Activation Of Opiate Receptors Research Articles

Liquid—liquid extraction of membranes from calf brain using conventional and centrifugal counter-current distribution techniques

Neural membranes isolated from calf brain have been partitioned in aqueous two-phase systems containing dextran and polyethyleneglycol. When the partition was repeated several times, using counter-current distribution technique, the distribution of the membranes between the upper phase and the interface changed in a non-ideal manner and in favour of the interface. By using a centrifugal counter-current distribution device the time for the experiment could be reduced by a factor of 7–8 and the distribution was similar to what could be expected for ideally behaving membranes. The time-dependent change of the membranes is discussed in terms of aggregation and lateral membrane perturbations. Despite this effect a certain fractionation has been achieved as deduced from analysis of cholesterol content, opiate receptor activity and acetylcholinesterase activity along the counter-current distribution row of fractions. Compared to the starting material these activities were enriched some two-fold in certain fractions.

Central cardiovascular effects of κ agonists dynorphin-(1–13) and ethylketocyclazocine in the anaesthetized rat

Opiate κ agonists were administered into the cisterna magna of normotensive urethane-anaesthetized artificially ventilated rats: ethylketocyclazocine (78.7 nM) (EKC) and dynorphin-(1–13) (62.3 nM) produced significant and long-lasting decreases in both blood pressure and heart rate. High doses of naloxone (1 mg/kg i.v.) were required to partially antagonize these effects. The central cardiovascular effects of EKC and dynorphin-(1–13) were compared to those of fentanyl (3 nM), [D-Ala 2, Met 5]enkephalinamide (17 nM) and β-endorphin (2.9 nM) which induced increases in blood pressure and heart rate. These results suggest that opposite central cardiovascular effects could be induced by activation of various opiate receptors.

The disappearance of opioidergic regulation of gonadotropin secretion in postmenopausal women.

In postmenopausal women, the administration of beta-endorphin in repeated pulses (1-2.5 mg) at 1- to 2-h intervals or constant infusion (1 mg/h) for 3-6 h elicited the prompt release of PRL without concomitant change in LH levels. Infusion of an opiate receptor antagonist, naloxone (1.6 mg/h), for 6-7 h also had no effect on LH levels. Since substantial evidence indicates that endogneous opioid peptides exert an inhibitory role on GnRH-LH secretion and that the functional activity of opiate receptors appears to be ovarian steroid dependent, the present observation suggests that the hypersecretion of gonadotropin in the absence of ovarian steroid feedback may, in part, be causally related to a reduced opioid inhibition of GnRH-LH release.

Opposite central cardiovascular effects of various morphine-like drugs and opiate peptides in the rat

Opiate agonists were administered into the cisterna magna of normotensive urethane-anaesthetized artificially-ventilated rats. Increases in blood pressure and heart rate were produced by fentanyl (0.3-3.0 nmol). [D-ala]2-met-enkephalinamide (17 nmol), and beta-endorphin (2.9 nmol). On the contrary, ethylketocyclazocine (78.7 nmol) and dynorphin (1-13) (62.3 nmol) produced decreases in blood pressure and heart rate. The cardiovascular effects of ethylketocyclazocine and dynorphin (1-13) were antagonized by higher doses of naloxone than that required to counteract the effects of fentanyl. These results suggest that opposite central cardiovascular effects could be induced by activation of various opiate receptors.

A comparison of the effects of morphine, enkephalin, kyotorphin and D‐phenylalanine on rat central neurones

1 Morphine, Met-enkephalin, kyotorphin and D-phenylalanine have been applied by microiontophoresis to neurones in the globus pallidus and cerebral cortex of rats anaesthetized with urethane. 2 In the pallidum, most cells were inhibited by all the agonists, with a high correspondence between cells inhibited by Met-enkephalin and D-phenylalanine and by Met-enkephalin and kyotorphin. Whereas responses to Met-enkephalin were readily antagonized by naloxone, responses to kyotorphin and D-phenylalanine were not. 3 In the cerebral cortex a high proportion of cells was excited by all four agonists and antagonism by naloxone was less consistent than in pallidum. 4 It is concluded that the naloxone-reversible analgesic effects of kyotorphin and D-phenylalanine may be mediated indirectly, rather through an activation of opiate receptors.

Differential modification of opiate receptor activity by arylsulfatase treatment

Treatment with the enzyme arylsulfatase in vivo selectively attenuated the effect of analgesia induced by morphine, β-endorphin or ethylketocyclazocine but not that induced by Sandoz FK33824 or D-ala 2-D-leu 5-enkephalin. The effect on morphine analgesia was indicated both by an increased morphine ED 50 in the presence of a fixed dose of naloxone and by a decreased naloxone ED 50 in the presence of a fixed dose of morphine. Arylsulfatase treatment in vivo also selectively affected in vitro ligand binding; B max values of the low affinity binding site of dihydromorphine, naloxone, D-ala 2-D-leu 5-enkephalin, D-ala 2-met 5-enkephalinamide and ethylketocyclazocine were decreased significantly while the B max values of the high affinity sites as well as the K D values of both the high and low affinity sites were affected little or not at all. The data suggest that the change induced by the enzyme may have been due to the alteration of certain constituents of the low affinity opiate binding site.

Current perspectives in the pharmacopsychiatry of depression and mania

The observation that opiates and endorphins exert euphorogenic effects in normal probands points to a possible involvement of endorphins in different types of affective disorders. There are several powerful arguments that the activation of particular central opiate receptors (e.g. by “opium cure”, β-endorphin, partial agonists, release of endorphins via electroconvulsion) exerts curative effects in endogenous depression. Results from a double-blind investigation of the possible antidepressant action of the opiate partial agonist buprenorphine in patients with endogenous depression revealed a strong antidepressant effect of this substance. A series of anticonvulsants, possibly acting via a GABA-ergic-like mechanism (valproate, dipropylacetamide, carbamazepine and oxcarbazepine), have recently been shown by different groups to possess antimanic and also, partially, antidepressant properties. Furthermore, a synergistic mode of action in the prophylaxis of manic episodes has been observed as concerns valproate and lithium. On the other hand, there is some evidence from both in vitro and in vivo animal experiments that chronic application of lithium results in a modification of the GABA-turnover. The present paper reviews the present state of knowledge concerning the concept of a GABA-dependent regulation of affective states.

The effect of morphine on 5-hydroxytryptamine synthesis and metabolism in the striatum, and several discrete hypothalamic regions of the rat brain.

The effects of morphine on 5-hydroxytryptamine (5-HT) synthesis (accumulation of 5-hydroxytryptophan following inhibition of aromatic L-amino acid decarboxylase) and metabolism (concentration of 5-HT and its primary metabolite, 5-hydroxyindole-3-acetic acid [5-HIAA]) were determined in discrete nuclei of the rat brain using high performance liquid chromatography coupled with electrochemical detection (LCEC). Morphine (10 mg/kg, s.c.) increased 5-HT synthesis in the medial preoptic (MPO), suprachiasmatic (SCN) and arcuate (AN) nuclei as well as the striatum (ST) 1 hour following its administration. 5-HT synthesis in the median eminence (ME) was not affected at any time examined. A lower dose of morphine (5 mg/kg) also stimulated 5-HT synthesis in the AN. Although steady state concentrations of 5-HT were not greatly affected by morphine administration, the concentration of 5-HIAA in the AN, MPO, and ST increased following morphine (10 mg/kg, s.c., 1 hour). The increase in 5-HT synthesis observed in the MPO, SCN, AN, and ST 1 hour following morphine involved the activation of opiate receptors as administration of an opiate receptor antagonist, naloxone, blocked this effect. These results, indicate that morphine causes an increase in 5-HT synthesis and metabolism via an opiate receptor-mediated mechanism in the AN, MPO, SCN, and ST but not in the ME.

A Subclassification of Multiple Opiate Receptors by Means of Selective Tolerance Development

The chronic activation of opiate receptors results in the development of tolerance. On theoretical grounds, the appearance of cross-tolerance between different opioids should imply that these compounds exhibit an identical pattern on receptor activation. Tests with isolated tissue preparations, e.g. the guinea-pig ileum, made highly tolerant in vivo to the mu-agonist morphine proved that these were of almost unchanged sensitivity to different types of receptor agonists such as kappa-agonists. Thus, the ability to selectively induce tolerance on particular types of opiate receptors represents a reliable tool for the differentiation and characterization of multiple opiate receptors. Moreover, this technique facilitated the demonstration of subtypes of the already known opiate receptors.

Coffee contains potent opiate receptor binding activity.

Opiate receptor-active peptide fragments (exorphins) have been identified recently in casein and gluten hydrolysates, and morphine has been found in bovine and human milk. To determine whether similar peptides or alkaloids occur in other foodstuffs, we have screened potential sources using a rat brain homogenate assay to detect opiate receptor activity. We report here that instant coffee powders from a variety of manufacturers compete with tritiated naloxone for binding to opiate receptors in the rat brain membrane preparations, with no significant difference between normal and decaffeinated coffee. The receptor binding activity resembles that seen with opiate antagonists, in that there was no change in the half-maximal effective dose (ED50) in the presence of 100 mM Na+; on bioassay, the activity was similarly shown to be antagonistic and specific for opiate-induced inhibition of twitch. Preliminary characterization of the activity reveals that it has a molecular weight (MW) in the range 1,000-3,500, is heat-stable, ether-extractable, not modified by enzymatic digestion with papain, and clearly separable from caffeine and morphine on TLC. As its concentration in an average cup of coffee is five times the ED50, these data suggest that drinking coffee may be followed by effects mediated via opiate receptors, as well as effects of caffeine.

Photoaffinity probes for opiate receptors: Synthesis and properties of a nitro-azido-derivative of 14-β-aminomorphinone

The potential photoaffinity ligand 14-β-( o -nitro, p -azido)-cinnamoyl-amino-N-cyclopropylmethylnormorphinone (NAM) and its derivative NOM, lacking the p -azido function, were synthesised and their opiate receptor activity determined in isolated tissue preparations. The ligands showed slow receptor kinetics. NAM was a pure competitive antagonist of met 5-enkephalin responses in MVD while its antagonism of normorphine responses in GPI appeared non-competitive and non-reversible. In radioligand binding assays NOM completely and irreversibly blocked specific binding of 3-H-DHM. Partial blockade of 3H-DADL specific binding was reversible by washing. No binding of NOM to ▪ sites was observed. The slow receptor kinetics of NAM preclude its use as a photoaffinity ligand but suggest that a chemically more stable derivative may have a role as a pseudocovalent blocker of μ-receptors.

Opiates and classical conditioning: selective abolition of conditioned responses by activation of opiate receptors within the central nervous system.

It has previously been shown that opiates produce selective abolition of aversively motivated classically conditioned responses in the rabbit. The experiments reported here show that these effects are mediated by specific activation of opiate receptors within the central nervous system in that this central activation is both necessary and sufficient to produce opiate-induced abolition of conditioned responding. Further characterization suggests that selective activation through opiate-mu-receptor interactions within the periaqueductal gray/periventricular region of the fourth ventricle may be critical in mediating this abolition.

Opioid dependence and cross-dependence in the isolated guinea-pig ileum.

The development of opioid dependence and tolerance attributed to selective types of opiate receptors was studied in the isolated ileum of guinea pigs chronically exposed to specific opioids. These investigations were based on reports that in this preparation highly tolerant opiate receptors may coexist with opiate receptors of almost unchanged sensitivity. Thus, the ilea were set up in vitro and tested for tolerance and dependence. Apparently precipitation of the withdrawal contracture, indicating dependence, proved a more sensitive parameter than the phenomenon of tolerance. Maximal dependence was determined at rather low degrees of tolerance (5 to 10 fold). The intensity of the withdrawal contracture failed to increase as opiate tolerance did. Furthermore, the experiments failed to present evidence for the existence of selective dependence at specific opiate receptor types. These findings may suggest multiple adaptational mechanisms upon chronic activation of opiate receptors. One mechanism may be responsible for the development of dependence and a low degree of tolerance, whilst a further increase of tolerance may be associated with changes at the opiate binding site level.

The development of opiate tolerance may dissociate from dependence

Experiments on opiate sensitive peripheral tissue preparations such as the mouse vas deferens and the guinea-pig ileum have demonstrated the ability to induce very high degrees of selective tolerance towards particular opiate agonists. Interestingly, the highly tolerant mouse vas deferens failed to display any sign of dependence as judged by the inability of naloxone to precipitate a withdrawal sign. In analogy, the present studies on the guinea-pig ileum revealed a striking dissociation in the degree of tolerance and dependence developed. Opiate receptor binding studies on both tissues point to distinct differences in the opiate-induced effector mechanisms. It is concluded that adaptational changes upon chronic opiate receptor activation may occur at multiple sites within the effector system of the opiate receptor.

Effect of opiate receptor agonists on striatally-mediated head turning: An [formula omitted] model of opiate delta receptor activation?

Electrical stimulation of the striatum in conscious rats elicits a contralateral head-turn. Modification of this response by opiate drugs injected sub-cutaneously or into the ipsilateral globus pallidus has been investigated. Morphine, normorphine and ethylketocyclazocine had no effect on the head-turn latency but met-enkephalin and D-Ala 2, D-Leu 5-enkephalin, injected into the pallidum, and small doses of etorphine injected subcutaneously, produced a dose-related slowing of the response. Large doses of the opiate receptor antagonist naloxone antagonised the effect of etorphine. These results suggest the presence of opiate delta receptors in the rat globus pallidus and this model may therefore prove useful for studying the effects of other agents acting at the delta receptor. Following the work of Martin, Eades, Thompson, Huppler and Gilbert, 1976 and Lord, Waterfield, Hughes and Kosterlitz, 1977, it is now generally accepted that there are multiple opiate receptors which can be characterised by their response to a range of agonists and by the effectiveness of antagonists in reversing some of these actions. However, although mu, kappa and sigma opiate effects can be measured in vivo (Martin et al., 1976), there is no published model for studying effects at the opiate delta-receptor in vivo . In the search for such a model, a number of opiate agonist drugs were evaluated in the striatally-evoked head-turn model described by Slater, Lee, Longman and Crossman (1980). This model was thought to be particularly appropriate because of the proposed existence of a striopallidal enkephalin pathway in the rat brain (Cuello and Paxinos, 1978) and the demonstration of the release of enkephalin from slices of rat globus pallidus in vitro (Iversen, Iversen, Bloom, Vargo and Guilleman, 1978) and from cat globus pallidus in vivo (Cesselin, Soubrie, Bourgoin, Artaud, Reisine, Michelot, Glowinski and Hamon, 1981). However, in contrast to the findings reported by Slater and Longman (1980), the results of the present study demonstrate that a slowing of the head turn response was specifically produced by opiate delta-receptor agonist compounds and that this effect was abolished by large doses of naloxone.

Circulating opioids: Possible physiological roles in central nervous function

Evidence is reviewed regarding the release of endorphins by such diverse conditions as stress, long distance running, acupuncture, sexual activity, suggestion and ritualistic dancing ceremonies. Additional evidence is cited regarding possible physiological roles of endorphins in antinociception, socialization, euphoria, some mental disorders, drive states and vegetative functions. The concentration of this latter type of evidence is on conditions during which endorphins seem to be exerting effects on a number of different systems together (for example, euphoria is almost always accompanied by analgesia), and the possibility is suggested that the activation of a number of functions together may be due to a global activation of opiate receptors throughout the CNS. A possible basis for this global activation arises from results from this laboratory indicating the presence of a blood-borne opioid hormone, secreted by the pituitary or by an endocrine gland under pituitary control, which is capable of passing from the blood into the CNS. This diffuse endorphinergic system, which is complementary to the well-established endorphinergic neuronal systems in the CNS, thus derives its property of global action on opiate receptors by the diffuse means by which the hormone reaches its target sites, i.e., by passing through the blood brain barrier. Thus, while each specific endorphin-mediated function can be activated by the activation of its respective neural pathway, it is proposed that the hormonal endorphinergic mechanism is activated to produce a global response provoked by conditions to which a more generalized response, including physiological and behavioural changes, is most appropriate.

Kyotorphin (tyrosine-arginine): Further evidence for indirect opiate receptor activation

Analgesia, opiate receptor binding, and neurochemical effects of kyotorphin (tyrosine-arginine) were studied in the rat. It was found that while kyotorphin, in vivo , causes naloxone reversible analgesia, and affects dopamine metabolism and acetylcholine turnover in the same manner as do morphine and other opiate agents, the dipeptide does not bind to mu, delta or kappa opiate receptors in vitro . Taken together, these data support the concept that there is an indirect action of kyotorphin on opiate receptors.

Climacteric flushing: clinical and endocrine response to infusion of naloxone.

Six postmenopausal women with frequent attacks of flushing were studied by measuring plasma luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin and noradrenaline concentrations at regular and frequent intervals and at the time of each of 82 flushes. The hormone measurements were made on a control day and on the second day during infusion of either naloxone (22 micrograms/min) or saline. The perception of a flush was associated with a significant increase of plasma LH concentrations. There were no significant changes in plasma FSH, prolactin or noradrenaline concentrations. Naloxone infusion resulted in a highly significant reduction in the frequency of flushes and in the number of LH pulses. We conclude that flushing and its neuro-endocrine correlates are related to activation of opiate receptors. Naloxone may provide the basis for a non-steroidal treatment of climacteric flushing attacks.

Naloxone-lnduced Lethargy-Reply

Volavka and Jacquet raise some issues of interest. It is true that at the inception of our studies the working hypothesis was that an excess or deficiency in opiate receptor activity might be responsible for mania and depression, respectively. This hypothesis was helpful in the beginning, but it has proved to be much too simplistic to effectively explain the potential role the opioid peptides may play in the pathophysiology of affective disorder. Therefore, we have abandoned this hypothesis. As Volavka and Jacquet point out, were the original hypothesis to be correct, naloxone hydrochloride should accentuate depression and exogenous opiates or opioids should exacerbate mania. We have preliminary data that have bearing on this issue. We have studied eight acutely manic patients who were administered 10 mg of methadone hydrochloride intramuscularly in a crossover, double-blind study. Systematic observations of the patients' behavior before and after placebo or methadone administration revealed a

The effects of some dopamine antagonists on cholinergic mechanisms in the guinea-pig ileum.

1. Activation of prejunctional muscarinic and opiate receptors on cholinergic neurones of the guinea-pig ileum by acetylcholine (ACh) or morphine was seen as an inhibition of twitch responses to transmural electrical stimulation. These inhibitory responses were antagonised by metoclopramide and DL308-IT but no other antagonist used blocked the morphine responses apart from sulpiride in very high concentrations (100 muM and above). 2 Prejunctional and postjunctional responses to ACh on the guinea-pig ileum were measured in the presence of increasing concentrations of some muscarinic and dopamine antagonists. Atropine, hyoscine, chlorpromazine, cis-flupenthixol and thioridazine showed no selectivity in their antagonism of either response to ACh. Benzhexol selectivity antagonised the postjunctional response whereas metoclopramide and DL308-IT showed greater selectivity for the prejunctional effect which they antagonised competitively. Sulpiride, a compound chemically related to metoclopramide, blocked neither the prejunctional nor postjunctional ACh responses. Clozapine antagonised the postjunctional ACh responses whilst apparently potentiating the prejunctional inhibition by ACh. 3 It was concluded that pharmacological differences may exist between prejunctional and postjunctional muscarinic receptors for ACh and that the antipsychotic actions of dopamine antagonists are unlikely to involve blockade of prejunctional ACh or opiate receptors.