Current status of opioid addiction treatment and related preclinical research

Opioid use disorders (OUDs) are diseases of the brain with behavioral, psychological, neurobiological, and medical manifestations. Vulnerability to OUDs can be affected by factors such as genetic background, environment, stress, and prolonged exposure to μ-opioid agonists for analgesia. Two standard-of-care maintenance medications, methadone and buprenorphine-naloxone, have a long-term positive influence on health of persons with opioid addiction. Buprenorphine and another medication, naltrexone, have also been approved for administration as monthly depot injections. However, neither medication is used as widely as needed, due largely to stigma, insufficient medical education or training, inadequate resources, and inadequate access to treatment. Ongoing directions in the field include (i) personalized approaches leveraging genetic factors for prediction of OUD vulnerability and prognosis, or for targeted pharmacotherapy, and (ii) development of novel analgesic medicines with new neurobiological targets with reduced abuse potential, reduced toxicity, and improved effectiveness, especially for chronic pain states other than cancer pain.

Self administration of oxycodone by adolescent and adult mice affects striatal neurotransmitter receptor gene expression

Illicit use of prescription opioid analgesics (e.g., oxycodone) in adolescence is a pressing public health issue. Our goal was to determine whether oxycodone self administration differentially affects striatal neurotransmitter receptor gene expression in the dorsal striatum of adolescent compared to adult C57BL/6J mice. Groups of adolescent mice (4 weeks old, n=12) and of adult mice (11 weeks old, n=11) underwent surgery during which a catheter was implanted into their jugular veins. After recovering from surgery, mice self administered oxycodone (0.25 mg/kg/infusion) 2 h/day for 14 consecutive days or served as yoked saline controls. Mice were sacrificed within 1h after the last self-administration session and the dorsal striatum was isolated for mRNA analysis. Gene expression was analyzed with real time PCR using a commercially available neurotransmitter receptor PCR array containing 84 genes. We found that adolescent mice self administered less oxycodone than adult mice over the 14 days. Monoamine oxidase A (Maoa) and neuropeptide Y receptor 5 mRNA levels were lower in adolescent mice than in adult mice without oxycodone exposure. Oxycodone self administration increased Maoa mRNA levels compared to controls in both age groups. There was a positive correlation of the amount of oxycodone self administered in the last session or across 14 sessions with Maoa mRNA levels. Gastrin-releasing peptide receptor mRNA showed a significant Drug × Age interaction, with point-wise significance. More genes in the dorsal striatum of adolescents (19) changed in response to oxycodone self administration compared to controls than in adult (4) mice. Overall, this study demonstrates that repeated oxycodone self administration alters neurotransmitter receptors gene expression in the dorsal striatum of adolescent and adult mice.

Bidirectional translational research: Progress in understanding addictive diseases

The focus of this review is primarily on recent developments in bidirectional translational research on the addictions, within the Laboratory of the Biology of Addictive Diseases at The Rockefeller University. This review is subdivided into major interacting aspects, including (a) Investigation of neurobiological and molecular adaptations (e.g., in genes for the opioid receptors or endogenous neuropeptides) in response to cocaine or opiates, administered under laboratory conditions modeling chronic patterns of human self-exposure (e.g., chronic escalating "binge"). (b) The impact of such drug exposure on the hypothalamic-pituitary-adrenal (HPA) axis and interacting neuropeptidergic systems (e.g., opioid, orexin and vasopressin). (c) Molecular genetic association studies using candidate gene and whole genome approaches, to define particular systems involved in vulnerability to develop specific addictions, and response to pharmacotherapy. (d) Neuroendocrine challenge studies in normal volunteers and current addictive disease patients along with former addicts in treatment, to investigate differential pharmacodynamics and responsiveness of molecular targets, in particular those also investigated in the experimental and molecular genetic approaches as described above.

Nicotine addiction: insights from recent animal studies

RATIONALE

Recent preclinical behavioral and neurobiological research has characterized important behavioral features and has identified neurobiological substrates that may underlie nicotine reinforcement and addiction.

OBJECTIVE

To examine recent advances on nicotine exposure in preclinical models, from three perspectives: (a) the chronopharmacokinetics of nicotine, (b) behavioral studies on nicotine reinforcement, withdrawal, and reinstatement/relapse, and (c) effects of nicotine on neurobiological substrates after repeated exposure.

RESULTS

Preclinical studies can be used to operationally model selected aspects of nicotine reinforcement, withdrawal, and reinstatement or relapse. These may be used to investigate the functional in vivo consequences of acute and long-term changes in neuronal acetylcholine receptor populations that follow nicotine exposure. Behavioral studies focusing on distinct stages of nicotine exposure (e.g., active reinforcement vs. cessation or reinstatement) may also be used in parallel with studies on dopaminergic function, a proposed substrate for the reinforcing effects of nicotine, and of opioid receptor function, a possible site of neuroadaptations secondary to nicotine exposure.

CONCLUSIONS

While no single current animal model may capture the experience of human smoking or nicotine addiction, increasingly, separate animal models are capturing the full spectrum of behavioral and neurobiological dimensions of this complex condition.

GR89,696: a potent kappa-opioid agonist with subtype selectivity in rhesus monkeys

GR89,696 is a synthetic kappa-opioid receptor agonist, recently reported to have an agonist profile consistent with selectivity at the proposed "kappa(2)" subtype. The present studies evaluated the effects of GR89,696 in vitro (i.e., in radioligand binding and [(35)S]guanosine-5'-O-(3-thio)triphosphate assays) and in vivo in rhesus monkeys, in assays used to study kappa-opioid agonists (i.e., thermal antinociception, sedation and muscle relaxation, diuresis, and increases in serum prolactin levels, as well as ethylketocyclazocine and U69,593 discrimination). Furthermore, the sensitivity of GR89,696 to naltrexone and nor-binaltorphimine (nor-BNI) antagonism was compared with that of U50,488 and U69,593, ligands selective for the proposed "kappa(1)" subtype. Overall, GR89,696 displayed the profile of a highly potent kappa-opioid agonist, following parenteral administration in rhesus monkeys. GR89,696 was less sensitive than U50,488 and U69,593 to naltrexone or nor-BNI antagonism, consistent with an action through the proposed kappa(2) receptor subtype.

kappa-Opioid receptor agonist-induced prolactin release in primates is blocked by dopamine D(2)-like receptor agonists

Kappa-opioid receptor agonists may have pharmacotherapeutic potential in the management of psychostimulant abuse, due to their ability to modulate dopamine receptor systems involved in drug reinforcement. kappa-Opioid receptor agonists also modulate dopamine receptor function in the hypothalamic tuberoinfundibular system, which has inhibitory control over an anterior pituitary hormone, prolactin. Prolactin levels may thus be a "biomarker" for the ability of kappa-opioid receptor agonists (e.g., (+)-(5 alpha,7 alpha,8 beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U69,593)) to modulate a dopamine receptor system in vivo in primates. The effectiveness of dopamine D(2)-like receptor agonists (quinpirole and (+/-)-7-hydroxy-dipropylaminotetralin (7-OH-DPAT); 0.0032-0.1 mg/kg) in preventing U69,593-induced prolactin release was studied in intact female rhesus monkeys. Quinpirole and 7-OH-DPAT inhibited U69,593-induced prolactin release (ID(50) values: 0.013 and 0.0072 mg/kg, respectively). However, the dopamine D(1)-receptor agonist (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazapine (SKF 82958; 1 mg/kg) did not inhibit U69,593-induced prolactin release under the same conditions. In contrast, the largest doses of quinpirole or 7-OH-DPAT presently studied (0.1 mg/kg), did not decrease sedation caused by U69,593 (0.01, 0.032 mg/kg), a prominent effect of centrally penetrating kappa-opioid receptor agonists. The sedative effect of U69,593 (0.032 mg/kg) was prevented by naltrexone (0.32 mg/kg), consistent with kappa-opioid receptor mediation of this effect. These studies suggest that prolactin release is a valid biomarker for the ability of kappa-opioid receptor agonists to modulate dopamine D(2)-like receptor function, and may also be used to quantify dopamine D(2)-like receptor agonist potency in primates.

Intracisternal nor-binaltorphimine distinguishes central and peripheral kappa-opioid antinociception in rhesus monkeys

Systemic administration of nor-binaltorphimine (nor-BNI) produces a long-lasting kappa-opioid receptor (kappaOR) antagonism and has kappa(1)-selectivity in nonhuman primates. The aim of this study was to establish the pharmacological basis of central kappaOR antagonism in rhesus monkeys (Macaca mulatta). After intracisternal (i.c.) administration of small doses of nor-BNI, the duration and selectivity of nor-BNI antagonism were evaluated against two kappaOR agonists, (trans)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50,488) and bremazocine. Thermal antinociception was measured in the warm water (50 degrees C) tail-withdrawal assay and sedation was evaluated by observers blind to treatment conditions. Following i.c. pretreatment with 0.32 mg nor-BNI, a 5- to 10-fold rightward shift of the U50,488 baseline dose-effect curve was observed in antinociception. In contrast, this dose of nor-BNI only produced an insignificant 2-fold shift against bremazocine. Pretreatment with a smaller dose (0.032 mg) of nor-BNI produced a 3-fold shift of U50, 488, which lasted for 7 days, but failed to alter the potency of bremazocine. This differential antagonism profile of i.c. nor-BNI also was observed in sedation ratings. In addition, the centrally effective dose of nor-BNI (0.32 mg), when administered s.c. in the back, did not antagonize either U50,488- or bremazocine-induced antinociception and sedation. After i.c. pretreatment with the same dose, nor-BNI also did not antagonize the peripherally mediated effect of U50,488 against capsaicin-induced thermal nociception in the tail. These results indicate that i.c. nor-BNI produces central kappaOR antagonism and support the notion of two functional kappaOR subtypes in the central nervous system. Moreover, it provides a valuable pharmacological basis for further characterizing different sources of kappaOR-mediated effects, namely, from central or peripheral nervous system receptors.

Apparent efficacy of kappa-opioid receptor ligands on serum prolactin levels in rhesus monkeys

These studies investigated whether serum prolactin levels could be a quantitative marker of the apparent efficacy of kappa-opioid receptor ligands in primates. The effects of s.c. bremazocine and U50,488 (trans-(+/-)-3, 4-Dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid e; agonists), nalorphine (partial agonist) and nalmefene (antagonist) on prolactin levels were studied in intact female rhesus monkeys. The above compounds, except nalmefene, increased prolactin levels, and their actions conformed to sigmoidal dose-effect curves. The rank order of the compounds' maximum effects in this neuroendocrine endpoint is similar to that in cloned kappa-receptors in vitro, and in a presently studied thermal antinociception assay in vivo. Prolactin may therefore be a quantitative marker of the apparent efficacy of kappa-opioid receptor ligands in primates.

Effects of E-2078, a stable dynorphin A(1-8) analog, on sedation and serum prolactin levels in rhesus monkeys

RATIONALE

The dynorphins are endogenous opioid peptides with relative binding selectivity for kappa-receptors. It is unclear whether the dynorphins share the pharmacological profile observed with synthetic kappa-agonists in primates.

OBJECTIVE

The main objective of this study was to compare the effects of s.c. E-2078, a stable dynorphin A(1-8) analog, with two synthetic kappa-opioid ligands, spiradoline (a reference arylacetamide kappa-agonist) and ICI204,448 (a "peripherally selective" kappa-agonist) in behavioral and neuroendocrine endpoints in rhesus monkeys.

METHODS

Dose-effect curves were determined for s.c. E-2078, spiradoline and ICI204,448 in causing overt sedation and muscle relaxation (as detected in observational rating scales), in increasing latency to retrieve and consume a food pellet and in increasing serum levels of the anterior pituitary hormone, prolactin, in intact female rhesus monkeys.

RESULTS

E-2078 and ICI204,448 (0.1-3.2 mg/kg) caused increases in sedation and muscle relaxation scores, but were less potent and apparently less effective than spiradoline (0.001-0.1 mg/kg) up to the highest doses presently studied. All three agonists were equieffective and approximately equipotent in increasing the latency to retrieve and consume a pellet. Furthermore, E-2078 (0.001-0.032 mg/kg) was equipotent and equieffective with spiradoline in increasing serum prolactin levels, whereas ICI204,448 was less potent, but slightly more effective than the former two agonists. The effects of E-2078 on serum prolactin levels were surmountably antagonized by quadazocine (1 mg/kg) and naltrexone (0.1 mg/kg).

CONCLUSIONS

The present studies show that serum prolactin levels are a highly sensitive, quantitative endpoint to study the potency and effectiveness of systemically administered E-2078, and show that the dynorphins may be potent and effective in causing some, but not all, the effects that are observed after the administration of synthetic kappa-agonists.

Effects of systemically administered dynorphin A(1-17) in rhesus monkeys

The effects of i.v. dynorphin A(1-17) and its main nonopioid biotransformation fragment, dynorphin A(2-17), were compared in rhesus monkeys with those of the selective kappa-opioid agonist, U69, 593, in assays of operant behavior, thermal antinociception, and neuroendocrine function (prolactin release). Dynorphin A(1-17) (0. 1-3.2 mg/kg i.v.) and U69,593 (0.001-0.032 mg/kg s.c.) decreased rates of schedule-controlled (fixed ratio 20) food-reinforced responding, whereas dynorphin A(2-17) (1-3.2 mg/kg i.v.) was ineffective. Pretreatment studies with the opioid antagonist quadazocine (0.32 mg/kg s.c.) revealed that the operant effects of dynorphin A(1-17) were not mediated by kappa- or micro-opioid receptors. A different profile was observed in the warm water tail withdrawal assay of thermal antinociception, where both dynorphin A(1-17) and A(2-17) (0.032-3.2 mg/kg i.v., n = 4) were modestly effective in 50 degrees C water, and both were ineffective in 55 degrees C water. By comparison, U69,593 (0.032-0.18 mg/kg s.c.) was maximally effective in 50 degrees C water and partially effective in 55 degrees C. kappa-opioid agonists increase serum levels of prolactin in animals and humans. Dynorphin A(1-17) (ED(50) = 0.0011 mg/kg i.v.), similar to U69,593 (ED(50) = 0.0030 mg/kg i.v.), was very potent in increasing serum prolactin levels in follicular phase female rhesus monkeys, whereas dynorphin A(2-17) (0.32 mg/kg i.v.) was ineffective. The effects of dynorphin A(1-17) and U69,593 on serum prolactin were both antagonized by quadazocine (0.32 mg/kg s.c.) in a surmountable manner, consistent with opioid receptor mediation. The present studies show that serum prolactin levels are a sensitive quantitative endpoint to study the systemic effects of the endogenous opioid peptide, dynorphin A(1-17), in primates.

Effects of (+)-HA-966, CGS-19755, phencyclidine, and dizocilpine on repeated acquisition of response chains in pigeons: systemic manipulation of central glycine sites

The effects of i.m. injections of (+)-HA-966, a glycine-site antagonist at the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor, its enantiomer (-)-HA-966, the competitive glutamate antagonist CGS-19755, the uncompetitive glutamate antagonists phencyclidine and dizocilpine, and the micro opioid agonist morphine were evaluated in a repeated acquisition task in pigeons. All of the drugs produced dose-dependent decreases in rates of responding. The NMDA receptor and channel blockers and (+)-HA-966 appeared to have a greater effect on acquisition than did morphine at doses that did not fully suppress responding. The rate suppression and learning impairment produced by a large dose of (+)-HA-966 (100 mg/kg) were completely prevented by coadministration of the glycine-site agonist D-serine (560 mg/kg) but not by its enantiomer, L-serine (1000 mg/kg). D-Serine, however, produced incomplete antagonism of the effects of dizocilpine and phencyclidine and failed to alter those of CGS-19755. These findings provide evidence that reducing the activity of the NMDA subtype of the glutamate receptor through pharmacological action at any of three sites produces similar decrements in acquisition, and those produced through antagonism of the glycine site are differentially sensitive to the glycine-site agonist D-serine.

Activation of peripheral kappa opioid receptors inhibits capsaicin-induced thermal nociception in rhesus monkeys

8-Methyl-N-vanillyl-6-nonenamide (capsaicin) was locally applied in the tail of rhesus monkeys to evoke a nociceptive response, thermal allodynia, which was manifested as reduced tail-withdrawal latencies in normally innocuous 46 degrees C water. Coadministration of three kappa opioid ligands, U50,488 (3.2-100 microgram), bremazocine (0.1-3.2 microgram), and dynorphin A(1-13) (3.2-100 microgram), with capsaicin in the tail dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by a small dose of an opioid antagonist, quadazocine; (0.32 mg), applied in the tail; however, this dose of quadazocine injected s.c. in the back did not antagonize local U50,488. Comparing the relative potency of either agonist or antagonist after local and systemic administration confirmed that the site of action of locally applied kappa opioid agonists is in the tail. In addition, local nor-binaltorphimine (0.32 mg) and oxilorphan (0.1-10 microgram) antagonist studies raised the possibility of kappa opioid receptor subtypes in the periphery, which indicated that U50,488 produced local antinociception by acting on kappa1 receptors, but bremazocine acted probably on non-kappa1 receptors. These results provide functional evidence that activation of peripheral kappa opioid receptors can diminish capsaicin-induced allodynia in primates. This experimental pain model is a useful tool for evaluating peripherally antinociceptive actions of kappa agonists without central side effects and suggests new approaches for opioid pain management.

Systemic effects of E-2078, a stabilized dynorphin A(1-8) analog, in rhesus monkeys

RATIONALE

E-2078 ([N-methyl-Tyr1, N-methyl-Arg7, D-Leu8] dynorphin A(1-8) ethylamide) is a dynorphin A(1-8) analog with a reduced tendency to be biotransformed, when compared to the unmodified opioid peptide. E-2078 has been found to produce kappa-opioid agonist effects in vivo in rodents.

OBJECTIVE

In the present studies, we investigated whether systemically administered E-2078 could produce kappa-agonist effects in rhesus monkeys, in tests of antinociception, diuresis and ethyl-ketocyclazocine (EKC) discrimination.

METHODS

E-2078 (0.32-18 mg/kg, SC, IM or IV) was tested in the warm water (50 degrees, 55 degrees C) tail withdrawal assay of thermal antinociception. The diuretic effects of E-2078 (0.056-1.8 mg/kg, SC) were also compared to those of the kappa-agonist, U69,593 (0.01-0.32 mg/kg, SC). Lastly, the effects of E-2078 (0.1-3.2 mg/kg, SC or IV) were studied in rhesus monkeys trained to discriminate EKC (0.0056 mg/kg SC) from vehicle, in a food-reinforced operant procedure.

RESULTS

E-2078 did not produce thermal antinociception in rhesus monkeys following SC or IM administration, up to the largest doses presently studied (i.e., 18 and 10 mg/kg, respectively). E-2078 caused thermal antinociception by the IV route, but this effect was not apparently mediated by kappa- or mu-opioid receptors, as shown by its insensitivity to quadazocine (1 mg/kg) pretreatment. However, SC E-2078 caused diuresis, and this effect was blocked by quadazocine pretreatment, consistent with mediation by kappa-opioid receptors. E-2078 generalized in EKC-discriminating monkeys, but only after the largest dose (3.2 mg/kg), and only following IV administration.

CONCLUSIONS

The present studies suggest that systemically administered E-2078 can produce some kappa-receptor mediated effects in rhesus monkeys, but its profile of action is not identical to non-peptidic kappa-agonists following all routes of administration, or across all experimental situations.

Analgesic, respiratory and heart rate effects of cannabinoid and opioid agonists in rhesus monkeys: antagonist effects of SR 141716A

This study characterized the antinociceptive, respiratory and heart rate effects of the cannabinoid receptor agonists Delta-9-tetrahydrocannabinol (Delta-9-THC) and WIN 55212 ((R)-(+)-2, 3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrol-[1,2,3-de]-1, 4-benzoxazin-6-yl)(1-naphtalenyl)methanone monomethanesulfonate), N-arachidonyl ethanolamide (anandamide) and the mu and kappa opioid receptor agonists heroin and U69593, alone and in conjunction with a cannabinoid receptor antagonist, SR 141716A [N-(piperidin-1-1-yl)-5-(4-chlorophenyl)-1(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride] and an opioid receptor antagonist, quadazocine, in rhesus monkeys (Macaca mulatta). Using 12 adult rhesus monkeys, latencies to remove the tail from a 50 degrees C water bath, respiration in 5% CO2 and heart rate were measured. When administered alone, SR 141716A (1.8, 5.6 mg/kg i.m.) did not alter nociception, respiration or heart rate. Delta-9-THC (0.1-10 mg/kg i.m.) and WIN 55212 (0.1-10 mg/kg i.m.) dose-dependently increased antinociception and dose-dependently decreased respiratory minute and tidal volumes and heart rate. These antinociceptive, respiratory and heart rate effects were reversed by SR 141716A but not by the opioid antagonist quadazocine (1 mg/kg i.m.). Anandamide (10 mg/kg i.m.) also produced antinociception. Heroin (0.01-10 mg/kg i.m.) and U69593 (0.01-3.2 mg/kg i.m.) also dose-dependently increased antinociception and decreased respiratory and heart rate measures; these effects were antagonized by quadazocine but not by SR 141716A. These results demonstrate selective and reversible antagonism of cannabinoid behavioral effects by SR 141716A in rhesus monkeys.

The role of peripheral mu opioid receptors in the modulation of capsaicin-induced thermal nociception in rhesus monkeys

Capsaicin produces burning pain, followed by nociceptive responses, such as allodynia and hyperalgesia in humans and rodents. In the present study, when administered subcutaneously into the tail of rhesus monkeys, capsaicin (0.01-0.32 mg) dose-dependently produced thermal allodynia manifested as reduced tail-withdrawal latencies in 46 degrees C water, from a maximum value of 20 sec to approximately 2 sec. Coadministration of selective mu opioid agonists, fentanyl (0.003-0.1 mg) and (D-Ala2,N-Me-Phe4, Gly5-ol)-enkephalin (0.001-0.03 mg), dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by small doses of opioid antagonists, quadazocine (0.03 mg) and quaternary naltrexone (1 mg), applied locally in the tail. However, these doses of antagonists injected s.c. in the back did not antagonize local fentanyl. Comparing the relative potency of either agonist or antagonist after local and systemic administration confirmed that the site of action of locally applied mu opioid agonists is in the tail. These results provide evidence that activation of peripheral mu opioid receptors can diminish capsaicin-induced allodynia in primates. This experimental pain model could be a useful tool for evaluating peripherally acting antinociceptive agents without central side effects and enhance new approaches to the treatment of inflammatory pain.

kappa-Opioid receptor binding populations in rhesus monkey brain: relationship to an assay of thermal antinociception

The binding characteristics of the kappa opioid ligands [3H]U69,593 and [3H]bremazocine, the mu opioid ligand [3H][D-ala2,N-Me-Phe4,glycol5]enkephalin and the delta opioid ligand [3H]p-Cl-[D-pen2,5]enkephalin were studied in rhesus monkey brain membranes in saturation binding experiments and were followed by competition binding experiments with a variety of peptidic and nonpeptidic opioid ligands. The [3H]U69,593 sites appeared to be a subset of kappa opioid receptors (kappa-1 receptors: Kd, 1.2 nM; Bmax, 66 fmol/mg). [3H]Bremazocine (in the presence of mu and delta receptor-masking agents), bound to a larger population of kappa receptors (kappa-all: Kd, 0.39 nM; Bmax, 227 fmol/mg), which presumably included the aforementioned kappa-1 sites. Competition binding experiments revealed that the presently defined kappa-1 sites were similar to previously reported sites in other mammalian species, particularly in terms of the higher kappa-1 selectivity observed with arylacetamide (e.g., U50,488) vs. benzomorphan kappa agonists (e.g., ethylketocyclazocine). The kappa-selective antagonist norbinaltorphimine (nor-BNI) displayed a very small (2.3-fold) selectivity for kappa-1 vs. kappa-all sites. This led to the prediction that in rhesus monkeys (n = 3), systemically administered nor-BNI (10 mg/kg s.c.) should have a very moderate degree of antagonist selectivity for the antinociceptive effects of a putative kappa-1-agonist, the arylacetamide U50,488 (0.1-3.2 mg/kg s.c.), vs. those of the benzomorphan kappa agonist ethylketocyclazocine (0.01-056 mg/kg s.c.). This prediction was confirmed in vivo because nor-BNI (10 mg/kg) caused a robust and long lasting (up to 21 days) antagonism of the antinociceptive effects of U50,488 and a small but significant antagonism of ethylketocyclazocine. The arylacetamide congener Cl-977 (enadoline), which displayed an 11-fold kappa-1 vs. kappa-all binding selectivity, was not sensitive to nor-BNI pretreatment. This indicates that the kappa subtype-binding profile of an agonist is not necessarily predictive of its sensitivity to nor-BNI in vivo. Overall, the present results suggest that at least two functional kappa receptor populations may be present in rhesus monkey brain.

Differentiation of kappa opioid agonist-induced antinociception by naltrexone apparent pA2 analysis in rhesus monkeys

Naltrexone (NTX) exhibited approximately 3-fold higher affinity for sites labeled by [3H]U69,593 (putative kappa 1-selective ligand) than [3H]bremazocine (non-selective ligand) in the presence of mu and delta receptor blockade in monkey brain membranes. This led us to test an hypothesis that NTX could display in vivo antagonist selectivity for kappa 1-versus non-kappa 1-mediated effects. Six opioid agonists were characterized by NTX apparent pA2 analysis in a 50 degrees C water tail-withdrawal assay in rhesus monkeys. Constrained NTX pA2 values (95% confidence limits) were: alfentanil, 8.66 (8.47-8.85); ethylketocyclazocine, 7.97 (7.93-8.01); U69,593, 7.64 (7.49-7.79); U50,488, 7.55 (7.42-7.67); bremazocine, 6.92 (6.73-7.12); enadoline, 6.87 (6.69-7.05). Pretreatment with clocinnamox, an irreversible mu antagonist, confirmed that mu receptors were not involved in the antinociception produced by the kappa agonists, U69,593, U50,488, bremazocine and enadoline; however, both mu and kappa receptors mediated the antinociceptive effects of ethyl-ketocyclazocine. The apparent NTX pA2 profile of opioid agonists correlated highly with the radioligand binding studies, which indicates that U69,593 and U50,488 produced antinociception by acting on kappa-1 receptors, whereas bremazocine and enadoline probably acted via non-kappa-1 receptors. This study provides further functional evidence of kappa opioid receptor multiplicity in primates and suggests that NTX may be a useful tool to study this phenomenon in vivo.

Dynorphin A (1-8) analog, E-2078, crosses the blood-brain barrier in rhesus monkeys

E-2078 is a dynorphin A (1-8) analog, [N-methyl-Tyr1, N-methyl-Arg7-D-Leu8] dynorphin A (1-8) ethylamide. Its ability to cross the blood-brain barrier was examined in rhesus monkeys using matrix-assisted laser desorption/ionization mass spectrometry. In vivo studies were carried out by i.v. injecting E-2078, 10 mg/kg, a dose that had been found to be antinociceptive, to rhesus monkeys. Blood and cerebrospinal fluid samples were collected at various time points after the injection. It was found that E-2078 was stable in vivo in rhesus monkey blood. No biotransformation products were detected in the blood. Mass spectrometric analysis of the cerebrospinal fluid samples collected after E-2078 injection detected the presence of E-2078, indicating that E-2078 had crossed the blood-brain barrier. These findings are consistent with the possibility that systemically administered E-2078 could produce centrally mediated behavioral and physiological effects.

Dynorphin A (1-8) analog, E-2078, is stable in human and rhesus monkey blood

E-2078 is a dynorphin A (1-8) analog, [N-methyl-Tyr1, N-methyl-Arg7-D-Leu8] Dyn A (1-8) ethylamide. Its biochemical stability against enzymatic degradation in vitro in human and rhesus monkey blood, and in vivo in rhesus monkey blood was studied using matrix-assisted laser desorption/ionization mass spectrometry. In vitro studies were carried out in freshly drawn human and rhesus monkey blood, incubated at 37 degrees C for various time periods. In vivo studies were conducted by E-2078 i.v. injection to rhesus monkeys, and blood samples were collected at various time points after the injection. It was found that E-2078 was stable against enzymatic degradation in vitro in freshly drawn human and rhesus monkey blood. Minor biotransformation products from E-2078, such as E (1-4), E (1-5) and E (3-6), were detected in vitro in some human and rhesus monkey blood, but they made up less than 5% of the total starting E-2078 peptide. No biotransformation products were detected in the blood samples from in vivo studies. The apparent half-life of elimination of E-2078 in vivo from the rhesus monkey blood was determined to be 44.0 min.

In vitro biotransformation of dynorphin A (1-17) is similar in human and rhesus monkey blood as studied by matrix-assisted laser desorption/ionization mass spectrometry

Dynorphin A (1-17) [Dyn A (1-17)] is an endogenous opioid peptide. In vitro biotransformation of Dyn A (1-17) in human and rhesus monkey blood was studied by matrix-assisted laser desorption/ionization mass spectrometry. Biotransformation was observed to produce various opioid and nonopioid dynorphin A peptides. In this study, in vitro Dyn A (1-17) biotransformation at physiological temperature (37 degrees C) was found to be very similar in human and rhesus monkey blood, although Dyn A (1-17) processing occurred at a faster rate in vitro in monkey blood than in human blood. One dominant pathway in this biotransformation was the slow removal of tyrosine at position one from Dyn A (1-17) to yield the dominant product, Dyn A (2-17). Further slow biotransformation of Dyn A (2-17) also occurred. Another major pathway of Dyn A (1-17) biotransformation is cleavage of the peptide linkage between Arg(6) and Arg(7) to produce the opioid peptide, Dyn A (1-6), and the nonopioid peptide, Dyn A (7-17). These two peptides had a short lifetime in blood, undergoing rapid biotransformation. Our results indicate that the rhesus monkey may be a good model for further in vivo pharmacological and neurobiological studies.

Methoclocinnamox: agonist and antagonist effects of a novel long-lasting opioid in rhesus monkeys

The present studies characterized the agonist and antagonist effects of methoclocinnamox, a novel codeinone, in the warm-water (50 degrees C and 55 degrees C) tail-withdrawal assay of thermal antinociception in rhesus monkeys. Methoclocinnamox (0.1-1.0 mg/ kg) was fully effective in the warm-water tail-withdrawal assay in 50 degrees C but not 55 degrees C water, similar to previously studied mu-opioid partial agonists. Consistent with this, methoclocinnamox (0.32 mg/kg), at a time when it acted as an agonist in 50 degrees C water, was an antagonist of the higher efficacy mu-agonist fentanyl in 55 degrees C water. The agonist effects of methoclocinnamox were antagonized by quadazocine (1.0 mg/kg), but to a lesser degree than would be expected for competitive antagonism at mu-receptors. However, the nonequilibrium mu-selective antagonist clocinnamox (0.32 mg/kg) completely blocked the antinociceptive effect of methoclocinnamox. After its agonist effects had waned (typically < 24 hr), methoclocinnamox (0.1-1.0 mg/ kg) caused long-lasting and nonparallel shifts in the dose-effect curves of the mu-agonist morphine, whereas even at the highest dose (1.0 mg/kg) methoclocinnamox did not antagonize the kappa-agonist U50,488. It is concluded that methoclocinnamox exhibits initial opioid agonist effects, followed by prolonged, insurmountable, mu-antagonist effects.

Nitric oxide synthase inhibitors produce phencyclidine-like behavioral effects in pigeons

The present study assessed the ability of nitric oxide synthase (NOS) inhibitors to produce PCP-like behavioral effects in pigeons. Food-restricted pigeons were trained to discriminate between PCP (1.0 mg/kg, i.m.) from saline in a two-key operant procedure. NOS inhibitors 7-nitroindazole (7-NI) and N omega-nitro-L-arginine methyl ester (L-NAME) produced PCP-like discriminative stimulus effects. 7-NI (17.8 mg/kg, i.m.) completely generalized to PCP. L-NAME (320-1000 mg/kg) produced partial generalization to PCP. D-NAME, the enantiomer of L-NAME, did not produce PCP-appropriate behavior. L-NAME was approximately 200-times more potent i.c.v., but did not fully generalize to PCP. Both NOS inhibitors were effective in producing catalepsy, which is an effect commonly produced by competitive and uncompetitive NMDA antagonists. 7-NI (32 mg/kg) produced catalepsy in all subjects, whereas L-NAME (3200 mg/kg) produced catalepsy in 50% of the subjects, D-NAME did not produce catalepsy. Pretreatment with L-arginine (32-3200 mg/kg) prevented the PCP-like discriminative stimulus and cataleptic effects of 7-NI (17.8-32 mg/kg), demonstrating that 7-NI produced PCP-like effects through blockade of NO synthesis. The current studies reveal that NOS inhibitors induced two behavioral actions, discriminative stimulus effects and catalepsy, that are very selective for NMDA antagonists in pigeons.

Clocinnamox antagonism of opioid suppression of schedule-controlled responding in rhesus monkeys

The antagonist effects of clocinnamox were evaluated against opioid agonists, acting at mu, kappa and delta-receptors, in rhesus monkeys (n = 3-4) responding under a fixed-ratio 30 (FR 30) schedule for food delivery. Clocinnamox (0.032-0.1 mg/kg) dose-dependently antagonized fentanyl (0.001-0.32 mg/kg) after either a 3-h or 1-day pretreatment; there was substantial recovery of agonist potency by 1 week after clocinnamox. Etonitazene (0.0001-0.01 mg/kg) was also antagonized by clocinnamox (0.1 mg/kg), but to a lesser extent than fentanyl. The smaller extent of antagonism was not due to the appearance of non mu-opioid response-decreasing effects of etonitazene, since the competitive antagonist quadazocine (0.1 mg/kg) shifted the etonitazene dose-effect curve in the presence of clocinnamox (0.1 mg/kg). Clocinnamox (0.1-0.32 mg/kg) did not antagonize the rate-suppressing effects of the delta-agonist BW373U86 (0.0.01-1.0 mg/kg) or the kappa-agonist U69,593 (0.001-0.032 mg/kg). These results are consistent with previous in vivo and in vitro evidence that characterized clocinnamox as an insurmountable antagonist, with selectivity for mu- over kappa- and delta-receptors.

Antinociceptive effects of cocaine/opioid combinations in rhesus monkeys

This study characterized the antinociceptive effects of cocaine alone and in combination with mu, delta, and kappa opioids in rhesus monkeys. The shaved tails of four rhesus monkeys were exposed to warm water (42, 46, 50, 54, and 58 degrees C), and tail withdrawal latencies (20 sec maximum) from each temperature were determined. The temperature that produced a tail withdrawal latency of 10 sec (T10) was interpolated, and drug-induced changes in the T10 value (delta T10) were calculated. Dose-dependent increases in delta T10 were produced by cocaine (0.032-1.8 mg/kg), the high efficacy mu agonist fentanyl (0.001-0.1 mg/kg), the intermediate efficacy mu agonist morphine (0.1-18 mg/kg), the low efficacy mu agonist nalbuphine (1-32 mg/kg), and the kappa agonist U69,593 (0.0032-0.1 mg/kg). The delta agonist BW373U86 (0.56 mg/kg) produced no effect. Relative maximum effects, determined from the maximum delta T10 values produced by each drug, were fentanyl > or = (5,7,8 beta)-N-methyl-N[2-(1-pyrrolidinyl)1-oxaspiro[4,5]dec-8- yl]benzeneacetamide > morphine > nalbuphine > or = cocaine > BW373U86. When individual doses of cocaine (0.1-1.8 mg/kg) and morphine (0.32-10.0 mg/kg) were combined, cocaine produced a dose-dependent increase in the effects of each dose of morphine, and the antinociceptive effects of most cocaine/morphine combinations were significantly greater than the antinociceptive effects of either cocaine or morphine alone. Cocaine (1.8 mg/kg) was also combined with nalbuphine (1.0, 10 mg/kg), fentanyl (0.001, 0.032 mg/kg), BW373U86 (0.56 mg/kg) and U69,593 (0.0032-0.056 mg/kg). Cocaine/nalbuphine combinations produced effects markedly greater than either drug alone.

Agonist and antagonist effects of dynorphin A-(1-13) in a thermal antinociception assay in rhesus monkeys

The agonist and antagonist effects of intravenously administered dynorphin A-(1-13) were characterized in the warm water (50 and 55 degrees C) tail withdrawal assay of antinociception in rhesus monkeys. The peptide dose-dependently elevated tail withdrawal latencies in 50 degrees C water, but was ineffective in 55 degrees C water. The antinociceptive effect of dynorphin was surmountably antagonized by quadazocine (0.1 mg/kg) and nor-binaltorphimine (3.2 mg/kg), but was not antagonized by clocinnamox (0.1 mg/kg); this pattern of antagonism is consistent with a kappa-opioid receptor mediated effect. Pretreatment with dynorphin A-(1-13) (0.032-3.2 mg/kg) antagonized the antinociceptive effects of U50,488 and U69,593 in 55 degrees C water, suggesting a low efficacy action of the peptide at the receptors activated by these kappa agonists. However, dynorphin A-(1-13) (3.2 mg/kg) did not antagonize other kappa agonists: bremazocine (0.018-0.056 mg/kg) and enadoline (0.0056-0.018 mg/kg). Taken together, these dynorphin A-(1-13) findings support the notion of functional kappa-opioid receptor subtypes, and it appears that dynorphin A-(1-13) has limited efficacy at one of these sites. Finally, dynorphin A-(1-13) (0.32 mg/kg) also antagonized the antinociceptive effects of the mu-agonist etonitazene (0.0018-0.01 mg/kg).

Effects of morphine and ketorolac on thermal allodynia induced by prostaglandin E2 and bradykinin in rhesus monkeys

When administered s.c. in the tail, both prostaglandin E2 (PGE2; 1.58-158.0 micrograms) and bradykinin (BK; 0.01-1.0 microgram) produced a dose-dependent allodynia in a warm-water tail-withdrawal assay in rhesus monkeys. PGE2 (A50 = 5.3 +/- 0.15 microgram) was 143-fold less potent than BK (A50 = 0.037 +/- 0.012 microgram) in producing allodynia at 42 degrees C. However, PGE2 (15.8 micrograms) was longer acting than an equieffective dose of BK (0.1 microgram), and the highest dose of PGE2 (158.0 micrograms) was the only treatment to produce allodynia when 38 degrees C water was used as the thermal stimulus, suggesting that PGE2 was a more efficacious allodynic agent than BK. Morphine (0.1-3.2 mg/kg) administered s.c. in the back completely blocked the allodynic effects of both BK (0.1 microgram) and PGE2 (15.8 micrograms), although morphine was more than twice as potent against BK (A50 = 0.26 +/- 0.085 mg/kg) than against PGE2 (A50 = 0.65 +/- 0.14 mg/kg). The effects of morphine were antagonized by the opioid antagonist quadazocine (0.1 mg/kg), indicating that morphine's effects were mediated by opioid receptors. The nonsteroidal anti-inflammatory drug ketorolac (0.32-10.0 mg/kg) administered s.c. in the back completely blocked the allodynic effects of BK (A50 = 0.60 +/- 0.095 mg/kg) but did not alter allodynia induced by PGE2. The antiallodynic effects of ketorolac against BK were not antagonized by quadazocine (1.0 mg/kg), indicating that these effects were not mediated by mu or kappa opioid receptors. Furthermore, relative to morphine, ketorolac displayed a slower onset and a longer duration of action. These findings suggest that the allodynic effects of BK in this procedure were mediated entirely by cyclooxygenase products of arachidonic acid metabolism, such as PGE2.

BW373U86, a delta-opioid receptor agonist, reverses bradykinin-induced thermal allodynia in rhesus monkeys

The synthetic delta-opioid receptor agonist BW373U86 (0.18-0.56 mg/kg s.c.) was studied in rhesus monkeys with a warm-water, tail-withdrawal assay, designed to detect bradykinin (0.1 microgram) and prostaglandin E2 (5-15.8 micrograms)-induced thermal allodynia. BW373U86 dose-dependently reversed bradykinin allodynia, but was ineffective against prostaglandin E2 allodynia. The BW373U86 dose-effect curve was shifted to the right by the delta-opioid receptor-selective antagonist naltrindole (1.0 mg/kg) but not by the mu-opioid receptor-selective antagonist quadazocine (0.1 mg/kg). The present findings add to the conditions in which delta-opioid receptor-mediated behavioral effects have been detected in primates, and suggest that delta-opioid agonists may be of therapeutic interest in the treatment of some types of hyperalgesic conditions.

Butorphanol: characterization of agonist and antagonist effects in rhesus monkeys

The effects of butorphanol were studied in assays of antinociception, respiratory depression, sedation, diuresis and reinforcing effects in rhesus monkeys, and opioid binding in monkey brain. Butorphanol (0.003-0.1 mg/kg s.c.) was effective in the warm-water tail withdrawal assay in 50 degrees C water but not in 55 degrees C. Over a similar dose range, butorphanol caused substantial respiratory depression, without an obvious plateau. Constrained quadazocine apparent pA2 analysis on the respiratory depressant and antinociceptive effects of butorphanol yielded different values between the two assays (respiratory depression pA2 = 6.61; antinociception pA2 = 8.26). Butorphanol (0.1 mg/kg) antagonized the antinociceptive effects of etonitazene in 55 degrees C water, but caused a nonparallel leftward shift in the U50,488 dose-effect curve; both effects were probably due to butorphanol's intermediate efficacy at mu receptors. Butorphanol (0.0001-0.003 mg/kg per injection i.v.) was self-administered; unlike other mu opioid agonists, its maximum effect was depressed after pretreatment with quadazocine (0.01-1.0 mg/kg). Butorphanol (0.003-0.32 mg/kg) was devoid of substantial sedative or muscle relaxant effects, as measured by observational rating scales. Butorphanol (0.01-0.1 mg/kg s.c.), unlike U50,488 (0.01-0.32 mg/kg) did not cause diuresis. Kappa agonist or antagonist effects of butorphanol were not detected in the present studies. This profile is consistent with butorphanol's binding characteristics in rhesus monkey brain which indicated 12-fold mu:kappa selectively and 34-fold mu:delta selectivity.

Antinociceptive and respiratory effects of nalbuphine in rhesus monkeys

Antinociceptive and respiratory effects of nalbuphine and other opioids were studied in rhesus monkeys. In a thermal, tail withdrawal assay, the kappa agonist enadoline and the mu agonists alfentanil and fentanyl produced maximum antinociceptive effects in all subjects and over a wide range of temperatures, whereas nalbuphine produced antinociceptive effects in only some subjects and only when the water temperature was < or = 50 degrees C. Naltrexone antagonized the antinociceptive effects of nalbuphine, alfentanil and enadoline; however, the magnitude of antagonism was not equal among agonists. In subjects that did not show an antinociceptive response to nalbuphine, nalbuphine (3.2-10.0 mg/kg) antagonized the antinociceptive effects of fentanyl but not enadoline. The irreversible opioid antagonist clocinnamox produced a parallel shift to the right in the nalbuphine dose-effect curve 1 hr after administration and decreased the maximum effect produced by nalbuphine 24 and 48 hr after administration. Nalbuphine had modest respiratory-depressant effects in monkeys breathing air and attenuated hyperventilation produced by 5% CO2. In contrast, alfentanil had marked respiratory-depressant effects in monkeys breathing air or 5% CO2 in air and these effects were antagonized by nalbuphine. Taken together, these results suggest nalbuphine has low efficacy at mu opioid receptors; however, quantitative differences between alfentanil and nalbuphine indicate a second (non-enadoline sensitive) receptor might also be important for the antinociceptive effects of nalbuphine.

Behavioral effects of the systemically active delta opioid agonist BW373U86 in rhesus monkeys

The behavioral effects of (+-)-4-((alpha R*)-alpha-((2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl)-3- hydroxybenzyl)-N,N-diethylbenzamide dihydrochloride (BW373U86), a nonpeptidic, systemically active, delta opioid agonist, were examined in rhesus monkeys. BW373U86, the mu agonist alfentanil and the kappa agonist U69,593 [(5 alpha,7 alpha,8 beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro- (4,5)dec-8-yl)benzeneacetamide] all produced a dose-dependent suppression of response rates maintained under a fixed ratio 30 schedule of food presentation. The rate-suppressing effects of BW373U86 lasted 1 to 2 hr and were no longer apparent after 4 hr. The selective delta antagonist naltrindole (NTI) antagonized the effects of BW373U86 with relatively high potency (pKB = 6.5) and the antagonist effects of NTI against BW373U86 lasted approximately 4 hr. NTI was less potent in antagonizing alfentanil (pKB = 5.1) and the highest dose of NTI examined (10.0 mg/kg) did not antagonize U69,593. BW373U86 did not generalize to the discriminative stimulus effects of the mu agonist alfentanil or the kappa agonist ethylketocyclazocine. BW373U86 also did not produce antinociceptive effects in the warm-water tail-withdrawal procedure, significant respiratory depressant effects in monkeys breathing either air or 5% CO2 or reinforcing effects in a self-administration procedure. The highest dose of BW373U86 examined (1.78 mg/kg) produced convulsions in one monkey. The high relative potency of NTI to antagonize the rate-suppressing effects of BW373U86 was consistent with the characterization of BW373U86 as a systemically active, delta-selective agonist in rhesus monkeys. Under the conditions evaluated in the present study, the delta receptors to which BW373U86 binds do not appear to mediate antinociceptive, respiratory depressant or reinforcing effects in monkeys.

Differential effects of systemically administered nor-binaltorphimine (nor-BNI) on kappa-opioid agonists in the mouse writhing assay

The opioid antagonist effects of systemically administered nor-binaltorphimine (nor-BNI) were evaluated against the kappa agonists CI-977, U69,593, U50,488, ethylketocyclazocine (EKC), Mr2034 and bremazocine, the mu agonist morphine and the alkaloid delta agonist BW-373U86 in the acetic acid-induced writhing assay in mice. All eight agonists completely and dose-dependently inhibited writhing. Antagonism of CI-977 was apparent 1 h after administration of 32 mg/kg nor-BNI, peaking after 4 h and was maintained for at least 4 weeks; no antagonist effects of nor-BNI were apparent after 8 weeks. Nor-BNI (32 mg/kg) caused little or no antagonism of morphine or BW-373U86 at 1 h and none at 24 h after nor-BNI administration. Subsequently, dose-effect curves for CI-977, U50,488, U69,593, EKC, Mr2034 and bremazocine were determined 24 h after pretreatment with 3.2, 10 and 32 mg/kg nor-BNI. Pretreatment with 3.2 mg/kg nor-BNI produced significant antagonism of all six kappa agonists, suggesting that their antinociceptive effects were mediated at least in part by nor-BNI-sensitive kappa receptors. At higher doses, nor-BNI dose-dependently shifted the agonist dose-effect curves of CI-977, U50,488, U69,593 and bremazocine, but not those of EKC and Mr2034, suggesting that the latter compounds may be producing effects via nor-BNI-insensitive receptors. Mu receptor involvement was demonstrated following a 24 h pretreatment with 32 mg/kg beta-FNA in combination with nor-BNI, which significantly increased the degree of antagonism of Mr2034 and EKC from that seen with nor-BNI alone.2+ off

In vivo determination of mu opioid receptor turnover in rhesus monkeys after irreversible blockade with clocinnamox

In a warm-water tail withdrawal antinociception assay performed at 45, 50 and 55 degrees C in the rhesus monkey, the irreversible opioid antagonist clocinnamox at a dose of 0.1 mg/kg s.c. produced an acute rightward shift of the dose-response curves of the selective mu opioid agonists alfentanil and morphine at all tested temperatures. In addition, clocinnamox depressed the maxima of the dose-response curves for both agonists at 50 and 55 degrees C. Analysis of these data according to Furchgott as modified by Black and Leff showed that clocinnamox acutely decreased mu opioid receptors available for alfentanil by 88%; receptor numbers returned to control levels with a half-life of 6.3 days. Assessment of receptor population changes after clocinnamox administration with either alfentanil or morphine gave essentially identical results: 2 to 4 weeks after clocinnamox, the receptor population not only returned to preclocinnamox levels, but actually showed an overshoot. In contrast, apparent values of alfentanil affinity; its efficacy, e; the theoretically obtainable maximum effect of the mu opioid antinociceptive system, Em; and the stimulus-response transducing factor, n; did not change significantly over time. Alfentanil showed a 29-fold higher affinity than morphine, the respective KA values being 0.84 mg/kg for alfentanil and 24 mg/kg for morphine. The efficacy of alfentanil was always 2- to 3-fold higher than that of morphine for any temperature tested, the efficacies of both mu opioid agonists being higher at lower temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Kappa opioid antagonist effects of systemically administered nor-binaltorphimine in a thermal antinociception assay in rhesus monkeys

The effects of subcutaneously administered nor-binaltorphimine (nor-BNI; 1.0 and 3.2 mg/kg) were examined in the warm-water (50 degrees C and 55 degrees C) tail-withdrawal assay in rhesus monkeys (n = 3). Nor-BNI alone produced variable antinociceptive effects in 50 degrees C water up to 3.5 hr after administration but was completely ineffective against the 55 degrees C stimulus. Pretreatment with nor-BNI under conditions where it was devoid of antinociceptive effects produced rightward shifts in dose-effect curves for the kappa opioid agonist U50,488 for as long as 14 and 21 days after 1.0 and 3.2 mg/kg of nor-BNI, respectively. Under conditions when U50,488 dose-effect curves were shifted, nor-BNI (3.2 mg/kg) also caused rightward shifts in the antinociceptive dose-effect curves of the kappa agonist U69,593 but not in those of the mu agonist alfentanil or the kappa agonists [5R-(5,7,8,beta)]N-methyl-N-[7- (1-pirrolidinyl)1-oxaspiro[4,5]dec-8-yl]4-benzofuranaceta mide, bremazocine, ethylketocyclazocine and Mr2033. It is concluded that under the present conditions, nor-BNI acts as a selective kappa opioid antagonist with an extremely long duration of action. These findings are also consistent with the notion that nor-BNI may antagonize only compounds acting at a subtype of kappa opioid receptor.

Opioid thermal antinociception in rhesus monkeys: receptor mechanisms and temperature dependency

The antinociceptive effects of the opioid agonists etonitazene and alfentanil, as well as the agonist/antagonists nalbuphine, [(1)-beta-2'-hydroxy-2,9-dimethyl-5-phenyl-6,7-benzomorphan (GPA 1657)] and profadol were studied in the warm water (48 degrees and 55 degrees C) tail-withdrawal assay in rhesus monkeys. Etonitazene and alfentanil produced dose-dependent increases in tail-withdrawal latency up to the maximum possible latency of 20 sec in 48 degrees and 55 degrees C water. Nalbuphine, GPA 1657 and profadol produced the maximum possible effect only at 48 degrees C, and were ineffective at 55 degrees C. The opioid antagonist quadazocine produced a dose-dependent antagonism of all agonists except profadol. In a Schild plot analysis, apparent pA2 values for quadazocine with alfentanil, etonitazene and nalbuphine were homogeneous (7.3-7.7 mol/kg), suggesting their effects were probably mediated by mu opioid receptors. The apparent pA2 value for GPA 1657 was significantly lower (6.2 mol/kg), suggesting GPA 1657 may have produced antinociception by a non mu receptor-mediated mechanism. The selective delta antagonist naltrindole (0.32-1.0 mg/kg) antagonized the antinociceptive effect of GPA 1657. The kappa-selective antagonist nor-binaltorphimine (nor-BNI, 3.2 mg/kg) caused a small rightward shift in the GPA 1657 dose-effect curve. Nalbuphine, GPA 1657 or profadol produced a rightward shift in the alfentanil dose-effect curve in 55 degrees C water, consistent with possible low-efficacy mu agonist effects of these compounds. These studies suggest agonists may be differentiated based on antinociceptive effectiveness, receptor selectivity and intrinsic efficacy in the rhesus monkey tail-withdrawal procedure.

Prostaglandin E2-induced thermal hyperalgesia and its reversal by morphine in the warm-water tail-withdrawal procedure in rhesus monkeys

Four monkeys were seated in primate restraint chairs and the terminal 10 cm of their shaved tails were dipped into water maintained at a series of temperatures ranging from 38-54 degrees C. The latency to tail withdrawal from several temperatures was measured and temperature-effect curves for each monkey were generated. When administered s.c. into the tail, prostaglandin E2 (PGE2; 0, 1.58, 5.0 and 15.8 micrograms) produced a dose-dependent hyperalgesia manifested as dose-dependent leftward shifts in the temperature-effect curves. This hyperalgesia peaked 15 to 45 min after administration and lasted for approximately 2 hr. PGE2-induced hyperalgesia was mediated locally, because administration of 15.8 micrograms of PGE2 into the back had no effect on the temperature-effect curve. The hyperalgesic effects of PGE2 were reversed potently by morphine (0.32-3.2 mg/kg), and the effects of morphine were antagonized in a surmountable manner by both the opioid antagonist quadazocine (0.1 mg/kg) and by systemic administration of the charged opioid antagonist quaternary naltrexone (3.2 mg/kg). These results indicate that PGE2 produces thermal hyperalgesia in rhesus monkeys and also suggests that this hyperalgesia may be reversed by activation of peripheral opioid receptors. PGE2-induced hyperalgesia in the warm-water tail-withdrawal procedure may provide a useful assay for evaluating the effects of pharmacological and nonpharmacological treatments on hyperalgesia associated with inflammation in primates.

Effects of clonidine, dexmedetomidine and xylazine on thermal antinociception in rhesus monkeys

The antinociceptive effects of the s.c. administration of the alpha-2 agonists clonidine (0.0032-1.0 mg/kg), dexmedetomidine (0.001-0.032 mg/kg) and xylazine (0.1-3.2 mg/kg) were examined in the warm-water tail withdrawal assay in rhesus monkeys. The three agonists were dose-dependently effective in this assay; their potency order being dexmedetomidine > clonidine > xylazine. The alpha-2 antagonist idazoxan (0.1-3.2 mg/kg) caused dose-dependent and roughly parallel rightward shifts in the dose-effect curves for the three agonists. Apparent pA2 analysis with idazoxan yielded homogeneous values for the three agonists, supporting the notion that similar receptors mediate their antinociceptive effects. The opioid antagonist quadazocine (1.0 mg/kg) did not antagonize the antinociceptive effects of clonidine and xylazine, indicating that opioid receptors do not participate in the effects of the compounds in this assay. At dose ranges found to be effective in the antinociceptive assay, clonidine, dexmedetomidine and xylazine also dose-dependently caused sedation, muscle relaxation, bradycardia and moderate respiratory depression. The sedative, muscle relaxant and respiratory depressant effects of xylazine could be antagonized by idazoxan, suggesting that these effects may be mediated through alpha-2 receptors. These data indicate that the three imidazoline alpha-2 agonists, clonidine, dexmedetomidine and xylazine are effective s.c. in the warm-water tail withdrawal assay in rhesus monkeys, but only at doses that produce other behavioral and physiological effects.

Apparent pA2 analysis on the respiratory depressant effects of alfentanil, etonitazene, ethylketocyclazocine (EKC) and Mr2033 in rhesus monkeys

Respiratory depression is a limiting factor in the therapeutic use of opioid analgesics. It has been suggested that respiratory depression is mediated by mu rather than kappa receptors and may involve a decrease in central nervous system sensitivity to hypercapnia. This study investigated opioid receptor mechanisms underlying respiratory depression in unanesthetized rhesus monkeys (n = 3) breathing air or 5% CO2 in air into a pressure displacement head plethysmograph. Apparent pA2 analyses of s.c. quadazocine (a mu-selective antagonist) were carried out on the effects of cumulative doses of s.c. bremazocine, ethylketocyclazocine (EKC) and (+/-)-(1-R/S,5-R/S,2 = R/S)-5,9-dimethyl-2'-hydroxy-2- tetrahydrofurfuryl-6,7-benzomorphan (Mr2033) (compounds with kappa agonist effects in other in vivo assays), alfentanil and etonitazene (compounds with mu agonist effects in other in vivo assays). Alfentanil, bremazocine, EKC and Mr2033 were approximately equipotent in causing dose-dependent depression of respiratory minute volume of CO2-stimulated and air respiration, whereas etonitazene was approximately 10-fold more potent than the above compounds. Dose-effect curves for respiratory frequency, tidal volume and respiratory minute volume for all of the agonists except bremazocine were shifted to the right by increasing quadazocine doses. Together with data previously obtained in drug discrimination and analgesia assays, results of the present study demonstrating homogeneous pA2 values for quadazocine with alfentanil, etonitazene, EKC and Mr2033 strongly suggest that the latter two compounds decrease respiratory function in rhesus monkeys by acting on mu receptors.

The effect of NMDA antagonists in the radial arm maze task with an interposed delay

N-Methyl-d-aspartate (NMDA) receptors mediate the triggering of hippocampal long-term potentiation (LTP), a current physiological model of memory. This model was tested in the rat through the effect of (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801, a novel noncompetitive NMDA antagonist) on the radial arm maze (RAM) task with a 15-minute delay interposed at the midpoint choice. In two separate experiments, substereotypical drug doses of MK-801 and phencyclidine (a dissociative anesthetic with NMDA antagonist properties) were given intraperitoneally, before the trial or at the start of the delay. "Efficiency" was impaired in both tasks, but near-instantaneous use of encoded information seemed to be unaffected. This evidence would support a proposed role for NMDA-mediated pathways (and possibly LTP) in delayed stages of memory formation or use.

A novel NMDA antagonist, MK-801, impairs performance in a hippocampal-dependent spatial learning task

N-Methyl-d-aspartate (NMDA) receptors have been implicated with the triggering of long-term potentiation, a currently studied physiological model of learning and memory. The compound (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801) has recently been classified as a potent and selective NMDA antagonist acting at the associated ion channel. After determination of the highest intraperitoneal dose of MK-801 at which increases in activity (measured in photocell activity cages and 3-arm maze) were not observed (0.2 mg/kg), rats that had been previously trained to obtain food pellets in an 8-arm radial maze up to criterion were tested with 0.1 and 0.2 mg/kg doses. Dose-related decreases in "efficiency" in the task were found. The present findings support the suggestion that NMDA antagonists cause impairments in "working memory" and also support the status of long-term potentiation as a physiological model of memory.