A thermostable bacterial cocaine esterase rapidly eliminates cocaine from brain in nonhuman primates.

A long-acting, thermostable bacterial cocaine esterase (CocE) has been identified that rapidly degrades cocaine with a K(M) of 1.33+0.085 µM. In vivo evaluation of CocE has shown protection against convulsant and lethal effects of cocaine in rodents, confirming the therapeutic potential of CocE against cocaine overdose. However, the current study is the first to evaluate the effects of CocE on cocaine brain levels. Positron emission tomogrpahy neuroimaging of [(11)C]cocaine was used to evaluate the time course of cocaine elimination from brain in the presence and absence of CocE in nonhuman primates. Systemic administration of CocE eliminated cocaine from the rhesus-monkey brain approximately three times faster than control conditions via peripheral actions through attenuating the input function from blood plasma. The efficiency of this process is sufficient to alleviate or prevent adverse central nervous system effects induced by cocaine. Although the present study used tracer doses of cocaine to access brain clearance, these findings further support the development of CocE for the treatment of acute cocaine toxicity.

Interaction of 5-HT2A and 5-HT2C receptors in R(-)-2,5-dimethoxy-4-iodoamphetamine-elicited head twitch behavior in mice.

Drug-elicited head-twitch behavior is a useful model for studying hallucinogen activity at 5-HT(2A) receptors in the mouse. Chemically diverse compounds active in this assay yield biphasic dose-effect curves, but there is no compelling explanation for the "descending" portion of these functions. A set of experiments was designed to test the hypothesis that the induction of head-twitch behavior is mediated by agonist actions at 5-HT(2A) receptors, whereas the inhibition of head-twitch behavior observed at higher doses results from competing agonist activity at 5-HT(2C) receptors. The effects of the phenethylamine hallucinogen R(-)-2,5-dimethoxy-4-iodoamphetamine (DOI) on head-twitch behavior were studied over a range of doses in the mouse, generating a characteristic biphasic dose-response curve. Pretreatment with the selective 5-HT(2A) antagonist (+)-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol (M100907) shifted only the ascending limb of the DOI dose-effect function, whereas pretreatment with the nonselective 5-HT(2A/2C) antagonist 3-{2-[4-(4-fluorobenzoyl)piperidin-1-yl]ethyl}quinazoline-2,4(1H,3H)-dione (ketanserin) produced a parallel shift to the right in the DOI dose-response curve. Administration of the 5-HT(2C) agonist S-2-(chloro-5-fluoro-indol-l-yl)-1-methylethylamine (Ro 60-0175) noncompetitively inhibited DOI-elicited head-twitch behavior across the entire dose-effect function. Finally, pretreatment with the selective 5-HT(2C) antagonists 6-chloro-5-methyl-1-[(2-[2-methylpyrid-3-yloxy]pyrid-5yl)carbamoyl]indoline (SB242084) or 8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulfonamido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4,5]decane-2,4-dione hydrochloride (RS 102221) did not alter DOI-elicited head-twitch behavior on the ascending limb of the dose-response curve but shifted the descending limb of the DOI dose-response function to the right. The results of these experiments provide strong evidence that DOI-elicited head-twitch behavior is a 5-HT(2A) agonist-mediated effect, with subsequent inhibition of head-twitch behavior being driven by competing 5-HT(2C) agonist activity.

14 beta-O-cinnamoylnaltrexone and related dihydrocodeinones are mu opioid receptor partial agonists with predominant antagonist activity.

14-O-Cinnamoyl esters of naltrexone (6) were synthesized and evaluated in isolated tissue assays in vitro and in vivo in mouse antinociceptive assays. Their predominant opioid receptor activity was mu receptor (MOR) antagonism, but the unsubstituted cinnamoyl derivative (6a) had partial MOR agonist activity in vitro and in vivo. When compared to the equivalent 14-cinnamoylaminomorphinones (5), the cinnamoyloxy morphinones (6) as MOR antagonists had a shorter duration of action and were less effective as pseudoirreversible antagonists. The antinociceptive activity of the cinnamoyloxycodeinones (7) was not significantly greater than that of the morphinones (6), but they exhibited no evidence of any pseudoirreversible MOR antagonism. In both respects, these profiles differed from those of the equivalent 14-cinnamoylaminocodeinones (4).

A comparison of the physiological, behavioral, neurochemical and microglial effects of methamphetamine and 3,4-methylenedioxymethamphetamine in the mouse.

3,4-Methylenedioxymethamphetamine (MDMA) and methamphetamine (METH) are amphetamine analogues with similar persistent neurochemical effects in the mouse which some have described as neurotoxicity. We attempted to identify dose regimens of MDMA and METH with similar effects on behavioral and physiological variables in the mouse, then quantified the effects of these dose regimens on neurochemistry and microglial markers. Four discrete injections of saline, MDMA (10, 20, or 30 mg/kg), or METH (5 or 10 mg/kg) were administered to mice at 2 h intervals. Body weight was quantified immediately before each injection, and 2 h after the last injection, while core temperature and locomotor activity were continuously monitored via radiotelemetry. Mice were killed 72 h after the final injection and brains were rapidly dissected on ice. Dopamine content in various brain regions was quantified via high pressure liquid chromatography (HPLC), and microglial activation was assessed by saturation binding of the peripheral benzodiazepine receptor (PBR) ligand 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide ([(3)H]PK11195). Specific dose regimens of MDMA and METH induced similar reductions in body weight, depletions of dopamine and its metabolites, and similar hyperthermic and locomotor stimulant effects, but only METH activated microglia in striatum. These results suggest that repeated high doses of MDMA and METH that produce hyperthermia, locomotor stereotypy, weight loss and neurochemical depletion are not consistently accompanied by microglial activation. The finding that METH, but not MDMA, induces microglial effects in the striatum consistent with neurotoxicity might imply different mechanisms of toxic action for these two psychostimulants.

Relative reinforcing effects of cocaine, remifentanil, and their combination in rhesus monkeys.

Human polydrug abusers often take combinations of opioids and stimulants, but it is not clear why. Behavioral economics with demand curve analysis is uniquely able to separate two of the possibilities: that the drug combination increases the reinforcing potency of the component drugs or that the drug combination is a more effective reinforcer than either drug alone. Rhesus monkeys self-administered a range of doses of cocaine, remifentanil, and combinations of the drugs through indwelling intravenous catheters; the number of responses required for each drug infusion increased across drug-availability sessions. Combining small doses of cocaine and remifentanil that by themselves resulted in very low rates of responding yielded rates of responding that were higher than the maximum maintained by any dose of the constituent drugs. Nevertheless, demand curve analysis demonstrated that the drug combination was equally elastic as the component drugs, indicating that it was not more effective as a reinforcer than either cocaine or remifentanil alone. This suggests that enhanced self-administration of this particular drug combination is due primarily to the drug enhancement of the potency of the other drug.

Hallucinogen-like actions of 5-methoxy-N,N-diisopropyltryptamine in mice and rats.

Few studies have examined the effects of 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT) in vivo. In these studies, 5-MeO-DIPT was tested in a drug-elicited head twitch assay in mice where it was compared to the structurally similar hallucinogen N,N-dimethyltryptamine (N,N-DMT) and challenged with the selective serotonin (5-HT)2A antagonist M100907, and in a lysergic acid diethylamide (LSD) discrimination assay in rats where its subjective effects were challenged with M100907 or the 5-HT 1A selective antagonist WAY-100635. Finally, the affinity of 5-MeO-DIPT for three distinct 5-HT receptors was determined in rat brain. 5-MeO-DIPT, but not N,N-DMT, induced the head twitch responses in the mouse, and this effect was potently antagonized by prior administration of M100907. In rats trained with LSD as a discriminative stimulus, there was an intermediate degree (75%) of generalization to 5-MeO-DIPT and a dose-dependent suppression of response rates. These interoceptive effects were abolished by M100907, but were not significantly attenuated by WAY-100635. Finally, 5-MeO-DIPT had micromolar affinity for 5-HT 2A and 5-HT 2C receptors, but much higher affinity for 5-HT 1A receptors. 5-MeO-DIPT is thus effective in two rodent models of 5-HT2 agonist activity, and has affinity at receptors relevant to hallucinogen effects. The effectiveness with which M100907 antagonizes the behavioral actions of this compound, coupled with the lack of significant antagonist effects of WAY-100635, strongly suggests that the 5-HT 2A receptor is an important site of action for 5-MeO-DIPT, despite its apparent in vitro selectivity for the 5-HT 1A receptor.

Antinociceptive, hypothermic, hypotensive, and reinforcing effects of a novel neurotensin receptor agonist, NT69L, in rhesus monkeys.

Neurotensin (NT) is a tridecapeptide found in the nervous system, as well as elsewhere in the body. It has anatomic and functional relationships to dopaminergic neurons in brain. NT has been implicated in the actions of antipsychotic drugs and psychostimulants, and animal studies suggest that neurotensin directly injected into brain has reinforcing effects. Previously, we showed that one of our brain-penetrating analogs of neurotensin, NT69L (N-methyl-L-Arg, L-Lys, L-Pro, L-neo-Trp, L-tert-Leu, L-Leu), has many pharmacological effects in rats including antinociception, hypothermia, and blockade of the hyperactivity caused by psychostimulants (cocaine, D-amphetamine, and nicotine). Since these studies in rats suggest that this compound may have clinical use in humans, we were interested to know what effects NT69L had in primates. NT69L caused a potent antinociceptive effect against capsaicin (0.1 mg)-induced allodynia in 46 degrees C water in rhesus monkeys, inducing 40% of the maximal possible effect at an intravenous dosage of 0.03 mg/kg; its hypotensive effects precluded evaluation of higher dosages. Core temperature measured by rectal probe was modestly reduced at 0.01 and 0.03 mg/kg. In an intravenous self-administration procedure, NT69L was without reinforcing effects at any dose, including those that caused other pharmacological effects, and did not alter cocaine-maintained behavior when administered as a pretreatment.

Reinforcing and discriminative stimulus effects of 1-benzylpiperazine and trifluoromethylphenylpiperazine in rhesus monkeys.

1-Benzylpiperazine (BZP) and 1-(3-trifluoromethylphenyl)piperazine (TFMPP) are two designer drugs that are often sold in combination tablets via the internet. The discriminative stimulus properties and reinforcing effects of these compounds have not previously been assessed in laboratory primates. In this regard, the reinforcing effects of BZP and TFMPP (alone, and in combination) were assessed via intravenous self-administration in rhesus monkeys previously trained to self-administer cocaine, while the discriminative stimulus effects of these compounds were determined in rhesus monkeys trained to discriminate amphetamine (AMPH) from saline. BZP was an effective reinforcer in self-administration tests, and appeared to induce long-lasting direct effects on behavior following sessions where BZP intakes were large. Additionally, BZP occasioned AMPH-appropriate responding in a dose-dependent manner, and produced full generalization in all monkeys tested. In contrast, TFMPP was not self-administered by any subjects and occasioned essentially no AMPH-appropriate responding at any dose tested. Non-contingent TFMPP administration had direct effects on behavior and abolished subsequent cocaine-maintained responding. Similarly, self-administration of various ratios of BZP:TFMPP combinations engendered less responding than did BZP alone. The present results suggest that BZP has abuse liability of the amphetamine type, but that such effects are not shared by TFMPP.

Transient reinforcing effects of phenylisopropylamine and indolealkylamine hallucinogens in rhesus monkeys.

Relatively few studies have assessed the reinforcing effects of hallucinogenic compounds, and no such studies have attempted to engender contingent responding for these compounds in animals with behavioral histories that include experience with serotonergically mediated reinforcing effects. The objectives of the present study were to investigate the capacity of several hallucinogenic compounds to maintain self-administration behavior in rhesus monkeys with a previous history of 3,4-methylenedioxymethamphetamine (MDMA) self-administration, and to compare these effects across a range of doses of drugs from two structural classes (indolealkylamines and phenylisopropylamines). The results indicate that no compound generated reliable responding and that no subject ever self-administered 4-iodo-2,5-dimethoxyphenylisopropylamine (DOI) at rates above those engendered by contingent saline. However, 3 out of 4 subjects did respond at rates between 0.75 and 3.0 responses/s in one or more sessions where N,N-dimethyltryptamine (DMT), mescaline or psilocybin were available. During some of these sessions in which self-administration was maintained, animals earned a majority of all available infusions and appeared intoxicated by the end of the session. This pattern of transient self-administration may indicate that these compounds have weak reinforcing effects, or mixed reinforcing and aversive effects.

Characterization of scratching responses in rats following centrally administered morphine or bombesin.

The aim of this study was to characterize scratching behavior elicited by central administration of morphine or bombesin in rats, and to determine the role of opioid receptors in scratching induced by both pruritogenic agents. Central administration included intracisternal (i.c.), intrathecal (i.t.), and intracerebroventricular (i.c.v.) routes. Scratching events made with hind paws were counted by observers blinded to treatment conditions. Intracisternal morphine (0.01-0.1 microg) produced dose-dependent increases in scratching; the maximum response to i.c. morphine 0.1 microg was approximately 500 scratches within a 1-hour period. Neither i.t. nor i.c.v. morphine significantly increased scratching. Bombesin (0.01-0.32 microg) elicited robust scratching following i.c. administration. The maximum response to i.c. bombesin 0.32 microg was approximately 4000 scratches within a 1-hour period. Both i.t. and i.c.v. bombesin produced profound scratching at similar doses. Antagonist studies confirmed that mu-opioid receptors selectively mediate i.c. morphine-induced scratching. However, selective mu-, kappa-, and delta-opioid antagonists did not attenuate i.c. bombesin-induced scratching. These results demonstrate that morphine and bombesin elicit scratching through different receptor mechanisms, at different central sites, and to different degrees.

Effects of the delta-opioid receptor agonist SNC80 on learning relative to its antidepressant-like effects in rats.

Delta-opioid receptor agonists produce decreases in immobility in the forced swim test, suggesting that these compounds have antidepressant-like activity. There is also the possibility that these compounds decrease immobility in the forced swim test by disrupting learning processes that occur during the swim, or with successive swim exposures, thus falsely identifying them as having "antidepressant" potential. This study investigated the effects of the delta-opioid receptor agonist, SNC80, on responding in a repeated-acquisition procedure and in the forced swim test in rats, and the effects were compared directly to those of scopolamine, a compound known to disrupt memory and learning. SNC80 disrupted acquisition of a response sequence (learning) and produced a significant antidepressant-like effect in the forced swim test. Scopolamine, however, produced larger decrements in learning without producing behavioral changes consistent with an antidepressant-like profile of action. These results suggest that SNC80 produces antidepressant-like activity through a mechanism independent of its disruptive effects on learning.

Ultra-long antagonism of kappa opioid agonist-induced diuresis by intracisternal nor-binaltorphimine in monkeys.

Kappa opioid receptor (KOR) agonists such as U-50488H and bremazocine are analgesics and diuretics. In monkeys, the selective KOR antagonist, nor-binaltorphimine (nor-BNI), produces a long-lasting antagonism of the antinociceptive effects of U-50488H but not those of bremazocine, suggesting that KOR-mediated antinociception may occur through two distinct KORs. The aim of this study was to characterize the antagonist effect of nor-BNI against the diuretic effects of U-50488H and bremazocine in monkeys. Urine outputs were collected over 3 h subsequent to i.m. administration of KOR agonists. Both U-50488H (0.032-1 mg/kg) and bremazocine (0.00032-0.01 mg/kg) dose-dependently increased urine output and the diuretic effect reached a plateau at higher doses. The maximum effect of either U-50488H or bremazocine was approximately 15 ml/kg/3 h of urine. Pretreatment with intracisternal nor-BNI 0.32 mg significantly blocked both U-50488H (0.18 mg/kg)- and bremazocine (0.0032 mg/kg)-induced diuresis for 20 weeks. However, the same dose of nor-BNI 0.32 mg given subcutaneously was not effective. These results demonstrate that central KOR mediate KOR agonist-induced diuresis in monkeys. More important, this study provides functional evidence for a homogenous population of KOR underlying KOR-mediated diuresis and illustrates a unique pharmacological profile of nor-BNI-induced ultra-long KOR antagonism in vivo.

Studies of micro-, kappa-, and delta-opioid receptor density and G protein activation in the cortex and thalamus of monkeys.

The aim of this study was to investigate the relative density of micro -, kappa-, and delta-opioid receptors (MOR, KOR, and DOR) and guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding stimulated by full agonists in cortical and thalamic membranes of monkeys. The binding parameters [Bmax (femtomoles per milligram)/Kd (nanomolar)] were as follows: [3H][d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) (MOR; 80/0.7), [3H]U69593 [(5alpha,7alpha,8beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl) benzeneacetamide] (KOR; 116/1.3), and [3H][d-Pen2,d-Pen5]-enkephalin (DPDPE) (DOR; 87/1.3) in the cortex; [3H]DAMGO (147/0.9), [3H]U69593 (75/2.5), and [3H]DPDPE (22/2.0) in the thalamus. The relative proportions of MOR, KOR, and DOR in the cortex were 28, 41, and 31% and in the thalamus were 60, 31, and 9%. Full selective opioid agonists, DAMGO (EC50 = 532-565 nM) and U69593 (EC50 = 80-109 nM) stimulated [35S]GTPgammaS binding in membranes of cortex and thalamus, whereas SNC80 [(+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethyl-benzamide] (DOR; EC50 = 68 nM) was only active in cortical membranes. The magnitudes of [35S]GTPgammaS binding stimulated by these agonists were similar in the cortex, ranging from 17 to 25% over basal binding. In the thalamus, DAMGO and U69593 increased [35S]GTPgammaS binding by 44 and 23% over basal, respectively. Opioid agonist-stimulated [35S]GTPgammaS binding was blocked selectively by antagonists for MOR, KOR, and DOR. The amount of G protein activated by agonists was highly proportional to the relative receptor densities in both regions. These results distinguish the ability of opioid agonists to activate G proteins and provide a functional correlate of ligand-binding experiments in the monkey brain. In particular, the relative densities of opioid receptor binding sites in the two brain areas reflect their functional roles in the pharmacological actions of opioids in the central nervous system of primates.

Effect of opioid receptor antagonists on hypothalamic-pituitary-adrenal activity in rhesus monkeys.

Some opioid antagonists increase the release of adrenocorticotropic hormone (ACTH) and cortisol in humans and, therefore, may indicate that endogenous opioids modulate hypothalamic-pituitary-adrenal axis activity. The type of opioid receptor that may be related to these endocrine effects is unknown. The purpose of this experiment was to evaluate the ability of different opioid antagonists to increase ACTH and cortisol plasma levels in rhesus monkeys. Eight monkeys received intramuscular injections of various antagonists: 0.0032-1.0 mg/kg naltrexone, 0.1-3.2 mg/kg naltrindole (delta-selective), 0.032-0.32 mg/kg clocinnamox (mu-selective), and 1-3.2 mg/kg nor-binaltorphimine (kappa-selective). Naltrexone, 0.1-1.0 mg/kg, increased ACTH levels, whereas naltrindole and clocinnamox failed to increase ACTH levels. Nor-binaltorphimine, 1-3.2 mg/kg, increased ACTH concentrations on the day of injection, but not at a time when other assays continue to demonstrate kappa-antagonism (24 h). Cortisol concentrations generally followed the same pattern as the ACTH concentrations, but the incremental differences in cortisol release between doses were less clear. Thus, opioid modulation of ACTH and cortisol plasma levels is not clearly associated with a particular opioid receptor. Although the kappa-antagonist increased ACTH and cortisol release on the day of injection, some evidence suggests that this endocrine effect may be due to mechanisms other than those mediated by the kappa-receptor. Alternatively, the naltrexone-induced increase of ACTH and cortisol plasma levels may be caused by activity at multiple opioid receptors or some uncharacterized receptor. Finally, the increased release of ACTH and cortisol may be a response to naltrexone's aversive properties.

Observing responses maintained by stimuli associated with cocaine or remifentanil reinforcement in rhesus monkeys.

RATIONALE: The stimuli associated with drug reinforcement may be particularly relevant to drug abuse and relapse. OBJECTIVES: The study measured behavior maintained by conditioned reinforcing stimuli in an observing response procedure. METHODS: The experiment was conducted with rhesus monkeys in three stages: 1) discriminative control was established by reinforcing responding on one lever with either intravenous cocaine or remifentanil in the presence of one stimulus and extinguishing the response in the presence of another stimulus, 2) discriminative control was suspended by not presenting the stimuli, and 3) a final stage was implemented wherein the stimuli from the first stage were presented only when one or more responses were made on a second (observing) lever. RESULTS: Under FR1 conditions, observing responses were maintained at low rates, but increased markedly when the response requirement was increased. CONCLUSIONS: The procedure maintained observing responses quite well and may be useful to an analysis of conditioned reinforcement based on drug reinforcement.

Determinants of the behavioral effects of opioids and their antagonists: contributions of the Laboratory of Psychobiology.

RATIONALE: The behavioral pharmacology of opioids has been influenced significantly by the research and writings of Drs. Peter B. Dews, Roger T. Kelleher, and William H. Morse, their colleagues, and their students. OBJECTIVE: Their conceptual and methodological approach to the topic is reviewed briefly, and three areas of research are described to provide an empirical perspective. RESULTS: The objective of determining the general effects of opioids on behavior is described; the effects of opioids on schedule-controlled behavior and punished behavior are described and compared to non-opioids. The differential effects of opioid antagonists on responding reinforced by different stimuli are also presented. CONCLUSION: The conceptual and methodological approach taken by the group, as well as their discoveries in the behavioral pharmacology of opioids, will continue to exert a positive influence on the field.

Reaction routes in biochemical reaction systems: algebraic properties, validated calculation procedure and example from nucleotide metabolism.

Elementary flux modes (direct reaction routes) are minimal sets of enzymes that can operate at steady state, with all irreversible reactions used in the appropriate direction. They can be interpreted as component pathways of a (bio)chemical reaction network. Here, two different definitions of elementary modes are given and their equivalence is proved. Several algebraic properties of elementary modes are then presented and proved. This concerns, amongst other features, the minimal number of enzymes of the network not used in an elementary mode and the situations where irreversible reactions are replaced by reversible ones. Based on these properties, a refined algorithm is presented, and it is formally proved that this algorithm will exclusively generate all the elementary flux modes of an arbitrary network containing reversible or irreversible reactions or both. The algorithm is illustrated by a biochemical example relevant in nucleotide metabolism. The computer implementation in two different programming languages is discussed.

Relative reinforcing strength of three N-methyl-D-aspartate antagonists with different onsets of action.

The potential contribution of onset and duration of pharmacological action to the reinforcing strength of three intravenously delivered N-methyl-D-aspartate antagonists was evaluated in this study. The onsets and durations of action of ketamine, phencyclidine, and dizocilpine were evaluated by observation and tabulation of their behavioral effects in rhesus monkeys after i.v. administration. The reinforcing effects of each drug were tested in a paradigm in which the fixed ratio requirements for i.v. drug injection were increased systematically. The peak observable effect of ketamine occurred immediately after its administration. There were some immediately observable effects of phencyclidine, although the peak effect of phencyclidine was delayed for 3 to 10 min. Dizocilpine had few immediate effects and a peak effect 32 min after administration. Ketamine had the shortest duration of action, followed by phencyclidine and dizocilpine. Analysis of demand curves and response output curves that were normalized to account for potency differences among the drugs revealed that ketamine and phencyclidine were equally effective as reinforcers, and they were both much stronger reinforcers than was dizocilpine. The data therefore suggest that a fast onset of action increases the reinforcing strength of drugs, although duration of action may play a role as well.

Relative reinforcing effects of three opioids with different durations of action.

The role of duration of action on the relative reinforcing effects of three opioid drugs (fentanyl, alfentanil, and remifentanil) was evaluated. Duration and onset of action were determined using measures of respiratory depression and antinociception after i.v. administration. Effects on minute volume of respiration indicated that each of the three opioids had immediate onsets of action after i.v. administration. Fentanyl's duration of suppression of respiration and antinociception was longer than that of alfentanil, which was longer than that of remifentanil. Reinforcing strength was measured in i.v. self-administration studies in which the fixed ratio resulting in drug administration was increased from one session to the next. Comparisons were made of the behavioral economic variables P(max) and area under the demand curve (O(max)). Remifentanil maintained higher rates of responding than did alfentanil, and alfentanil maintained higher rates of responding than did fentanyl. When normalized demand functions were compared, however, the drugs did not differ significantly from each other in terms of P(max) or O(max). These data agree with those of others who have suggested that duration of action is not an important contributor to drugs' reinforcing strength.

Orphanin FQ inhibits capsaicin-induced thermal nociception in monkeys by activation of peripheral ORL1 receptors.

1. Orphanin FQ (OFQ), an endogenous peptide for ORL1 receptors, has been identified. Although the actions of OFQ have much in common with those of opioid peptides at the cellular level, behavioral studies in rodents seem conflicting. 2. The aim of this study was to investigate the potential pronociceptive or antinociceptive function of peripheral ORL1 receptors in primates. Experiments were conducted to verify whether local administration of OFQ can attenuate capsaicin-induced nociception and whether peripheral ORL1 receptors selectively mediate the local action of OFQ in monkeys. 3. Capsaicin (100 microg) was administered subcutaneously in the tail to locally evoke a nociceptive response (thermal allodynia/hyperalgesia), which was manifested as a reduced tail-withdrawal latency in normally innocuous 46 degreeC warm water. 4. Co-administration of OFQ (1--30 microg) with capsaicin in the tail dose-dependently inhibited thermal nociception. However, a locally effective dose of OFQ (30 microg), when applied in the back, did not inhibit capsaicin-induced nociception. 5. OFQ-induced local antinociception was antagonized by a small dose (10 microg) of J-113397, a selective ORL1 receptor antagonist, in the tail. Similarly, s.c. administration of 10 microg of J-113397 in the back did not antagonize local antinociception of OFQ. 6. In addition, s.c. administration of either OFQ or J-113397 in the tail alone did not change its thermal nociceptive threshold. Local administration of opioid receptor antagonists selective for mu, kappa, and delta opioid receptors did not antagonize OFQ-induced local antinociception. Local administration of J-113397 also did not interfere with the local actions of mu, kappa, and delta opioid agonists in the tail. 7. These results provide the first functional evidence that activation of peripheral ORL1 receptors produces thermal antinociception in primates and this action is independent of antinociception produced at classical opioid receptors.