The kappa-opioid U-50,488H suppresses the initiation of nocturnal spontaneous drinking in normally hydrated rats

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

The role of the dynorphin-kappa opioid system in the reinforcing effects of drugs of abuse

BACKGROUND

Initial hypotheses regarding the role of the kappa opioid system in drug addiction suggested that kappa receptor stimulation had anti-addictive effects. However, recent research suggests that kappa receptor antagonists may reverse motivational aspects of dependence. In the present review, we revisit the studies that measured the effects of kappa receptor ligands on the reinforcing and rewarding effects of drugs and postulate underlying neurobiological mechanisms for these effects to elaborate a more complex view of the role of kappa receptor ligands in drug addiction.

RESULTS

The review of studies indicates that kappa receptor stimulation generally antagonizes the acute reinforcing/rewarding effects of drugs whereas kappa receptor blockade has no consistent effect. However, in a drug dependent-like state, kappa receptor blockade was effective in reducing increased drug intake. In animal models of reinstatement, kappa receptor stimulation can induce reinstatement via a stress-like mechanism. Results in conditioned place preference/aversion and intracranial self-stimulation indicate that kappa receptor agonists produce, respectively, aversive-like and dysphoric-like effects. Additionally, preclinical and postmortem studies show that administration or self-administration of cocaine, ethanol, and heroin activate the kappa opioid system.

CONCLUSION

kappa receptor agonists antagonize the reinforcing/rewarding effects of drugs possibly through punishing/aversive-like effects and reinstate drug seeking through stress-like effects. Evidence suggests that abused drugs activate the kappa opioid system, which may play a key role in motivational aspects of dependence. Kappa opioid systems may have an important role in driving compulsive drug intake.

Opposing effects of intra-nucleus accumbens mu and kappa opioid agonists on sensory specific satiety

Mu opioid (MOP) agonists acting in the nucleus accumbens (NAcc) robustly enhance consumption of palatable foods. In addition, the effect on consumption of palatable foods produced by MOP agonists acting in the NAcc depends on both recent flavor exposure and the availability of a choice between different-flavored foods. In contrast, kappa opioid (KOP) agonists have variable effects on feeding and KOP agonists have MOP opposing behavioral actions when microinjected at several brain sites. We previously demonstrated that NAcc MOP agonists reverse the devaluation (satiety) effect of pre-feeding for a given flavor; in fact, NAcc MOP agonists selectively increase consumption of a recently sampled food. In contrast, in the present study, we found that the selective KOP agonist U50488 injected into the NAcc of rats reduced consumption of a recently sampled flavor while increasing consumption of the flavor that was not pre-fed. Intra-NAcc U50488 did not affect overall consumption or flavor preference in the absence of pre-feeding. The present data, in conjunction with our previous findings, highlight the robust and opposing role of NAcc MOP and KOP opioid receptors in palatability-based food choice and consumption and raise the possibility that an endogenous KOP agonist acting in the NAcc contributes to the phenomenon of sensory specific satiety.

Reduced alcohol consumption in mice lacking preprodynorphin

Many studies suggest a role for endogenous opioid peptides and their receptors in regulation of ethanol intake. It is commonly accepted that the kappa-opioid receptors and their endogenous ligands, dynorphins, produce a dysphoric state and therefore may be responsible for avoidance of alcohol. We used mutant mice lacking preprodynorphin in a variety of behavioral tests of alcohol actions. Null mutant female, but not male, mice showed significantly lower preference for alcohol and consumed lower amounts of alcohol in a two-bottle choice test as compared with wild-type littermates. In the same test, knockout mice of both sexes showed a strong reduction of preference for saccharin compared to control mice. In contrast, under conditions of limited (4 h) access (light phase of the light/dark cycle), null mutant mice did not show any differences in consumption of saccharin, but they showed significantly reduced intake of sucrose. To determine the possible cause for reduction of ethanol preference and intake, we studied other ethanol-related behaviors in mice lacking the preprodynorphin gene. There were no differences between null mutant and wild-type mice in ethanol-induced loss of righting reflex, acute ethanol withdrawal, ethanol-induced conditioned place preference, or conditioned taste aversion to ethanol. These results indicate that deletion of preprodynorphin leads to substantial reduction of alcohol intake in female mice, and suggest that this is caused by decreased orosensory reward of alcohol (sweet taste and/or palatability).

The selective kappa-opioid receptor agonist U50,488H attenuates voluntary ethanol intake in the rat

Non-selective opioid receptor antagonists are increasingly used in the treatment of alcohol dependence. The clinical effects are significant but the effect size is rather small and unpleasant side effects may limit the benefits of the compounds. Ligands acting at mu- and/or delta- receptors can alter the voluntary intake of ethanol in various animal models. Therefore, the attenuating effects of selective opioid receptor ligands on ethanol intake may be of clinical interest in the treatment of alcoholism. The objective of this study was to examine the effects of a selective kappa-receptor agonist, U50,488H on voluntary ethanol intake in the rat. We used a restricted access model with a free choice between an ethanol solution (10% v/v) and water. During the 3-days baseline period, the rats received a daily saline injection (1 ml/kg, i.p.) 15 min before the 2 h access to ethanol. The animals had free access to water at all times. The control group received a daily saline injection during the 4-days treatment-period, whereas the treatment groups received a daily dose of U50,488H (2.5, 5.0 or 10 mg/kg per day). Animals treated with U50,488H dose-dependently decreased their ethanol intake. The effect of the highest dose of U50,488H was reduced by pre-treatment with the selective kappa-antagonist nor-binaltorphimine (nor-BNI). These results demonstrate that activation of kappa-opioid receptors can attenuate voluntary ethanol intake in the rat, and the data suggest that the brain dynorphin/kappa-receptor systems may represent a novel target for pharmacotherapy in the treatment of alcohol dependence.

Modulation of food intake by the kappa opioid U-50,488H: evidence for an effect on satiation

The main goal of the present study was to test the hypothesis that the prophagic effect of the kappa opioid agonist U-50,488H (U50) is primarily due to an effect on satiation. In Experiment 1, the feeding effects of U50 (2.0 and 4.0 mg/kg, i.p.) was tested in animals with ad libitum access to ground food and to three sucrose solutions (1, 4, and 20%). In Experiment 2, a classical "one-bottle" test was utilized to test for the effect of U50 (4.0 mg/kg, i.p.) on the intake of five different sucrose solutions (1, 4, 16, 32, and 40%) over a 30-min period. Finally, in Experiment 3 we evaluated the effect of U50 (2.0, 4.0, and 6.0 mg/kg, i.p.) on extracellular dopamine (DA) concentration in the nucleus accumbens. In Experiment 1, U50 enhanced the intake of ground food but not of sucrose. In Experiment 2, U50 increased the intake of high concentration sucrose solutions whereas it decreased that of low concentration solutions. In Experiment 3, U50 produced a dose-dependent decrease in DA concentrations in the absence but not in the presence of food. The most likely explanation for the present results is that U50 enhances feeding by activating mechanisms that block satiety and satiation. In contrast, we found little evidence for an effect of U50 on palatability.

Glucocorticoids have state-dependent stimulant effects on the mesencephalic dopaminergic transmission

An increase in the activity of mesencephalic dopaminergic neurons has been implicated in the appearance of pathological behaviors such as psychosis and drug abuse. Several observations suggest that glucocorticoids might contribute to such an increase in dopaminergic activity. The present experiments therefore analyzed the effects of corticosterone, the major glucocorticoid in the rat, both on dopamine release in the nucleus accumbens of freely moving animals by means of microdialysis, and on locomotor activity, a behavior dependent on accumbens dopamine. Given that glucocorticoids have certain state-dependent neuronal effects, their action on dopamine was studied in situations differing in dopaminergic tonus, including during the light and dark phases of the circadian cycle, during eating, and in groups of animals differing in their locomotor reactivity to novelty. Dopaminergic activity is increased in the dark period, further increased during food-intake, and is higher in rats defined as high responders to novelty than in low responders. Corticosterone, peripherally administered in a dose that approximates stress-induced plasma concentrations, increased extracellular concentrations of dopamine, and this increase was augmented in the dark phase, during eating, and in high responder rats. Corticosterone had little or no effects in the light phase and in low responder rats. Corticosterone also stimulated locomotor activity, an effect that paralleled the release of dopamine and was abolished by neurochemical (6-hydroxydopamine) depletion of accumbens dopamine. In conclusion, glucocorticoids have state-dependent stimulant effects on mesencephalic dopaminergic transmission, and an interaction between these two factors might be involved in the appearance of behavioral disturbances.

Kappa opioid inhibition of morphine and cocaine self-administration in rats

Two kappa agonists, U50,488 and spiradoline, produced dose-related acute decreases in both morphine and cocaine self-administration in rats; higher doses of both agents were required to decrease rates of bar-pressing for water. On the day after kappa agonist administration, both agents produced extinction-like patterns of responding in many rats self-administering morphine or cocaine but not in rats responding for water. Two days after their administration, both U50,488 and spiradoline produced significant decreases in both morphine and cocaine intake; some rats continued to show decreases in drug self-administration for 5-6 days. Although the kappa antagonist nor-binaltorphimine (10 mg/kg s.c.) had no effect itself on either morphine or cocaine self-administration, it fully antagonized the effects of U50,488 (10 m/kg i.p.). The results suggest that although endogenous kappa opioid systems may not tonically modulate mechanisms involved in drug reinforcement, pharmacological activation of kappa pathways may be a novel and effective pharmacological approach to treating both opioid and stimulant addiction.

Physiological and environmental aspects of drinking stimulated by chronic exposure to amphetamine in rats

1. To examine whether the hyperdipsic response to chronic administration of d,l-amphetamine (AMPH) is associated with modification of salt appetite, rats were allowed to choose between tap water and a 1.7% NaCl solution. 2. Under AMPH rats preferred water to saline throughout the experiment. 3. By testing rats in a distinct test cage environmental influences on AMPH-mediated hyperdipsia were also evaluated. 4. In the test cage hyperdipsia was suppressed, but preference for tap water was preserved. 5. Finally, the role of alpha 2-adrenoceptors in the drinking response to AMPH was evaluated by studying the effects of clonidine and yohimbine on water intake. 6. We conclude that AMPH-induced preference for tap water over saline is unrelated to hyperdipsia but, being also induced by yohimbine, it may depend on noradrenergic mechanisms.

Endogenous opiates: 1992

This paper is the fifteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1992 involving the behavioral, non-analgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Ontogeny of prodynorphin gene expression in the rat hypothalamus

Opioid peptides, deriving from prodynorphin, proenkephalin and proopiomelanocortin genes, have been shown to modulate brain development. Prodynorphin gene expression was studied here by in situ hybridization in the developing rat hypothalamus using oligodeoxynucleotide probes. Prodynorphin mRNA-synthetizing cells were observed in the ventromedial hypothalamic nucleus, the supraoptic and the paraventricular nuclei from embryonic days 16, 18 and 21, respectively. We detected no transient expression of prodynorphin gene in the rat hypothalamus. Prodynorphin mRNA-containing cells were also observed prenatally in the striatum, the cortex, the hippocampus and the amygdala. When compared with data from the literature, our results suggest that translation may immediately follow transcription of prodynorphin gene in the supraoptic nucleus. The presence of prodynorphin mRNA in the developing rat hypothalamus also raises the possibility of an involvement of prodynorphin-derived peptides in developmental processes and/or in the maturation of adult neural regulations.

Rats orally self-administer corticosterone

Corticosterone, the major glucocorticoid in the rat, may modulate the reinforcing properties of addictive drugs as well as act as a positive reinforcer for intravenous self-administration. Since glucocorticoids are generally administered to humans via the oral route, we examined the ability of corticosterone to induce oral self-administration in the rat. In a first experiment, animals with free access to food could choose between a corticosterone solution and water. Three doses (25, 50 and 100 micrograms/ml) were tested. The group receiving the 100 micrograms/ml dose was also submitted to an extinction followed by a reversal test. In a second experiment, we examined whether the reinforcing properties of corticosterone could induce drinking independently of food intake. In the pre-test phase rats had access to food only during a fixed period of the day (11.00 h to 14.00 h). Corticosterone solution (200 micrograms/ml) or tap water were available during this period, with free access to tap water for the rest of the day. During the test period, access to food was shifted forward in time, while the availability of the corticosterone solution remained the same. The first experiment showed that rats preferred a corticosterone solution to tap water, developing self-administration in a dose-dependent manner. This preference could be extinguished, but was regained during the reversal phase. In the second experiment, animals that had access to the corticosterone solution drank more than rats that had access to water in the absence of food. These results indicate that corticosterone has reinforcing properties after oral administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of the prophagic but not of the antidipsogenic effect of U-50, 488H after chronic amphetamine

Two groups of rats were treated with seven daily injections of either saline or d-amphetamine (3 mg/kg IP). On the 2 days following the last injection, rats were tested according to a counterbalanced experimental design, each animal receiving, immediately prior to the beginning of the dark phase, saline on one day and the highly selective kappa-opioid agonist trans- +/- 3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene- acetamide methanesulfonate hydrate [U-50,488H (U50)] on the other. A microcomputer-controlled data acquisition system was used for the structural analysis of the feeding and drinking responses to amphetamine and U50. U50 enhanced feeding and depressed drinking in the first hour. The increased food intake was probably the result of the effect of U50 on the development of satiation and duration of satiety. Chronic amphetamine potentiated the prophagic effect but not the antidipsogenic effect of U50. The structural analysis demonstrated that the characteristics of the prophagic effect of U50 were amplified but not changed.