Monoaminergic drugs and directly observable signs of LAAM withdrawal in rhesus monkeys

Combined Treatment with Morphine and Δ9-Tetrahydrocannabinol in Rhesus Monkeys: Antinociceptive Tolerance and Withdrawal

Opioid receptor agonists are effective for treating pain; however, tolerance and dependence can develop with repeated use. Combining opioids with cannabinoids can enhance their analgesic potency, although it is less clear whether combined treatment alters opioid tolerance and dependence. In this study, four monkeys received 3.2 mg/kg morphine alone or in combination with 1 mg/kg Δ(9)-tetrahydrocannabinol (THC) twice daily; the antinociceptive effects (warm water tail withdrawal) of morphine, the cannabinoid receptor agonists WIN 55,212 [(R)-(1)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate] and CP 55,940 (2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl) cyclohexyl]-5-(2-methyloctan-2-yl)phenol), and the κ opioid receptor agonist U-50,488 (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzenacetamide methanesulfonate) were examined before, during, and after treatment. To determine whether concurrent THC treatment altered morphine dependence, behavioral signs indicative of withdrawal were monitored when treatment was discontinued. Before treatment, each drug increased tail withdrawal latency to 20 seconds (maximum possible effect). During treatment, latencies did not reach 20 seconds for morphine or the cannabinoids up to doses 3- to 10-fold larger than those that were fully effective before treatment. Rightward and downward shifts in antinociceptive dose-effect curves were greater for monkeys receiving the morphine/THC combination than monkeys receiving morphine alone. When treatment was discontinued, heart rate and directly observable withdrawal signs increased, although they were generally similar in monkeys that received morphine alone or with THC. These results demonstrated that antinociceptive tolerance was greater during treatment with the combination, and although treatment conditions were sufficient to result in the development of dependence on morphine, withdrawal was not markedly altered by concurrent treatment with THC. Thus, THC can enhance some (antinociception, tolerance) but not all (dependence) effects of morphine.

Cross-tolerance to cannabinoids in morphine-tolerant rhesus monkeys

RATIONALE

Opioids remain the drugs of choice for treating moderate to severe pain, although adverse effects limit their use. Therapeutic utility might be improved by combining opioids with other drugs to enhance analgesic effects, but only if adverse effects are not similarly changed.

OBJECTIVE

Cannabinoids have been shown to enhance the antinociceptive potency of opioids without increasing other effects; this study examined whether the effectiveness of cannabinoids is altered in morphine-dependent monkeys.

METHODS

Four monkeys received up to 10 mg/kg morphine twice daily. Changes in the antinociceptive effects of opioid receptor agonists (morphine, U50,488) and cannabinoid receptor agonists (WIN 55,212, CP 55,940, and Δ(9)-tetrahydrocannabinol [THC]) were determined by measuring the latency for monkeys to remove their tails from 40, 50, 54, and 58 °C water.

RESULTS

Before treatment, all drugs increased tail withdrawal latency from warm (54 °C) water. Chronic morphine treatment decreased the potency of each drug; the magnitude of rightward shift in dose-effect curves was greatest for morphine, WIN 55,212 and CP 55,940 with at least sixfold shifts for each drug during treatment. Discontinuation of morphine treatment resulted in signs that are indicative of withdrawal, including increased heart rate, decreased daytime activity, and tongue movement.

CONCLUSION

Tolerance developed to the antinociceptive effects of morphine and cross-tolerance developed to cannabinoids under conditions that produced modest physical dependence. Compared with the doses examined in this study, much smaller doses of opioids have antinociceptive effects when given with cannabinoids; it is possible that tolerance will not develop to chronic treatment with opioid/cannabinoid mixtures.

Affective and neuroendocrine effects of withdrawal from chronic, long-acting opiate administration

Although the long-acting opiate methadone is commonly used to treat drug addiction, relatively little is known about the effects of withdrawal from this drug in preclinical models. The current study examined affective, neuroendocrine, and somatic signs of withdrawal from the longer-acting methadone derivative l-alpha-acetylmethydol (LAAM) in rats. Anxiety-like behavior during both spontaneous and antagonist-precipitated withdrawal was measured by potentiation of the startle reflex. Withdrawal elevated corticosterone and somatic signs and blunted circadian variations in baseline startle responding. In addition, fear to an explicit, Pavlovian conditioned stimulus (fear-potentiated startle) was enhanced. These data suggest that anxiety-like behavior as measured using potentiated startle responding does not emerge spontaneously during withdrawal from chronic opiate exposure - in contrast to withdrawal from acute drug exposure - but rather is manifested as exaggerated fear in response to explicit threat cues.

The fatty acid amide hydrolase inhibitor URB 597: interactions with anandamide in rhesus monkeys

BACKGROUND AND PURPOSE

The fatty acid amide hydrolase inhibitor URB 597 increases brain anandamide levels, suggesting that URB 597 could enhance the behavioural effects of anandamide. The goal of the current study was to examine and characterize the in vivo pharmacology of URB 597 alone and in combination with anandamide and Δ⁹-tetrahydrocannabinol (Δ⁹ -THC) in two drug discrimination assays in rhesus monkeys.

EXPERIMENTAL APPROACH

The effects of URB 597 alone and in combination with anandamide were investigated in one group of monkeys (n= 4) that discriminated Δ⁹-THC (0.1 mg·kg⁻¹ i.v.) from vehicle, and in another group (n= 5) receiving chronic Δ⁹-THC (1 mg·kg⁻¹ 12 h⁻¹ s.c.) that discriminated the cannabinoid antagonist rimonabant (1 mg·kg⁻¹ i.v.).

KEY RESULTS

Intravenous anandamide fully substituted for, and had infra-additive effects with, Δ⁹-THC. URB 597 (up to 3.2 mg·kg⁻¹ i.v.) did not substitute for or modify the effects of Δ⁹-THC but markedly increased the potency (32-fold) and duration of action of anandamide. The rimonabant discriminative stimulus in Δ⁹-THC-treated monkeys (i.e. Δ⁹-THC withdrawal) was attenuated by both Δ⁹-THC (at doses larger than 1 mg·kg⁻¹ per 12 h) and anandamide but not by URB 597 (3.2 mg·kg⁻¹). URB 597 did not increase the potency of anandamide to attenuate the rimonabant-discriminative stimulus.

CONCLUSIONS AND IMPLICATIONS

URB 597 enhanced the behavioural effects of anandamide but not other CB₁ agonists. However, URB 597 did not significantly enhance the attenuation of Δ⁹-THC withdrawal induced by anandamide. Collectively, these data suggest that endogenous anandamide in primate brain does not readily mimic the behavioural effects of exogenously administered anandamide.

Some effects of dopamine transporter and receptor ligands on discriminative stimulus, physiologic, and directly observable indices of opioid withdrawal in rhesus monkeys

RATIONALE

Some monoamine uptake inhibitors (e.g., cocaine) attenuate the subjective and discriminative stimulus effects of opioid withdrawal.

OBJECTIVE

This study examined a role for dopamine transporters and receptors as targets for drugs to modify the discriminative stimulus effects of opioid withdrawal and further examined a subset of these drugs for their capacity to modify some directly observable and physiologic indices of withdrawal.

MATERIALS AND METHODS

Rhesus monkeys receiving 2 mg/kg/day of L: -alpha-acetylmethadol discriminated the opioid antagonist naltrexone (0.0178 mg/kg s.c.).

RESULTS

The naltrexone discriminative stimulus was attenuated not only by the mu agonist morphine but also by the dopamine D(2)-like receptor agonists bromocryptine and quinpirole. In contrast, the naltrexone discriminative stimulus was not consistently modified by the non-selective, D(1)- and D(2)-like agonist apomorphine or by uptake inhibitors with high selectivity for dopamine transporters (GBR 12909, RTI 113, and RTI 177). In the same monkeys, naltrexone dose dependently decreased body temperature, increased breathing frequency, and induced directly observable signs (grimacing, salivation, and unusual posture). Hypothermia, hyperventilation, and signs of withdrawal were significantly attenuated by morphine and not by quinpirole.

CONCLUSIONS

Attenuation of opioid withdrawal by D(2)-like receptor agonists that have lower efficacy than dopamine, and not by uptake inhibitors with selectivity for dopamine transporters, suggests that magnitude of receptor stimulation (e.g., efficacy) and selectivity at dopamine receptors are important factors in the modulation of opioid withdrawal. Attenuation of the naltrexone discriminative stimulus by drugs that inhibit both dopamine and serotonin uptake (e.g., cocaine) could result from an inhibitory effect of serotonin on dopamine. The role of dopamine in opioid withdrawal appears to be restricted to subjective (i.e., not somatic) indices [corrected]

Mechanisms of withdrawal-associated increases in heroin self-administration: pharmacologic modulation of heroin vs food choice in heroin-dependent rhesus monkeys

Opioid withdrawal can produce a constellation of physiological and behavioral signs, including an increase in opioid self-administration. Different mechanisms mediate different withdrawal signs, and the present study used pharmacologic tools to assess mechanisms underlying withdrawal-associated increases in opioid reinforcement. Five rhesus monkeys were rendered heroin dependent via daily 21-h heroin self-administration sessions. One hour after each heroin self-administration session, monkeys chose between heroin (0-0.1 mg/kg per injection) and food (1 g pellets) during 2-h choice sessions. Under these conditions, heroin maintained a dose-dependent increase in heroin choice, such that monkeys responded primarily for food when low heroin doses were available (0-0.01 mg/kg per injection) and primarily for heroin when higher heroin doses were available (0.032-0.1 mg/kg per injection). Periods of spontaneous withdrawal were intermittently introduced by omitting one 21-h heroin self-administration session, and test drugs were administered during these withdrawal periods. Untreated withdrawal robustly increased heroin choice during choice sessions. Withdrawal-associated increases in heroin choice were completely suppressed by the mu opioid agonist morphine (0.032-0.32 mg/kg/h, i.v.), but not by the alpha-2 noradrenergic agonist clonidine (0.01-0.1 mg/kg/h, i.v.), the dopamine/norepinephrine releaser amphetamine (0.032-0.1 mg/kg/h, i.v.), or the kappa-opioid antagonist 5'-guanidinonaltrindole (1.0 mg/kg, i.m.). The corticotropin-releasing factor 1 antagonist antalarmin (1.0-10 mg/kg per day, i.m.) produced a morphine-like suppression of withdrawal-associated increases in heroin choice in one of three monkeys. These results suggest that mechanisms of withdrawal-associated increases in the relative reinforcing efficacy of opioid agonists may be different from mechanisms of many other somatic, mood-related, and motivational signs of opioid withdrawal.

Antagonist-precipitated and discontinuation-induced withdrawal in morphine-dependent rhesus monkeys

RATIONALE

Upon discontinuation of chronic opioid treatment, withdrawal typically peaks in 1-3 days and decreases markedly within 1 week; however, persistent physiological changes have been reported long after other signs have waned.

OBJECTIVE

The goal of this study was to compare the discriminative stimulus, directly observable signs, and physiological effects of withdrawal in morphine-treated monkeys.

MATERIALS AND METHODS

Monkeys received 5.6 mg/kg/12 h morphine and discriminated 0.0178 mg/kg naltrexone while responding under a fixed-ratio 5 schedule of stimulus-shock termination. Drug discrimination, behavioral observation, and telemetry were used to monitor the emergence of withdrawal, as well as any persistent changes, following discontinuation of morphine treatment.

RESULTS

Naltrexone dose (0.001-0.032 mg/kg, s.c.) was positively related with indices of withdrawal. In the discrimination study, monkeys responded on the naltrexone lever 1-5 days following discontinuation of treatment; thereafter, they responded exclusively on the saline lever. After discontinuation of morphine, the frequency of observable signs peaked within 2-3 days and most were not significantly increased after 5 days. In contrast, increased heart rate and body temperature persisted for 14 days, returning to values obtained prior to discontinuation by 21 days.

CONCLUSIONS

To the extent that discriminative stimulus effects of withdrawal in nonhumans are predictive of subjective reports of withdrawal in humans, these data indicate that effective treatments for opioid dependence must address not only the short-term subjective components of withdrawal but also, and perhaps more importantly, lingering behavioral and physiological effects that might contribute to relapse long after chronic drug use is discontinued.

Thienorphine: receptor binding and behavioral effects in rhesus monkeys

Thienorphine is an oripavine with long-lasting antinociceptive effects in mice that are thought to be mediated by mu-opioid receptors. This study examined the receptor binding of thienorphine in cell membrane homogenates and its behavioral effects in rhesus monkeys (Macaca mulatta). Affinity and potency were determined using radioligand displacement and stimulation of guanosine 5'-O-(3-[35S]thio)triphosphate binding in C6 (mu, delta) and Chinese hamster ovary (kappa) cell membranes. Thienorphine displayed high affinity for kappa-, mu-, and delta-opioid receptors with K(i) values of 0.14, 0.22, and 0.69 nM, respectively. Thienorphine partially stimulated kappa-opioid (75%) and mu-opioid (19%) receptors and not delta-opioid receptors. Thienorphine dose-dependently increased tail-withdrawal latency for 50 degrees C water and not 55 degrees C water with effects lasting for more than 7 days. The kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) (3.2 mg/kg) and a large dose (1.0 mg/kg) of naltrexone prevented thienorphine-induced antinociception. Thienorphine enhanced the antinociceptive effects of morphine and U50,488 [trans-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide] with 50 degrees C water; with 55 degrees C water, thienorphine enhanced the effects of morphine and attenuated the effects of U50,488. In other monkeys, thienorphine decreased responding in both components of a multiple schedule of food presentation and stimulus shock termination for up to 8 days; naltrexone and nor-BNI partially prevented these rate-decreasing effects. In morphine-treated monkeys discriminating naltrexone, thienorphine, and U50,488 neither substituted for nor modified the naltrexone discriminative stimulus. Thienorphine and U50,488 produced the same directly observable signs. These results show that thienorphine has long-lasting effects that seem to be mediated by low-efficacy agonism at kappa-opioid receptors, both in vitro and in vivo.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).