In vivo opiate receptor binding of oripavines to mu, delta and kappa sites in rat brain as determined by an ex vivo labeling method

Quantitative Assessment of Pulmonary Targeting of Inhaled Corticosteroids Using Ex Vivo Receptor Binding Studies

The goal of locally acting inhaled corticosteroids is to achieve distinct pulmonary effects with reduced systemic side effects. The present work using an ex vivo receptor binding model in rats was interested in assessing pulmonary targeting for several commercially available corticosteroids by monitoring receptor occupancies in the lung and systemic organs (liver, kidney, spleen, and brain) after intravenous (IV) injection or intratracheal (IT) instillation of a dry powder administration at a dose of 100 μg/kg. Pulmonary targeting, defined as the difference in cumulative receptor occupancies (AUC_E) between the lung and kidney after pulmonary delivery, differed across the investigated corticosteroids (ΔAUC_E range, 33 ± 46 to 143 ± 52% *h) with the highest degree found for corticosteroids with high systemic clearance and pronounced lipophilicity (presumably allowing a long pulmonary residence time). Additionally, this study demonstrated differences in the receptor occupancies across systemic organs. Using kidney receptor occupancies as the comparator, liver receptor occupancies were reduced (ΔAUC_E range: - 157 ± 43 to 178 ± 42% *h) after IV and IT administration for corticosteroids with high intrinsic clearance, while they were increased for corticosteroid prodrugs due to hepatic activation. Spleen receptor occupancies were increased after IT (ΔAUC_E range: 33 ± 35 to 135 ± 28% *h), but not after IV administration. This was especially true for slowly dissolving drugs. Reduced brain uptake was also observed for ciclesonide (CIC) and des-ciclesonide (desCIC), two compounds previously not investigated. In summary, ex vivo receptor binding studies represent a powerful tool to assess the fate of ICSs.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

A non-rewarding, non-aversive buprenorphine/naltrexone combination attenuates drug-primed reinstatement to cocaine and morphine in rats in a conditioned place preference paradigm

Concurrent use of cocaine and heroin is a major public health issue with no effective relapse prevention treatment currently available. To this purpose, a combination of buprenorphine and naltrexone, a mixed very-low efficacy mu-opioid receptor agonist/kappa-opioid receptor antagonist/nociceptin receptor agonist, was investigated. The tail-withdrawal and the conditioned place preference (CPP) assays in adult Sprague Dawley rats were used to show that naltrexone dose-dependently blocked the mu-opioid receptor agonism of buprenorphine. Furthermore, in the CPP assay, a combination of 0.3 mg/kg buprenorphine and 3.0 mg/kg naltrexone was aversive. A combination of 0.3 mg/kg buprenorphine and 1.0 mg/kg naltrexone was neither rewarding nor aversive, but still possessed mu-opioid receptor antagonist properties. In the CPP extinction and reinstatement method, a combination of 0.3 mg/kg buprenorphine and 1.0 mg/kg naltrexone completely blocked drug-primed reinstatement in cocaine-conditioned rats (conditioned with 3 mg/kg cocaine, drug prime was 3 mg/kg cocaine) and attenuated drug-primed reinstatement in morphine-conditioned rats (conditioned with 5 mg/kg morphine, drug prime was 1.25 mg/kg morphine). These data add to the growing evidence that a buprenorphine/naltrexone combination may be protective against relapse in a polydrug abuse situation.

Buprenorphine-mediated transition from opioid agonist to antagonist treatment: state of the art and new perspectives

Constant refinement of opioid dependence (OD) therapies is a condition to promote treatment access and delivery. Among other applications, the partial opioid agonist buprenorphine has been studied to improve evidence-based interventions for the transfer of patients from opioid agonist to antagonist medications. This paper summarizes PubMed-searched clinical investigations and conference papers on the transition from methadone maintenance to buprenorphine and from buprenorphine to naltrexone, discussing challenges and advances. The majority of the 26 studies we examined were uncontrolled investigations. Many small clinical trials have demonstrated the feasibility of in- or outpatient transfer to buprenorphine from low to moderate methadone doses (up to 60-70 mg). Results on the conversion from higher methadone doses, on the other hand, indicate significant withdrawal discomfort, and need for ancillary medications and inpatient treatment. Tapering high methadone doses before the transfer to buprenorphine is not without discomfort and the risk of relapse. The transition buprenorphine-naltrexone has been explored in several pilot studies, and a number of treatment methods to reduce withdrawal intensity warrant further investigation, including the co-administration of buprenorphine and naltrexone. Outpatient transfer protocols using buprenorphine, and direct comparisons with other modalities of transitioning from opioid agonist to antagonist medications are limited. Given its potential salience, the information gathered should be used in larger clinical trials on short and long-term outcomes of opioid agonist-antagonist transition treatments. Future studies should also test new pharmacological mechanisms to help reduce physical dependence, and identify individualized approaches, including the use of pharmacogenetics and long-acting opioid agonist and antagonist formulations.

Analgesic efficacy of buprenorphine in the presence of high levels of SDF-1α/CXCL12 in the brain

Although morphine is often the best option for treating acute and chronic severe pain, its analgesic activity can be blocked in situations in which there are elevated levels of chemokines. Indeed, recently we have shown that elevated brain levels of the chemokine stromal cell-derived growth factor-1alpha (SDF-1α/CXCL12, the ligand of the HIV co-receptor CXCR4) diminish the antinociceptive effect of morphine. The purpose of the present study was to investigate whether such an effect is restricted to morphine or extends to other opioid medications such as buprenorphine. A sterilized stainless-steel C313G guide cannula was implanted into the periaqueductal grey (PAG), a brain region critical to the processing of pain signals, and a primary site of action of many analgesic compounds. The cold-water (-3°C) tail-flick test (CWT) was used to measure antinociception. Rats were pretreated with SDF-1α/CXCL12 administered into the PAG, and the antinociceptive actions of buprenorphine were measured. Direct infusion of SDF-1α/CXCL12 into the PAG failed to alter the antinociceptive action of buprenorphine. The presence of SDF-1α/CXCL12 in the PAG differentially alters the antinociceptive function of opioid medications. While it was able to diminish the antinociception induced by morphine (Adler et al., 2006), SDF-1α/CXCL12 did not affect the buprenorphine-induced antinociception. Buprenorphine appears to be more effective in the presence of high levels of SDF-1α/CXCL12 in the brain (which frequently occurs during neuroinflammatory conditions).

Buprenorphine is a weak partial agonist that inhibits opioid receptor desensitization

Buprenorphine is a weak partial agonist at mu-opioid receptors that is used for treatment of pain and addiction. Intracellular and whole-cell recordings were made from locus ceruleus neurons in rat brain slices to characterize the actions of buprenorphine. Acute application of buprenorphine caused a hyperpolarization that was prevented by previous treatment of slices with the irreversible opioid antagonist beta-chlornaltrexamine (beta-CNA) but was not reversed by a saturating concentration of naloxone. As expected for a partial agonist, subsaturating concentrations of buprenorphine decreased the [Met](5)enkephalin (ME)-induced hyperpolarization or outward current. When the ME-induced current was decreased below a critical value, desensitization and internalization of mu-opioid receptors was eliminated. The inhibition of desensitization by buprenorphine was not the result of previous desensitization, slow dissociation from the receptor, or elimination of receptor reserve. Treatment of slices with subsaturating concentrations of etorphine, methadone, oxymorphone, or beta-CNA also reduced the current induced by ME but did not block ME-induced desensitization. Treatment of animals with buprenorphine for 1 week resulted in the inhibition of the current induced by ME and a block of desensitization that was not different from the acute application of buprenorphine to brain slices. These observations show the unique characteristics of buprenorphine and further demonstrate the range of agonist-selective actions that are possible through G-protein-coupled receptors.

The role of the opioid receptor-like (ORL1) receptor in motor stimulatory and rewarding actions of buprenorphine and morphine

We have previously shown that the ability of buprenorphine to activate the opioid receptor-like (ORL1) receptor compromises its antinociceptive effect. Furthermore, morphine has been shown to alter the level of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the ORL1 receptor, raising the possibility that the endogenous OFQ/N/ORL1 receptor system may be involved in the actions of these opioids. Thus, using mice lacking the ORL1 receptor and their wild-type littermates, the present study assessed the role of the ORL1 receptor in psychomotor stimulant and rewarding actions of buprenorphine and morphine. Morphine (5, 10 mg/kg) dose-dependently increased motor activity and induced conditioned place preference. However, the magnitude of each response was comparable for the mutant mice and their wild-type littermates. In contrast, buprenorphine (1 mg/kg) induced greater motor stimulation in ORL1 receptor knockout mice as compared with their wild-type littermates. Further, single conditioning with buprenorphine (3 mg/kg) induced place preference in mutant mice but not in their wild-type littermates. The results of binding assay showed that buprenorphine concentration-dependently (0-1000 nM) displaced specific binding of [(3)H]-OFQ/N in brain membrane of wild-type mice. Together, the present results suggest that the ability of buprenorphine to interact with the ORL1 receptor modulates its acute motor stimulatory and rewarding effects.

Opioid Imaging

Many breakthrough scientific discoveries have been made using opioid imaging. Developments include the application of ever higher resolution whole-brain positron emission tomography (PET) scanners, the availability of several radioligands, the combination of PET with advanced structural imaging, advances in modeling macroparameters of PET ligand binding, and large-scale statistical analysis of imaging datasets. Suitable single-photon emission computed tomography (SPECT) tracers are lacking, but with the increase in the number of available PET (or PET/CT) cameras and cyclotrons thanks to the clinical successes of PET in oncology, PET may become widespread enough to overcome this. In the coming decade, there should be a more widespread application of the available techniques to patients and an impact in clinical medicine.

Low sensitivity of the positron emission tomography ligand [11C]diprenorphine to agonist opiates

Previously, we reported minimal opioid receptor occupancy following a clinical dose of the micro-opioid agonist, methadone, measured in vivo using positron emission tomography (PET) with [(11)C]diprenorphine and subsequently used rats to obtain experimental data in support of a high receptor reserve hypothesis (Melichar et al., 2005). Here, we report on further preclinical studies investigating opioid receptor occupancy with oxycodone (micro- and kappa-receptor agonist), morphine (micro-receptor agonist), and buprenorphine (partial agonist at the micro-receptor and antagonist at the delta- and kappa-receptors), each given at antinociceptive doses. In vivo binding of [(11)C]diprenorphine was not significantly reduced after treatment with the full agonists but was reduced by approximately 90% by buprenorphine. In addition, given that [(11)C]diprenorphine is a non-subtype-specific PET tracer, there was no regional variation that might feasibly be interpreted as due to differences in opioid subtype distribution. The data support minimal competition between the high-efficacy agonists and the non-subtype-selective antagonist radioligand and highlight the limitations of [(11)C]diprenorphine PET to monitor in vivo occupancy. Alternative means may be needed to address clinical issues regarding opioid receptor occupancy that are required to optimize treatment strategies.

Upregulation of opioid receptor binding following spontaneous epileptic seizures

Animal and limited human data suggest an important anticonvulsant role for opioid peptides and their receptors. We aimed to provide direct human in vivo evidence for changes in opioid receptor availability following spontaneous seizures. We scanned nine patients within hours of spontaneous temporal lobe seizures and compared their postictal binding of the non-subtype selective opioid receptor PET radioligand [11C]diprenorphine (DPN), quantified as a volume-of-distribution (VD), with interictal binding and with binding changes in 14 healthy controls, controlling for a range of behavioural variables associated with opioid action. A regionally specific increase of opioid receptor availability was evident in the temporal pole and fusiform gyrus ipsilateral to the seizure focus following seizures (Z 5.01, P < 0.001, 16 432 mm3). Within this region, there was a negative correlation between VD and log10 time since last seizure (r = -0.53, P < 0.03), compatible with an early increase and gradual return to baseline. [11C]DPN VD did not undergo systematic changes between time points in controls. This study provides direct human in vivo evidence for changes in opioid receptor availability over a time course of hours following spontaneous seizures, emphasizing an important role of the opioid system in seizure control.

Buprenorphine duration of action: mu-opioid receptor availability and pharmacokinetic and behavioral indices

BACKGROUND

Buprenorphine (BUP) is effective in the treatment of opioid dependence when given on alternating days, probably as a result of long-lasting occupation of micro opioid receptors (microORs). This study examined the duration of action of BUP at microORs and correlations with pharmacokinetic and pharmacodynamic outcomes in 10 heroin-dependent volunteers.

METHODS

Availability of microOR (measured with positron emission tomography and [(11)C]-carfentanil), plasma BUP concentration, opioid withdrawal symptoms, and blockade of hydromorphone (HYD; heroin-like agonist) effects were measured at 4, 28, 52, and 76 hours after omitting the 16 mg/d dose of BUP in a study reported elsewhere.

RESULTS

Relative to heroin-dependent volunteers maintained on BUP placebo, whole-brain microOR availability was 30%, 54%, 67%, and 82% at 4, 28, 52, and 76 hours after BUP. Regions of interest showed similar effects. Plasma concentrations of BUP were time dependent, as were withdrawal symptoms, carbon dioxide sensitivity and extent of HYD blockade. Availability of microOR was also correlated with BUP plasma concentration, withdrawal symptoms, and HYD blockade.

CONCLUSIONS

Together with our previous findings, it appears that microOR availability predicts changes in pharmacokinetic and pharmacodynamic measures and that about 50%-60% BUP occupancy is required for adequate withdrawal symptom suppression (in the absence of other opioids) and HYD blockade.

Opioid Imaging

Many breakthrough scientific discoveries have been made using opioid imaging, particularly in the fields of pain, addiction and epilepsy research. Recent developments include the application of ever higher resolution whole-brain positron emission tomography (PET) scanners, the availability of several radioligands, the combination of PET with advanced structural imaging, advances in modeling macroparameters of PET ligand binding, and large-scale statistical analysis of imaging datasets. Suitable single-photon emission computed tomography (SPECT) tracers are lacking, but with the increase in the number of available PET (or PET/CT) cameras and cyclotrons thanks to the clinical successes of PET in oncology, PET may become widespread enough to overcome this limitation. In the coming decade, we hope to see a more widespread application of the techniques developed in healthy volunteers to patients and more clinical impact of opioid imaging.

Reversibility of opioid receptor occupancy of buprenorphine in vivo

The slow association and incomplete dissociation of buprenorphine from opioid receptors observed in vitro have been suggested to reduce the accessibility of opioid receptors in vivo. If so, it might be expected that buprenorphine continues to occupy opioid receptors long after the antinociceptive activity has dissipated. To examine this hypothesis, buprenorphine (46.4 microg/kg i.v.) was administered to rats 1, 2, 4 or 8 h before isolation of their forebrain membranes and the maximal binding capacity (Bmax) for [3H]-[D-Ala2, N-methyl-Phe4-Gly5-ol]-enkephalin ([3H]DAMGO) was determined to measure the number of mu-opioid receptor binding sites remaining. Extent and duration of the reduction of Bmax by buprenorphine (ED50 11.2 microg/kg 1 h post-application) correlated with the antinociceptive activity in the rat tail flick (ED50 16.4 microg/kg i.v. 1 h post-application). At 8 h after administration there was still residual antinociception but no further attenuation of Bmax was detectable. Thus receptor occupancy by buprenorphine does not cause impairment of mu-opioid receptor accessibility beyond the duration of its antinociceptive activity. Therefore, no impairment of antinociception in the case of an opioid switch is to be expected.

Characterization of Opioid-Binding Sites in Zebrafish Brain

The pharmacological profile of opioid-binding sites in zebrafish brain homogenates has been studied using radiolabeled binding techniques. The nonselective antagonist [(3)H]diprenorphine binds with high affinity (K(D) = 0.27 +/- 0.08 nM and a B(max) = 212 +/- 14.3 fmol/mg protein), displaying two different binding sites with affinities of K(D1) = 0.08 +/- 0.02 nM and K(D2) = 17.8 +/- 9.18 nM. The nonselective agonist [(3)H]bremazocine also binds with high affinity to zebrafish brain membranes but only displays one single binding site with a K(D) = 1.1 +/- 0.09 nM and a B(max) = 705 +/- 19.3 fmol/mg protein. Competition binding assays using [(3)H]diprenorphine and several unlabeled ligands were performed. The synthetic selective agonists for mammalian opioid receptors DPDPE ([DPen(2),D-Pen(5)]-enkephalin), DAMGO ([D-Ala(2),NMe-Phe(4),Gly(5)-ol]-enkephalin), and U69,593 [(5alpha,7alpha,8beta)-(+)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide] failed to effectively displace [(3)H]diprenorphine binding, whereas nonselective ligands and the endogenous opioid peptides such as dynorphin A showed good affinities in the nanomolar range, although several of the endogenous peptides only displaced approximately 50% of the specifically bound [(3)H]diprenorphine. Our results provide evidence that, although the selective synthetic compounds for mammalian receptors do not fully recognize the opioid-binding sites in zebrafish brain, the activity of the endogenous zebrafish opioid system might not significantly differ from that displayed by the mammalian opioid system. Hence, the study of zebrafish opioid activity may contribute to an understanding of endogenous opioid systems in higher vertebrates.

Binding Profile of the Endogenous Novel Heptapeptide Met-Enkephalin-Gly-Tyr in Zebrafish and Rat Brain

Zebrafish is considered a model organism, not only for the study of the biological functions of vertebrates but also as a tool to analyze the effects of some drugs or toxic agents. Five opioid precursor genes homologous to the mammalian opioid propeptide genes have recently been identified; one of these, the zebrafish proenkephalin, codes a novel heptapeptide, the Met-enkephalin-Gly-Tyr (MEGY). To analyze the pharmacological properties of this novel ligand, we have labeled it with tritium ([(3)H]MEGY). In addition, we have also synthesized two analogs: (d-Ala(2))-MEGY (Y-d-Ala-GFMGY) and (d-Ala(2), Val(5))-MEGY (Y-d-Ala-GFVGY). The binding profile of these three agents has been studied in zebrafish and rat brain membranes. [(3)H]MEGY presents one binding site in zebrafish, as well as in rat brain membranes, although it shows a slight higher affinity in zebrafish brain. The observed saturable binding is displaced by naloxone, thus confirming the opioid nature of the binding sites. Competition binding assays indicate that the methionine residue is essential for high-affinity binding of MEGY and probably of other peptidic agonists in zebrafish, whereas the change of a Gly for a d-Ala does not dramatically affect the ligand affinity. Our results show that the percentage of [(3)H]MEGY displaced by all the ligands studied is higher than 100%, thus inferring that naloxone (used to determine nonspecific binding) does not bind to all the sites labeled by [(3)H]MEGY. Therefore, we can deduct that some of the MEGY binding sites should not be considered classical opioid sites.

Buprenorphine: considerations for pain management

New effective analgesics are needed for the treatment of pain. Buprenorphine, a partial mu-opioid agonist which has been in clinical use for over 25 years, has been found to be amenable to new formulation technology based on its physiochemical and pharmacological profile. Buprenorphine is marketed as parenteral, sublingual, and transdermal formulations. Unlike full mu-opioid agonists, at higher doses, buprenorphine's physiological and subjective effects, including euphoria, reach a plateau. This ceiling may limit the abuse potential and may result in a wider safety margin. Buprenorphine has been used for the treatment of acute and chronic pain, as a supplement to anesthesia, and for behavioral and psychiatric disorders including treatment for opioid addiction. Prolonged use of buprenorphine can result in physical dependence. However, withdrawal symptoms appear to be mild to moderate in intensity compared with those of full mu agonists. Overdoses have primarily involved buprenorphine taken in combination with other central nervous system depressants.

Broad analgesic profile of buprenorphine in rodent models of acute and chronic pain

Buprenorphine is a potent opioid analgesic clinically used to treat moderate to severe pain. The present study assessed its analgesic efficacy in a broad range of rodent models of acute and chronic pain. In the phenylquinone writhing, hot plate, and tail flick mouse models of acute pain, full analgesic efficacy was obtained (ED50 values: 0.0084-0.16 mg/kg i.v.). Full analgesic efficacy was also obtained in yeast- and formalin-induced inflammatory pain (ED50 values: 0.0024-0.025 mg/kg i.v., rats and mice) and in mustard-oil-induced spontaneous pain, referred allodynia, and referred hyperalgesia in mice (ED50 values: 0.018-0.025 mg/kg i.v.). Buprenorphine strongly inhibited mechanical and cold allodynia in mononeuropathic rats, as well as mechanical hyperalgesia and cold allodynia in polyneuropathic rats (ED50 values: 0.055 and 0.036 mg/kg i.v. and 0.129 and 0.038 mg/kg i.p., respectively). It is concluded that buprenorphine shows a broad analgesic profile and offers the opportunity to treat different pain conditions, including neuropathic pain.

Relative efficacy of buprenorphine, nalbuphine and morphine in opioid-treated rhesus monkeys discriminating naltrexone

Efficacy is one determinant of whether a drug is an agonist or an antagonist under a particular set of conditions. Relative efficacy among the micro opioid receptor (MOR) ligands buprenorphine, nalbuphine, and morphine was examined in monkeys dependent on morphine (3.2 mg/kg/day) or l-alpha-acetylmethadol (LAAM) (1.0 mg/kg twice daily) and that discriminated naltrexone (0.0178 mg/kg) from saline. In morphine-treated monkeys, buprenorphine and not nalbuphine substituted for naltrexone. When administered before naltrexone in morphine-treated monkeys, morphine and nalbuphine shifted the naltrexone dose-effect curve to the right, while buprenorphine shifted the naltrexone dose-effect curve to the left. Under conditions of acute morphine deprivation, naltrexone-lever responding was slightly attenuated by buprenorphine and markedly attenuated by nalbuphine and morphine. In LAAM-treated monkeys, buprenorphine substituted completely for naltrexone in only one monkey, while nalbuphine and morphine failed to substitute in any monkey. When administered before naltrexone in LAAM-treated monkeys, buprenorphine, nalbuphine, and morphine dose dependently shifted the naltrexone dose-effect curve to the right, with the exception of one monkey in which buprenorphine shifted the naltrexone dose-effect curve to the left. These results demonstrate that a low efficacy MOR ligand can exert agonist or antagonist actions in the same animal depending on immediate pharmacologic history. The qualitatively different effects of buprenorphine in morphine- and LAAM-treated monkeys might be related to magnitude of dependence insofar as dependence can determine the efficacy required for agonist activity. Thus, buprenorphine has markedly different effects across different levels of opioid dependence.

Buprenorphine: an analgesic with an expanding role in the treatment of opioid addiction

Buprenorphine, a long-acting opioid with both agonist and antagonist properties, binds to mu-opioid (OP(3)), kappa-opioid (OP(2)), delta-opioid (OP(1)), and nociceptin (ORL-1) receptors. Its actions at these receptors have not been completely characterized, although buprenorphine is generally regarded as a mu-opioid receptor partial agonist and a kappa-opioid receptor antagonist. Its pharmacology is further complicated by an active metabolite, norbuprenorphine. Although buprenorphine can be used as an analgesic agent, it is of greater importance in the treatment of opioid abuse. Because of its partial agonist activity at mu-opioid receptors and its long half-life, buprenorphine has proven to be an excellent alternative to methadone for either maintenance therapy or detoxification of the opioid addict. Although buprenorphine may ultimately prove to be superior to methadone in the maintenance of the pregnant addict, its effects on the developing fetus must be carefully evaluated.

Effects of perinatal buprenorphine and methadone exposures on striatal cholinergic ontogeny

The effects of exposure to various doses of buprenorphine, methadone or water during the perinatal period were studied on striatal cholinergic development in the rat. Rats were exposed to buprenorphine (0.3 or 3.0 mg/kg/day), methadone (9 mg/kg/day) and/or water prenatally, postnatally or both pre- and postnatally via maternally implanted osmotic minipumps. The effects of buprenorphine varied with the dose used. There were some similarities between the effects of perinatal buprenorphine and perinatal methadone, such as a reduction in striatal acetylcholine (ACh) content in 4-day-old pups exposed prenatally to methadone or buprenorphine (0.3 mg/kg/day). However, differences were also observed between the effects of the two drugs. Unlike methadone, the 0.3-mg/kg/day dose of buprenorphine produced a sex-related increase in striatal ACh in male postnatal day (PND) 21 pups. The 3-mg/kg/day dose of buprenorphine produced a completely different range of results, such as decreased striatal ACh content in 4-day-old pups exposed to the drug postnatally and in 21-day-old pups exposed both pre- and postnatally. Differences in the effects of the two drugs may be related to the different affinities and efficacies of the drugs at different opioid receptor subtypes.

Buprenorphine substitution ameliorates spontaneous withdrawal in fentanyl-dependent rat pups

Iatrogenic physical dependence has been documented in human infants infused i.v. with fentanyl or morphine to maintain continuous analgesia and sedation during extracorporeal membrane oxygenation and mechanical ventilation. Many infants are slowly weaned from the opioid. However, this approach requires extended hospital stays. Little is known about the potential benefits of substitution therapy to prevent abstinence. Therefore, the hypothesis was tested that s.c. and p.o. buprenorphine substitution would ameliorate spontaneous withdrawal in fentanyl-dependent rat pups. Analgesia in the tail-flick test was used to indicate behaviorally active doses of buprenorphine in opioid-naïve postnatal day 17 rats. Other postnatal day 14 rat pups were surgically implanted with osmotic minipumps that infused saline (1 microL/h) or fentanyl (60 microg/kg/h) for 72 h. Vehicle or buprenorphine was administered s.c. or p.o. before the initiation of spontaneous withdrawal brought about the removal of the osmotic minipumps. The major withdrawal signs of wet-dog shakes, jumping, wall climbing, forepaw tremor, and mastication were counted during a 3-h period of withdrawal. The major scored sign, scream on touch, was assessed every 15 min for 3 h. Injection of naloxone after the 3-h observation did not reveal any residual dependence. Subcutaneous buprenorphine administration significantly ameliorated all signs of withdrawal. Surprisingly, p.o. buprenorphine was nearly as efficacious as the s.c. route of administration. These results indicate that buprenorphine substitution therapy may be effective in fentanyl-dependent human infants.

Differential binding properties of oripavines at cloned mu- and delta-opioid receptors

This study examines the possibility that oripavine opioid receptor agonists bind equally to both high and low affinity states of the mu-opioid receptor. Studies were performed in C6 cells expressing mu- or delta-opioid receptors; high and low agonist affinity states of the receptors were defined by the absence and presence, respectively of Na+ ions and the GTP analog Gpp(NH)p. At the mu-opioid receptor dihydroetorphine and etorphine were full agonists, buprenorphine had moderate efficacy while diprenorphine was an antagonist. At the delta-opioid receptor, dihydroetorphine, etorphine, and diprenorphine had moderate efficacy while buprenorphine was an antagonist. The binding affinities of the oripavines at the mu-opioid receptor decreased only one to 2-fold in the presence of NaCl and Gpp(NH)p. In contrast, decreases in oripavine affinity at the delta-opioid receptor correlated with delta-opioid receptor efficacy. The ability of oripavine agonists to bind with high affinity to the low agonist affinity state of the nu-opioid receptor may explain the high potencies of these compounds in vivo.

Characterization of [3H]-diprenorphine binding in Rana pipiens: observations of filter binding enhanced by naltrexone

Initial studies were undertaken to examine the properties of [3H]-diprenorphine binding to Rana pipiens whole brain tissue using naltrexone for the definition of nonspecific binding. Saturation analysis demonstrated the binding of [3H]-diprenorphine to be saturable with a K(D) value of 0.65 nM and a Bmax value of 287.7 fmol/mg protein. Unlabeled diprenorphine dose-dependently displaced [3H]-diprenorphine from a single noninteractive site in competition studies which yielded a Ki of 0.22 nM. However, control studies in the absence of tissue revealed significant binding of [3H]-diprenorphine to the filter alone. Interestingly, [3H]-diprenorphine in the presence of unlabeled naltrexone as well as with unlabeled naloxone showed significantly greater binding to the filter than did [3H]-diprenorphine alone. Given this observation of increased nonspecific binding, an artificially low Bmax value would be expected. It is our hypothesis that the unlabeled nonspecific drug forms a complex with [3H]-diprenorphine preventing it from being effectively washed through the filter or the unlabeled drug itself is blocking the flow of [3H]-diprenorphine through the filter. The latter is unlikely however as other binding studies done in our lab using the radioligand [3H]-naloxone with unlabeled naltrexone do not show significant binding to the filter.

Detection of adenosine receptor antagonists in rat brain using a modified radioreceptor assay

The present study describes a modified radioreceptor binding assay using brain homogenate or serum from drug treated animals as the 'competing drug' in a conventional in vitro radioligand binding assay. Method validation involved measurement of the brain and serum concentration of three adenosine receptor antagonists following systemic administration, using a [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) binding assay. The intrinsic [3H]DPCPX binding capacity of test samples was abolished by protein denaturation (80 degrees C, 15 min) and, endogenous ligand was depleted enzymatically, prior to determination of drug concentration. Brain and serum concentrations of the adenosine A1 receptor antagonist, DPCPX increased in a dose related manner when measured 20 min after intraperitoneal injection. Estimated brain concentrations were 13.8, 87.7 and 288 nM following injection of 0.01, 0.1 and 1.0 mg/kg DPCPX, and serum concentrations were 26.5, 195 and 1370 nM respectively. A time dependent decrease in both brain and serum concentration was noted 20-180 min following injection of 1.0 mg/kg DPCPX. The peripheral adenosine receptor antagonists, 1,3-dipropyl-8-p-sulphophenylxanthine (DPSPX; 5.6 mg/kg) and 8-(p-sulphophenyl)theophylline (8-PST; 20 mg/kg), were not detected in brain tissue 20 min after intraperitoneal injection, despite serum concentrations of 56 and 52 microM respectively. This assay provides a useful and versatile method for determining the central penetration of neuroactive drugs.

Etorphines: mu-opioid receptor-selective antinociception and low physical dependence capacity

Comparative analgesic studies revealed that dihydroetorphine was more potent than etorphine in the tail-flick and hot-plate tests, respectively and nearly equipotent in the phenylquinone assay. Both compounds were short acting. Studies with selective opioid receptor antagonists beta-funaltrexamine, nor-binaltorphimine and naltrindole revealed that both etorphines were mu-selective agonists. Presumptive evidence for competitive antagonism of these compounds with naloxone was provided by Schild regressions with slopes of near unity. In a suppression test in rhesus monkeys maximally dependent on morphine, dihydroetorphine and etorphine dose-dependently replaced morphine. Drug-naive simians chronically exposed to frequent, intermittent and escalating doses of dihydroetorphine for 42 days showed few withdrawal signs when challenged with large doses of naloxone or were abruptly withdrawn from this drug. The results suggest that these atypical opioids may be useful in the clinical treatment of pain and opiate drug abuse.

Alterations in opioid receptor binding in Parkinson's disease patients with levodopa-induced dyskinesias

Levodopa-induced dyskinesias remain a major challenge in the therapeutic management of Parkinson's disease (PD). Their etiology is unknown although dysfunction of striatal opioid transmission has been implicated in experimental models of PD. To determine whether the opioid system is involved in human dyskinetic PD, we measured in vivo opioid receptor binding in PD patients with and without levodopa-induced dyskinesias, using positron emission tomography (PET) and the opioid receptor ligand [11C]diprenorphine. Striatal and thalamic/occipital uptake ratios were calculated using a region of interest (ROI) approach. In addition, we used statistical parametric mapping (SPM) and images reflecting the volume of distribution of [11C]diprenorphine to assess changes in cerebral receptor binding on a voxel-by-voxel basis. By using the ROI approach, we found significantly reduced striatal and thalamic opioid binding in dyskinetic, but not in nondyskinetic, PD patients. The SPM approach confirmed reduced availability in these areas and, in addition, showed decreased cingulate and increased prefrontal opioid receptor binding in the dyskinetic patients. Our findings confirm that altered opioid transmission is part of the pathophysiology of levodopa-induced dyskinesias in PD and support further investigation into the role of opioid agents in the management of these involuntary movements.

[Met]enkephalin in the spinal cord is involved in the antinociception induced by intracerebroventricularly-administered etorphine in the mouse

We have recently reported that the antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of both mu- and epsilon-opioid receptors and the activation of both monoaminergic and opioidergic descending pain control systems. [Xu J. Y. et al. (1992) J. Pharmac. exp. Ther. 263, 246-252]. Since the opioid epsilon-receptor-mediated antinociception induced by beta-endorphin is mediated by the release of [Met]enkephalin and subsequent stimulation of delta-opioid receptors in the spinal cord, the present studies were designed to determine if beta-endorphin-like action is also involved in etorphine-induced antinociception. The tail-flick test was used to assess the antinociceptive response performed in male ICR mice. Etorphine at doses from 5 to 20 ng given i.c.v. produced a dose-dependent inhibition of the tail-flick response. The inhibition of the tail-flick response induced by etorphine given i.c.v. was antagonized by intrathecal pretreatment for 60 min with antiserum against [Met]enkephalin (10 microg), but not with antiserum against [Leu]enkephalin (10 microg) or dynorphin A (1-13) (10 microg). Desensitization of delta-opioid receptors in the spinal cord by intrathecal pretreatment with [Met]enkephalin (5 microg) for 60 min attenuated i.c.v. administered etorphine-induced tail-flick inhibition. However, intrathecal pretreatment with [Leu]enkephalin (5 microg) or dynorphin A (1-17) (0.1 microg) for 60 min did not attenuate i.c.v. administered etorphine-induced tail-flick inhibition. The results indicate that antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of the epsilon-opioid receptor at the supraspinal sites and by the release of [Met]enkephalin, which subsequently stimulates delta-opioid receptors in the spinal cord.

Opioid receptor imaging and displacement studies with [6-O-[11C] methyl]buprenorphine in baboon brain

Buprenorphine (BPN) is a mixed opiate agonist-antagonist used as an analgesic and in the treatment of opiate addiction. We have used [6-O-[11C]methyl]buprenorphine ([11C]BPN) to measure the regional distribution in baboon brain, the test-retest stability of repeated studies in the same animal, the displacement of the labeled drug by naloxone in vivo, and the tissue distribution in mice. The regional distribution of radioactivity in baboon brain determined with PET was striatum > thalamus > cingulate gyrus > frontal cortex > parietal cortex > occipital cortex > cerebellum. This distribution corresponded to opiate receptor density and to previously published data (37). The tracer uptake in adult female baboons showed no significant variation in serial scans in the same baboon with no intervention in the same scanning session. HPLC analysis of baboon plasma showed the presence of labeled metabolites with 92% +/- 2.2% and 43% +/- 14.4% of the intact tracer remaining at 5 and 30 min, respectively. Naloxone, an opiate receptor antagonist, administered 30-40 min after tracer injection at a dose of 1.0 mg/kg i.v., reduced [11C]BPN binding in thalamus, striatum, cingulate gyrus, and frontal cortex to values 0.25 to 0.60 of that with no intervention. There were minimal (< 15%) effects on cerebellum. Naloxone treatment significantly reduced the slope of the Patlak plot in receptor-containing regions. These results demonstrate that [11C]BPN can be displaced by naloxone in vivo, and they affirm the feasibility of using this tracer and displacement methodology for short-term kinetics studies with PET. Mouse tissue distribution data were used to estimate the radiation dosimetry to humans. The critical organ was the small intestine, with a radiation dose estimate to humans of 117 nrad/mCi.

An open trial of low dose buprenorphine in treating methadone withdrawal

Buprenorphine (BPN) is a prescription analgesic with mixed opioid agonist and antagonist properties. This pilot study conducted detailed case studies with 15 methadone dependent patients. The study sought to determine whether repeated low doses (0.15 to 0.3 mg) of sublingual BPN would relieve opioid withdrawal symptoms. Subjects developed mild to moderate withdrawal symptoms within 26 to 31 hours of methadone discontinuation. Once in withdrawal, the subjects received 0.15 mg of BPN sublingually. A second dose of 0.15 mg was administered in an hour, and a 0.30 mg dose in 2 hours, if the subject obtained no relief of withdrawal symptoms. In 6 subjects a low dose of 0.15 to 0.30 mg sublingual BPN resulted in the disappearance of subjective and objective withdrawal symptoms within 10 minutes to 2.5 hours. Four others had brief, partial relief of symptoms. Five subjects failed to experience any relief of withdrawal symptoms after a total of 0.6 mg BPN administered over 3 hours. One nonresponder suffered what appeared to be a severe precipitated withdrawal reaction similar to that which can be produced in addicts by a naloxone challenge. The 4 Caucasian responders required 1 to 2 hours to respond to BPN, whereas the 2 African-American responders required only 10 to 20 minutes. Low (analgesic) doses of BPN were sufficient to treat all methadone withdrawal symptoms in 6 of 15 subjects. There may be ethnic differences in response to BPN. Low dose BPN may play a role in carefully monitored heroin detoxification treatment.

Investigation of the opioid system in absence seizures with positron emission tomography

The neuroanatomical and pathophysiological basis of primary generalised absences is uncertain. Administration of endogenous opioids has been shown to result in absence-like seizures in animal models. Positron emission tomography scans were performed in eight patients with primary generalised epilepsy and eight control subjects. Regional cerebral blood flow was measured interictally with C15O2, after which a 90 minute dynamic study with the opioid-receptor ligand 11C-diprenorphine was performed. Serial absences were precipitated by hyperventilation for 10 minutes, starting 30-40 minutes after injection of diprenorphine. Absences, with generalised spike-wave discharges on the EEG, occurred for between 10% and 51% of the provocation period. No individual (normal or patient) had any interictal focal abnormalities of cerebral blood flow. After provocation of serial absence seizures, there was increased diprenorphine elimination from the association cortex, but not from the thalamus, basal ganglia, or cerebellum, compared with control subjects and patients scanned without provocation of absences. It was possible to simulate the observed increased diprenorphine elimination following seizures in cerebral cortex using a two tissue compartment model, with an estimated 15-41% decrease in the specific tracer uptake rate constant (k3). These results suggest that endogenous opioids are released in the association cortex at the time of serial absences, lead to increased receptor occupancy, and may have an important role in the pathophysiology of generalised absences.

Binding of angiotensin antagonists to rat liver and brain membranes measured ex vivo

1. The effects of the angiotensin antagonists GR117289, losartan and Sar1Ala8-angiotensin II on the ex vivo binding of [125I]-Sar1Ile8-angiotensin II to rat liver and cortex/hippocampus (Cx/H) membranes have been investigated. 2. GR117289 (0.1-30 mg kg-1, s.c., 2 h pretreatment) caused a dose-dependent reduction in [125I]-Sar1Ile8-angiotensin II binding to both liver and cortex/hippocampus membranes. 3. Administration of a submaximal dose of GR117289 (1 mg kg-1, s.c.) indicated that the peak inhibition of binding in the liver occurred within 0.5 h, whereas the peak inhibition of binding in the Cx/H occurred 2 h after drug treatment. 4. The effect of GR117289 was long lasting. Binding was still reduced in the Cx/H 48 h after drug treatment (10 mg kg-1, s.c.) but had returned to normal 72 h after drug treatment. In the liver binding was still reduced 72 h after treatment with the same dose. 5. Losartan (1-30 mg kg-1, s.c.) was equipotent with GR117289 in its ability to reduce liver binding, but was less effective at inhibiting binding to central receptors. 6. The non-peptide antagonist Sar1Ala8-angiotensin II (3 and 10 mg kg-1) reduced binding in the liver but not in the Cx/H membranes. 7. These results suggest that, unlike the peptide antagonist Sar1Ala8-angiotensin II, the non-peptide angiotensin antagonists, GR117289 and losartan, are able to cross the blood brain barrier and occupy central angiotensin II receptors.

In vivo occupancy of histamine H3 receptors by thioperamide and (R)-alpha-methylhistamine measured using histamine turnover and an ex vivo labeling technique

In the brain, the H3 type of histamine receptor has a pre-synaptic autoreceptor inhibitory role which regulates neuronal release and synthesis of histamine. To examine the interaction of the selective H3 receptor antagonist thioperamide with H3 receptors in the brain in vivo, we have used a functional and non-functional measurement of H3 receptor occupancy. In three species (rat, guinea-pig and mouse) peripheral administration of thioperamide caused dose-related increases in histamine turnover in the cerebral cortex (whole brain was examined in the mouse) and, in the same tissues, inhibited the ex vivo binding of the selective H3 receptor agonist [3H](R)-alpha-methylhistamine ([3H]-RAMH). The peak effect of thioperamide to inhibit ex vivo binding of [3H]RAMH was observed approximately 30 min after i.p. administration, whilst the maximum increase in histamine turnover did not occur until after at least 100 min. At a pretreatment time of 30 min, the ED50 of thioperamide to inhibit ex vivo binding of [3H]RAMH binding in the rat, guinea-pig and mouse brain was found to be 2.0 +/- 0.2, 4.8 +/- 0.6 and 2.6 +/- 0.3 mg/kg (mean +/- SEM, N = 4), respectively. We have also examined the effect of peripheral administration of RAMH on ex vivo binding of [3H]RAMH in rat cortex. Qualitatively and quantitatively similar results to those of thioperamide were observed following i.p. administration of RAMH to rats (ED50 = 3.9 +/- 0.4 mg/kg, mean +/- SEM, N = 4). An effect of RAMH on histamine turnover in rat cortex could not be determined as this compound displayed significant cross-reactivity with the antibodies used in the radioimmunoassay to measure histamine and telemethylhistamine. These data indicate that, following peripheral administration, both thioperamide and RAMH penetrate the brain where they can subsequently interact with H3 receptors. It would appear that binding of thioperamide to H3 receptors is linked with a concomitant increase in histamine turnover in the brain. In conclusion, the ex vivo binding technique, particularly when coupled with measurement of histamine turnover, should provide a valuable means for investigating the ability of any peripherally administered compound to cross the blood-brain barrier and subsequently interact with histamine H3 receptors.

In vivo distribution of opioid receptors in man in relation to the cortical projections of the medial and lateral pain systems measured with positron emission tomography

In vivo opioid receptor binding in the cortical projections of the medial (cingulate and prefrontal cortex) and lateral pain system (primary somatosensory cortex) in male volunteers has been quantitated using [11C]diprenorphine and positron emission tomography. High levels of opioid receptor binding were seen in the cortical projections of the medial pain system in the cingulate and prefrontal cortex as has previously been observed in post-mortem studies. However, a focal reduction of opioid receptor binding was observed and quantitated in the primary motor/sensory strip when compared to surrounding parietal cortex. This new finding suggests that the medial pain system is likely to be more susceptible to exogenous and endogenous opioid neuromodulation than the so-called lateral pain system.

Quantification of human opiate receptor concentration and affinity using high and low specific activity [11C]diprenorphine and positron emission tomography

[11C]Diprenorphine, a weak partial opiate agonist, and positron emission tomography were used to obtain noninvasive regional estimates of opiate receptor concentration (Bmax) and affinity (Kd) in human brain. Different compartmental models and fitting strategies were compared statistically to establish the most reliable method of parameter estimation. Paired studies were performed in six normal subjects using high (769-5,920 Ci/mmol) and low (27-80 Ci/mmol) specific activity (SA) [11C]diprenorphine. Two subjects were studied a third time using high SA [11C]diprenorphine after a pretreatment with 1-1.5 mg/kg of the opiate antagonist naloxone. After the plasma radioactivity was corrected for metabolites, the brain data were analyzed using a three-compartment model and nonlinear least-squares curve fitting. Linear differential equations were used to describe the high SA (low receptor occupancy) kinetics. The k3/k4 ratio varied from 1.0 +/- 0.2 (occipital cortex) to 8.6 +/- 1.6 (thalamus). Nonlinear differential equations were used to describe the low SA (high receptor occupancy) kinetics and the curve fits provided the konf2 product. The measured free fraction of [11C]diprenorphine in plasma (f1) was 0.30 +/- 0.03, the average K1/k2 ratio from the two naloxone studies was 1.1 +/- 0.2, and the calculated free fraction of [11C]diprenorphine in the brain (f2) was 0.3. Using the paired SA studies, the estimated kinetic parameters, and f2, separate estimates of Bmax and Kd were obtained. Bmax varied from 2.3 +/- 0.5 (occipital cortex) to 20.6 +/- 7.3 (cingulate cortex) nM. The average Kd (eight brain regions) was 0.85 +/- 0.17 nM.

Positron emission tomography as a research tool in the investigation of psychiatric and psychological disorders

The principles of positron emission tomography (PET) are described, and illustrations of how these can be applied to clinical psychiatric questions relating to schizophrenia and depression are delineated. The metabolic changes in the frontal lobes which have been described in both depression and schizophrenia and depression are reviewed and discussed. More recent PET techniques allow several serial measurements of changes in regional blood flow in response to either a pharmacological challenge or a specific psychological task. This method provides a promising new approach to the study of the dopaminergic system in schizophrenia. New tracer methods of quantitating changes in in vivo concentrations of opioid receptors allow direct pharmacological access to the endogenous opioid system in the brain. Observations of regional cortical differences in opioid receptor concentration in relation to the medial and lateral pain systems are described. In addition, changes in receptor occupancy during sleep using [11C]diprenorphine and changes in the mu-specific tracer [11C]carfentanil in temporal lobe epilepsy are discussed.

Compartmental analysis of diprenorphine binding to opiate receptors in the rat in vivo and its comparison with equilibrium data in vitro

The regional binding of the opiate receptor ligand diprenorphine has been examined in rat brain both in vivo and in vitro. The time course of total label in specific brain regions was followed up to 2 h after intravenous bolus injection of [3H]diprenorphine, with or without a pulse chase of unlabelled diprenorphine at 30 min. In addition, total label was measured 30 min after injection of labelled diprenorphine at nontracer concentrations over a range of specific activities. Total data sets for each region were fitted simultaneously to a compartmental model to give estimates of maximal binding capacity (Bmax), the second-order apparent association rate constant, and the first-order dissociation rate constant of the receptor-ligand complex. The model incorporated the use of a reference region with low specific binding (cerebellum). The binding of diprenorphine to rat brain homogenates was measured in vitro under equilibrium conditions at 37 degrees C, pH 7.4, in the presence and absence of naloxone, to give corresponding regional estimates of Bmax and the half-saturation constant Kd. The results showed a close correlation between in vitro and in vivo regional estimates of Bmax over a wide range. There were no significant interregional differences either in Kd in vitro or in the Kd derived from the in vivo analysis, although in vitro and in vivo estimates differed by an order of magnitude. This work was carried out as part of a validation study with a view to the application of the compartmental model to data obtained in vivo in humans using positron emission tomography, when successive studies over a range of specific activities are not feasible.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of [11C]diprenorphine and [11C]carfentanil binding to opiate receptors in humans by positron emission tomography

The kinetics and regional distribution of [11C]carfentanil, a mu-selective opiate receptor agonist, and [11C]diprenorphine, a nonselective opiate receptor antagonist, were compared using paired positron emission tomography studies in two normal volunteers. Kinetics of total radioactivity (counts/mCi/pixel) was greater for [11C]diprenorphine than [11C]carfentanil in all regions. [11C]Carfentanil binding (expressed as the total/nonspecific ratio) reached near equilibrium at approximately 40 min, whereas [11C]diprenorphine showed a linear increase until approximately 60 min. Kinetics of specific binding demonstrated significant dissociation of [11C]carfentanil from opiate receptors, whereas little dissociation of [11C]diprenorphine was observed during the 90-min scan session. Regional distributions of [11C]carfentanil and [11C]diprenorphine were qualitatively and quantitatively different: Relative to the thalamus (a region with known predominance of mu-receptors), [11C]diprenorphine displayed greater binding in the striatum and cingulate and frontal cortex compared to [11C]carfentanil, consistent with labeling of additional, non-mu sites by [11C]diprenorphine. We conclude from these studies that [11C]diprenorphine labels other opiate receptor subtypes in addition to the mu sites selectively labeled by [11C]carfentanil. The nonselective nature of diprenorphine potentially limits its usefulness in defining abnormalities of specific opiate receptor subtypes in various diseases. Development of selective tracers for the delta- and kappa-opiate receptor sites, or alternatively use of unlabeled inhibitors to differentially displace mu, delta, and kappa subtypes, will help offset these limitations.

Cardiovascular effects of etorphine in rats

1. Cardiovascular effects of intravenously administered etorphine were investigated in mechanically ventilated normotensive rats under pentobarbitone anaesthesia. 2. Etorphine (0.1-2 micrograms kg-1) induced a dose-related bradycardia and hypotension which was prevented by pretreatment with naloxone (0.1 mg kg-1). 3. After bilateral vagotomy etorphine (1 microgram kg-1) produced a pressor effect which was prevented by prazosin (0.5 mg kg-1), but unaltered by adrenalectomy. 4. The bradycardia due to etorphine was abolished by bilateral vagotomy, but only partially reduced by atropine (1 mg kg-1) and still evident after propranolol (1.5 mg kg-1). 5. Etorphine was without effect on blood pressure in the pithed rat, although there was a small bradycardia which was not seen after naloxone. 6. The data presented indicate that etorphine produces an opioid receptor-mediated stimulation of both vagal (partially cholinergic) and sympathetic outflow and a direct cardiodepressant effect.

Investigation of the antinociceptive activity of buprenorphine in sheep

1. Buprenorphine given intravenously (6 micrograms kg-1) was examined for its antinociceptive activity in unrestrained sheep using devices to measure thermal and mechanical thresholds. 2. The plasma levels of buprenorphine following intravenous injection over the time period of the antinociceptive testing were measured using a radioimmunoassay. 3. Buprenorphine produced a clear antinociceptive effect lasting for up to three and a half hours when measured by the thermal threshold test, but no detectable antinociception in the mechanical test. 4. The plasma levels of buprenorphine indicated that the drug was rapidly distributed in a manner not dissimilar to that reported in man, although individual animals showed a wide variation in some parameters. 5. When plasma levels of the drug were high (less than 700 pg ml-1) during the first sixty minutes, no antinociceptive activity in the thermal test could be detected, which may be due to the slow receptor kinetics shown by this drug.

In vivo binding of putative delta-selective opioid antagonists to central opiate receptors

The in vivo opiate receptor binding of three putative delta-selective opioid antagonists, the irreversible oripavine ligand NIH 10236 and two enkephalin analogs, ICI 154,129 and ICI 174,866, was studied in rat brain. Following i.c.v. injection, potent and apparently irreversible opiate receptor binding was observed for NIH 10236 with similar affinity for mu, delta and kappa sites. However, opiate receptor binding was not evident either after s.c. injection of NIH 10236 or ICI 154,129, or following i.c.v. administration of ICI 154,129 or ICI 174,864. The receptor sites that mediate the pharmacological effects of these drugs should be reevaluated.