Multiple opioid receptors: a little about their history and some implications related to evolution

Differential tolerance to the antipentylenetetrazol activity of benzodiazepines in flurazepam-treated rats

Rats were treated for one week with flurazepam (FZP). After an additional two days with no treatment, each rat was injected with one of seven benzodiazepines (BZs). Several different doses of each BZ were evaluated. Ten min later, 100 mg/kg pentylenetetrazol (PTZ) was injected, IP, and convulsive activity was recorded. Rats treated for a week with FZP were tolerant to ataxia induced by each of the seven BZs tested. There was a dose-dependent anti-PTZ effect for each BZ. Whether or not tolerance to the anti-PTZ effect was found depended on the particular BZ used. Tolerance was found for four of the drugs: diazepam, clobazam, flurazepam and desalkylflurazepam. However, no tolerance was found to the anti-PTZ actions of midazolam, triazolam or clonazepam. Brain BZ levels were measured by the ability of brain extracts to displace specifically bound [3H]flunitrazepam in vitro. There was no significant effect of one week of flurazepam treatment. It was proposed that differences among BZs in their interactions with receptors allowed some to circumvent the mechanism responsible for tolerance to the anti-PTZ effect.

Opioid-induced respiratory depression and analgesia may be mediated by different subreceptors

Use of selective delta opioid antagonists provide evidence that the delta receptor within the brain seems an integrated part in the mediation of respiratory depression induced by a potent analgesic like fentanyl. Low doses of the delta antagonists RX-8008M (3-6 micrograms/kg) as well as ICI 174,864 (3-6 micrograms/kg) reversed fentanyl-related respiratory depression (arterial blood gases) in the unanesthetized canine. Opioid-induced blockade of afferent sensory nerve volleys (amplitude height of the somatosensory-evoked potential) could be reversed only by a high dose (9 micrograms/kg) of RX-8008M. Depression of amplitude height of the SEP could not be reversed by ICI 174,864 over the whole dose range (3-6-9 micrograms/kg). In comparison, naloxone (1-5-10 micrograms/kg) not only reversed depression of PaO2, it also reversed the blockade of afferent sensory nerve impulses in the low (5-micrograms/kg)-dose range. A highly selective delta antagonist may have a therapeutic value in reversing opioid-related respiratory depression, resulting in little or no attenuation of analgesia.

Phencyclidine receptors and N-methyl-D-aspartate antagonism: electrophysiologic data correlates with known behaviours

Using cortical wedges and isolated frog spinal cords, the potency of a series of psychoactive phencyclidine (PCP) and sigma receptor ligands as antagonists of N-methyl-D-aspartate (NMDA) has been compared with their potency in neurochemical and behavioural studies. Phencyclidine receptor, but not sigma or kappa, ligands were selective antagonists of NMDA on both preparations. Combination studies suggested that dissociative anaesthetics and sigma benzomorphans act at the same site. The relative potencies of the drugs as NMDA antagonists correlated well with their potency in PCP receptor binding studies in vitro and in PCP discrimination studies in vivo.

Pharmacological adjuncts in the treatment of opioid and cocaine addicts

Addiction consists of a complex neuropharmacologic behavioral cycle. The positive reinforcement of the drug and the negative reinforcement of withdrawal serve to drive the behavior of obtaining and ingesting the drug. The pharmacological adjuncts that are available today work by interfering with one or another part of the cycle. The alpha 2-adrenergic agonists, such as clonidine and guanabenz, act to block noradrenergic activity in the locus coeruleus and therefore block the negative reinforcement of opioid withdrawal. Naltrexone, on the other hand, works by preventing the positive reinforcement of administered opioids by preventing them from binding to the opioid receptor. In the case of cocaine addiction, most of the adjuncts currently in use focus on decreasing the severity of the immediate withdrawal symptoms. These agents potentiate dopaminergic transmission and in so doing tend to counter the dopamine depletion effect of prolonged cocaine use. Bromocriptine is the best known and probably the most effective member of this class. It may be that neuroleptics or naltrexone will prove to decrease reinforcement of cocaine use. However, the hazards of long-term neuroleptic use make it unlikely that they will be widely used for this purpose. Desipramine and perhaps other antidepressants may have a special role in treating cocaine addiction: They may prove to have some long-term prophylactic value and prevent relapse in recovering addicts. This ability may stem from the antidepressant action or it may involve a more specific action on dopaminergic transmission. These pharmacological agents may be very effective for certain purposes, such as relieving withdrawal symptoms. However, since they only act on one part of the addiction cycle, they can never be considered complete treatment by themselves. Of course, the use of all of these agents requires the voluntary cooperation of the patient. Therefore, the basis of the treatment of addictive illness continues to be the peer group and other types of interpersonal interactions. However, these pharmacological adjuncts may serve to make treatment easier, shorter and less expensive, and they may improve overall success rates. Consequently, they are of great value in the present and they can serve as models for the development of more effective agents in the future.

Identification and characterization of the opiate receptor in the ciliated protozoan, Tetrahymena

Tetrahymena, a ciliated protozoan, is a highly specialized, differentiated eukaryotic organism. It is known to possess many informational substances, including beta-endorphin (beta E). We wished to investigate the possibility that this organism possesses a functional opiate receptor which might be similar to the well-characterized opiate receptor in the rat brain. Binding assays using both living cells and membrane preparations, verified stereospecific, saturable, reversible 125I-beta E binding. This binding was displaceable by various opiates chosen to represent each of the putative opiate subtypes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of a disuccinimidyl suberate cross-linked 125I-beta E-receptor complex revealed a pattern of bands which consistently included bands at 110, 58-55, and 29 kDa. These bands, which were all displaceable by the classical antagonist, naloxone, as well as by other opiates, are thought to be prototypic for various opiate receptor subtypes. Limited proteolysis in SDS-PAGE showed that the 110 kDa band could be fragmented into 58-55 and 29 kDa bands and that the 58 kDa band could generate a 29 kDa fragment. The limited digest fragments of the 110, 58-55 doublet and 29 kDa bands were remarkably similar to those generated from the rat brain receptor. Analytical isoelectric focusing of digitonin solubilized 125I-beta E-receptor complexes showed the isoelectric points (pI) from both the rat and Tetrahymena were identical (pI 4.6). Chemotactic experiments with the intact Tetrahymena, demonstrated that these unicellular animals migrated toward a 10(-9) M beta E gradient. Chemotaxis was blocked by (-)-naloxone but not (+)-naloxone, suggesting a stereospecific opiate receptor-mediated response. We conclude that Tetrahymena possesses a functional opiate receptor (recognition molecule) very similar to the opiate receptor of the rat brain.

Lipids covalently bonded to pteridines and to nucleotides may bind neurotransmitters, and may be cleaved into second messengers for opiates

By hypothesis: 1.) lipid-pteridines and lipid-nucleotides, while fixed in the external side of membranes at the ends of nerve cells, bind neurotransmitters and perhaps opiates; 2.) during their normal metabolism in animals they are cleaved by phospholipases into pteridines and nucleotides which act as 2nd messengers for opiates. These 2nd messengers, some with tri-iodo-thyronine, act by sandwiching about circulating neurotransmitters; 3.) dinucleotides containing neopterin are specific 2nd messengers for steroids; and 4.) lipid-RNAs are the molecular locations in the brain which contain memories.

Anesthesiology in the 21st century: analgesic, sedative and anesthetic focusing

The specialty of anesthesiology is on the verge of a major technological evolution (revolution) which will utilize the computative and logic powers of the computer as well as the recent and rapid advances in magnetic imaging and laser and drug receptor technology to introduce the concept of anesthetic drug focusing by the beginning of the next century. This will result in the need for only a few molecules of future anesthetic compounds because they will only be necessary at a limited number of receptors in a few neurons or nuclei in the brain and/or spinal cord. Analgesic, sedative and anesthetic action may not be dependent on any drug but rather be produced solely by a wave form or energy source focused via a computer on the appropriate receptors in the brain, spinal cord or peripheral nerves. Anesthetic drug focusing will enable anesthetic depth to be ideal, anesthetic action to be localized to only those cells, tissues or organs where it is desired and anesthetic onset and termination to be measured in micro-seconds.

Effects of enkephalins, morphine, and naloxone on pial arteries during perivascular microapplication

The effects of the opiate receptor agonists, enkephalins and morphine, and the antagonist, naloxone, on cerebrovascular resistance vessels was investigated in situ by employing perivascular microapplication. Feline pial arteries with a resting diameter of 66-294 micron were tested. Vascular diameter was measured using television image splitting. Concentration-response curves revealed no change of diameter when Leu-enkephalin, D-Ala2-Leu-enkephalinamide, D-Ala2-Met-enkephalinamide, and morphine were applied in concentrations of 10(-11)-10(-5) M. Considering the concentrations of enkephalins that have been found in natural cerebrospinal fluid or that can be expected in the vicinity of enkephalinergic synapses, the data obtained with the lower concentrations indicate that enkephalins are probably not important for the regulation of pial arterial resistance. At 10(-4) M only the dilation (4.3%) elicited by D-Ala2-Leu-enkephalinamide was statistically significant (p less than 0.01). All four agonists at 10(-3) M induced significant dilatations varying between 5.4 and 13.6%. Naloxone exerted no vascular effect per se at 10(-5) and 10(-4) M but a dilatation of 15.3% at 10(-3) M. The latter can be explained by a partial agonist action. During simultaneous administration, naloxone (10(-4) M) reduced the dilatations induced by 10(-4) and 10(-3) M D-Ala2-Leu-enkephalinamide dose dependently. This indicates that mu- and delta-opioid receptors, probably located at the vascular smooth muscle cell, were involved in the mediation of the dilatation induced by the highest concentrations of the compounds.

Pharmacological advances in addiction treatment

In the past 20 years significant advances in the pharmacological treatment of opioid dependence have been made, and research in this area is continuing. Therapeutic applications and current research in the use of pharmacological agents in maintenance therapy, treatment with narcotic antagonists, and narcotic detoxification are discussed. In addition, an overview is presented of recent developments in opioid pharmacology and of recently developed novel pharmacological agents which may prove useful in the future treatment and/or prevention of opioid dependence.

(+)-[3H]SKF 10,047, (+)-[3H]ethylketocyclazocine, mu, kappa, delta and phencyclidine binding sites in guinea pig brain membranes

Binding of the opiates (+)-[3H]SKF 10,047 [N-allylnormetazocine; (+)-[3H]SKF] and (+)-[3H]ethylketocyclazocine [(+)-[3H]EKC] were compared to mu, kappa and delta and phencyclidine (PCP) receptor binding in guinea pig brain membranes. (+)-[3H]SKF and (+)-[3H]EKC binding were not blocked by naloxone, and had different drug selectivity compared to mu, kappa and delta binding sites. The number of binding sites, drug selectivity and region distribution in brain were similar for (+)-[3H]SKF and (+)-[3H]EKC. Sigma opiates that are associated with psychotomimetic activities, such as pentazocine, cyclazocine, SKF 10,047 and bremazocine, were potent inhibitors of (+)-[3H]SKF and (+)-[3H]EKC binding. Haloperidol was the most potent inhibitor of (+)-[3H]SKF binding. Haloperidol and sigma opiates demonstrated biphasic displacement of [3H]PCP binding, suggesting that [3H]PCP labelled two sites. PCP had a similar affinity for both (+)-[3H]SKF and [3H]PCP binding sites in the presence of 100 mM NaCl. The highest concentrations of (+)-[3H]SKF and (+)-[3H]EKC bindings sites were in the hypothalamus, anterior pituitary, midbrain, pons and medulla.

Exogenous and endogenous opioid-induced enhancements of naloxone's effects on serum luteinizing hormone levels in the male rat

We have previously shown that brief periods of exposure to opiate alkaloid drugs markedly enhance the subsequent effects of the opiate antagonist, naloxone, on serum luteinizing hormone (LH) levels in the male rat. In the present studies, we have found that this phenomenon is not simply a property of opiate drugs, but can be produced by a metabolically stable analog of an endogenously occurring opioid peptide, methionine enkephalin (FK 33-824). These findings suggest that alterations in the sensitivity of those opioid receptors involved in LH-releasing hormone (LHRH) generated in our experimental paradigm may occur under in vivo conditions, particularly since it now appears that endogenous opioids are released in an episodic manner like most neurotransmitter/neuromodulators. We also attempted to more fully characterize the factors responsible for the development of opiate-induced enhancements of naloxone's effects on LH. We found that this effect was produced only by those doses of morphine which initially suppressed serum LH levels, followed by a "rebound" increase in the gonadotropin 6-8 h later. A modest facilitation of LHRH-evoked increases in serum LH was also observed, but our data suggest that this represents only a minor component of opiate-induced enhancements of naloxone's effects. These data indicate that hypothalamic or suprahypothalamic sites are the major loci involved, but no differences in the uptake or regional distribution of naloxone in brain have been previously found, as a function of morphine pretreatment, nor were we able to demonstrate any alterations in opiate binding sites in the hypothalamus or whole brain. Thus, the mechanisms involved in this effect remain unclear.

Loss of sensitivity to morphine induced by prolonged ACTH treatment

The effect of long term ACTH treatment on some actions of morphine were studied. The effect of ACTH administration was compared to that induced by acute dexamethasone injection. ACTH caused a delayed inhibition of the morphine induced increase in growth hormone secretion demonstrable 24 hr after the last hormone injection. The morphine induced increase of striatal DOPAC (3,4-dihydroxyphenylacetic acid) content was also inhibited by ACTH treatment, however, neither the analgesia, nor the hypermotility caused by morphine were affected. Dexamethasone did not alter significantly the responsiveness to morphine. It is concluded that the prolonged exposure to ACTH presumably causes a corticosterone-mediated loss of responsiveness of functionally restricted opiate sensitive mechanisms in the central nervous system.

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.

The anticonvulsant effect of opioids and opioid peptides against maximal electroshock seizures in rats

Opioids and opioid peptides influence the threshold to a seizure which is a model of petit mal epilepsy (Cowan, Geller and Adler, 1979). The present authors investigated representative opioid compounds in a model of a grand mal seizure, maximal electroshock (MES). Although all of the opioids and opioid peptides tested blocked tonic hindlimb extension, they divided into two groups, based on their ability to decrease the total length of the tonic component of the maximal electroshock seizure and their sensitivity to blockade by naloxone. The first group contained morphine, meperidine, methadone, ethylketocyclazocine (EK), D-ala2-met-enkephalinamide, D-ala2-leu5-enkephalin and beta-endorphin. The compounds in this group caused a decrease in the length of the tonic component that was dose-related, with the maximum decrease amounting to approx. 40%. The effect was blocked by the prior administration of 1 mg/kg of naloxone. The second group contained the partial agonists, pentazocine and cyclazocine. These opioids also caused a dose-related decrease in the length of the tonic component and, in the largest doses, the tonic component of the convulsion was completely blocked. Naloxone, in doses as large as 10 mg/kg, did not appreciably reverse the action of either drug.

Methadone induced physical dependence in the rat

Although the morphine withdrawal syndrome has been well described in the rat, a syndrome having similar characteristics has not been demonstrated following chronic methadone treatment. In this study we describe the behavioral effects produced by naloxone (4 mg/kg sc) following 72 hours of continuous iv infusion of methadone, (12.2 micrograms/kg/min), morphine (12.2 to 97.9 micrograms/kg/min) or saline. The cessation of methadone or morphine but not saline treatment followed by naloxone resulted in graded signs including wet dog shakes, escape attempts, self-stimulation and body weight loss and quantal signs including diarrhea, ear blanching, exophthalmos, ptosis, tachypnea and teeth chattering. These results indicate that this mode of methadone administration produces physical dependence characterized by a morphine-like withdrawal syndrome in the rat.

Naloxone-inaccessible sigma receptor in rat central nervous system

It has been postulated that the psychotomimetic effects of opiates of the benzomorphan series are due to their activity at the sigma receptor. Therefore, the binding of (+/-)-[3H]ethylketocyclazocine ( [3H]EKC), a benzomorphan, to synaptosomal membranes of rat central nervous tissue was studied. Surprisingly, high concentrations of naloxone, a mu, delta, and kappa receptor antagonist, only inhibited about 80% of the specifically bound [3H]EKC in the spinal cord. This suggested that the remaining 20% of the binding sites were not mu, delta, or kappa. The Scatchard plot of the binding of [3H]EKC was nonlinear but became linear in the presence of naloxone (1 microM), suggesting a single class of naloxone-inaccessible receptor sites. This biochemically readily distinguishable receptor type bound the dextrorotatory isomer of EKC stereoselectively. The sigma agonist N-allylnormetazocine [(+)-SKF 10,047] stereoselectively competed with the binding of [3H]EKC to this naloxone-inaccessible binding site. A number of opiates that have psychotomimetic activity also competed for binding to this binding site. This binding site is designated as sigma binding site according to the nomenclature originally suggested by Martin et al. [Martin, W. R., Eades, C. G., Thompson, J. A., Huppler, R. E. & Gilbert, P. E. (1976) J. Pharmacol. Exp. Ther. 197, 517-532]. The drug selectivity and regional distribution of this sigma binding site in the rat central nervous system are different from that of the mu and delta opioid receptors and phencyclidine receptors. The concentration of the sigma binding site is highest in the spinal cord, pons and medulla, and cerebellum.

A novel opioid peptide, leumorphin, acts as an agonist at the kappa opiate receptor

The primary structure of the common precursor of porcine beta-neo-endorphin and dynorphin (preproenkephalin B) has shown the existence of a third leucine-enkephalin (leu-enkephalin) sequence with a C-terminal extension of 24 amino acids. This nonacosapeptide, named leumorphin, was approximately 70 times more potent than leu-enkephalin in inhibiting the contraction of the myenteric plexus-longitudinal muscle preparation of the guinea pig ileum. This action of leumorphin, like those of beta-neo-endorphin and dynorphin, was antagonized less effectively by naloxone than that of leu-enkephalin, but more effectively by Mr2266, an antagonist relatively specific for the kappa type opiate receptor. The inhibitory action of leumorphin or beta-neo-endorphin on the contraction of the guinea pig ileum muscle strip was reduced in a dose-dependent manner by pretreatment with dynorphin and vice versa. Leumorphin as well as beta-neo-endorphin and dynorphin inhibits the contraction of the rabbit vas deferens which is known to have only the kappa type opiate receptor. This action was also effectively antagonized by Mr2266. It is concluded that leumorphin has potent opioid activity and acts at the kappa receptor, like other opioid peptides derived from preproenkephalin B.

The dissociative anaesthetics, ketamine and phencyclidine, selectively reduce excitation of central mammalian neurones by N-methyl-aspartate

The interaction of two dissociative anaesthetics, ketamine and phencyclidine, with the responses of spinal neurones to the electrophoretic administration of amino acids and acetylcholine was studied in decerebrate or pentobarbitone-anaesthetized cats and rats. Both ketamine and phencyclidine selectively blocked excitation by N-methyl-aspartate (NMA) with little effect on excitation by quisqualate and kainate. Ketamine reduced responses to L-aspartate somewhat more than those of L-glutamate; the sensitivity of responses to these two putative transmitters was between that to NMA on one hand and that to quisqualate or kainate on the other. On Renshaw cells, ketamine and phencyclidine reduced responses to acetylcholine less than those to NMA but more than those to quisqualate or kainate. Dorsal root-evoked synaptic excitation of Renshaw cells was reduced to a greater extent than that following ventral root excitation. Intravenous ketamine, 2.5-20 mg/kg, and phencyclidine, 0.2-0.5 mg/kg, also selectively blocked excitation of neurones by NMA. Ketamine showed no consistent or selective effect on inhibition of spinal neurones by electrophoretically administered glycine or gamma-aminobutyricacid (GABA). The results suggest that reduction of synaptic excitation mediated via NMA receptors contributes to the anaesthetic/analgesic properties of these two dissociative anaesthetics.

Modification by GABA-ergic agents and clonidine of morphine-induced convulsions and toxicity in rats

Intracerebroventricular (i.c.v.) administration of morphine produced dose-dependent behavioural changes in rats, the highest dose producing violent tonic convulsions. Naloxone (up to 10 mg/kg i.p., or 100 micrograms i.c.v.) failed to reverse morphine-induced toxicity. The conventional anticonvulsant drugs also failed to offer protection to morphine-induced seizures. GABA-ergic substances, GABA, piracetam and diazepam, on the other hand, protected the animals against morphine-induced convulsions and toxicity. Similarly, clonidine pretreatment also had a protective action against morphine-induced convulsions.

Differential epileptogenic potentials of selective mu and delta opiate receptor agonists

By using electroencephalographic (EEG) and electromyographic recordings in anaesthetized and free-moving rats, two opioid peptides, known as selective agonists for mu and delta opiate receptors, respectively, were examined for their epileptogenic properties. The delta receptor peptide (DSTLE, 4.6-18.6 nmol, intraventricularly, ivt), a putative delta opiate agonist, produced a dose-related increase of myoclonic contractions (MC) with epileptic discharges in anaesthetized rats and severe wet dog shakes, with occasionally falling down, in free-moving animals. Morphiceptin, a specific mu opiate agonist, used in equimolar doses and under the same experimental conditions, had a significantly less pronounced effect on the number of MC and epileptiform EEG phenomena. Similarly, DSTLE (18.6 nmol) injected in the CA2 area of the hippocampus, a region with a nearly equal distribution of mu and delta opiate receptors, induced epileptic discharges in anaesthetized and free-moving rats, while an equimolar dose of morphiceptin had no significant effect. It is suggested that the epileptiform activity of opioid peptides is mainly due to an activation of delta opiate receptors in the central nervous system.

Comparison of analgesic potencies of mu, delta and kappa agonists locally applied to various CNS regions relevant to analgesia in rats

Analgesic potencies of relatively selective agonists for mu, delta and kappa subtypes of opioid receptors, morphine, [D-Ala2,D-Leu5]-enkephalin (DADL) and ethylketocyclazocine (EKC), respectively, were examined with the tail-pinch test in the rat, when microinjected into the brain stem regions relevant to analgesia such as the nucleus reticularis paragigantocellularis (NRPG), nucleus raphe magnus (NRM) and periaqueductal gray matter (PAG), and into the lumbar subarachnoid space (LSS). Morphine, DADL and EKC produced dose-dependent analgesic effects at the NRPG, NRM, PAG and LSS. The ED50 values indicated that morphine was more potent than DADL at the NRPG and LSS but less at the NRM and PAG. EKC was the weakest at all the injection sites. Further, naloxone (0.1 mg/kg, s.c.) significantly antagonized the analgesic effect of morphine but not that of DADL or EKC at the NRPG. These findings suggest that mu, delta and kappa subtypes of opioid receptors mediate analgesia, and that the extent of contribution of each subtype to the production of analgesia differs among the CNS sites examined in this study.

Ontogeny of benzomorphan-selective (kappa) sites: a computerized analysis

In an investigation of the postnatal development of kappa opiate receptors, the affinity and capacity of 0.5 nM [3H]-ethylketocyclazocine (EKC) binding in crude rat brain homogenates was measured by displacement with unlabeled EKC, morphine, or D-ala2-D-leu5-enkephalin (DADL). Displacement curves were analyzed using a weighted, non-linear regression, curve fitting computer program. At all stages of development, [3H]-EKC binding fit a two site model significantly better than a one site model. Affinities of EKC, morphine, or DADL for the high affinity [3H]-EKC binding site did not change during the postnatal period. The density of the high affinity [3H]-EKC binding site increased linearly with age, whereas the levels of the low affinity site rose more rapidly during the second week.

Interaction of synthetic opioid metenkephalin peptide analogs, Lilly 127623 and FK 33-824 with indole hallucinogens: antagonism of N,N-dimethyltryptamine- and LSD-induced disruption of food-rewarded bar pressing behavior in the rat

The selected opioid metenkephalin synthetic peptide analogs Lilly (LY) 127623 and FK 33-824 were tested for behavioral dose effects and potential interaction with N,N-dimethyltryptamine (DMT) and lysergic acid diethylamide-25 (LSD) in adult male Holtzman rats trained on a positive reinforcement fixed-ratio 4 (FR-4) behavioral bar pressing schedule, i.e., a reward of 0.01 ml sugar-sweetened evaporated milk was earned on every fourth bar press. DMT (3.2 mg/kg) and LSD (0.1 mg/kg), administered IP following a 0.9% NaCl 15-20-min control pretreatment, disrupted established food-rewarded FR-4 bar pressing in a consistent and reproducible manner. Animals pretreated IP with predetermined behaviorally noneffective doses of LY 127623 (0.01-0.32 mg/kg) and FK 33-824 (0.001-0.01 mg/kg) 15-20 min prior to receiving DMT demonstrated significant antagonism to DMT-induced disruption of FR-4 bar pressing, while doses of 0.10-0.32 mg/kg LY 127623 and 0.00032-0.0032 mg/kg FK 33-824 significantly antagonized LSD-induced behavioral effects.

Differential effects of naloxone and diprenorphine on defensive behaviour in rats

Two opiate antagonists, naloxone and diprenorphine, were used to examine the proposed involvement of endogenous opioid mechanisms in the modulation of shock-induced defensive behaviour patterns in rats. Naloxone was found to exert a biphasic influence on defensive fighting, with small dose (0.1 mg/kg) facilitation and large dose (10 mg/kg) inhibition. This compound also induced a dose-dependent inhibitory effect on shock-elicited threat whilst facilitated escape/avoidance behaviour was observed with the largest doses only. None of these influences could be attributed to alterations in electric shock thresholds. In contrast to the effects of naloxone, diprenorphine (0.1-10 mg/kg) was largely ineffective in modifying shock-induced defensive behaviour. These data suggest that endogenous opioid mechanisms may not be involved in the effects of naloxone on defensive fighting and cast some doubt upon the general hypothesis of the involvement of endorphins in the regulation of innate defence patterns.

A comparison of the receptor constants of morphine and ethylketocyclazocine for analgesia and inhibition of gastrointestinal transit in the rat

The efficacies and dissociation constants of proposed mu and kappa receptor agonists (morphine and ethylketocyclazocine, respectively) were compared using the method of partial irreversible blockade (with buprenorphine) and Stephenson's theory of drug action. While there was good agreement between the dissociation constant (KA) of morphine in analgesia (3.3 x 10(-5) M) and in inhibition of gastrointestinal transit (1.1 x 10(-5) M), the KA of ethylketocyclazocine differed by an order of magnitude in these endpoints (3.2 x 10(-6) M and 6.7 x 10(-5) M, respectively). The efficacies of morphine were found to be similar for the two effects studied (4.23 and 5.26), while those for ethylketocyclazocine differed markedly (2.06 and 10.39). The fraction of receptors remaining unblocked after buprenorphine was consistent for the test but not for the agonist, indicating a different distribution of receptors for the two endpoints. Our results strongly suggest that morphine induces analgesia, and slows transit in the small intestine, through the same type of receptor. The same conclusion cannot be drawn for ethylketocyclazocine.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

The effects of stress on central nervous system concentrations of the opioid peptide, dynorphin

Dynorphin is an opioid peptide distributed throughout the central nervous system. Using a highly specific and sensitive radioimmunoassay for dynorphin we have examined the effect of stress on ir-dynorphin levels in the cortex and hypothalamus of the rat. Stresses related to food ingestion, i.e. starvation (72 hr), mild tail-pinch and insulin (10 U/kg) induced hypoglycemia all produced alterations in ir-dynorphin levels in the cortex. In contrast, restraint stress and 10-minute swim stress produced no changes in ir-dynorphin levels in either the hypothalamus or the cortex. Two hour exposure at 4 degrees C resulted in a fall in ir-dynorphin levels in the hypothalamus. Taken together with previously reported pharmacological effects of dynorphin-(1-13), these results suggest a possible physiological role for dynorphin in appetite and temperature regulation.

Effects of systemic naloxone upon ventrobasal thalamus neuronal responses in arthritic rats

This study deals with the effect of various doses of systemic naloxone (10 microgram, 300 microgram, 1 mg/kg) upon activities of 21 ventrobasa thalamus neurons recorded in 20 rats rendered arthritic by injection of Freund's adjuvant into the tail. These neurons presented reproducible responses to movement and/or mild lateral pressure on a joint and were recorded for at least 30 min after naloxone administration. Several neurons (5) were tested with two doses. After intravenous injection of naloxone at the dose of 10 microgram/kg (10 cases) there was a rapid decrease of the responses. The maximum effect occurred at 15 min when the mean value expressed as a percentage of the control was 46.20 +/- 8.51% (n = 10, P less than 0.001). Recovery could be considered as complete at 30 min. At the dose of 300 microgram/kg (9 cases), the decrease in the responses was less important, variable from one neuron to another but significant between 5 and 20 min (mean = 67.43 +/- 9.00% at 20 min, n = 7, P less than 0.01). At the dose of 1 mg/kg (7 cases), there was no significant modification of the response. Spontaneous firing rate of the neurons was slightly but significantly increased after injection of the two highest doses and unmodified after the lowest. The relationship between the depressive effect produced by low doses of naloxone upon the neuronal responses, and the 'bi-directional' analgesic-hyperalgesic action of the drug, demonstrated in these suffering rats, is discussed.