affect the place preference induced by the dopamine (DA) receptor agonist apomorphine (1 mg/kg, s.c.). In addition, morphine (3 mg/kg, s.c.) significantly increased the levels of the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the limbic forebrain (nucleus accumbens and olfactory tubercle) but not in the striatum, implying that activation of the mesolimbic DA system may play an important role in the morphine-induced place preference in mice. Pretreatment with U-50,488H significantly reduced the morphine-induced elevation of DA metabolites in the limbic forebrain.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral effects of the mu-opioid peptide agonists DAMGO, DALDA, and PL017 on locomotor activities

The relative role of central mu-opioid receptor agonists Tyr-D-Ala-Gly-N-Methyl-Phe-Gly-ol (DAMGO), Tyr-D-Arg-Phe-Lys (DALDA), and Tyr-Pro-MePhe-D-Pro (PL017) (0.00, 0.01, 01, or 1.0 micrograms, ICV) on behavior was investigated in rats for 60 min in activity monitors. DAMGO (0.1 and 1.0 micrograms) and PL017 (1.0 micrograms) resulted in biphasic effects, inhibition followed by hyperactivity for linear locomotor, whereas the 0.01-micrograms dosage was associated with hyperactivity. On the other hand, DALDA (0.1 and 1.0 micrograms) suppressed locomotor activity over the 60-min session.

Behavioral effects of opioid peptide agonists DAMGO, DPDPE, and DAKLI on locomotor activities

The effects of the mu-selective agonist DAMGO (ICV doses of 0.00, 0.01, 0.1, or 1.0 micrograms), the delta-selective agonist DPDPE (ICV doses of 0.00, 0.1, 1.0, or 10.0 micrograms), and the kappa-selective agonist DAKLI (ICV doses of 0.00, 0.01, 0.1, or 1.0 micrograms) were tested in rats for 60 min in an activity monitor. The durations in seconds of linear locomotor time, rearing time, stereotypy time, and margin time (thigmotaxis) were measured during six 10-min time blocks. DAMGO (0.1 and 1.0 micrograms) resulted in biphasic effects, inhibition followed by hyperactivity for linear locomotor, rearing, and stereotypy times, and an inhibition of thigmotaxis. DPDPE (10.0 micrograms) was associated with monophasic potentiation of linear locomotor activity and mixed effects in stereotypy times. DAKLI did not effect horizontal, rearing, or margin times; only stereotypy times resulted in mixed effects of DAKLI. The differential behavioral profiles were discussed in reference to the three opioid receptor subtypes.

Opioid peptides in the rabbit carotid body: identification and evidence for co-utilization and interactions with dopamine

The rabbit carotid body is a catecholaminergic organ that contains dopamine and norepinephrine in a proportion of nearly 5:1. Chronic (15 days) carotid sinus nerve denervation or superior cervical ganglionectomy did not modify the carotid body dopamine content (5-6 nmol/mg of protein, equivalent to 250 pmol per carotid body), but sympathectomy reduced by approximately 50% the norepinephrine content. The carotid body has also a very high content of opioid activity (250 equivalent pmol of Leu-enkephalin/mg of protein) as measured by a radioreceptor assay that detects preferentially delta-opioid activity. In the carotid body the degree of opioid posttranslational processing to low-molecular-weight peptides (mostly Leu- and Met-enkephalin) is nearly 80%. HPLC identification of opioid peptides revealed that the sequences of Met- and Leu-enkephalin were in a proportion of nearly 6:1, indicating that the main opioid precursor in the carotid body is proenkephalin A. Chronic denervations of the carotid body did not modify the levels or the degree of opioid precursor processing. Acute hypoxic exposure of the animals (8% O2 in N2; 3 h) resulted in a parallel decrease of dopamine and opioid activity, without any change in the degree of opioid processing. Norepinephrine levels were not affected by hypoxia. These findings suggest corelease of dopamine and opioids during natural hypoxic stimulation. In agreement with the analytical data, [D-Ala2, D-Leu5]enkephalin, but not [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, reduced the in vitro release of dopamine induced by low PO2, a high external K+ concentration, and dinitrophenol. Naloxone augmented the release response elicited by low PO2 stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic prenatal methadone exposure alters central opioid mu-receptor affinity in both fetal and maternal brain

The effects of chronic prenatal methadone exposure (6.3-9.0 mg/kg/day) via osmotic minipumps to pregnant dams on fetal and maternal brain opioid mu-receptors were assessed on gestation day 20 and day 7 postnatally. By using the 3H-DAMGO binding assay, it was shown that chronic methadone treatment (gestation days 7-20) did not affect mu-receptor capacity in both fetal and maternal brains during gestation day 20, nor when tested 7 days after delivery. However, this chronic exposure decreased mu-receptor affinity in both fetal and maternal brain homogenates when determined on day 20 of pregnancy. Scatchard analysis of binding data in both tissues indicated that the methadone-induced increase in KD returned to control values when tested 7 days after delivery. The change in mu-receptor affinity was not due to competition between 3H-DAMGO and residual methadone. Extensive washing of the brain homogenates failed to alter the affinity of the receptor but decreased the concentration of the residual methadone. This decrease in receptor affinity was also observed in extensively washed brain tissue from female adult rats treated acutely with methadone (9.0 mg/kg, IP) or when brain homogenates were exposed to methadone (50 ng/ml) in vitro. Thus, these data suggest that methadone alters mu-receptor affinity by some unknown mechanism.

An immunocytochemical analysis of the lateral geniculate complex in the pigeon (Columba livia)

The lateral geniculate complex (GL) of pigeons was investigated with respect to its immunohistochemical characteristics, retinal afferents, and the putative transmitters/modulators of its neurons. The distributions of serotonin-, choline acetyltransferase-, glutamic acid decarboxylase-, tyrosine hydroxylase-, neuropeptide Y- (NPY), substance P- (SP), neurotensin- (NT), cholecystokinin- (CCK), and leucine-enkephalin- (L-ENK) like immunoreactive perikarya and fibers were mapped. Retinal projections were studied following injections of Rhodamine-B-isothiocyanate into the vitreous. Transmitter-specific projections onto the visual Wulst and the optic tectum were studied by simultaneous double-labelling of retrograde tracer molecules and immunocytochemical labelling. The GL can be divided into three major subdivisions, the n. geniculatus lateralis, pars dorsalis (GLd; previously designated as the n. opticus principalis thalami, OPT), the n. marginalis tractus optici (nMOT), and the n. geniculatus lateralis, pars ventralis (GLv). All three subdivisions are retinorecipient. The GLd can be further subdivided into at least five components differing in their immunohistochemical characteristics: n. lateralis anterior (LA); n. dorsolateralis anterior thalami, pars lateralis (DLL), n. dorsolateralis anterior thalami, pars magnocellularis (DLAmc); n. lateralis dorsalis nuclei optici principalis thalami (LdOPT); and n. suprarotundus (SpRt). The LdOPT consists of an area of dense CCK-like and NT-like terminals of probable retinal origin. Three subnuclei (DLL, DLAmc, SpRt) were shown to project to the visual Wulst. Cholinergic and cholecystokinergic relay neurons participated in this projection. The nMOT occupies a position between the GLd and GLv and encircles the rostral pole of n. rotundus and the LA. It is characterized mainly by medium sized NPY-like perikarya which were shown to project onto the ipsilateral optic tectum. Bands of NPY-like fibers in the tectal layers 2, 4, and 7 could at least in part be due to this projection of the nMOT. Most of the antisera used revealed transmitter/modulator-specific fiber systems in the GLv which often showed a layer-specific distribution. Perikaryal labelling was only obtained with glutamic acid decarboxylase. On the basis of its chemoarchitectonics, topography, and connectional pattern, the GLd complex of pigeons is most directly equivalent to the mammalian GLd. However, although the different subdivisions of the avian GLd may represent functionally different channels within the thalamofugal pathway similar to the lamina-specific differentiation within the mammalian geniculostriate projection, direct comparison of subnuclei of birds and mammals is not justified at this time. The nMOT appears similar to the intergeniculate leaflet (IGL) and the avian GLv clearly corresponds in many features to the mammalian GLv.

Delta receptor involvement in morphine suppression of noxiously evoked activity of spinal WDR neurons in cats

Morphine has been considered to be primarily a mu opiate receptor agonist. The present study was designed to determine if opiate receptor subtypes in addition to mu contribute to morphine analgesia at the level of the spinal cord. Extracellular activity of single wide dynamic range (WDR) neurons in the feline lumbar spinal cord were studied. Intrathecal administration of DAGO (selective mu agonist) or DPDPE (selective delta agonist) suppressed the noxiously (51 degrees C radiant heat) evoked activity of WDR neurons. Pretreatment with spinal beta-FNA (selective mu antagonist) antagonized the suppressive effects of spinal DAGO, but not that of DPDPE. Two doses of spinal morphine (200 and 400 micrograms) suppressed the noxiously evoked activity of WDR neurons confirming our previous report. Following beta-FNA pretreatment, the suppressive effects of morphine were reduced, however, when ICI 174,864 (selective delta antagonist) was co-administered with morphine on the spinal cord of the animals pretreated by beta-FNA, there was an even greater reduction in the neuronal suppression by morphine. Intravenous ICI 174,864 also reversed the suppressive effects of morphine in beta-FNA pretreated animals. beta-FNA antagonism of spinal morphine is evidence of the well-known mu receptor-mediating antinociception. However, antagonism by ICI 174,864 of morphine suppression in beta-FNA-pretreated animals demonstrates that morphine is capable of suppressing noxiously evoked activity of WDR neurons as a result of an interaction with delta receptors in addition to mu receptors at the level of spinal cord.

A monoclonal antibody recognizing kappa- but not mu- and delta-opioid receptors

A monoclonal antibody (mAb), KA8 that interacts with the kappa-opioid receptor binding site was generated. BALB/c female mice were immunized with a partially purified kappa-opioid receptor preparation from frog brain. Spleen cells were hybridized with SP2/0AG8 myeloma cells. The antibody-producing hybridomas were screened for competition with opioid ligands in a modified enzyme-linked immunosorbent assay. The cell line KA8 secretes an IgG1 (kappa-light chain) immunoglobulin. The mAb KA8 purified by affinity chromatography on protein A-Sepharose CL4B was able to precipitate the antigen from a solubilized and affinity-purified frog brain kappa-opioid receptor preparation. In competition studies, the mAb KA8 decreased specific [3H]ethylketocyclazocine ([3H]EKC) binding to the frog brain membrane fraction in a concentration-dependent manner to a maximum to 72%. The degree of the inhibition was increased to 86% when mu- and delta-opioid binding was suppressed by 100 nM [D-Ala2,NMe-Phe4,Gly-ol]-enkephalin (DAGO) and 100 nM [D-Ala2,L-Leu5]-enkephalin (DADLE), respectively, and to 100% when mu-, delta-, and kappa 2-sites were blocked by 5 microM DADLE. However, the mu-specific [3H]DAGO and the delta-preferring [3H]DADLE binding to frog brain membranes cannot be inhibited by mAb KA8. These data suggest that this mAb is recognizing the kappa- but not the mu- and delta-subtype of opioid receptors. The mAb KA8 also inhibits specific [3H]naloxone and [3H]EKC binding to chick brain cultured neurons and rat brain membranes, whereas it has only a slight effect on [3H]EKC binding to guinea pig cerebellar membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological profile of various kappa-agonists at kappa-, mu- and delta-opioid receptors mediating presynaptic inhibition of neurotransmitter release in the rat brain

1. The potency, relative efficacy and selectivity of a series of kappa-opioid receptor agonists at the mu-, delta- and kappa-opioid receptors mediating inhibition of electrically-induced (radiolabelled) neurotransmitter release from superfused rat brain slices was determined. 2. With regard to their potencies at kappa-receptors mediating inhibition of striatal [3H]-dopamine release, the highest pD2 value (8.7) was found for bremazocine and the lowest (7.1) for U50488; the pD2 values for ethylketocyclazocine (EKC), tifluadom, U69593 and PD117302 were between 8.0 and 8.3. There were no marked differences between the relative efficacies of the kappa-agonists (maximum inhibition being 60-70%). In contrast to the other kappa-agonists, at a concentration of 1 microM, PD117302 caused a significant (25-40%) increase of the spontaneous efflux of tritium. 3. None of the kappa-agonists significantly affected striatal [14C]-acetylcholine (ACh) release, with the exception of a slight inhibitory effect of EKC. The delta-receptor-mediated inhibitory effect of [D-Ala2, D-Leu5]enkephalin (DADLE) on [14C]-ACh release was antagonized in a concentration-dependent manner by bremazocine (0.1 and 1.0 microM) and also partially by EKC (1 microM), but not by the other kappa-agonists. The pA2 value for bremazocine as an antagonist at the delta-receptors involved was 8.0, compared to 7.6 for naloxone. 4. None of the kappa-agonists significantly affected cortical [3H]-noradrenaline (NA) release, with the notable exception of tifluadom, which strongly inhibited release by activating mu-receptors. The mu-receptor-mediated inhibitory effect of Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) on [3H]-NA release was antagonized in a concentration-dependent manner by bremazocine and EKC, but not by the other K-agonists. The pA2 value for bremazocine as an antagonist at the mu-receptors involved was 8.2, compared to 8.6 for naloxone. 5. Thus, whereas U69593 and PD1 17302 display high potency and selectivity towards K-opioid receptors, the potent benzomorphan K-agonists bremazocine and EKC also appear to be strong mu-opioid receptor antagonists.

Distribution of mu, delta, and kappa opioid receptor binding sites in the brain of the one-day-old domestic chick (Gallus domesticus): an in vitro quantitative autoradiographic study

Three highly specific opioid ligands--[D-Ala2,Gly-ol]-enkephalin (DAGO) for mu (mu) receptor sites, [D-Pen2,D-Pen5]-enkephalin (DPDPE) for delta (delta) sites, and U-69593 for kappa (kappa) sites--were used to determine the regional distribution of the three major subtypes of opioid receptor binding sites in the brains of 1-day-old domestic chicks by the technique of quantitative receptor autoradiography. Whilst there was a degree of heterogeneity in the binding levels of each of the ligands, some notable similarities existed in the binding of the mu and kappa ligands in several forebrain regions, and in the optic tectum of the midbrain where mu and delta binding was very high. In the forebrain there was a high level of binding of mu and kappa ligands in the hyperstriatum, and for the mu ligand there was a very distinct lamination of binding sites in hyperstriatum accessorium, intercalatum supremum, dorsale and ventrale. Levels of binding of the mu and kappa ligands were also high in nucleus basalis, and (for mu only) in the neostriatum. The distribution of binding of the delta specific ligand in the forebrain showed marked differences to that of mu and kappa, being particularly low in the hyperstriatum and neostriatum. Very high levels of labelling of delta binding sites were, however, found in the nucleus rotundus. Binding of the three ligands was generally low or absent in the cerebellum and medulla, apart from a distinct labelling of the granule cell layer by the mu-ligand. A kinetic analysis was made of the binding of the three ligands to whole forebrain sections using scintillation counting methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Method for isolation of kappa-opioid binding sites by dynorphin affinity chromatography

A kappa-opioid receptor subtype was purified from a digitonin extract of frog brain membranes, using affinity chromatography. The affinity resin was prepared by coupling dynorphin (1-10) to AH Sepharose 4B. The purified receptor binds 4,750 pmol [3H]ethylketocyclazocine (EKC) per mg protein (5,600-fold purification over the membrane-bound receptor) with a Kd of 9.1 nM. The addition of cholesterol-phosphatidylethanolamine (2:1) enhanced 3.6-fold the binding activity of the purified material, which gives a purification very close to the theoretical. The purified receptor protein exhibits high affinity for kappa-selective ligands. The purified fraction shows one major band (65,000 Mr) in sodium dodecyl sulfate (SDS) gel electrophoresis.

Kappa opioid receptors stimulate phosphoinositide turnover in rat brain

The effects of various subtype-selective opioid agonists and antagonists on the phosphoinositide (PI) turnover response were investigated in the rat brain. The kappa-agonists U-50,488H and ketocyclazocine produced a concentration-dependent increase in the accumulation of IP's in hippocampal slices. The other kappa-agonists Dynorphin-A (1-13) amide, and its protected analog D[Ala]2-dynorphin-A (1-13) amide also produced a significant increase in the formation of [3H]-IP's, whereas the mu-selective agonists [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephalin and morphine and the delta-selective agonist [D-Pen2,5]-enkephalin were ineffective. The increase in IP's formation elicited by U-50,488H was partially antagonized by naloxone and more completely antagonized by the kappa-selective antagonists nor-binaltorphimine and MR 2266. The formation of IP's induced by U-50,488H varies with the regions of the brain used, being highest in hippocampus and amygdala, and lowest in striatum and pons-medulla. The results indicate that brain kappa- but neither mu- nor delta-receptors are coupled to the PI turnover response.

Opioid receptor subtypes in the supraoptic nucleus and posterior pituitary gland of morphine-tolerant rats

Morphine, given acutely, inhibits oxytocin secretion in adult female rats, but chronic intracerebroventricular infusion for five to six days induces tolerance and dependence in the mechanisms regulating oxytocin secretion. One explanation for tolerance could be that there is a loss of opioid receptors. To test this hypothesis cryostat sections of selected brain regions and the pituitary, from six control and six intracerebroventricular morphine-infused rats, were processed for quantitative in vitro receptor autoradiography. [3H]Etorphine or [3H](-)-bremazocine were used as ligands, and DAGO, DPDPE and U50,488H as selective displacers from mu-, delta-, and kappa-receptors, respectively. Control incubations had naloxone determined specificity. The supraoptic nucleus (site of oxytocin-secreting magnocellular perikarya) contained both mu- and kappa-receptors in control rats (mean +/- S.E.M. binding of mu-selective [3H]etorphine was 91.8 +/- 25.4 fmol/mg of tissue, and of kappa-selective [3H](-)-bremazocine was 130.4 +/- 25.6 fmol/mg). Chronic morphine treatment caused a 83.9% decrease in binding in mu-selective conditions (P less than 0.05), but no significant change in kappa-selective binding. In the median preoptic nucleus (which projects to the supraoptic nucleus) mean +/- S.E.M. binding of [3H]etorphine decreased by 77.0% (P less than 0.01) in chronic morphine-treated rats, from the control value of 76.2 +/- 9.8 fmol/mg of tissue. In the posterior pituitary gland (site of the terminals of the oxytocin-secreting magnocellular perikarya) binding with [3H](-)-bremazocine in controls was over 90% lower than in the supraoptic nucleus. No changes followed chronic morphine treatment. Thus chronic morphine exposure reduces the numbers of available mu-receptors in the supraoptic nucleus, and of opioid receptors in the median preoptic nucleus, perhaps accounting for morphine-tolerance in relation to oxytocin secretion.

Naloxone-reversible monocyte dysfunction in patients with chronic fatigue syndrome

We studied monocyte function in 35 consecutive patients with chronic fatigue syndrome (CFS) and 25 healthy controls. Eighty-five per cent of the patients showed monocyte dysfunction characterized by marked reduction in the number of monocytes displaying immunoreactive cytoskeletal vimentin filaments, a low phagocytosis index, and a reduced expression of HLA-DR antigens. These values increased dramatically after incubation of the patients' monocytes with the opioid antagonist naloxone. Other immunological abnormalities also noted in the patients were low lymphocyte blastogenesis and diminished numbers of monocytes displaying receptors for Fc of IgG (FcR) and C3b (CR1). These findings suggest that an increased opioid activity acting through a classical receptor mechanism is active on monocytes from a high proportion of patients with CFS and that this represents a novel example of immunomodulation by opioid peptides in human disease. We suggest that endogenous opioids are involved in the pathogenesis of the chronic fatigue syndrome.

Methionine-enkephalin stimulates in vitro proliferation of human peripheral lymphocytes via delta-opioid receptors

The influence of methionine-enkephalin (Met-Enk) on in vitro proliferation of human peripheral lymphocytes was investigated. Met-Enk, in the concentration range 10(-12) to 10(-4) M enhanced the proliferative response to suboptimal concentrations of concanavalin A of human peripheral lymphocytes and to cells in the absence of mitogen. The response to Met-Enk in the absence of mitogen was not influenced by the presence of fetal calf serum: similar levels of enhancement were seen in cultures supplemented with 10% autologous serum. Enhancement of proliferation was blocked in a concentration-dependent manner by both naloxone and the delta specific antagonist ICI-174864. The sensitivity to the antagonistic influence of ICI-174864 suggests strongly that the stimulatory influence of Met-Enk on human lymphocyte proliferation in the absence of mitogen is mediated via the delta-opioid receptor.

Opioid peptides modulate the organization of vimentin filaments, phagocytic activity, and expression of surface molecules in monocytes

It is theorized that intermediate filaments are important in the modulation of membrane activity and cell motility; however, their functions are unknown. The assembly and organization of these filaments are under hormonal regulation. We investigated in human monocytes the in vitro effects of Met-enkephalin, Leu-enkephalin, and beta-endorphin on the expression of immunoreactive cytoskeletal vimentin filaments. We simultaneously examined their effect on the phagocytosis of Candida albicans and on the membrane display of surface molecules. The three opioid peptides markedly reduced the expression of vimentin filaments, the phagocytic activity, and the display of HLA-DR molecules at concentrations of 10(-6), 10(-8), and 10(-10) M. On the other hand, the intravenous administration of fentanyl, a synthetic opiate agonist, to patients undergoing surgery induced similar changes in monocytes. In other experiments, 10(-8) M beta-endorphin also decreased the expression of CR3 but did not influence the display of CD13, a surface protein of unknown function. Expression of vimentin filaments correlated directly with the display of HLA-DR antigens and CR3 and with the phagocytic activity. The results of this paper indicate that opiates and opioids, neuropeptides known to be released during stress, can directly depress several monocyte functions. Furthermore, from these data it may be speculated that intermediate filaments may regulate the membrane expression of some surface molecules and the phagocytic process.

Characterization of zeta (zeta): a new opioid receptor involved in growth

Endogenous opioid systems (i.e., opioids and opioid receptors) are known to play a role in neural cancer. Using [3H]-[Met5]enkephalin, a potent ligand involved in growth, specific and saturable binding was detected in homogenates of S20Y neuroblastoma transplanted into A/Jax mice; the data fit a single binding site. Scatchard analysis yielded a Kd of 0.49 nM and a binding capacity of 5.32 fmol/mg protein. Binding was dependent on protein concentration, time, temperature, and pH, and was sensitive to Na+ and guanine nucleotides. Optimal binding required protease inhibitors, and pretreatment of the tumor homogenates with trypsin markedly reduced [3H]-[Met5]enkephalin binding, suggesting that the binding site was proteinaceous in character. Displacement experiments indicated that [Met5]enkephalin was the most potent displacer of [3H]-[Met5]enkephalin; other ligands selective for mu, delta, kappa, epsilon, and sigma were not highly competitive. Given the functional significance of [Met5]enkephalin as a potent regulator of normal and abnormal growth, and that the receptor recognized by [Met5]enkephalin does not resemble any previously described, the present study has demonstrated the presence of a new opioid receptor termed zeta (zeta) (from the Greek 'Zoe', life) related to the proliferation of cells and tissues.

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)

Regulation of endogenous acetylcholine release from mammalian brain slices by opiate receptors: hippocampus, striatum and cerebral cortex of guinea-pig and rat

The effects of opiate agonists on acetylcholine release from hippocampal, striatal and cerebral cortical slices were tested; tissue from rat was compared to that from guinea-pig. The results show that opiate receptors in each of these areas can alter the evoked release of acetylcholine from nerve terminals; however, there are species and tissue differences with respect to the apparent subtype of opiate receptor effective. In the hippocampus and striatum of the two species studied, opiates caused a dose-dependent decrease in evoked acetylcholine release from tissue slices but in the guinea-pig kappa-selective agonists were effective, and mu or delta agonists were not, whereas in the rat, mu-, but not delta- or kappa-selective drugs were effective. Opiates also altered acetylcholine release from the frontal, parietal and occipital cortex of both of these species. In all three regions of the guinea-pig cortex, kappa and delta agonists were active and in the parietal cortex mu agonists were also active; rat cortical slices showed similar results except that delta agonists were not effective. The inhibitory effects of the opiate agonists were effectively antagonized by the non-selective opiate antagonist naloxone and by the calcium channel agonists, BAY K 8644 or YC-170. In addition, the effects of the opiate drugs tested in this study on acetylcholine release were confined to evoked release, that is, spontaneous acetylcholine release was not affected. The results suggest that in guinea-pig and rat brain, opiate receptors regulate acetylcholine release, and that, although the subtypes of opiate receptors involved in this effect are different in the two species and in different tissues from the same species, the effect results from a common mechanism that involves alterations of calcium influx into the nerve terminals during depolarization.

Kappa opioid binding sites in the dog cerebral cortex and spinal cord

In the dog cerebral cortex and spinal cord, [3H]bremazocine and [3H]U69593 both bound with high affinity to an apparent single population of binding sites under kappa-selective conditions. In the cortex similar Bmax values for both radioligands in the saturation studies and the high affinity of the kappa-selective agents PD117302 and U69593 for both [3H]bremazocine and [3H]U69593 labelled sites in the competition studies suggested a predominance of U69593-sensitive sites previously described as kappa 1 in the guinea-pig and rat brain. The lower slope values for the inhibition curves of PD117302 and U69593 against [3H]bremazocine but not against [3H]U69593 suggested that [3H]bremazocine could also be binding to a relatively minor proportion of additional, possibly kappa 2, sites while [3H]U69593 would appear to be selective for the kappa 1 site. In contrast, in the dog spinal cord, [3H]U69593 appeared to recognize only a proportion (approximately 35%) of the [3H]bremazocine labelled binding site. The significantly lower affinities and slope values of U69593 and PD117302 against [3H]bremazocine were consistent with the additional sites representing the k2 (benzomorphan) sites previously described in guinea-pig and rat spinal cord. Alternatively, the low (micromolar) affinity of the mu-selective ligand, [D-Ala2, MePhe4, Gly-ol5]enkephalin, implied that these additional sites might not be kappa 2 but possibly a low affinity mu site normally expressed under more physiological conditions.

The distribution of opioid binding subtypes in the bovine adrenal medulla

Autoradiography has been used to examine the distribution of opioid binding subtypes in the bovine adrenal gland. Specific opioid binding sites were restricted to the adrenal medulla. Kappa sites, labelled with [3H]bremazocine (in the presence of excess unlabelled mu and delta ligands), were highly concentrated over nerve tracts. These nerve tract associated binding sites were sensitive to competition by the endogenous opioid, dynorphin (1-13). Specific [3H]bremazocine binding sites were also found over the adrenal medullary chromaffin tissue. These binding sites were concentrated over the peripheral, adrenaline-containing region of the medulla and were sensitive to competition by diprenorphine but not dynorphin (1-13). Delta opioid sites, labelled with [3H][D-Ala2,D-Leu5] enkephalin (in the presence of excess unlabelled mu ligand) were selectively localized to the central, noradrenaline-containing region of the adrenal medulla. Mu opioid sites, labelled with [3H][D-Ala2, NMePhe4,Gly-ol5]enkephalin, were low in number and distributed throughout the adrenal medulla. These studies demonstrate that mu, delta and two distinct kappa opioid binding sites are differently distributed within the bovine adrenal medulla and suggest possible new sites of action for the adrenal medullary opioid peptides.

Selective protection of benzomorphan binding sites against inactivation by N-ethylmaleimide. Evidence for kappa-opioid receptors in frog brain

Selective binding of [3H]bremazocine and [3H]-ethylketocyclazocine to kappa-opioid receptor sites in frog (Rana esculenta) brain membranes is irreversibly inactivated by the sulfhydryl group alkylating agent N-ethylmaleimide (NEM). Pretreatment of the membranes with kappa-selective compounds [ethylketocyclazocine (EKC), dynorphin (1-13), or U-50,488H] but not with [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO; mu specific ligand) or [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DADLE; delta specific ligand) strongly protects the binding of the radioligands against NEM inactivation. These results provide more evidence for the existence of kappa-opioid receptors in frog brain. The relatively high concentrations of NEM that are needed to decrease the specific binding of [3H]bremazocine together with the observation of an almost complete protection of its binding sites by NaCl suggest that bremazocine may act as an opioid antagonist in frog brain.

Characterization of kappa opioid receptors in the rabbit ear artery

Quantitative characterization of the kappa opioid receptor in the rabbit ear artery was carried out using three kappa-selective agonist compounds, dynorphin-(1-13), U-69593 and ethylketocyclazocine. Kinetic analysis was performed using the antagonist, MR 2266. Two other in vitro preparations were studied for comparison: the mouse was deferens and rabbit was deferens. To avoid mu receptor action in the mouse was deferens the irreversible mu receptor antagonist, beta-funaltrexamine, was used. It was demonstrated that, using the highly selective kappa agonist compound U-69593, Ke values for MR 2266 obtained in the three assay systems were not significantly different. These results suggest that kappa receptors present in these three tissues share identical properties.

Interaction of 3,8-diazabicyclo (3.2.1) octanes with mu and delta opioid receptors

A series of 3,8-diazabicyclo (3.2.1) octanes (DBO) (1) substituted at the nitrogen atoms by acyl and aralkenyl groups, were tested in in vitro binding assays towards mu and delta opioid receptors. The most representative terms (1a, 1d, 1g, 1j,) were also evaluated for the analgesic potency in vivo by the hot plate method. Among the compounds tested the most potent was the p.nitrocinnamyl DBO (1d) which displayed a mu/delta selectivity and an analgesic activity respectively 25 and 17 fold those of morphine. On the contrary, the m.hydroxycinnamyl DBO (1g) was markedly less active as agonist than the parent 1a, thus suggesting that structure 1 interacts with opioid receptors in a different fashion than morphine. Compound 1j isomer of 1a which is provided with high mu affinity, but lower analgesic potency, was found to possess a mixed agonist-antagonist activity.

Dynorphin1-17 reduces the inhibitory actions of mu- and delta-selective opioid agonists in cortical neurons of the rat in vivo

Spontaneous and L-glutamate-evoked neuronal activity was recorded extracellularly from neocortical neurons of rats. Opioid agonists with preference for different receptor types were applied microiontophoretically or pneumatically from multibarrelled micropipettes. Morphine (mu-selective), [D-Ala2,D-Leu5]enkephalin (delta-selective; DADL) and the kappa-selective agonist dynorphin1-17 (DYN 17) suppressed spontaneous and evoked neuronal activity in a naloxone-reversible manner. In a substantial number of neurons the inhibitory effect of DADL and morphine was reduced or abolished by DYN 17. This antagonistic action was often observed with DYN 17 levels that did not influence the discharge activity by itself. The physiological significance of this observation remains to be elucidated.

Influence of N-allyl-normetazocine on acetylcholine release from brain slices: involvement of muscarinic receptors

The effects of (+/-)N-allyl-normetazocine on the release of acetylcholine from different areas of guinea-pig and rat brain were investigated. 1. The drug did not modify the electrically (2 Hz) evoked tritium efflux from guinea-pig cerebral cortex, thalamus and caudate nucleus slices, preloaded with 3H-choline 0.1 mumol/l and superfused with Krebs solution containing hemicholinium-3 10 mumol/l. 2. (+/-)N-allyl-normetazocine 10 mumol/l enhanced the evoked 3H efflux from guinea-pig brain slices superfused with Krebs solution containing physostigmine 30 mumol/l or oxotremorine 0.3-1 mumol/l; the effect was naloxone-insensitive and was abolished by atropine 0.15 mumol/l, but not by pirenzepine 1 mumol/l. 3. (+/-)N-allyl-normetazocine 5 mumol/l enhanced the electrically evoked release of endogenous acetylcholine as well, in a naloxone-insensitive way. 4. Both (+/-) and (+)N-allyl-normetazocine were without effect on 3H efflux from rat caudate nucleus slices electrically stimulated at 0.2 Hz frequency, after preloading with 3H-choline and during superfusion with hemicholinium-3. 5. The results are discussed in view of the antimuscarinic properties of the drug.

Opioid receptors inhibit the adenylate cyclase in guinea pig cochleas

The effects of mu- and delta-preferring agonists on adenylate cyclase activity have been investigated in vitro in homogenates of guinea pig cochleas. Morphine, Leu-enkephalin, D-Ala2, N-methyl-Phe4, Gly-ol5-enkephalin (DAGO) and D-Ser2-Leu-enkephalin-Thr (DSLET) each inhibited the synthesis of cyclic AMP. This effect was reversed by naloxone which had a greater affinity in blocking the effect of the mu-preferring agonists (morphine, DAGO) than in blocking the effect of the delta-preferring agonists (Leu-enkephalin, DSLET). Finally, no additive effects were observed when various combinations of two agonists were used. These results indicate that opioid receptors exist in the guinea pig cochlea and that they are negatively linked to adenylate cyclase. The different affinities shown by naloxone to reverse the inhibition induced by the mu- and delta-preferring agonists suggest that morphine and DAGO act through mu-receptors, whereas Leu-enkephalin and DSLET act through delta-receptors. Since no additive effects have been found when combining two different agonists, it can be hypothesized that the mu- and delta-receptors are coupled to the same pool of adenylate cyclase. It may be proposed from these findings that in vivo enkephalins inhibit the synthesis of cyclic AMP via mu- and delta-receptors. However, whether this effect occurs at a presynaptic level (within opioid-containing olivocochlear varicosities) or at the postsynaptic level (within dendrites of the primary auditory neurons) remains to be determined.

Photoaffinity labeling of opioid receptor of rat brain membranes with 125I(D-Ala2, p-N3-Phe4-Met5)enkephalin

A photoreactive (D-Ala2, p-N3-Phe4-Met5)enkephalin derivative was prepared, iodinated with carrier-free 125I, and then purified by high-performance liquid chromatography. The purified radioactive photoprobe was monoiodinated at the amino terminal tyrosine residue. This radioactive photoprobe was used to photoaffinity label membranes prepared from the rat brain (minus cerebellum) and the spinal cord. The photolabeled membranes were analyzed by sodium dodecyl sulfate gel electrophoresis. A 46,000-Da protein was specifically photolabeled in these membrane preparations. The photolabeling of this protein was inhibited by peptides related to enkephalin but not by unrelated substance P or gastrin tetrapeptide. A concentration-dependent inhibition of the photolabeling of the 46,000-Da protein was observed in the presence of competing ligands specific for the mu-, delta-, and kappa-opioid receptors. These data demonstrate that the radioactive photoprobe labels the mu-, delta-, and kappa-opioid receptors. Although there is no evidence available to show that the 46,000-Da protein is identical in all the cases, our data strongly suggest that it is a binding protein common to all of the opioid receptor subtypes.