The role of CB1 receptors in sweet versus fat reinforcement: effect of CB1 receptor deletion, CB1 receptor antagonism (SR141716A) and CB1 receptor agonism (CP-55940).

It is well established that Cannabis sativa can increase appetite, particularly for sweet and palatable foods. In laboratory animals, cannabinoid CB1 receptor antagonism decreases motivation for palatable foods, and most recently, the CB1 receptor antagonist SR141716A, or rimonabant (Acomplia), was reported to produce weight loss in obese human subjects. Indeed, the endocannabinoid system plays a select role in the rewarding properties of palatable foods, and this is well characterized in laboratory animals with sweet sucrose solutions. In the present study, CB1 knockout mice (CB1 KO) and wild-type littermate mice (WT) were trained to respond for a complex sweet as well as a pure fat reinforcer under a progressive ratio (PR) schedule, to determine whether motivation to consume different palatable foods is tonically regulated by CB1 receptors. To assess sweet reinforcement, several concentrations of the liquid nutritional drink, Ensure, were presented under the PR schedule. For fat reinforcement, several concentrations of corn oil (emulsified in 3% xanthan gum) were made available. Additionally, to compare the result of genetic invalidation of the CB1 receptor to antagonism of the CB1 receptor system, the effect of SR141716A (3.0 mg/kg) on responding for Ensure and corn oil were also assessed using the PR schedule. We also assessed the effect of the CB1 agonist CP-55940 (30 microg/kg) on responding for Ensure and corn oil. CB1 KOs took significantly longer to acquire operant responding maintained by Ensure, and responding for Ensure under the PR schedule was significantly reduced in CB1 KOs as well as in WTs pretreated with SR141716A, as compared to WT controls. Additionally, pretreatment with the CB1 agonist CP-55940 increased responding for Ensure. In contrast, responding for corn oil during acquisition and under the PR schedule was not significantly different in CB1 KOs versus wild-type mice. However, SR141716A did reduce responding for corn oil in WTs, and CP-55940 significantly increased responding for corn oil. Taken together, these results suggest that CB1 receptors are preferentially involved in the reinforcing effects of a complex sweet, as compared to a pure fat, reinforcer. These data also suggest, however, that antagonism of CB1 receptors with SR141716A is sufficient to attenuate the reinforcing effect of Ensure and corn oil, while activation of the central CB1 system is sufficient to enhance Ensure and corn oil reinforcement.

Relative opioid efficacy is determined by the complements of the G protein-coupled receptor desensitization machinery.

G protein-coupled receptor regulation by G protein-coupled receptor kinases and beta-arrestins can lead to desensitization and subsequent internalization of the receptor. In in vitro and cellular systems, beta-arrestins do not seem to play a major role in regulating micro opioid receptor (microOR) responsiveness. Removal of the betaarrestin2 (betaarr2) gene in mice leads paradoxically to enhanced and prolonged microOR-mediated antinociception. The betaarr2 knockout (betaarr2-KO) mice also fail to develop morphine antinociceptive tolerance in the hot-plate test, further indicating that the betaarr2 protein plays an essential role in microOR regulation in vivo. In this study, the contribution of betaarr2 to the regulation of the microOR was examined in both human embryonic kidney 293 cells and in betaarr2-KO mice after treatment with several opiate agonists. A green fluorescent protein tagged betaarr2 was used to assess receptor-betaarr2 interactions in living cells. Opiate agonists that induced robust betaarr2-green fluorescent protein translocation produced similar analgesia profiles in wild-type and betaarr2-KO mice, whereas those that do not promote robust betaarr2 recruitment, such as morphine and heroin, produce enhanced analgesia in vivo. In this report, we present a rationale to explain the seemingly paradoxical relationship between beta-arrestins and microOR regulation wherein morphine-like agonists fail to promote efficient internalization and resensitization of the receptor.

Three-choice discrimination in pigeons is based on relative efficacy differences among opioids.

RATIONALE: Drug discrimination assays can provide important information on receptor selectivity and relative efficacy to guide the classification and characterization of opioid agonists. OBJECTIVES: A three-choice discrimination was established among high efficacy opioid agonist morphine, low efficacy opioid agonist nalbuphine, and saline to examine the conditions under which differences in relative efficacy might serve as a basis for stimulus control. METHODS: Seven White Carneau pigeons were trained to discriminate among 5.6 mg/kg nalbuphine, 3.2 mg/kg morphine, and saline under fixed ratio 30 (FR30) schedules of food reinforcement. Substitution and antagonism experiments were then conducted with mu, kappa, and delta opioids and naltrexone, respectively and the percent responding appropriate to the training stimuli was determined. RESULTS: Low, intermediate, and high doses of morphine produced > or = 80% saline-, > or = 60% nalbuphine-, and > or = 96% morphine-appropriate responding, respectively. Low and high doses of nalbuphine produced > or = 80% saline- and nalbuphine-appropriate responding, respectively. In substitution tests, low doses of fentanyl and etorphine produced partial nalbuphine-appropriate responding (20-60%) and high doses produced > or = 60-80% morphine-appropriate responding. Intermediate doses of buprenorphine and dezocine produced > or = 60-80% nalbuphine-appropriate responding and high doses produced > or = 80% morphine-appropriate responding. The lower efficacy agonists butorphanol, nalorphine, and levallorphan produced > or = 40-80% nalbuphine-appropriate responding. The kappa agonists spiradoline and U50,488 produced approximately > or = 50% nalbuphine-appropriate responding whereas d-amphetamine, saline, and delta agonists BW373U86 and SNC 80 produced > or = 80% saline-appropriate responding. Naltrexone produced > or = 80% saline-appropriate responding and reversed the stimulus effects of morphine and nalbuphine. CONCLUSIONS: The discrimination between morphine and nalbuphine in pigeons is predominantly based on the relative efficacy differences between morphine, a higher-efficacy mu agonist and nalbuphine, a lower-efficacy mu agonist.

N-methyl-D-aspartate receptor antagonists potentiate the antinociceptive effects of morphine in squirrel monkeys.

Data from rodent antinociception models indicate that N-methyl-D-aspartate (NMDA) receptor antagonists do not produce antinociception alone or potentiate morphine antinociception, but do attenuate the development of morphine tolerance. This study examined the antinociceptive effects of the noncompetitive NMDA receptor antagonist dizocilpine, the competitive NMDA receptor antagonist (-)-6-phosphonomethyl-decahydroisoquinoline-3-carboxylic acid (LY235959), and the glycine-site antagonist (+)-(1-hydroxy-3-aminopyrrolidine-2-one) [(+)-HA-966], alone and in combination with morphine in a squirrel monkey titration procedure. In this procedure, shock (delivered to the tail) increased in intensity every 15 s from 0.01 to 2.0 mA in 30 increments. Five lever presses during any given 15-s shock period produced a 15-s shock-free period after which shock resumed at the next lower intensity. Morphine (0.3-3.0 mg/kg i.m.) dose-dependently increased the intensity below which monkeys maintained shock 50% of the time (median shock level; MSL). In contrast, dizocilpine (0.003-0.1 mg/kg i.m.) produced only modest increases in MSL in some monkeys (three of five) at the highest dose tested. Neither LY235959 (0.1-1.0 mg/kg i.m.) or (+)-HA-966 (10-56 mg/kg i.m.) increased MSL in any monkey tested. Dizocilpine, LY235959, and (+)-HA-966, when administered in combination with doses of morphine (1.0 mg/kg, 1.7 mg/kg) that either produced no antinociception or produced very little antinociception, were all found to dose-dependently potentiate the antinociceptive effect of morphine. Importantly, although these NMDA antagonists in combination with morphine produced marked increases in MSL, these combinations did not alter response rate, demonstrating that the potentiation was not due to nonspecific motor effects.

Attenuation of mu-opioid tolerance and cross-tolerance by the competitive N-methyl-D-aspartate receptor antagonist LY235959 is related to tolerance and cross-tolerance magnitude.

Although NMDA receptor antagonists attenuate the development of morphine tolerance, it is not clear whether NMDA receptor antagonists also prevent tolerance and cross-tolerance to other mu-opioid agonists and, if so, whether prevention is related to the efficacy of the agonist used to examine tolerance. A rat tail-withdrawal procedure was used to test the antinociceptive effects of the mu-opioids etorphine, morphine, and dezocine before and after twice-daily subcutaneous injections with either 0. 003 mg/kg etorphine, 10 mg/kg morphine, or 3.0 mg/kg dezocine, each administered alone or in combination with 3.0 mg/kg of the competitive NMDA antagonist LY235959. After chronic etorphine, the etorphine, morphine, and dezocine curves were shifted rightward 1.0-, 2.2-, and 3.4-fold, respectively. LY235959 prevented cross-tolerance to morphine and dezocine. After chronic morphine, the etorphine and morphine curves were shifted rightward 2.5- and 2. 9-fold, respectively, and the dezocine curve was flattened. LY235959 prevented morphine tolerance and cross-tolerance to etorphine and reduced the magnitude of cross-tolerance to dezocine. After chronic dezocine, the etorphine, morphine, and dezocine curves were shifted rightward 4.1-, 3.5-, and 9.6-fold, respectively. LY235959 did not prevent but reduced the magnitude of tolerance and cross-tolerance. In a separate experiment, the following rank order of efficacy was determined from the magnitudes of rightward shift in each dose-effect curve after administration of 1.0 mg/kg of the irreversible antagonist clocinnamox: etorphine > morphine > dezocine. These data show that differences in tolerance magnitude are related to opioid efficacy and that attenuation of mu-opioid tolerance and cross-tolerance by LY235959 depends upon the magnitude of opioid tolerance.

Role of morphine maintenance dose in the development of tolerance and its attenuation by an NMDA receptor antagonist.

RATIONALE: Current research shows that N-methyl-d-aspartate (NMDA) receptor antagonists attenuate the development of morphine tolerance in rodent antinociceptive assays. OBJECTIVE: The purpose of this study was to determine the role of morphine maintenance dose in the attenuation of morphine tolerance by the competitive NMDA receptor antagonist, LY235959. METHODS: A rat warm-water tail-withdrawal procedure was used to measure the antinociceptive effects of morphine and LY235959. In this procedure, the distal 8 cm of each rat's tail is immersed in 40 degrees (non-noxious) and 55 degrees C (noxious) water, and the latency to remove the tail is recorded. RESULTS: Morphine (0.3-10 mg/kg, SC) produced dose-dependent increases in tail-withdrawal latencies from the 55 degrees C water. Following determination of the morphine dose-effect curves, rats were administered chronically one of three doses of morphine (10, 20, or 40 mg/kg) either alone or in combination with LY235959 (1.0, 3.0, or 5.6 mg/kg, SC) twice daily for 7 days. Chronic administration of 10, 20, and 40 mg/kg morphine produced rightward shifts in the morphine dose-effect curves of approximately 3-, 6-, and 12-fold, respectively. When LY235959 (1.0-5.6 mg/kg) was co-administered with 10 mg/kg morphine, the development of morphine tolerance was attenuated in a dose-dependent manner, with complete prevention observed following 3.0 mg/kg LY235959. LY235959 (1.0, 3.0 mg/kg) also attenuated the development of tolerance to 20 and 40 mg/kg morphine; however, tolerance was not completely prevented. Administering 3.0 mg/kg LY235959 along with 20 and 40 mg/kg morphine was functionally equivalent to treating rats with half the amount of morphine. CONCLUSION: These data suggest that the maintenance dose of morphine, and thus the magnitude of tolerance, can determine the effectiveness of an NMDA receptor antagonist to attenuate morphine tolerance.

Immunomodulatory effects of morphine withdrawal in the rat are time dependent and reversible by clonidine.

RATIONALE: It is well established that opioids can modulate the immune status following both acute and chronic administration and that tolerance develops to some of these immunomodulatory effects. Few studies, however, have investigated opioid withdrawal-induced immunomodulation and the mechanism by which that process may be mediated. OBJECTIVES: The present study examines the immunomodulatory properties of morphine withdrawal alone and in the presence of the alpha(2)-adrenergic agonist, clonidine. METHODS: Rats drank a morphine solution for 20 days; withdrawal was induced on day 21 by replacing the morphine solution with plain tap water. Measurements of withdrawal-induced weight change and immunomodulation were obtained at several time points after withdrawal induction. Immune status was assessed by determining concanavalin A (Con-A), toxic shock syndrome toxin (TSST-1), and lipopolysaccharide (LPS)-stimulated splenocyte proliferation, splenic ConA-stimulated interferon (IFN)-gamma production, and splenic natural-killer (NK) cell activity. In a separate series of experiments, systemic injections of clonidine (0.001-0.01 mg/kg) were administered during a 12-h withdrawal episode and all measures of immune status were reassessed. RESULTS: Weight change was time dependent, with peak decreases in weight occurring 24 h following withdrawal induction. Rats also exhibited a time-dependent suppression of immune status in all assays except LPS-stimulated proliferation; immunomodulation was most evident 12 h following withdrawal induction. Clonidine dose dependently prevented withdrawal-induced suppression of Con-A and TSST-1-stimulated splenocyte proliferation, Con-A-stimulated splenocyte IFN-gamma production, and splenic NK cell activity. CONCLUSIONS: These findings demonstrate that opioid withdrawal significantly suppresses a subset of immune parameters and that these effects can be prevented by clonidine.

Discriminative stimulus effects of nalbuphine in nontreated and morphine-treated pigeons.

In the present study, the stimulus effects of the low efficacy agonist nalbuphine were examined under two conditions: nontreated and morphine treated. In the first experiment, five pigeons were trained to discriminate among 3.2 mg/kg morphine, 5.6 mg/kg nalbuphine, and saline. Nalbuphine produced nalbuphine-like responding. Low doses of morphine produced nalbuphine-like responding, whereas high doses produced morphine-like responding. Naltrexone produced saline-like responding and reversed the stimulus effects produced by the training doses of morphine and nalbuphine. Five different pigeons were treated daily with 10 mg/kg morphine (i.m.) and trained 6 h later to discriminate among 10 mg/kg morphine, 1.0 mg/kg nalbuphine and saline. In these pigeons, morphine produced morphine-like responding and nalbuphine produced nalbuphine-like responding. Morphine abstinence produced nalbuphine-like responding that was reversed by morphine. Additionally, naltrexone produced nalbuphine-like responding. These data suggest that the discrimination between morphine and nalbuphine in the nontreated and morphine-treated pigeons may be based on the relative efficacy differences between morphine, a higher efficacy mu-agonist, and nalbuphine a lower efficacy mu-agonist.

Naltrexone and beta-funaltrexamine antagonism of the antinociceptive and response rate-decreasing effects of morphine, dezocine, and d-propoxyphene.

RATIONALE: Patterns of competitive and insurmountable antagonism provide important data to guide the classification and characterization of different types of opioid agonists as well as infer the mechanism of action for agonists. OBJECTIVE: Experiments with the competitive antagonist, naltrexone, and the insurmountable antagonist, beta-funaltrexamine (beta-FNA), were conducted to determine whether the antinociceptive and rate-decreasing effects of the opioid agonists dezocine and d-propoxyphene are 1) mediated through muu opioid receptors in rats, and 2) differ from morphine with respect to relative efficacy. METHODS: The rat tail-withdrawal assay was used to measure antinociception and a fixed ratio 20 (FR20) schedule of food delivery was used to measure rate suppression. RESULTS: Naltrexone (0.01-1.0 mg/kg) was approximately equipotent as an antagonist of the antinociceptive and rate-decreasing effects of both morphine and dezocine and as an antagonist of the antinociceptive effects of d-propoxyphene. Naltrexone failed to block the rate-decreasing effects of d-propoxyphene. beta-FNA (5 and 10 mg/kg) also antagonized the antinociceptive and rate-decreasing effects of morphine and dezocine as well as the antinociceptive effects of d-propoxyphene. beta-FNA failed to produce a dose-dependent antagonism of the rate-decreasing effects of d-propoxyphene. CONCLUSIONS: These data suggest that the antinociceptive effects of morphine, dezocine, and d-propoxyphene and the rate-decreasing effects of morphine and dezocine are mediated through mu opioid receptors. Overall, high doses of beta-FNA produced a greater degree of antagonism of the behavioral effects of dezocine than morphine or d-propoxyphene, confirming other reports that dezocine is a lower efficacy agonist than morphine. Additionally, the degree of antagonism produced by beta-FNA was greater for the antinociceptive effects of all three compounds than for the rate-decreasing effects.

The competitive NMDA receptor antagonist LY235959 modulates the progression of morphine tolerance in rats.

A rat warm-water tail-withdrawal procedure was used to examine the effects of chronic administration of the competitive NMDA receptor antagonist LY235959 in morphine tolerant rats. Morphine dose-dependently increased tail-withdrawal latencies from 55 degree C water. When morphine (10 mg/kg) was administered twice-daily for 7 days, the morphine dose-effect curves shifted 0.3-0.5 log unit to the right. When morphine was administered for an additional 7 days, the morphine dose-effect curve shifted 0.4 log unit further to the right. Co-administration of LY235959 (1, 3, 10 mg/kg) along with morphine prevented the development of tolerance observed during the second week of chronic morphine administration. Although the highest dose of LY235959 (10 mg/kg) partially reversed tolerance in five of seven rats, tolerance was not reversed by lower doses of LY235959. These data suggest that NMDA receptor antagonists may effectively prevent the progressive development of morphine tolerance at doses that are not sufficient to reverse pre-established morphine tolerance.

Clocinnamox antagonism of the antinociceptive effects of mu opioids in squirrel monkeys.

The opioid agonists morphine, etorphine, buprenorphine and U50,488 were examined alone and in combination with the insurmountable opioid antagonist clocinnamox (C-CAM) in squirrel monkeys responding under a schedule of shock titration. In this procedure, shock intensity increased every 15 sec from 0.01 to 2.0 mA in 30 increments. Five lever presses during any given 15-sec shock period produced a 15-sec timeout, after which shock resumed at the next lower intensity. When given alone, each of these agonists increased the median intensity at which the monkeys maintained shock [median shock level (MSL)]. At the highest dose examined alone, each agonist produced maximal increases in MSL and, except buprenorphine, decreased response rates. C-CAM dose-dependently antagonized the effects of morphine, etorphine and buprenorphine on MSL. In the presence of the higher C-CAM doses, etorphine, morphine and buprenorphine did not produce maximal effects on MSL. The effects of U50,488 were not systematically altered when tested in combination with the highest C-CAM dose. In general, C-CAM was more potent and the duration of antagonism was slightly longer against buprenorphine than against morphine and etorphine. Quantitative analysis of these data according to an extended model of yielded the following apparent affinity and efficacy estimates, respectively: etorphine (0. 085 mg/kg, 117); morphine (49 mg/kg, 24) and buprenorphine (0.62 mg/kg, 7.1). Determination of the individual q values over time indicated that the receptor population recovers more quickly after C-CAM antagonism of etorphine than from C-CAM antagonism of either morphine or buprenorphine. These data suggest that C-CAM functions as a long-lasting antagonist of mu opioid agonist actions in a shock titration procedure and yields estimates of relative intrinsic efficacy with the rank order of etorphine > morphine > buprenorphine.

Differential tolerance to morphine's immunomodulatory effects following continuous administration.

Rats were continuously infused with either morphine or saline via an osmotic minipump for 20 consecutive days. Effects on immune status were assessed on the twentieth day of the chronic administration period following a bolus injection of morphine administered 1 h prior to sacrifice. The morphine injection suppressed measures of splenic natural killer (NK) cell activity, mitogen-stimulated T-cell proliferation, and gamma-interferon (IFN) production in rats that received saline via the minipump. In rats that received chronic morphine via the minipump, the morphine injection also suppressed mitogen-stimulated splenocyte proliferation and gamma-IFN production but did not suppress NK cell activity. These data indicate that chronic morphine administration via osmotic minipumps leads to differential tolerance to the immunomodulatory effects of morphine. These findings support previous results indicating differential tolerance development within the immune system following chronic morphine administration via the drinking water.

Comparison of the time course of morphine's analgesic and immunologic effects.

UNLABELLED: Morphine, an opioid analgesic commonly prescribed and abused, produces immune-altering effects. Whether morphine's antinociceptive and immunologic effects occur concurrently is unknown. Therefore, we investigated the time course of morphine's immunologic and antinociceptive effects. Rats were given a 15-mg/kg morphine injection (subcutaneously), and experimental assessments were taken at 30 min, 1 h, 2 h, 6 h, 12 h, and 24 h after treatment. Immune measures included natural killer (NK) cell activity, proliferation of splenic T and B lymphocytes, and cytokine production. Antinociception was assessed by using the tail withdrawal assay. Results show that morphine's immunomodulatory effects on NK cell activity begin within 30 min, continue for at least 12 h, and return to control values by 24 h. In contrast, proliferation of splenic T and B cells and interferon-gamma production are not altered within 30 min; maximal suppression occurs at 1 h, and recovery begins within 2 h. In all immune measures, therefore, maximal suppression is present at the 1-h time point, and recovery is complete within 24 h. Morphine induces antinociception 30 min to 2 h after drug administration; recovery is complete within 6 h. These results suggest the possibility that different mechanisms modulate morphine's immunologic and analgesic effects. IMPLICATIONS: Acute morphine treatment in rats produces immune alterations and antinociception. Although there are slight differences in morphine's maximal immunological and antinociceptive effects, morphine suppresses immune status at time points concordant with its antinociceptive effects. These effects should be considered when administering morphine to patients whose systems are immunocompromised.

Tolerance development to morphine-induced alterations of immune status.

A variety of in vitro immune measures were examined in groups of Lewis rats that chronically consumed either tap water or a 0.2, 0.4, or 0.6 mg/ml morphine drinking solution. Rats received a subcutaneous injection of either saline or 15 mg/kg morphine sulfate 1 h before sacrifice. In the drinking groups, the acute morphine injection significantly suppressed splenic natural killer (NK) cell activity, mitogen-stimulated splenic T- and B-cell proliferation and gamma-interferon (gamma-IFN) production. A single, acute injection of morphine did not suppress NK cell activity in rats that drank the two highest concentrations of morphine, whereas it did suppress the mitogen-stimulated splenic T- and B-cell proliferation and gamma-IFN production. These results suggest that rats that drank morphine for 20 days developed tolerance to morphine's suppressive effect on NK cell activity but not to other measures of immune status. Morphine drinking rats also developed tolerance to morphine's antinociceptive effects and revealed signs of physical dependence when the morphine solution was withdrawn or when naltrexone was administered.

Antagonism of the response rate-decreasing effects of meperidine and morphine by beta-funaltrexamine and naltrexone in squirrel monkeys.

Level pressing by squirrel monkeys was maintained under a fixed-ratio 30 schedule of food presentation. The response rate-decreasing effects of meperidine and morphine were examined alone and in combination with several doses of the irreversible, mu-selective opioid antagonist beta-funaltrexamine and the reversible, opioid antagonist naltrexone. beta-Funaltrexamine alone decreased response rates to greater than 50% of control at all doses in two of the four monkeys and at the highest dose in one monkey. In the monkeys in which beta-funaltrexamine decreased rates, beta-funaltrexamine either did not shift the meperidine or the morphine dose-effect curve or it shifted these curves to the left. In the monkeys in which beta-funaltrexamine alone did not decrease rates, it shifted the meperidine and the morphine dose-effect curves to the right. Naltrexone also shifted both the meperidine and morphine dose-effect curves rightward, although not in a dose-dependent manner. These data suggest that the rate-decreasing effects of meperidine and morphine in squirrel monkeys are altered by beta-funaltrexamine and naltrexone in a similar manner, providing additional evidence that the rate-decreasing effects of both meperidine and morphine are mediated by mu-opioid receptors.

Discriminative stimulus and subjective effects of opioids with mu and kappa activity: data from laboratory animals and human subjects.

Although a large and rich body of data is available regarding the discriminative stimulus effects of opioids in laboratory animals and human subjects, it has been difficult to reconcile the data obtained from these two different sources. Therefore, the purpose of this review is to bring together data from both animal and human laboratories and systematically to compare the discriminative stimulus effects of opioids, in particular those with activity at both mu and kappa opioid receptor types (i.e., the mixed action opioids). The data that can be collected from laboratory animals differ from the data that can be collected in human subjects. In general, the advantage of studies in laboratory animals is that they can investigate very broad dose ranges of opioids as well as some very selective opioids that are not available for investigation in human subjects. Although investigations in human subjects are limited by the compounds and doses available for examination, the advantage of these studies is that they can examine the subjective as well as the discriminative stimulus effects of opioids. Taken together, studies conducted in laboratory animals and human subjects indicate that the mixed action opioids are best classified as intermediate efficacy mu agonists with additional activity through other non-mu, possibly kappa opioid systems.

Acetylcholinesterase monoclonal antibody-induced sympathectomy: effects on immune status and acute morphine-induced immunomodulation.

The present study examined the role of the sympathetic nervous system (SNS) in immunomodulation by using the acetylcholinesterase monoclonal antibody (AChE mAb)-induced sympathectomy model. As part of this investigation, the effects of AChE mAb treatment on the immune alterations produced by acute morphine treatment also were explored. Experimental rats received tail vein injections of murine monoclonal IgG2b antibodies against rat brain acetylcholinesterase, which produce a destruction of cholinergic, sympathetic preganglionic neurons and a resultant decrease in sympathetic activity. Control rats received tail vein injections of murine IgG antibodies, which do not affect sympathetic preganglionic neurons or sympathetic activity. One week after antibody treatment, rats received a subcutaneous injection of 15 mg/kg morphine or the saline vehicle. One hour after the morphine or saline injections, rats were sacrificed and immune assays were conducted. AChE mAb treatment increased the mitogen-stimulated proliferation of splenic T cells and interleukin-2 (IL-2) production by stimulated splenocytes, indicating that these immune measures are sensitive to the AChE mAb-induced alteration in sympathetic function. Treatment with AChE mAb did not alter the mitogen-stimulated proliferation of splenic B cells or blood T cells, splenic natural killer (NK) cell activity, or the production of interferon-gamma (IFN-gamma) by stimulated splenocytes, indicating that these immune measures are relatively insensitive to the AChE mAb-induced alteration in sympathetic activity. The AChE mAb-induced alteration in sympathetic activity did not affect the suppressive effects of acute morphine treatment on the mitogen-stimulated proliferative response of splenic T and B cells and blood T cells, splenic NK cell activity, and the production of IFN-gamma and IL-2 by stimulated splenocytes.

The role of serotonergic receptors in the effects of mu opioids in squirrel monkeys responding under a titration procedure.

This experiment was conducted to determine whether drugs acting on brain serotonin modulate the effects of the mu opioid, morphine, as measured by the squirrel monkey shock titration procedure and, if so, whether serotonergic modulation is mediated via specific 5HT receptor subtypes. Under this procedure, electric shock was delivered to the monkey's tail and scheduled to increase once every 15 s from 0.01 to 2.0 mA in 30 steps. Five responses on a lever during the 15-s shock period terminated the shock for 15 s, after which the shock resumed at the next lower intensity. The intensity below which monkeys maintained shock 50% of the time (median shock level or MSL) and rate of responding (RR) in the presence of shock were determined under control conditions and after administration of morphine alone and in combination with various serotonergic compounds. Morphine increased median shock level and decreased rate of responding in a dose-dependent manner. These effects of morphine was attenuated by the 5HT1A receptor agonists, 8-OH-DPAT [(+)-8-hydroxy-2(di-n-propylamino tetralin HBr] and ipsapirone. The effects of morphine were not altered by the 5HT1A receptor antagonist, NAN-190 [1-(2-methoxyphenyl-4-[4-(2-phthalimido) butyl] piperazine HBr], and 5HT2 receptor antagonist, ketanserin, the 5HT3 receptor antagonist, MDL 72222 [3-tropanyl-3,5-dichlorobenzoate], the alpha 2 adrenergic antagonist, yohimbine, or the alpha2 adrenergic agonist, clonidine. These results suggest that 5HT1A receptors may be involved in the effects of morphine in the shock titration procedure, whereas 5HT2, 5HT3 and alpha 2 adrenergic receptors do not appear to play a role in morphine's effects in this procedure.