Effects of mixed-action kappa/mu opioids on cocaine self-administration and cocaine discrimination by rhesus monkeys

kappa-Opioid agonists may functionally antagonize some behavioral effects of cocaine, but the role of mixed kappa/mu receptor activity is unclear. The effects of three mixed kappa/mu agonists (MCL-101, (-)cyclorphan, and Mr2034) and one kappa-selective agonist (enadoline) on cocaine self-administration and cocaine discrimination were compared in rhesus monkeys. Acute treatment with all kappa agonists dose dependently reduced cocaine-maintained responding and produced a downward shift in the cocaine self-administration dose-effect curve (0.001-0.32 mg/kg/inj, i.v.). During 7 days of chronic treatment, (-)cyclorphan (0.0032-0.032 mg/kg/h) and MCL-101 (0.0032-0.032 mg/kg/h) each dose dependently reduced cocaine self-administration maintained by a dose near the peak of the cocaine self-administration dose-effect curve. MCL-101 (0.032 mg/kg/h) produced selective and sustained decreases in cocaine self-administration, whereas (-)cyclorphan (0.032 mg/kg/h) had selective but transient effects. In addition, these mixed kappa/mu agonists produced fewer side effects (some salivation) than the kappa-selective agonist (sedation, salivation, emesis). However, none of these kappa agonists substituted for or antagonized cocaine's discriminative stimulus effects in monkeys trained to discriminate cocaine (0.4 mg/kg, i.m.) from saline. Thus, kappa and mixed kappa/mu-opioid agonists may reduce cocaine self-administration without altering cocaine's discriminative stimulus effects. Mixed kappa/mu agonists appear to offer some advantages over selective kappa agonists as potential treatments for cocaine abuse.

Effects of heroin and its metabolites on schedule-controlled responding and thermal nociception in rhesus monkeys: sensitivity to antagonism by quadazocine, naltrindole and beta-funaltrexamine

Recent studies have reported differences in the receptor mechanisms and intrinsic efficacies of heroin and its metabolites 6-acetylmorphine and morphine in rodents. The present study examined the generality of these findings to rhesus monkeys using two behavioral procedures. In an assay of schedule-controlled behavior, response rates were recorded under a fixed-ratio 30 schedule of food presentation. In an assay of thermal nociception, tail-withdrawal latencies were measured from warm water (42-58 degrees C). Heroin, 6-acetylmorphine and morphine produced dose-dependent rate-decreasing and antinociceptive effects. Antagonism studies were conducted with the competitive mu-selective antagonist quadazocine, the competitive delta-selective antagonist naltrindole, and the irreversible mu-selective antagonist beta-funaltrexamine (beta-FNA). Quadazocine dose-dependently antagonized the effects of all three opioids. Quadazocine pA2 values were similar across drugs and assays (7.4-7.8) and similar to quadazocine pA2 values for antagonism of other mu agonists. In contrast, naltrindole did not alter the effects of any of the opioids. beta-FNA antagonized the rate-decreasing and antinociceptive effects of heroin and morphine. Dose-effect data for heroin- and morphine-induced antinociception alone and after beta-FNA treatment were used to estimate in vivo apparent efficacy values (tau). Tau values (95% confidence limits) were 8.1 (6.9-9.6) for heroin and 2.6 (2.5-2.9) for morphine, but this difference is relatively small. These results suggest that the rate-decreasing and antinociceptive effects of heroin, 6-acetylmorphine and morphine are mediated by pharmacologically similar populations of mu opioid receptors in rhesus monkeys. The in vivo apparent efficacy of heroin at mu receptors was similar to or only slightly greater than that of morphine.

Effects of chronic d-amphetamine treatment on cocaine- and food-maintained responding under a second-order schedule in rhesus monkeys

Effective treatment of opioid dependence with methadone and of tobacco dependence with nicotine illustrates the potential usefulness of agonist medications for drug abuse treatment. The monoamine-releaser d-amphetamine is one drug under consideration as an agonist pharmacotherapy for cocaine dependence. To assess the concordance between preclinical evaluations and ongoing clinical trials, the present study examined the effects of chronic treatment with saline or d-amphetamine on cocaine- and food-maintained responding in rhesus monkeys. Cocaine injections and food pellets were available under a second-order schedule during alternating daily sessions of cocaine and food availability. d-Amphetamine (0.01-0.1 mg/kg per h i.v. for 7 consecutive days) dose-dependently decreased self-administration of a unit dose of cocaine (0.01 mg/kg per injection) at the peak of the cocaine self-administration dose-effect curve. d-Amphetamine (0.032-0.1 mg/kg per h for 7 days) also decreased self-administration of a broad range of cocaine doses (0.0032-0.1 mg/kg per injection) and produced rightward and downward shifts in the cocaine dose-effect curve. Food-maintained responding was usually decreased less than cocaine self-administration, and few signs of toxicity were noted. To evaluate the effects of a longer treatment regimen, d-amphetamine (0.1 mg/kg per h) was administered for 28 consecutive days. d-Amphetamine nearly eliminated self-administration of cocaine (0.01 mg/kg per injection) throughout this treatment, whereas food-maintained responding returned to baseline levels after approximately 9 days. These preclinical findings are concordant with recent clinical studies and suggest that chronic d-amphetamine may selectively decrease cocaine-taking behavior in rhesus monkeys, possibly by producing a selective decrease in the reinforcing effects of cocaine.

Effects of chronic d-amphetamine treatment on cocaine- and food-maintained responding under a progressive-ratio schedule in rhesus monkeys

RATIONALE

d-Amphetamine is a candidate agonist medication for the treatment of cocaine dependence, and evaluation of d-amphetamine effects on abuse-related effects of cocaine in preclinical assays is warranted.

OBJECTIVE

This study was designed to assess the effects of chronic d-amphetamine treatment on cocaine- and food-maintained responding under a progressive-ratio schedule in rhesus monkeys. The effects of schedule manipulations on cocaine and food-maintained responding were also examined for comparison with d-amphetamine effects.

METHODS

Key-press responding under a progressive-ratio schedule resulted in the delivery of cocaine (0.032 mg/kg per injection) or 1 g food pellets. The effect of manipulating cocaine dose (saline, 0.001-0.1 mg/kg per injection) or the number of food pellets delivered (0, 1 and 4 pellets) was determined. Subsequently, three schedule parameters were manipulated: (1) starting ratio value, (2) increments of the ratio progression, and (3) duration of post-reinforcer time-outs when the ratio value was constant. Finally, the effects of 10-day treatment with d-amphetamine (0.01-0.1 mg/kg per hour) were examined.

RESULTS

Break points increased as a function of cocaine dose or the number of food pellets, and similar break points were maintained by delivery of 0.032 mg/kg per injection cocaine and 1 food pellet. Manipulation of schedule parameters produced similar effects on responding maintained by cocaine (0.032 mg/kg per injection) or food (1 pellet). In contrast, d-amphetamine produced a dose-dependent decrease in cocaine-maintained responding and had less consistent effects on food-maintained responding.

CONCLUSIONS

These results are consistent with the hypothesis that chronic treatment with d-amphetamine decreases cocaine self-administration in rhesus monkeys, possibly by attenuating the reinforcing effects of cocaine.

Diminished interleukin-6 response to proinflammatory challenge in men and women after intravenous cocaine administration

Cocaine abuse is associated with increased rates of infections, including human immunodeficiency virus, and cocaine has immunomodulatory effects in experimental animal and cellular models. When challenged by antigens, tissues release cytokine polypeptides that signal a complex balance of cellular and humoral immune responses. Placement of indwelling venous catheters also leads to surrounding tissue inflammation, mediated partially by local production and release of the proinflammatory cytokine, IL-6. Thus, catheter placement provides a model for examination of cocaine's immunological effects. Thirty healthy men and women with a history of cocaine use participated in this study of neuroendocrine and immunological responses to iv injection of 0.4 mg/kg cocaine or saline placebo. After injection, blood samples were collected from the antecubital vein of the opposite arm via an indwelling venous catheter at 2, 4, 8, 12, 16, 20, 30, 40, 60, 80, 120, 180, and 240 min. Cocaine, ACTH, cortisol, and dehydroepiandrosterone concentrations peaked at 8, 12, 40, and 20 min, respectively. Stimulation of IL-6 at 240 min was markedly reduced in subjects receiving cocaine compared with subjects receiving placebo (3.85 +/- 0.49 vs. 11.64 +/- 2.21 pg/ml; P = 0.0019, by two-tailed t test). Gender and menstrual cycle phase did not significantly influence most endocrine or IL-6 measures, although the small number of subjects limits the power of these comparisons. Because cocaine stimulates the hypothalamic-pituitary-adrenal axis, IL-6 suppression may be a consequence of corticosteroid release. Cocaine-induced suppression of proinflammatory IL-6 may mediate impaired host defenses to infections.

Delta opioid antagonist effects of buprenorphine in rhesus monkeys

Buprenorphine is an opioid with high affinity for delta, mu and kappa opioid receptors. The delta receptor-mediated effects of buprenorphine have not been studied. Thus, the present study examined the delta receptor-mediated effects of buprenorphine in rhesus monkeys. assays of receptor binding and agonist-stimulated GTP S binding confirmed that buprenorphine had high affinity for, and low efficacy at, delta receptors. In an assay of schedule-controlled responding for food presentation in four monkeys, buprenorphine produced little effect alone, but it antagonized the effects of the delta agonist SNC80, the mu agonist morphine and the kappa agonist U50,488. Buprenorphine was approximately 30-fold less potent as a delta antagonist than as a mu or kappa antagonist. In three monkeys trained to discriminate SNC80 from saline, buprenorphine alone produced only saline-appropriate responding, and buprenorphine pretreatment antagonized the discriminative stimulus effects of SNC80. In a fourth monkey, buprenorphine produced a partial substitution for SNC80 that could be blocked by the delta-selective antagonist naltrindole but not by the mu-selective antagonist quadazocine. These results indicate that, in rhesus monkeys, buprenorphine has very low efficacy at delta receptors, and that buprenorphine produces delta receptor-mediated effects with lower potency than it produces mu or kappa receptor-mediated effects.

Comparison of plasma cocaine levels during a "binge" pattern of cocaine administration in male and female rhesus monkeys

RATIONALE

Cocaine is often abused in a "binge" pattern, but little is known about changes in plasma cocaine concentrations or cocaine pharmacokinetics during administration of multiple cocaine doses. Moreover, the extent to which gender may influence plasma cocaine levels during a cocaine binge has not been studied in rhesus monkeys.

OBJECTIVES

To compare the effects of repeated injections of the same dose of cocaine (0.4 mg/kg or 0.8 mg/kg, i.v.) on plasma cocaine concentrations, cocaine pharmacokinetics and behavioral responses in male and female rhesus monkeys.

METHODS

Four injections of cocaine (0.4 mg/kg or 0.8 mg/kg, i.v.) were administered at 30-min intervals to five or six male and five or six female rhesus monkeys. Samples for plasma cocaine analysis were collected at 2, 4, 8, 16 and 24 min after the first three injections. After the fourth cocaine injection, additional samples were collected at 10-min intervals over 150 min.

RESULTS

Plasma cocaine peaks and nadirs increased monotonically after successive cocaine injections ( P<0.0001). Peak plasma cocaine levels measured at 2 min after cocaine administration were higher after 0.8 mg/kg cocaine than after 0.4 mg/kg cocaine ( P<0.006). There were no significant gender differences in time to peak plasma cocaine levels (t(max)), peak cocaine concentrations (C(max)) or half-life (t(1/2); min) at either dose of cocaine. Group average behavioral ratings were similar in males and females after each dose of cocaine.

CONCLUSIONS

Peak plasma cocaine concentrations increased progressively during low and high dose cocaine binge episodes, and there were no significant gender differences in cocaine pharmacokinetics. These findings demonstrate the feasibility of simulating "binge" patterns of cocaine administration in rhesus monkeys.

Kappa opioid antagonist effects of the novel kappa antagonist 5'-guanidinonaltrindole (GNTI) in an assay of schedule-controlled behavior in rhesus monkeys

RATIONALE

Opioid receptors are divided into three types: kappa, mu, and delta receptors. Receptor-selective antagonists are useful experimental tools for evaluation of opioid receptor-mediated processes. 5'-Guanidinonaltrindole (GNTI) was recently developed as a novel kappa-selective antagonist.

OBJECTIVES

To evaluate the potency, time course, and selectivity of GNTI's opioid antagonist effects in rhesus monkeys in an assay of schedule-controlled responding.

METHODS

Five rhesus monkeys were trained to respond under a fixed ratio 30 schedule of food reinforcement. The rate-decreasing effects of the kappa agonists U50,488 and U69,593, the mu agonist morphine, and the delta agonist SNC80 were examined alone and after pretreatment with GNTI (0.1 and 1.0 mg/kg i.m.; 1 h to 14 days).

RESULTS

U50,488, U69,593, morphine, and SNC80 dose-dependently decreased response rates in this procedure. GNTI produced a dose- and time-dependent antagonism of the rate-decreasing effects of U50,488. The kappa antagonist effects of GNTI had a slow onset and a long duration of action, and peak antagonist effects were observed after 24 h. A higher dose of 3.2 mg/kg GNTI eliminated responding in one monkey and was not studied further. The antagonist effects of GNTI were kappa selective, because 1.0 mg/kg GNTI also antagonized the effects of U69,593, but not those of morphine or SNC80.

CONCLUSIONS

These results suggest that GNTI is a potent and selective kappa antagonist with a slow onset and long duration of action in rhesus monkeys. Relative to the prototype kappa antagonist nor-binaltorphimine, GNTI may have some advantages as a tool for the study of kappa receptor-mediated processes.

Antagonism of the antinociceptive and discriminative stimulus effects of heroin and morphine by 3-methoxynaltrexone and naltrexone in rhesus monkeys

It has been suggested that heroin and morphine may act on different opioid receptor populations in rodents. In support of this hypothesis, the opioid antagonist 3-methoxynaltrexone was reported to be more potent as an antagonist of the antinociceptive effects of heroin than of morphine in mice and rats. To assess the generality of this finding across species and experimental endpoints, the present study compared the potencies of naltrexone and 3-methoxynaltrexone as antagonists of heroin and morphine in two behavioral assays in rhesus monkeys. In the thermal nociception study, tail-withdrawal latencies were measured from water heated to 50 degrees C. In the heroin discrimination study, monkeys were trained to discriminate 0.1 mg/kg heroin from saline in a two-key, food-reinforced drug discrimination procedure, and percentage of heroin-appropriate responding and response rates were measured. Both heroin and morphine produced dose-dependent antinociception, increases in percentage of heroin-appropriate responding, and decreases in response rates. Heroin was approximately 20-fold more potent than morphine. Naltrexone (0.032-0.1 mg/kg) was equipotent in antagonizing all effects of heroin and morphine (pA(2) values = 7.90-8.22). 3-Methoxynaltrexone (0.1-3.2 mg/kg) was also equipotent in antagonizing the antinociceptive, discriminative stimulus, and rate-suppressant effects of heroin and morphine; however, 3-methoxynaltrexone was approximately 100-fold less potent than naltrexone (pA(2)/pK(B) values = 5.96-6.36). These results suggest that heroin and morphine act on pharmacologically similar populations of opioid receptors in rhesus monkeys, and also indicate that 3-methoxynaltrexone does not differentially antagonize the effects of heroin and morphine in rhesus monkeys.

An inpatient study of the effects of buprenorphine on cigarette smoking in men concurrently dependent on cocaine and opioids

The effect of sublingual buprenorphine on cigarette smoking was examined in 23 adult men with DSM III-R diagnosis of concurrent opiate and cocaine dependence. After admission to a clinical research ward, subjects were detoxified with methadone (10-50 mg/day), then were drug-free for 6 days before random assignment to either 4 or 8 mg/day of buprenorphine. Gradually increasing daily sublingual doses of buprenorphine were administered for 5 days, then subjects were maintained on 4 or 8 mg/day of buprenorphine for 12 days. Each subject's preferred brand of cigarettes was available ad libitum throughout the study. Five responses (FR 5) on a key were required to earn each cigarette. The time and number of cigarettes were recorded by an automated cigarette dispenser. Subjects acquired significantly more cigarettes during the buprenorphine induction and maintenance phases (25.5+/-2.0) than during the drug-free phase (18.5+/-1.8; p<0.0002). During buprenorphine induction, the number of cigarettes acquired was positively correlated with increasing doses of buprenorphine (p<0.001) and the inter-cigarette interval was significantly shorter during buprenorphine maintenance than during drug-free conditions (p<0.001). These data showed that daily administration of the partial mu opioid agonist buprenorphine was associated with increased smoking in men concurrently dependent on opiates and cocaine. These findings are consistent with previous reports of opioid-cigarette interactions.

The effects of heroin on prolactin levels in male rhesus monkeys: use of cumulative-dosing procedures

There is considerable evidence that mu opioid receptors are involved in the regulation of anterior pituitary function. For example, in nonhuman primates and humans, mu agonists generally increase prolactin (PRL) levels. In contrast, mu antagonists decrease or have no effect on PRL levels. The goal of this study was to assess the potential utility of cumulative-dosing procedures to evaluate the endocrine effects of mu opioid receptor ligands. The effects of single and multiple, cumulative doses of the mu agonist heroin and the mu-selective antagonist quadazocine on PRL levels were investigated in four male rhesus monkeys. Cumulative dose-response curves were determined by infusing increasing drug doses at 60 min intervals over 290 min. Blood samples for PRL analysis were collected at 25 and 50 min after each cumulative infusion. Samples were collected at similar time points following single drug dose administration. Heroin (0.01-0.32 mg/kg, IV) administration dose-dependently increased PRL levels. Maximum levels of heroin-induced PRL levels were equivalent after single and cumulative doses. Quadazocine alone (0.032-1.0 mg/kg, IM) did not alter PRL levels significantly. However, quadazocine (0.1 mg/kg, IM) antagonized heroin-stimulated increases in PRL levels and produced a significant rightward shift in the heroin dose-effect curve. These data suggest that a cumulative-dosing procedure similar to that used in behavioral pharmacology may be useful to study the endocrine pharmacology of mu opioids in rhesus monkeys.

Effects of mu-opioid agonists on cocaine- and food-maintained responding and cocaine discrimination in rhesus monkeys: role of mu-agonist efficacy

Mu-opioid agonists decrease cocaine self-administration in laboratory studies and cocaine use by many cocaine- and opioid-dependent polydrug abusers. To assess the role of mu-agonist efficacy as a determinant of these effects, this study evaluated cocaine- and food-maintained responding by rhesus monkeys (Macaca mulatta) during chronic treatment with saline or the high-efficacy mu-agonist fentanyl (0.001-0.01 mg/kg/h), the intermediate-efficacy mu-agonist morphine (0.032-0.32 mg/kg/h), or the low-efficacy mu-agonists nalbuphine (0.1-1.0 mg/kg/h) and butorphanol (0.0032-0.032 mg/kg/h). Responding was maintained by cocaine and food under a second order schedule of reinforcement during multiple daily sessions of cocaine and food availability. Saline and each opioid dose were administered continuously for 7 consecutive days during availability of each cocaine dose. All four mu-agonists produced dose-dependent and sustained decreases in cocaine self-administration across a range of cocaine doses (0.0032-0.1 mg/kg/injection). Nalbuphine and butorphanol produced the greatest decreases in cocaine self-administration and the smallest effects on food-maintained responding. Morphine and fentanyl produced smaller decreases in cocaine self-administration, and undesirable effects precluded evaluation of higher fentanyl and morphine doses. Decreases in cocaine self-administration produced by nalbuphine and butorphanol probably did not reflect a general blockade of cocaine's abuse-related effects, because nalbuphine and butorphanol did not block the discriminative stimulus effects of cocaine in monkeys trained to discriminate 0.4 mg/kg cocaine from saline in a food-reinforced drug discrimination procedure. These results suggest that low-efficacy mu-agonists may decrease cocaine self-administration to a greater degree and with fewer undesirable effects than high-efficacy mu-agonists.

Effects of gonadal steroid hormone treatments on opioid antinociception in ovariectomized rhesus monkeys

RATIONALE

Gonadal steroid hormones altered opioid antinociception under some conditions in rodents, and we reported previously that chronic estradiol enhanced kappa but not mu opioid antinociception in ovariectomized rhesus monkeys. Sex differences have also been observed in the antinociceptive effects of opioid agonists. These findings suggest that gonadal hormones may modulate opioid antinociception.

OBJECTIVES

To extend our previous studies of estradiol by examining the effects of progesterone alone, estradiol in combination with progesterone, and testosterone alone on opioid antinociception in ovariectomized rhesus monkeys.

METHODS

Opioid effects were studied during chronic treatment with vehicle (sesame oil) or with progesterone alone (P; 0.32 mg/kg per day), a combination of progesterone+estradiol (P+E; 0.32 mg/kg per day P + 0.002 mg/kg per day E), or testosterone alone (T; 0.32 mg/kg per day). Opioid antinociception in a warm-water tail-withdrawal procedure was examined with the selective kappa opioid agonist U50,488, the selective mu agonist morphine, and the two mixed-action opioids butorphanol and nalbuphine.

RESULTS

The steroid treatment regimens produced physiological levels of progesterone and estradiol similar to peak levels observed during the luteal phase of the menstrual cycle and physiological levels of testosterone similar to those observed in intact males. Treatment with P, P+E, or T did not alter baseline thermal nociception. P+E significantly increased the potency of U50,488 at 50 degrees C but not at 54 degrees C. Gonadal hormone treatments had little or no effect on antinociception produced by morphine, butorphanol, or nalbuphine at either temperature.

CONCLUSIONS

These findings further suggest that chronic treatment with physiological levels of gonadal hormones may modulate the antinociceptive effects of U50,488 in ovariectomized rhesus monkeys.

Temporal concordance of cocaine effects on mood states and neuroendocrine hormones

Cocaine stimulates the release of adrenocorticotropin hormone (ACTH) and cortisol in both clinical and preclinical studies, but the temporal sequence of cocaine-induced changes in other hormones and affective states is unclear. The purpose of this study was to analyze the pattern and temporal concordance of cocaine-induced changes in ACTH, cortisol, dihydroepiandrosterone (DHEA), epinephrine, heart rate and subjective reports of euphoria. Six healthy men who met Diagnostic and Statistical Manual-IV (DSM-IV) criteria for cocaine abuse provided informed consent for participation. Cocaine (0.4 mg/kg) or saline placebo was infused intravenously over 1 min under double-blind conditions. Euphoria, ACTH, epinephrine and heart rate increased significantly within 8 to 12 min after i.v. cocaine administration in all subjects (P<0.01-0.001). Moreover, the increases in euphoria, ACTH, epinephrine and heart rate each were significantly correlated with increases in plasma cocaine levels (P<0.001). Euphoria increased significantly within 2 min after i.v. cocaine injection, as plasma cocaine levels were increasing, and peak euphoria was reported at 10 min (P<0.001). Peak ACTH values were measured at 8.7 (+/-0.8) min after cocaine injection (P<0.01). Peak levels of epinephrine were measured at 10 (+/-1) min after cocaine injection (P<0.05). Peak increases in heart rate occurred at 11.7 (+/-1.1) min after cocaine injection (P<0.05). Peak levels of cortisol and DHEA were measured at 36 (+/-4.0) and 28.7 (+/-4.3) min after cocaine injection (P<0.01 and P<0.01). The temporal concordance between cocaine-induced stimulation of ACTH, epinephrine and subjective euphoria suggests that these hormonal changes are significant concomitants of the abuse-related effects of cocaine. The similarities between these hormonal profiles, the subjective effects of cocaine and the effects of "stress" are discussed.

Interactions between cocaine and dopamine agonists on cardiovascular function in squirrel monkeys

Conscious squirrel monkeys were treated i.v. with cocaine and various dopamine agonist drugs. Cocaine produced a dose-dependent increase in blood pressure, heart rate, and the rate-pressure product (RPP). The dopamine D1 receptor agonist (+/-)-6-chloro-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF 82958) produced effects comparable to cocaine. The D1 agonist (+/-)-6-chloro-7, 8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF 81297) also produced increases in blood pressure and heart rate but was much less potent than either cocaine or SKF 82958. The partial D1 agonist (+/-)-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SKF 77434) did not significantly affect any cardiovascular parameters. The D2 agonist quinpirole slightly decreased blood pressure and increased heart rate. As such, the RPP only slightly increased. The selective dopamine uptake inhibitor 1-[2-[bis-(4-fluorphenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR 12909) produced increases in blood pressure, heart rate, and RPP, but again these effects were smaller and only seen at doses higher than cocaine. Effects similar to those with GBR 12909 were seen with the dopamine autoreceptor antagonist cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin (UH 232). The combination of GBR 12909, SKF 82958, or SKF 77434 with cocaine produced effects that were clearly subadditive. The effects of quinpirole in combination with cocaine were comparable to, or lower than, those of cocaine alone on blood pressure and RPP. The effects on heart rate were additive. Only UH 232 produced additive effects with cocaine for all three measures. As dopamine agonists have been proposed as potential treatments for cocaine abuse, these results suggest that dopamine D1 agonists and uptake inhibitors can be used safely in combination with cocaine. Caution may be warranted, however, with the use of dopamine autoreceptor antagonists in the treatment of cocaine abuse.

Mixed kappa agonists and mu agonists/antagonists as potential pharmacotherapeutics for cocaine abuse: synthesis and opioid receptor binding affinity of N-substituted derivatives of morphinan

A series of new N-substituted derivatives of morphinan was synthesized and their binding affinity for the three opioid receptors (mu, delta, and kappa) was determined. A paradoxical effect of N-propargyl (MCL-117) and N-(3-iodoprop-(2E)-enyl) (MCL-118) substituents on the binding affinities for the mu and kappa opioid receptors was observed. All of these novel derivatives showed a preference for the mu and kappa versus delta binding.

Effects of indatraline and buprenorphine on self-administration of speedball combinations of cocaine and heroin by rhesus monkeys

The simultaneous intravenous (i.v.) administration of heroin and cocaine, called a "speedball," is often reported clinically, and identification of effective pharmacotherapies is a continuing challenge. We hypothesized that treatment with combinations of a dopamine reuptake inhibitor, indatraline, and a mu partial agonist, buprenorphine, might reduce speedball self-administration by rhesus monkeys more effectively than either drug alone. Speedballs (0.01 mg/kg/inj cocaine + 0.0032 mg/kg/inj heroin) and food (1 g banana pellets) were available in four daily sessions on a second-order schedule of reinforcement [fixed ratio (FR)4; variable ratio (VR)16:S]. Monkeys were treated for 10 days with saline or ascending dose combinations of indatraline (0.001-0.032 mg/kg/day) and buprenorphine (0.00032-0.01 mg/kg/day). Two combinations of indatraline (0.32 and 0.56 mg/kg/day) + buprenorphine (0.10 and 0.18 mg/kg/day) significantly reduced speedball self-administration in comparison to the saline treatment baseline (p <.01-.001), whereas the same doses of each compound alone had no significant effect on speedball-maintained responding. Daily treatment with 0.56 mg/kg/day indatraline + 0.18 mg/kg/day buprenorphine produced a significant downward shift in the speedball dose-effect curve (p <.01) and transient changes in food-maintained responding. These findings suggest that medication mixtures designed to target both the stimulant and opioid component of the speedball combination may be an effective approach to polydrug abuse treatment.

Antinociceptive effects of delta-opioid agonists in Rhesus monkeys: effects on chemically induced thermal hypersensitivity

The effects of SNC80 and other structurally related delta-opioid receptor agonists were assessed under conditions of chemically induced hypersensitivity to thermal stimuli in four rhesus monkeys. The shaved tail of each monkey was exposed to warm water (38, 42, 46, and 50 degrees C), and the tail-withdrawal latency from each temperature was recorded. The effects of drugs on the temperature that produced a 10-s tail-withdrawal latency (the T(10) value) were examined. Capsaicin (0.01-0.32 mg) injected into the tail of monkeys dose dependently decreased the T(10), indicating that capsaicin increased sensitivity to thermal stimuli. A dose of 0.1 mg of capsaicin decreased the T(10) from 48.0 to 42.1 degrees C (a -5.9 degrees C change) 15 min after injection. SNC80 (1.0-10.0 mg/kg s.c.) dose dependently blocked the capsaicin-induced decrease in the T(10), and 10.0 mg/kg SNC80 fully blocked the effects of capsaicin. The delta-selective antagonist naltrindole (0.1-1.0 mg/kg) dose dependently antagonized the effects of SNC80, whereas a mu-selective dose of the opioid antagonist quadazocine (0.1 mg/kg) did not. Two other delta-selective agonists, SNC162 (1.0-10.0 mg/kg) and SNC243A (1.0-10.0 mg/kg), also dose dependently blocked capsaicin-induced thermal hypersensitivity. In contrast, neither SNC67 (10.0 mg/kg), which is the (-)-enantiomer of SNC80, nor the nonsteroidal anti-inflammatory drug (NSAID) ketorolac (1.0-10.0 mg/kg) modified the effects of capsaicin. SNC80 was also effective in reversing thermal hypersensitivity induced by prostaglandin E(2) (0.0158 mg) and Freund's complete adjuvant (10% concentration). These findings suggest that delta-agonists have antinociceptive effects in primates under conditions of chemically induced thermal hypersensitivity and might be effective under a broader range of conditions than clinically available NSAIDs.

Effects of cocaine on luteinizing hormone in women during the follicular and luteal phases of the menstrual cycle and in men

Cocaine stimulates luteinizing hormone (LH) release in rhesus monkeys and in men, but its effects on LH in women are unknown. Cocaine (0.2 and 0.4 mg/kg i.v.) was administered to groups of follicular and luteal phase women (N = 22) and to men (N = 12) to examine the influence of gender and menstrual cycle phase on cocaine and LH interactions. All subjects met American Psychiatric Association Diagnostic and Statistical Manual IV criteria for cocaine abuse, and menstrual cycle phase was verified by estradiol and progesterone measures. Baseline LH levels were equivalent between groups. Peak cocaine levels did not differ significantly between men and women and averaged between 87 +/- 21 and 124 +/- 18 ng/ml after 0.2 mg/kg cocaine and between 227 +/- 22 and 287 +/- 21 ng/ml after 0.4 mg/kg cocaine. The lower dose of cocaine (0.2 mg/kg) significantly increased LH levels in men (P < 0.001) but not in women at either phase of the menstrual cycle. The higher dose of cocaine (0.4 mg/kg) stimulated significant increases in LH in men (P < 0.001) and in women at both phases of the menstrual cycle (P < 0.004-0.001). Although cocaine's effects on LH in women were dose-dependent, there were no significant differences as a function of menstrual cycle phase. LH remained significantly elevated longer in men (32 min) than in women (8 and 12 min). This gender difference in cocaine's potency in stimulating LH was unexpected.

Effects of GABA agonists and GABA-A receptor modulators on cocaine discrimination in rhesus monkeys

RATIONALE

Dopaminergic systems thought to mediate the abuse-related effects of cocaine are under inhibitory control by GABAergic systems. These findings suggest that GABA agonists may attenuate some abuse-related effects of cocaine.

OBJECTIVE

To assess the effects of GABA receptor agonists and GABA-A receptor modulators on cocaine discrimination in rhesus monkeys.

METHODS

Rhesus monkeys were trained to discriminate 0.4 mg/kg cocaine from saline in a two-key, food-reinforced drug discrimination task. The effects of the GABA-A agonist muscimol, the GABA-B agonist baclofen, the barbiturate GABA-A receptor modulator pentobarbital, and the benzodiazepine GABA-A modulators triazolam and imidazenil were examined alone and as pretreatments to cocaine. For comparison, the effects of pentobarbital pretreatment on the cocaine-like discriminative stimulus effects of amphetamine were also examined.

RESULTS

When administered alone, the GABA agonists and GABA-A receptor modulators produced primarily saline-appropriate responding. When administered as pretreatments to cocaine, pentobarbital attenuated the discriminative stimulus effects of cocaine in all monkeys tested, and the high efficacy benzodiazepine agonist triazolam attenuated cocaine's effects in three of five monkeys. Muscimol, baclofen and the low efficacy benzodiazepine agonist imidazenil did not alter cocaine's discriminative stimulus effects. Although pentobarbital blocked the effects of the monoamine reuptake blocker cocaine, it did not alter the cocaine-like effects of the monoamine releaser amphetamine.

CONCLUSIONS

These results are consistent with the hypothesis that GABA-A receptor modulators attenuate the discriminative stimulus effects of cocaine in rhesus monkeys by decreasing the activity of dopaminergic systems. Direct GABA receptor agonists may be less effective in blocking the abuse-related effects of cocaine in rhesus monkeys.

The effects of cocaine on basal and human chorionic gonadotropin-stimulated ovarian steroid hormones in female rhesus monkeys

Cocaine stimulates gonadotropin (luteinizing hormone) release from the anterior pituitary in humans and in rhesus monkeys, but its acute effects on ovarian steroid hormones are unknown. The acute effects of cocaine and placebo on estradiol and progesterone were studied in 13 drug-naive female rhesus monkeys during the mid-follicular (days 8-10) and the mid-luteal (days 21-23) phases of the menstrual cycle. Each monkey was her own control under cocaine and placebo conditions. Samples for ovarian steroid hormone analysis were collected before and at 15-min intervals for 300 min after cocaine or placebo administration. In follicular phase females, estradiol levels increased significantly within 15 min after cocaine (0.8 mg/kg i.v.) administration (P <.008) but did not change after placebo administration. Estradiol remained significantly above baseline for 45 min (P <.002-0.02). In contrast, in mid-luteal phase females, estradiol did not change after cocaine or placebo administration. Basal progesterone levels did not change after cocaine or placebo administration in either mid-follicular or mid-luteal phase females. After hCG (500 I.U. i.m.) was administered to mid-luteal phase females, cocaine (0.4 and 0.8 mg/kg i.v.) and placebo administration did not increase or decrease estradiol or progesterone. One implication of these findings is that cocaine-induced increases in follicular phase estradiol levels could disrupt folliculogenesis and contribute to the menstrual cycle abnormalities observed during chronic cocaine self-administration.

Interactions between kappa opioid agonists and cocaine. Preclinical studies

Kappa opioid agonists inhibit dopamine release from mesolimbic dopaminergic neurons and attenuate some behavioral effects of cocaine in rodents. Evidence that kappa opioid agonists may act as functional antagonists of cocaine led us to examine their interactions with cocaine's abuse-related effects in rhesus monkeys. In cocaine self-administration studies, four arylacetamides (U50,488, enadoline, (-) spiradoline and PD117302) and four benzomorphans (ethylketocyclazocine [EKC], bremazocine, Mr2033 and cyclazozine) each were administered as continuous infusions over 10 days. EKC, Mr2033, bremazocine, U50,488 and enadoline produced significant dose-dependent and sustained decreases in cocaine self-administration and also decreased food-maintained responding at some doses. Emesis and sedation were occasionally observed during the first two days of kappa agonist treatment, but tolerance developed rapidly to these effects. Cyclazocine, PD117302 and spiradoline did not significantly alter cocaine self-administration. The behavioral effects of EKC and U50,488 were antagonized by both the kappa opioid antagonist nor-binaltorphimine and the non-selective opioid antagonist naloxone. In general, compounds with mixed activity at both kappa and mu opioid receptors (e.g. EKC, Mr2033) decreased cocaine self-administration more consistently and with fewer or less severe undesirable side effects than more selective kappa agonists (e.g. U50,488, spiradoline). Although several kappa agonists decreased cocaine self-administration, EKC and U50,488 did not consistently block the discriminative stimulus effects of cocaine in monkeys trained to discriminate cocaine from saline. The extent to which kappa agonist-induced decreases in cocaine self-administration reflect an antagonism of cocaine's abuse-related effect remains to be determined.

Effects of dopamine D1-like and D2-like agonists in rats trained to discriminate cocaine from saline: influence of experimental history

Effects of D1-like and D2-like agonists were compared in rats (Rattus norvegicus) with differing levels of experience (24 or 9 mo) in a cocaine discrimination procedure (5.6 mg/kg cocaine; fixed-ratio 20 schedule of food presentation). Cocaine, d-amphetamine, and D2-like agonists (quinelorane, 7-OH-DPAT) dose-dependently substituted for cocaine in both groups of rats. In contrast, D1-like agonists (SKF 82958, SKF 77434) substituted for cocaine only in rats with less discrimination experience. Pretreatment with D2-like agonists increased the stimulus effects of low cocaine-doses in both groups, whereas D1-like agonists produced these effects only in rats with less discrimination experience. The data suggest that the stimulus effects of cocaine overlap with those of D2-like agonists across a broader range of conditions than with those of D1-like agonists. Thus, D2-like receptors may play an especially important role in cocaine's behavioral effects.

Acute effects of nalmefene on LH, prolactin, and testosterone in male rhesus monkeys

The effects of the long-acting opioid antagonist, nalmefene [17-N-cyclopropylmethyl-3,14-beta-dihydroxy-4, 5-alpha-epoxy-6-methylene morphinan hydrochloride] on LH, T, and prolactin release in rhesus monkeys are unknown. The acute effects of nalmefene (0.01 and 0.10 mg/kg, IV) or placebo on LH, PRL, and T were studied, and samples were collected at 10-min intervals for 360 min to permit cluster analysis of pulsatile release patterns. LH increased significantly within 30 min after nalmefene, and remained significantly above baseline levels for 50 to 60 min (p < 0.05). Testosterone increased significantly within 70 to 80 min after nalmefene, and remained significantly above baseline for 60 min (p < 0.05). Although nalmefene antagonizes opioid agonists for 6-8 h, inhibitory feedback by testosterone appeared to limit the duration of its antagonism of endogenous opioid inhibition of LHRH and stimulation of LH. Nalmefene did not change LH or PRL pulse frequency or amplitude significantly in comparison to placebo administration.

Kappa opioid agonists as targets for pharmacotherapies in cocaine abuse

Kappa opioid receptors derive their name from the prototype benzomorphan, ketocyclazocine (1a) which was found to produce behavioral effects that were distinct from the behavioral effects of morphine but that were antagonized by the opioid antagonist, naltrexone. Recent evidence suggests that agonists and antagonists at kappa opioid receptors may modulate the activity of dopaminergic neurons and alter the neurochemical and behavioral effects of cocaine. Kappa agonists blocked the effects of cocaine in squirrel monkeys in studies of cocaine discrimination and scheduled-controlled responding. Studies in rhesus monkeys suggested that kappa opioids may antagonize the reinforcing effects of cocaine. These studies prompted the synthesis and evaluation of a series of kappa agonists related to the morphinan, L-cyclorphan (3a) and the benzomorphan, L-cyclazocine (2). We describe the synthesis and preliminary evaluation of a series of morphinans, structural analogs of cyclorphan 3a-c, the 10-keto morphinans 4a and b, and the 8-keto benzomorphan 1b, structurally related to ketocyclazocine (1a). In binding experiments L-cyclorphan (3a), the cyclobutyl (3b), the tetrahydrofurfuryl 3c and the 10-keto 4b analogs had high affinity for mu (mu), delta (delta) and kappa (kappa) opioid receptors. Both 3a and 3b were more selective for the kappa receptor than the mu receptor. However, 3b was 18-fold more selective for the kappa receptor in comparison to the delta receptor, while cyclorphan (3a) had only a 4-fold greater affinity for the kappa receptor in comparison to the delta receptor. The cyclobutyl compound 3b was found to have significant mu agonist properties, while 3a was a mu antagonist. All compounds were also examined in the mouse tail flick and writhing assay. Compounds 3a and 3b were kappa agonists. Correlating with the binding results, compound 3a had some delta agonist properties, while 3b was devoid of any activity at the delta receptor. In addition, compounds 3a and 3b had opposing properties at the mu opioid receptor. The cyclobutyl compound 3b was found to have significant mu agonist properties, while 3a was a mu antagonist.