Multiple opiate receptors: emerging concepts

Opioid receptor binding in rat spinal cord

The interaction of various radioligands with spinal opioid receptors has been characterized under variable experimental conditions. Binding to mu, delta, and kappa sites was measured in all (cervical, thoracic, lumbar) segments. The apparent affinity constant (K) of [3H]Ethylketocyclazocine (EKC) was similar in Tris, 2.09 (+/- 1.06) X 10(8) M-1, and phosphate buffer, 2.16 (+/- 0.02) X 10(8) M-1, when its interaction with delta and mu sites was blocked. Without blocking ligands, EKC binding was resolved in two components: K1 = 1.01 (+/- 0.21) X 10(9) M-1 and K2 = 0.95 (+/- 0.61) X 10(7) M-1. Likewise, the binding of [D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGO) or [D-Ala2, D-Leu5]-enkephalin (DADLE) alone was represented by a 2-site model. By adjusting the radioligand and receptor concentration or by the addition of blocking ligands, binding was represented by a 1-site model for DAGO, K = 4.35 (+/- 1.41) X 10(8) M-1, and DADLE, K = 2.44 (+/- 0.08) X 10(8) M-1.

Effect of morphine and naloxone on shock avoidance learning in headless cockroaches (Periplaneta americana L.)

Systemic administration of high doses of morphine (56 micrograms morphine/g body weight) or of naloxone (54 micrograms naloxone/g body weight) results in a significant improvement of shock avoidance behavior in headless cockroaches. In both cases the learning parameter stimulation time (time during which an animal receives shocks) is significantly decreased. The behavioral parameters, stimulation rate (activity) and mean stimulation duration (shock responsiveness), as the two factors of the stimulation time considered individually, do not significantly change with morphine. Only the responses of both parameters together give the significant decrease of stimulation time (improvement of shock avoidance behavior) as mentioned above. However, the administration of naloxone causes a significant shortening of the mean stimulation duration indicating that these animals avoid the shocks by learning to lift their legs more quickly out of electrified saline in order to terminate the shocks (escape learning). Combined administration of both drugs together also causes a significant improvement of the leg-lifting response even at lower doses (morphine: 0.56 microgram plus naloxone: 0.54 microgram/g; morphine: 0.0056 microgram/g plus naloxone: 0.0054 microgram/g). Although the various effective doses of these drugs administered in combination have similar effects on the stimulation time, they affect the behavioral parameters in different ways. These different actions on activity and shock responsiveness as well as the efficacy of doses smaller than those known from vertebrates are discussed as behavioral evidence of opiate receptors in the cockroach.

Opiate antagonist in traumatic shock

In experimental animal studies, opiate receptor antagonists (such as naloxone) and physiological opiate antagonists (thyrotropin-releasing hormone [TRH] have been used with some success to improve outcome and physiological variables following traumatic shock associated with hypovolemia, spinal cord trauma, and head injury. Naloxone at high doses (in the mg/kg range) improves blood pressure and survival following hypovolemic shock in some species subjected to fixed-pressure shock. Similarly, naloxone treatment in the same dose range improves blood pressure and outcome following traumatic spinal shock as well as shock associated with traumatic head injury in selected animal models. The high doses of naloxone required in these studies suggest that the beneficial effects may be due to actions at relatively naloxone-insensitive opiate receptors, such as the kappa-receptor. Changes in the putative kappa-receptor ligand dynorphin are found after hypovolemic shock and traumatic injury to the brain or spinal cord. Opiate receptor antagonists with increased selectivity for the kappa-receptor may be superior to naloxone in the treatment of these conditions. TRH or TRH analogs similarly improve blood pressure and outcome following hypovolemic or spinal shock. Clinical trials of naloxone (at high doses) in human spinal cord injury have begun, and there are plans for clinical trials of naloxone in human head trauma and of TRH in human spinal cord injury.

Perspective graphics as a means of portraying the distribution of radiolabelled ligands in the spinal cord--a pilot study using intrathecally administered 3H morphine

Tritium-labelled morphine sulphate was injected into the lumbar (L4-5) subarachnoid space of an adult male baboon. Three hours after injection, the animal was sacrificed. Using quantitative light microscopic autoradiographic mapping techniques, contour and perspective diagrams were prepared that described the position of radiolabel and by inference the distribution of morphine binding sites within the spinal cord. High concentrations of 3H was found in the medial regions of laminae I, II (substantia gelatinosa) and III of the dorsal horns. Smaller, but significant levels were seen bilaterally in the spinal anterolateral quadrant. Minimal 3H activity was seen in the remainder of the spinal cord with the lowest level being recorded in the spinal canal. Perspective graphics proved a precise and attractive method for locating the position and quantifying the concentration of radiolabel in baboon spinal cord.

Opiate-induced turning in rats after injection into the ventral tegmental area

Morphine and ethylketazocine caused ipsilateral circling when injected unilaterally into the ventral tegmental area (VTA) of rats. Systemic naloxone only slightly inhibited this effect while systemic diprenorphine completely prevented circling. Systemic haloperidol and alpha-methyl-p-tyrosine also blocked circling. Rats made tolerant to morphine still turned after morphine injection into the VTA. Levorphanol, dextrorphan, methadone, DADLE, dynorphin(1-13), SKF 10,047 and phencyclidine were inactive when injected unilaterally into the VTA of naive rats; naloxone and naltrexone alone also were inactive. The opiate-induced circling appears to involve a non-mu opiate receptor as well as a dopaminergic neuronal system.

Effects of a stable enkephalin analogue, (D-Met2,Pro5)-enkephalinamide, and naloxone on cortical blood flow and cerebral blood volume in experimental brain ischemia in anesthetized cats

The effects of intracarotid injection of the stable enkephalin analogue (D-Met2,Pro5)-enkephalinamide (ENK) and intravenous administration of naloxone on the cerebrocortical blood flow (dye dilution method) and cerebral blood volume (CBV) (photoelectric method) were investigated during unilateral brain ischemia in anesthetized cats. Both parameters were measured simultaneously in the intact and ischemic (middle cerebral artery occluded) hemispheres. An intracarotid injection of ENK 0.5 mg/kg induced a significant increase in cortical vascular resistance and a -87% decrease in cerebrocortical blood flow from 25 +/- 3 to 4 +/- 3 ml/100 g/min, without CBV alteration in the ischemic hemisphere. Naloxone (1 mg/kg i.v.), on the other hand, induced a marked two-fold increase in cerebrocortical blood flow and a significant elevation of CBV from 5.9 +/- 0.5 to 7.4 +/- 0.7 vol% in the ischemic hemisphere. No change in cerebrocortical blood flow or CBV was observed in the intact hemisphere either after ENK or after naloxone administration. Arterial blood gases and hematocrit remained unchanged. On the basis of the present findings, we conclude that besides other factors, endogenous opioid mechanisms may also participate in ischemic cerebrovascular reactions and the cerebral circulatory effects of naloxone probably reflect its opiate receptor blocking property and not simply its other non-opiate-related actions.

Emetic and antiemetic effects of opioids in the dog

The emetic and antiemetic effects of opioid agonists were studied in awake dogs. The mu-agonists morphine, fentanyl and methadone, in sedative doses, prevented the emetic response to apomorphine and copper sulphate; only morphine induced emesis, at doses lower than those required to prevent emesis. The delta-agonist [D-Ala2,Met5]enkephalinamide (DALA) and [Leu5]enkephalin induced emesis in some of the dogs studied but had no antiemetic activity. The kappa-agonists bremazocine and ethylketocyclazocine (EKC) did not induce emesis but, at sedative doses, prevented the emetic response to apomorphine. The emetic effect of DALA was antagonized by naloxone in some dogs; the antiemetic effect of morphine, bremazocine and EKC was blocked by both naloxone and MR 2266. The non-opioid sedatives diazepam, phenobarbital and xylazine, administered in sedative doses, did not prevent apomorphine-induced emesis. Our results suggest that a delta-receptor is involved in the emetic effect and a mu- and/or or kappa-receptor in the antiemetic effect of opioids.

Stimulation of food intake following opioid microinjection into the nucleus accumbens septi in rats

The involvement of opioid peptides in the regulation of food intake has been postulated. However, it is not known how they are involved in this regulation and which brain region is responsible for the mediation of their effects. We studied the effect of a microinjection of opioid agonists and antagonists into the nucleus accumbens septi (NAS) on the food intake in rats, as this area is known to be important for motivation. Male Wistar rats were implanted stereotaxically with guide cannulae. Rats were not allowed food prior to drug treatment and solutions (1 microliter) were microinjected bilaterally. Food intake was measured throughout a 2 hr period after the drug injection. Infusions into the NAS of 2, 5 and 10 nmol of morphine, D-ala2, D-Leu5-enkephalin (DADLE), and beta-endorphin (beta E), or of 5 and 10 nmol of alpha-neoendorphin (ANEO) induced a dose-dependent increase in the food intake. Dynorphin (DYN) also increased the food intake, but only at a 10 nmol dose. The new, highly selective delta agonist D-Pen2,5-enkephalin (DPDPE) induced a dose-dependent increase in the food intake. Naloxone in doses of 2 and 10 nmol antagonized the increased food intake induced by morphine, beta E, ANEO and DYN in a dose-dependent manner, but only partly antagonized the effect of DADLE on the food intake. The selective mu-receptor antagonist beta-funaltrexamine (beta-FNA), in a dose of 5 nmol completely blocked the increase in the food intake induced by morphine but not by DADLE.(ABSTRACT TRUNCATED AT 250 WORDS)

Phencyclidine-induced stereotyped behaviors after injection of ethylketocyclazocine, Mr 2266 and naltrexone in rats

The effects of ethylketocyclazocine (EKC), Mr 2266 and naltrexone on the stereotyped behaviors induced by an intraperitoneal injection of phencyclidine (PCP) were examined. PCP-induced turning, backpedalling, head weaving and sniffing were antagonized by pretreatment with EKC (0.25-4.0 mg/kg). While pretreatment with Mr 2266 (2.5 mg/kg), a kappa selective antagonist, and naltrexone (10 mg/kg), a mu selective antagonist, failed to affect the PCP-induced stereotypy, Mr 2266 antagonized the suppressing effect of EKC on PCP-induced stereotypy. Taken into consideration, this suggests that kappa opioid agonists such as EKC antagonize PCP-induced stereotyped behaviors through a kappa opioid mechanism, and that the mu opioid receptor may not play an important role in the PCP-induced stereotypy in rats.

beta-Funaltrexamine (beta-FNA) and the regulation of body and brain development in rats

The effects of beta-FNA, a highly selective and irreversible mu opioid receptor antagonist, in altering body and brain development in preweaning rats were determined. Animals given beta-FNA did not differ from controls in body weights, brain and cerebellar weights, macroscopic dimensions of the brain, the area of the cerebellum, or in organ weight. The dosage of beta-FNA utilized (5 mg/kg) blocked morphine-induced analgesia (2 mg/kg morphine sulfate, SC) for each injection period (i.e., 48 hr). In contrast to beta-FNA treatment, rats given naltrexone (50 mg/kg SC) in a regimen which completely blocked the opioid receptor throughout ontogeny exhibited marked increases in somatic and neurobiological growth. These results suggest that, in and by themselves, mu receptors selectively antagonized by beta-FNA do not play an important role in regulating development.

Phencyclidine-induced stereotyped behaviors after injection of morphine and N-allylnormetazocine (SKF 10,047) in rats

We investigated the effects of N-allylnormetazocine (SKF 10,047) and morphine on the stereotyped behaviors induced by the intraperitoneal injection of phencyclidine (PCP). PCP-induced turning and backpedalling were significantly potentiated by pretreatment with SKF 10,047 (10 mg/kg) but sniffing and head weaving were not. On the other hand, pretreatment with morphine dose-dependently attenuated PCP-induced sniffing and head weaving, but not turning and backpedalling. These results suggest that PCP-induced stereotypy may be mediated by not only a sigma opioid receptor but also some other receptors. In addition, each component of PCP-induced stereotypy may be controlled by different opioid systems and/or neuronal systems.

Interactions of phencyclidine with hippocampal circuitry: evidence for neuronal heterogeneity

The discovery of phencyclidine (PCP) receptors has stimulated the search for specific PCP antagonists. A direct product of this research is metaphit, an irreversible PCP ligand, which has recently been synthesized. In this study we examined the effects of metaphit on the responses of hippocampal neurons to PCP. On the basis of unfiltered action potential durations, hippocampal cells were divided into two groups, complex-spike cells and theta neurons. Local application of PCP caused inhibitions of the spontaneous firing rates of complex-spike cells. Metaphit, locally applied, antagonized approximately 50% of these responses, while the remaining responses were unaffected. In contrast, PCP caused increases in the spontaneous firing rates of theta cells and in almost all cases, these responses to PCP were attenuated by metaphit administration. These effects of metaphit were specific for PCP as the responses to locally applied norepinephrine were not altered by metaphit. The data suggest two mechanisms of action of PCP in the hippocampus. In addition, these mechanisms may be localized in part to different cell types.

Receptors on individual neurones

Membrane potential or ionic conductance of neurones of the mammalian central or peripheral nervous system maintained in vitro can be measured over periods of several hours. Drugs or transmitters which change potential or conductance can be applied repeatedly under equilibrium conditions, and pharmacological null methods used to characterize the receptors with which they interact. The method offers an advantage over ligand binding studies on nervous tissue because both agonist and antagonist affinities can be estimated on individual functioning cells. The results to date suggest the hypothesis that a given receptor subtype is always associated with the same change in ion conductance, and the corollary that distinct ion conductances affected by the same transmitter result from interactions with different receptor subtypes.

Distinct subtypes of the opioid receptor with allosteric interactions in brain membranes

The binding isotherms of opioid receptors in rat brain membranes with [3H]D-Ala2-D-Leu5-enkephalin ([3H]DADLE), [3H]dihydromorphine ([3H]DHM), and [3H]etorphine were analysed to show the effects of Mg2+, Na+, and guanine nucleotides. Four opioid receptor subtypes of delta, kappa, mu 1, and mu 2 specificities were differentiated, where necessary with the aid of specific displacing ligands. Both a guanine nucleotide [guanosine-5'-(beta, gamma-imido)triphosphate] and the cations (Na+, Mg2+) affect the affinity state of all four subtypes of the receptor. The opioid binding behaviour is found on detailed inspection to be complex, with cases of "half-of-the-sites" reactivity and of cooperativity. By their behaviour under the various ionic conditions noted, it was concluded that these subtypes are distinct, without the need to assume interconvertibility by such agents. The evidence suggests that the formation of heterologous kappa-delta or mu 1-mu 2 receptor complexes is required for stabilization of the high-affinity conformational state of the receptor. Important effects of cations in increasing the binding and regulating the equilibria of receptor association-dissociation were observed when these studies were conducted, not in the Tris-HCl buffer commonly used in opioid binding assays, but in N-tris[hydroxymethyl]-methyl-2-aminoethanesulphonate (K+) buffer (TES-KOH; 10 mM, pH 7.5): it was found that ionic species of Tris can substitute for divalent cations. Dithiothreitol effects on agonist binding in the presence and absence of the cations suggested that those cation effects involve the exchange of -SH/-SS- bonds between receptor subunits. All of the behaviour is interpreted in terms of a model involving association-dissociation equilibria of homologous and/or heterologous receptor subunits of an oligomeric opioid receptor structure.

Evidence for multiple opiate receptor involvement in different phencyclidine-induced unconditioned behaviors in rats

Rats were used for comparing the behavioral response profiles of phencyclidine (PCP) and d,1-N-allylnormetazocine (NANM), two drugs that are proposed to exert their effects through the "PCP/sigma" receptor. Phencyclidine (1.0-5.0 mg/kg) and NANM (2.5-10.0 mg/kg) induced dose-related increases in locomotion, sniffing, repetitive head movements, non-object directed mouth movements, and ataxia. Both drugs also increased food and water consumption during the latter portion of the drug response. Ingestive behaviors induced by PCP (2.5 mg/kg), as with eating and drinking stimulated by the mu-opiate morphine (2.0 mg/kg), were blocked by a relatively low dose of the opiate antagonist naloxone (0.5 mg/kg). Multiple injections of PCP (2.5 mg/kg for 4 days) or NANM (10.0 mg/kg for 4 days) augmented several measures of behavioral activation, including horizontal locomotion, rearing, and nonfocused sniffing, but did not significantly change stereotyped behaviors or ataxia. Reciprocal cross-sensitization of locomotor activation is indicated by the finding that the response to a challenge injection of PCP (2.5 mg/kg) or to NANM (10.0 mg/kg) after 4 days of treatment with the other drug closely resembled the enhanced locomotor response observed after the chronic treatment. Phencyclidine and NANM thus appear to exert many of their effects on unconditioned behavior through common mechanisms, including interaction with sigma receptors. In addition, these findings are consistent with previous suggestions that a mu-opiate receptor system may modulate some effects of PCP.

Design and interpretation of opiate antagonist trials in dementia

In view of the reports of possible beneficial effects of naloxone in dementia, rationales and strategies for studying endogenous opiate systems are reviewed. Important considerations in the design and interpretation of clinical investigations using naloxone are also reviewed. The nature and distribution of endogenous opiate systems are summarized from an historical perspective. Endogenous opiate systems are distributed throughout the central nervous system and play important roles in a variety of brain functions, including memory and learning. In view of this, several rationales are evident for studying endogenous opiate systems in dementia, since it is a syndrome in which structures known to contain opiate systems are disturbed, functions modulated by opiate systems are disturbed, and other neurotransmitter systems (functionally linked to endogenous opiate systems) are disturbed. Different strategies for studying endogenous opiate systems are reviewed, including examination of body fluids and pharmacologic challenge studies. Naloxone hydrochloride, a competitive opiate receptor antagonist, is a commonly used pharmacologic agent. The design of a multidose naloxone study of 12 dementia patients is discussed, with reference to the pharmacokinetics, pharmacodynamics, and specificity of naloxone as well as to the nature of the dependent measures selected for this study. No cognitive benefit was observed in this study. Behavioral arousal was observed at naloxone doses, with more evident psychomotor retardation at higher doses. These findings are contrasted with the results of naloxone challenges in other studies. The varying effects of naloxone within and across populations can be conceptualized in terms of the basic and clinical considerations previously discussed. The importance of dose-finding studies is stressed for this and other drug trials.

Possible mediation of catecholaminergic pathways in the antinociceptive effect of an extract of Cannabis sativa L

An extract of cannabis (5 and 15 mg/kg expressed as delta 9-THC) orally administered to rats caused an elevation of the nociceptive threshold (tail-flick latency and vocalization tests). Naloxone and naltrexone (blockers of mu-type opiate receptors) as well as MR 1452 (blocker of kappa opiate receptors) did not prevent the antinociceptive effect of cannabis when used at the dose of 2 mg/kg SC; only a high dose (10 mg/kg SC) of these narcotic antagonists partially blocked cannabis antinociception. ICI 154, 129, an antagonist of delta-type opiate receptors, failed to prevent the cannabis-induced rise in nociceptive threshold when used at a dose of 2 mg/kg SC but produced a significant effect at 10 mg/kg SC. While the role of opiate receptors does not seem fundamental to cannabis antinociception, the clear-cut effectiveness shown by 6-hydroxydopamine (a neurotoxin which causes a degeneration of catecholamine-containing terminals) in reducing cannabis antinociception is indicative of a participation of catecholamines in the phenomenon.

Dopamine and the action of opiates: a reevaluation of the dopamine hypothesis of schizophrenia. With special consideration of the role of endogenous opioids in the pathogenesis of schizophrenia

It is suggested that the antipsychotic efficacy of opioids in patients suffering from schizophrenia may result from an interaction of opioids with the dopaminergic system. The modulatory effect of opioids on dopaminergic functions has already been demonstrated in basic experiments: Anatomical and biochemical data reveal an interaction between opioid receptors and dopamine (DA) actions on dopaminergic nerve terminals, cell bodies, and afferent nerve endings. Endogenous enkephalin levels correlate well with the endogenous dopamine content in various brain areas. Systemic or iontophoretic administration of morphine alters the spontaneous activity of ventral tegmental dopaminergic neurons. Morphine and enkephalin effectively enhance pituitary prolactin release, whereas dopamine inhibits it. Opioid agonists effectively alter DA release, DA reuptake, and DA metabolism in the striatum and substantia nigra. In reverse, chronic neuroleptic treatment enhances the synthesis and release of pituitary beta-endorphin. Opioids affect contralateral rotation elicited by dopamine agonists in animals with unilateral lesions of the nigrostriatal pathway. Phencyclidine, a psychotropic drug that shares certain pharmacological characteristics with the putative sigma-opioid receptor ligand SKF 10,047, indirectly mimics the effects of dopamine agonists on prolactin release, release of acetylcholine, etc. It is suggested that an imbalance of opiate-DA interaction might be involved in the pathogenesis of schizophrenia. Consequently, clinical studies on the effects of opioids on psychotic symptoms should also examine opioid influence on dopaminergic functions in these patients.

beta-Funaltrexamine (beta-FNA) and neural tumor response in mice

The effects of beta-FNA, a highly selective and irreversible mu opioid receptor antagonist, in altering tumor response in A/Jax mice inoculated with S20Y cells were determined. Inoculation of neuroblastoma cells in control subjects resulted in 100% tumor incidence within 16 days, and mean and median survival times of 36 and 35 days, respectively, following tumor inoculation. Tumor incidence and survival times were comparable to controls for mice given chronic injections of 2 mg/kg and 10 mg/kg beta-FNA every 48 h beginning 2 days after tumor inoculation. Tumor growth was subnormal in the 10 mg/kg beta-FNA group. Both dosages of beta-FNA were found to block morphine-induced analgesia for 48 h. These results suggest that, in and by themselves, mu receptors selectively antagonized by beta-FNA do not play an important role in neuro-oncogenic events.

Regional heterogeneity of rat brain phencyclidine (PCP) receptors revealed by photoaffinity labeling with [3H] azido phencyclidine

Photoaffinity labeling of rat brain phencyclidine (PCP) receptors with [3H] azido phencyclidine ([3H]AZ-PCP) reveals the existence of five polypeptides which are specifically labeled by the affinity probe (Mr's 90,000, 62,000, 49,000, 40,000 and 33,000). These labeled components are unevenly distributed in rat brain. In the frontal cortex, thalamus and olfactory bulb, the major bands labeled are the Mr's 90 K and 62 K polypeptides; in the cerebellum most of the labeling is in the 90 K and 33 K bands; and in the hippocampus all but the Mr 40 K band are heavily labeled. Together with dexoxadrol/[3H]PCP competition binding data, which indicated the existence of high and low affinity dexoxadrol/PCP binding sites, these results suggest regional heterogeneity of PCP receptors. The regional distribution of the high affinity dexoxadrol binding sites correlates best with that of the Mr 90 K polypeptide.

Reversal of neurological deficits by levallorphan in patients with acute ischemic stroke

This study was conducted to examine the effect of the intramuscular injection of levallorphan tartrate (1.0 mg), a mixed agonist-antagonist opiate, on the neurological signs, symptoms, and vital signs in 19 patients with acute ischemic stroke. A temporary improvement of hemiplegia or hemiparesis was observed within several minutes after levallorphan injection in 13 of the patients. There were no significant alterations in blood pressure or pulse rate after injection. The findings indicate that levallorphan may have a temporary improving effect on neurological deficits in acute ischemic stroke. In addition, observation of the response to levallorphan may serve to predict the prognosis of the final neurological outcome in this type of patient.

Effects of phencyclidine in combination with morphine on the levels of met-enkephalin, dopamine, DOPAC and HVA in discrete brain areas of mice

The study investigated the interaction between phencyclidine (PCP) and morphine in affecting the levels of met-enkephalin, dopamine, DOPAC and HVA in mice. Morphine 5 mg/kg alone and PCP 10 mg/kg alone failed to change the levels of met-enkephalin in the midbrain and striatum. However, PCP in combination with morphine produced an increase in met-enkephalin levels and a decrease in HVA levels. In the midbrain, there was a direct relationship between the decrease in met-enkephalin levels and the increase in HVA levels. These results suggest that PCP may change the function in dopaminergic and enkephalinergic neuronal systems in the midbrain and/or striatum.

Rabbit anti-idiotypic antibodies raised against monoclonal anti-morphine IgG block mu & delta opiate receptor sites

Rabbit antibodies have been raised against murine monoclonal anti-morphine Fab fragments. Following affinity purification, these antibodies competitively inhibit morphine binding to anti-morphine monoclonal antibodies, yet do not recognize normal mouse IgGs, suggesting that the antibodies produced are anti-idiotypic with respect to the anti-morphine IgG. More importantly, the purified antibodies competitively inhibit binding of morphine, naloxone, and D-ala-2-D-leu-5-enkephalin to rat brain opiate receptors.

The kappa opioid receptor and food intake

Many studies have suggested a role of opioid receptors in the modulation of food intake. Several distinct classes of opioid receptors have been postulated. In an attempt to establish which opioid receptor(s) modulate feeding we studied the effect of the kappa agonist, bremazocine, on feeding and compared its effects to the preferential mu agonist, morphine, and the mixed kappa-sigma agonist, butorphanol and the kappa agonist, ethylketocyclazocine. Bremazocine increased feeding to the same extent as morphine and was less potent than the mixed agonist/antagonists. The bremazocine effect demonstrated a bell-shaped dose response curve. Daily administration of bremazocine or morphine enhances the effect on increasing food intake. However, this effect of daily injections on enhancing food intake is not present when animals receiving morphine are crossed over to bremazocine and vice versa. The bremazocine effect is enhanced by diprenorphine and not inhibited by naloxone. Low doses of the dopamine antagonist, haloperidol, enhance the bremazocine effect and higher doses inhibit it. Finally, using another kappa agonist, tifluadom, we showed that the effect on food intake is stereospecific. Our studies provided further evidence for a role for the kappa opioid receptor in feeding. However, they also suggest that more than one subpopulation of opioid receptors is involved in feeding modulation.

Phencyclidine-like effect of cyclazocine on pentylentetrazol-induced seizures in laboratory animals

The present work deals with an EEG and behavioural study of the effect of cyclazocine against the convulsions due to pentylentetrazol (PTZ) in mice, rats and rabbits. In rats, cyclazocine, at the high doses (15-25 mg/kg) prevents the tonic motor convulsions and EEG epileptiform "grand mal" seizure induced by PTZ. In rabbits and mice, cyclazocine inhibits the tonic motor convulsions without modifying either the spike-frequency or the duration of the PTZ-induced EEG seizures. Naloxone, even at high doses, was not able to block the anticonvulsive effects of cyclazocine on PTZ-induced convulsions in the rat. The effects of cyclazocine were compared to those of phencyclidine. These results confirm the multiple behavioural effects of cyclazocine and support the idea that both cyclazocine and phencyclidine, may act on the PCP/sigma receptor identified in binding studies.

Endogenous opioid immunoreactivity in rat spinal cord following traumatic injury

It has been postulated that endogenous opioids play a pathophysiological role in spinal cord injury, based on the therapeutic effects of the opiate receptor antagonist naloxone in certain experimental models. The high doses of naloxone required to exert a therapeutic action suggest that naloxone's effects may be mediated by non-mu opiate receptors, such as the kappa receptor. This notion is supported by recent pharmacological studies demonstrating that an opiate antagonist more active at kappa sites is effective and far more potent than naloxone in improving outcome after spinal cord injury. Moreover, dynorphin--postulated to be the endogenous ligand for the kappa receptor--is unique among opioids in producing hindlimb paralysis following intrathecal administration in the rat. In the present studies we have examined changes in endogenous opioid immunoreactivity following traumatic spinal cord injury in the rat. Dynorphin A was found to increase progressively with graded injury; changes were restricted to the injury segment and adjacent areas and were time dependent. Dynorphin A-(1-8) showed no marked changes. Methionine and leucine enkephalin were either unaltered or reduced at the injury site; changes were not well localized and were not clearly related to the injury variables. These findings provide further support for a potential pathophysiological role of prodynorphin-derived peptides in spinal cord injury.

Functional alteration of opioid receptor subtypes in the mice exhibited conditioned suppression in motility

Mice exhibit a marked suppression of motility (conditioned suppression) when placed in the same environment in which they had previously received the electric footshock. The present study was designed to investigate the functional change of opioid receptor subtypes in the conditioned suppression group using an opioid binding assay technique. In the synaptic membrane of the conditioned suppression group, the binding capacities of [3H]naloxone at high and low affinity binding sites and of [3H]phencyclidine at high affinity binding site were significantly increased compared to those of the control group. On the other hand, the binding capacity of [3H]ethylketocyclazocine at both affinity binding sites in the conditioned suppression group was not changed. These results suggest that the binding function of different opioid receptor subtypes may be altered differently by stress.

Opiate receptor interaction of a new analgesic drug 2-piperidinoethyl dibenzylglycolate

The pharmacological properties of a new analgesic drug, 2-piperidinoethyl dibenzylglycolate (PDG), have been demonstrated by classical tests. The technique of iontophoresis was used in order to compare the effects of PDG with those of Tyr-D-Ser-Gly-Phe-Leu-Thr (DSTLE), syndyphalin, morphine and naloxone (NAL) on hypothalamic neurones. PDG as other four substances evoked only inhibitory responses. Some neurones, on which were tested three substances, were sensitive to one, two or three of these substances. The differential responses so obtained suggested that PDG does not act on mu- and delta-receptors but on an unidentified receptor for which morphine and NAL have a high affinity as agonist. Structural requirements for activity on different receptors were also proposed on the basis of crystallographic data and the above results.

Nalbuphine

Nalbuphine is a potent analgesic with a low side effect and dependence profile in animals and man. Nalbuphine is distinguished from other agonist/antagonist analgesics in having greater antagonist activity and fewer behavioral effects at analgesic doses than pentazocine, butorphanol or buprenorphine. At equi-analgesic doses, nalbuphine is quantitatively similar to nalorphine in regard to its large ratio of antagonist to analgetic activity. Clinical studies have confirmed this balance of strong antagonist to analgesic activity. Nalbuphine has been shown to effectively antagonize the respiratory depressant activity of narcotic analgesics while concomitantly adding to their analgetic responses. Unlike nalorphine or pentazocine, nalbuphine produces few overt behavioral or autonomic effects in animals at doses over 300 times its analgesic range. These findings are confirmed by clinical results which show that nalbuphine produces few psychotomimetic effects, even at elevated dose levels, in contrast to nalorphine or pentazocine. Nalbuphine produces limited respiratory depression in animals and in man. Significant cardiovascular effects have not been found. Nalbuphine was found to produce significantly less inhibition of gastrointestinal activity than any of the clinically useful narcotic or agonist/antagonist analgesics tested in animals. Nalbuphine's analgetic effects are reversed by naloxone doses similar to those which reverse nalorphine's agonist effects. Results in this and other tests suggest that nalbuphine is primarily a kappa-agonist/mu-antagonist analgesic. Unlike pentazocine or buprenorphine, nalbuphine does not suppress the narcotic abstinence syndrome in partly-withdrawn morphine-dependent animals or man. Rather, due to nalbuphine's strong antagonist activity, analgesic-range doses of nalbuphine severely exacerbate the withdrawal syndrome in partly-withdrawn mice, monkeys and humans. Nalbuphine also precipitates a strong abstinence response in non-withdrawn morphine-dependent animals and man. In post-addict humans, analgesic-range doses of nalbuphine are perceived as minimally morphine-like, but higher doses are judged to be progressively more nalorphine-like (i.e. dysphoric), which further limits nalbuphine's abuse potential in drug-seeking individuals. Primary dependence studies have demonstrated that physical dependence is possible at high dose levels that produce marked side effects. Other studies show that dependence is unlikely to be of significance within nalbuphine's usual analgesic range. Six-month studies in patients with chronic pain have confirmed that analgesic tolerance or physical dependence is uncommon.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug discrimination studies

Opioid agonists and agonist/antagonists comprise a heterogeneous body of compounds that can be partitioned into at least three groups on the basis of their discriminative stimulus properties in several animal species: (1) stimulus effects similar to those of morphine or fentanyl and blocked completely by low doses of antagonists, such as naloxone and naltrexone; (2) stimulus effects similar to those of ethylketocyclazocine or nalorphine and blocked by higher doses of antagonists; (3) stimulus effects similar to those N-allylnormetazocine or phencyclidine and not blocked by antagonists. This diversity of stimulus properties is consistent with other evidence that multiple populations of receptors mediate the actions of opioids. In man, drugs in group 1 produce subjective effects that are entirely morphine-like and highly reinforcing whereas drugs in groups 2 and 3 produce dysphoric and psychotomimetic subjective effects. Thus, discriminative stimulus properties of opioids appear to reflect drug actions at the neuronal level that are directly relevant to potential for abuse in man.

Purification and characterization of the mu opiate receptor from rat brain using affinity chromatography

Opiate receptors have been solubilized from rat neural membranes and purified 500-fold (relative to the crude solubilized extract) by affinity chromatography. Active receptors were solubilized by using 3-[( 3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), a zwitterionic derivative of cholic acid. Affinity chromatography was carried out using Affi-Gel 401, a sulfhydryl derivative of agarose to which "hybromet," a newly synthesized opioid ligand with high affinity for the mu receptor, had been attached. Scatchard analysis of [3H]etorphine binding to the purified receptor revealed a single class of high-affinity sites (Kd = 1.4 nM; Bmax = 2800 fmol/mg of protein). Half-maximal binding was achieved at approximately equal to 1 nM. Activity was markedly inhibited by protein modifying reagents, findings which suggest that the sites are proteinaceous. Opiate binding activity was also inhibited by the guanyl nucleotide GTP. Electrophoresis of the purified material under denaturing conditions revealed three subunits of molecular weights 94,000, 44,000, and 35,000. The inhibitory guanyl nucleotide binding protein (Ni) implicated in opiate action has been shown to be comprised of two subunits of molecular weights 42,000 and 35,000. Thus, the opiate receptor may be an aggregate of multiple protein components that may include a guanyl nucleotide binding protein.

Highly selective photoaffinity labeling of mu and delta opioid receptors

We report the synthesis and photolabeling properties of two highly selective ligands for mu and delta opioid-binding sites: Tyr-D-Ala-Gly-MePhe (pN3)-Gly-ol (AZ-DAMGE) and Tyr-D-Thr-Gly-Phe (pN3)-Leu-Thr (AZ-DTLET). An irreversible inhibition of the electrically induced contractions of mouse vas deferens is caused by irradiation (at 254 nm) of the muscle strip in the presence of AZ-DTLET (1 nM). This phenomenon is antagonized only at large concentrations (10 microM) of naloxone, in accordance with the well-known lower selectivity of naloxone for delta sites. Competition experiments with [3H]DAMGE and [3H]DTLET on crude rat brain membranes showed that the azido photoprobes display a similar (AZ-DAMGE) and even a better (AZ-DTLET) selectivity than their respective parent compounds DAMGE and DTLET. Up to 25 nM, AZ-DTLET irreversibly and selectively photolabels the delta sites of crude rat brain homogenates. Due to its lower affinity AZ-DAMGE provides similar selective photolabeling of the mu sites but at higher concentrations (approximately equal to 0.3 microM). When [3H]DAMGE and [3H]DTLET were used as ligands for mu and delta binding subtypes, respectively, no important change in binding capacity and affinity of one receptor type was observed after photolabeling of the other.

Opioid control of the ruminant stomach motility: functional importance of mu, kappa and delta receptors

In sheep, the subcutaneous (SC) or intracerebroventricular (ICV) administration of the mu-type opioid agonists, fentanyl and morphine, evokes a blockade of the cyclic contractions of the reticulum. A similar inhibition of forestomach motility was recorded following the administration of the two enkephalin analogs, D-Ala2-Met5-enkephalinamide (DAMA) and D-Ala2-D-Leu5-enkephalin (DADLE) which are mixed mu - delta opioid agonists. In contrast, the reticular contractions were enhanced by the SC or ICV administration of the kappa type agonist, ethylketazocine (EKC) and U - 50 488 H. The proximal duodenum motor activity was transiently increased resulting in the occurrence of a phase III-like activity by these opioid agonists, regardless of the subtypes. The effects of the opioid agonists on reticular motility were prevented by the injection of naloxone but not by the quaternary parent compound methylnaloxone which does not cross the blood-brain barrier. The duodenal motor effects elicited by the opioid agonists were antagonized by both naloxone and methylnaloxone. The results suggest that the inhibition of the ruminant stomach motility is centrally mediated by mu - delta type opioid agonists and are consistent with opposite effects from kappa type opioid agonists. The stimulatory effect of peptide and non-peptide opioid agonists on the duodenum may result in part from direct opioid receptor-mediated actions on smooth muscle.

Opiate receptor supersensitivity produced by chronic naloxone treatment: dissociation of morphine-induced antinociception and conditioned taste aversion

In three separate experiments, rats were used to assess the effects of chronic administration of naloxone on specific binding of 3H-naloxone in various regions of the central nervous system (CNS) and on the efficacy of morphine to produce antinociception and a conditioned taste aversion. Chronic naloxone treatment increased opiate binding in medulla-pons, midbrain, hypothalamus, hippocampus, striatum, and prefrontal cortex, but not in either spinal cord or cerebellum. In those CNS regions exhibiting increased opiate binding, the duration of increased binding following termination of the naloxone treatment differed between regions. In conjunction with the increase in opiate binding, the efficacy of morphine to produce antinociception was potentiated, while the efficacy to produce a conditioned taste aversion was unchanged. Moreover, the administration of naloxone during behavioral testing blocked completely the antinociceptive effect, but not the aversive effect, of morphine. These results indicate that morphine-induced antinociception and conditioned taste aversion may be dissociated neuropharmacologically.

Attenuation of pharmacological effects and increased metabolism of phencyclidine in morphine tolerant mice

Mice were used for a study of the interaction between morphine and phencyclidine (PCP) in relation to lethality, motor incoordination, locomotor activity and rearing, together with the half-life of PCP, following continuous administration of morphine by pellet (75 mg base) implantation for 72 h and after removal of the pellets for 6 and 24 h. PCP induced motor incoordination and suppressed locomotor activity and rearing; these effects were enhanced in morphine 'pellet-implanted' mice and were attenuated in morphine 'pellet-removed' groups. The enhancing effect of morphine on the PCP responses was attributable more to the presence of residual morphine than to the alterations in its disposition. The morphine-induced increase in locomotor activity and analgesia was attenuated in PCP (40 mg/kg per day i.p. for 5 days) tolerant mice. The rate of decay of PCP in serum and brain or morphine pellet-implanted animals was not different; however, in the 24 h 'pellet-removed' group, the rate of decay of PCP was increased. The results indicate that there is a two-way cross-tolerance development between PCP and morphine. The phenomenon appears to involve both dispositional and functional adaptation mechanisms.

Naloxone in experimental spinal cord ischemia: dose-response studies

Temporary aortic occlusion produces a consistent degree of spinal cord injury in the unanesthetized rabbit. This 'spinal stroke' model was utilized to examine the potential therapeutic effects of the opiate antagonist naloxone in central nervous system ischemia. Naloxone treatment resulted in dose-related enhancement of motor recovery; greatest functional recovery was observed in rabbits treated with a dose of 2 mg/kg per h. This dose compares well with the high doses of naloxone shown to have a beneficial effect in other experimental models of stroke and spinal injury. In contrast, clinical stroke studies, which have been largely unsuccessful, have utilized naloxone doses which are several orders of magnitude lower than those successfully employed in experimental models.

Food intake: opioid/purine interactions

The exogenous opioids butorphanol tartrate (BT) and ethylketocyclazocine (EKC) have been reported to stimulate feeding in rats. In this study we evaluated the effects of purines (known to suppress feeding) and the adenosine antagonist, caffeine, on opioid induced feeding. Adenosine and inosine significantly suppressed BT and EKC induced feeding at various doses and time points. Caffeine enhanced food consumption was suppressed by various doses of naloxone, but was not suppressed by adenosine or inosine. Although caffeine itself induced further feeding, it did not enhance BT induced food consumption. Adenosine and inosine failed to suppress BT induced feeding when 12.5 mg/kg of caffeine was administered to the rats suggesting blockade of the adenosine receptor by caffeine. In contrast to 12.5 mg/kg caffeine, high dose caffeine (50 mg/kg) suppressed BT induced feeding over a 4 hour time period. Adenosine (50 mg/kg) and inosine (50 mg/kg) injected one hour after injection of BT and caffeine (50 mg/kg) reversed the suppressive effect of high dose caffeine in BT induced feeding. These studies indicate that opioid induced feeding can be suppressed by adenosine and inosine. Also, caffeine can reverse the suppressive effect of adenosine and inosine on feeding and vice versa. Naloxone's suppression of caffeine enhanced food consumption indicate that at least part of caffeine's effect on food intake may be mediated through an opioid mechanism.

Visualization of opiate receptor upregulation by light microscopy autoradiography

Light microscopy autoradiography has been used to visualize neuroanatomical patterns of brain opiate receptor upregulation in response to chronic naltrexone administration. Slide-mounted brain sections of frozen rat brain were labeled in vitro with dihydro[3H]morphine, a relatively selective mu opioid ligand. The greatest relative increases in opiate receptor density were observed in the nucleus accumbens, the amygdala, striatal patches, nuclei of the thalamus and hypothalamus, layers I and III of neocortex, substantia nigra compacta, midbrain periaqueductal gray regions, and the parabrachial nuclei of the brainstem. The substantia nigra reticulata, surrounding areas of striatal patches, and the locus ceruleus, were not affected by this drug treatment. These findings demonstrate that chronically administered naltrexone differentially regulates opiate receptors throughout the brain. In particular, three brain systems appear to be target areas of receptor upregulation : (i) the dopamine A9/A10 systems, (ii) the limbic system, and (iii) structures that receive input from afferent sensory pathways. Two possible mechanisms to account for this finding are (i) that the drug does not have uniform effects throughout the brain or (ii) that the receptors themselves may be associated with different functional systems. Receptor density changes are paralleled by increases in methionine-enkephalin content in the striatum, nucleus accumbens, periaqueductal gray, and hypothalamic areas of chronic naltrexone-treated rats relative to control rats. Thus opiate receptors and opioid peptides appear to be subject to regulatory mechanisms similar to those that modulate other neurotransmitters and their receptors. These results document in a visual manner brain patterns of opiate receptor upregulation .

Central cardiovascular actions of D-Ala2-Met5-enkephalinamide in the rat: effects of naloxone and nucleus reticularis gigantocellularis lesion

In pentobarbital-anesthetized rats, intraventricular administration of D-Ala2-Met5-enkephalinamide (D-Ala, 30, 100 or 300 nmol/kg) dose-dependently elicited a reduction in arterial pressure. This D-Ala-promoted hypotension was significantly antagonized by naloxone pretreatment (3 mg/kg, i.c.v.) and attenuated by bilateral focal nucleus reticularis gigantocellularis (NRGC) lesions. At higher doses, D-Ala also produced a delayed hypertension that was not only unaffected by naloxone or NRGC lesions, but was in fact potentiated by such pretreatments. We speculate that D-Ala may produce its cardiovascular effect by activating separate subclasses of opiate receptors, possibly at different neural substrates that include the NRGC.

Differential role of the opioid mu and delta receptors in the activation of prolactin (PRL) and growth hormone (GH) secretion by morphine in the male rat

Administration of naloxazone (50 mg/kg i.v.), an irreversible, selective and long acting antagonist of the mu 1 subclass of the opioid receptors, strongly reduced stimulation of PRL secretion by morphine (5.0 mg/kg i.v.) injected 24 hours later into conscious, unrestrained rats. In contrast, the effect of morphine on PRL release was unimpaired in rats treated 24 hours beforehand with either the reversible opioid antagonist naloxone (50 mg/kg i.v.), or the vehicle for naloxazone. A complete suppression of the PRL response to morphine (3.0 mg/kg i.v.) was observed in animals given intraventricular (IVT) injection of beta- funaltrexamine (beta-FNA, 2.5 micrograms), another selective, irreversible and long acting antagonist of the mu receptors, 24 hours beforehand. Neither naloxazone nor beta-FNA had any effect on the activation of GH secretion by morphine, which, however, was conspicuously reduced by ICI 154, 129, a preferential delta receptor antagonist, injected IVT (50 micrograms) 5 minutes before morphine. ICI 154, 129 had no effect on the PRL response to morphine. It is concluded that the PRL stimulating effect of morphine is mediated by the mu receptors, whereas activation of GH probably involves the delta sites.

Multiple opioid receptors in endotoxic shock: evidence for delta involvement and mu-delta interactions in vivo

The use of selective delta and mu opioid antagonists has provided evidence that delta opioid receptors within the brain mediate the endogenous opioid component of endotoxic shock hypotension. The selectivity of these delta and mu antagonists was demonstrated by their differing effects upon morphine analgesia and endotoxic hypotension. The mu antagonist beta-funaltrexamine, at doses that antagonized morphine analgesia, failed to alter shock, whereas the delta antagonist M 154,129: [N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH] (ICI) reversed shock at doses that failed to block morphine analgesia. Therefore, selective delta antagonists may have therapeutic value in reversing circulatory shock without altering the analgesic actions of endogenous or exogenous opioids. Additional data revealed that prior occupancy of mu binding sites by irreversible opioid antagonists may allosterically attenuate the actions of antagonists with selectivity for delta binding sites. For endogenous opioid systems, this observation provides an opportunity to link in vivo physiological responses with receptor-level biochemical interactions.

Differential effects of clonidine on pain, arterial blood pressure, and heart rate in the cat: lack of interactions with naloxone

In cats anesthetized with alpha-chloralose and urethane, intravertebral administration of clonidine (4 and 10 micrograms/kg) dose-dependently suppressed the jaw-opening reflex, arterial blood pressure, and heart rate. For a given dose, there was a differential degree of inhibition in the order of analgesia much greater than hypotension greater than bradycardia. Naloxone injections (0.4 and 1.0 mg/kg, i.vert.) essentially failed to antagonize these effects, suggesting the lack of involvement of the opiate receptors or endogenous opioids in these processes. Furthermore, pain suppression by clonidine appeared to be independent of the vasodepression and cardioinhibition it promoted. It is possible that neural mechanisms responsible for clonidine-induced antinociception, hypotension, and bradycardia are likely to have differential sensitivities to the imidazoline compound, regardless of whether they exist in separate central sites or in subpopulations of neurons within common neural substrates.

Derivatives of beta-casomorphins with high analgesic potency

Beta-casomorphin (5) Tyr-Pro-Phe-Pro-Gly, a partial sequence of bovine beta-casein with moderate opioid properties and mu-receptor affinity, was modified by substituting for the natural L-amino acids their D-analogs, and D-pipecolic acid, as well as by amidation of the C-terminal. Substitution of D-Pro or D-pipecolic acid for L-Pro4 considerably increased the analgesic action and the potency on guinea-pig ileum of beta-casomorphin (5) as well as of casomorphin [4] amide. The resulting D-Pro4 analogs Deprolorphin and Deproceptin which showed high analgesic potency after both intracerebroventricular and intravenous administrations. Also, the substitution of D-Phe for L-Phe3 enhanced, even though to a lesser degree, the antinociceptive action. Both naltrexone and naloxone completely blocked the effects in vivo and in vitro. The substitution of D-Pro for L-Pro2 abolished the opioid-like actions, while substituting D-pipecolic acid for L-Pro2 resulted in an increased analgesic effect of remarkably long duration. The correlation of analgesic action with the effects on isolated organs separates the L-Pro4-substituted derivatives and D-Phe3-CM(5) from the other modified casomorphins and morphine, indicating that the analgesic potency of the former was about ten times that of the latter group in the case of identical GPI-potency. This may involve different subpopulations of opiate mu-receptors.

Comparison of the effectiveness of different opioid peptides in suppressing heroin withdrawal

The effectiveness of beta-endorphin, dynorphin-(1-13), dynorphin-(1-10) amide, alpha-neoendorphin and [D-Ala2,D-Leu5]enkephalin in suppressing withdrawal in heroin addicts was compared in this study. Groups of six patients were stabilized overnight in the hospital and were treated with either saline or peptide when withdrawal symptoms began to appear the following morning. Withdrawal was scored before and after treatment by the patient himself and an independent observer. Peptides were administered in a bolus dose of 60 micrograms/kg body weight. The patient, the observer and the physician who administered the injection were all blind to the nature of the compound given. All treatments, including those with saline, produced an overall reduction of withdrawal score. However, by statistical analysis, only treatments with beta-endorphin, [D-Ala2,D-Leu5]enkephalin and dynorphin-(1-13) were effective in producing a significant decrease of withdrawal symptoms. The length of relief brought about by the different peptides varied from less than an hour to a maximum of 5 h in one case. The average period of relief brought about by beta-endorphin, dynorphin-(1-13) and [D-Ala2,D-Leu5]enkephalin was 44, 46 and 60 min, respectively. Of the five peptides administered [D-Ala2,D-Leu5]enkephalin produced the largest number of side-effects.