Attenuation of ethanol-induced conditioned taste aversion by naloxazone: behavioral evidence for an opiate receptor-mediated morphine-ethanol interaction

Methylnaltrexone attenuates taste aversion conditioned by low-dose ethanol

Previous research has shown that activation of a subset of peripheral opioid receptors located in the gut produce aversive effects as measured in the place and taste conditioning (CTA) paradigms. Endogenous opioid activity and tetrahydroisoquinolines (TIQs) are stimulated or formed after ethanol (EtOH) administration and both are known to activate opioid receptors. We therefore examined the hypothesis that a portion of the aversive effects of EtOH may be mediated through peripheral opioid receptors, activated by EtOH-induced opioids or TIQs. EtOH CTAs were slightly attenuated when animals were pretreated with the putative peripheral opioid receptor antagonist methylnaltrexone. By itself MNTX did not condition a taste preference or aversion. However, blood EtOH levels (BELs) in animals pretreated with MNTX were lower than those of saline-pretreated subjects, an effect that just reached statistical significance and was not present at specific EtOH doses. The results indicate that a portion of the aversive conditioning effects of EtOH (using a two-bottle CTA paradigm) may be receptor-mediated effects, exerted by an action on peripheral opioid receptors, but the specific mechanism of attenuation is unclear.

The effects of naloxone on expression and acquisition of ethanol place conditioning in rats

Naloxone has been shown to facilitate extinction of ethanol-induced conditioned place preference (CPP) in mice. The present-study extended these findings by examining naloxone's effect on the expression (Experiment 1) and acquisition (Experiment 2) of place conditioning with ethanol in rats. In Experiment 1, after place conditioning with ethanol (1.8 g/kg, I.P.), groups N0, N1.5, and N10 received 0, 1.5, or 10 mg/kg naloxone before testing. As expected, ethanol produced a robust conditioned place aversion (CPA). However, naloxone had no effect on expression of CPA. In contrast to studies with mice, the endogenous opioid system does not appear to be involved in the conditioned motivational effects of ethanol in rats. In Experiment 2, groups SE1 and SE2, NS(1.5), NE(1.5), and NE(10), received ethanol alone (1.2 g/kg), naloxone alone (1.5 mg/kg), naloxone 1.5 mg/kg plus ethanol, and naloxone 10 mg/kg plus ethanol during acquisition, respectively. All naloxone-treated groups exhibited CPA. Moreover, group NE(1.5) showed a stronger CPA than group NS(1.5). The CPA produced by coadministration of naloxone and ethanol was attributed to naloxone's effects on the neural processes underlying ethanol's unconditioned aversive effects, or to other nonspecific effects on ethanol's motivational properties.

Genetic differences in naloxone enhancement of ethanol-induced conditioned taste aversion

The influence of the opioid system on acquisition of an ethanol-induced conditioned taste aversion was examined in alcohol-preferring and avoiding inbred strains of mice (C57BL/6J and DBA/2J). Fluid-deprived mice from each strain received either ethanol alone, naloxone alone, or both ethanol and naloxone immediately after access to a novel tasting fluid. Naloxone alone (1 or 3 mg/kg) did not induce a conditioned taste aversion in either strain of mice. Administration of ethanol (1.5 g/kg) to DBA/2J mice produced a moderate taste aversion that was not affected by co-administration of naloxone. Although ethanol administered alone (3 g/kg) did not cause a taste aversion in C57BL/6J mice, the combination of ethanol and the higher dose of naloxone produced a significant taste aversion that increased across trials. A second experiment addressed the possibility that naloxone failed to enhance the ethanol-induced condition taste aversion in DBA/2J mice due to a "floor" effect on consumption. A lower ethanol dose (1 g/kg) was given alone or in combination with naloxone (1 or 3 mg/kg). Again, ethanol produced a moderate conditioned taste aversion that was not potentiated by naloxone. Subsequent conditioning with a high ethanol dose produced further suppression of intake, confirming that naloxone's failure to enhance aversion on earlier trials was not due to a "floor" effect. These data demonstrate a strain specific interaction between the aversive effect of ethanol and naloxone. More specifically, the results indicate that blockade of opioid receptors enhances the aversive effect of ethanol in C57BL/6J but not DBA/2J mice, suggesting that genetically determined differences in the endogenous opioid system of alcohol-preferring mice may mitigate ethanol's aversive effect.

Involvement of endogenous opioid mechanisms in the interaction between stress and ethanol

The involvement of endogenous opioid mechanisms in the interaction between stress and ethanol was investigated in the rat. Animals were pretreated with naltrexone (10 mg/kg) or saline 3 h before a second injection consisting of ethanol (1.0 g/kg) or saline. They were then restrained for 15 or 60 min or left in home cages for an equivalent amount of time. After restraint, animals were either subjected to an open-field test or decapitated to collect blood for corticosterone determinations. Locomotor depression was found to be induced by 15 but not 60 min restraint. In naltrexone-treated animals, however, 60 min restraint was also found to induce locomotor depression. Ethanol pretreatment was found to block the locomotor depression induced by 15 min restraint. Such an interaction was in turn antagonized by naltrexone. In the 15 min condition, stress and ethanol were also found to interact in their effects on plasma levels of corticosterone. Naltrexone did not alter any effects of the stressors on corticosterone levels. These results provide support for the involvement of endogenous opioid mechanisms in the interaction of stress and ethanol at a behavioural level.

Narcotic antagonism of seizures induced by a dopamine-derived tetrahydroisoquinoline alkaloid

This paper describes experiments designed to evaluate whether the narcotic antagonist naloxone significantly interferes with seizures induced by tetrahydroisoquinolines (TIQs). In these experiments we found that naloxone significantly reduced seizure scores induced by intra-cranially infusing mice with 50 micrograms of the dopamine-derived tetrahydroisoquinoline (TIQ) alkaloid, 6,7-dihydroxy TIQ. These findings support an opioid involvement in the actions of TIQs and may lead to further understanding of opioid-mediated novel excitatory receptors.

Effects of naloxazone on morphine-ethanol interaction in the conditioned taste aversion paradigm

The effects of naloxazone on the interaction between alcohol and morphine were examined in the conditioned taste aversion paradigm (CTA). Naloxazone injected 4.5 h before the pre-exposure drug significantly attenuated the pre-exposure effect of morphine on ethanol CTA as well as the effect of ethanol on morphine CTA. The results are discussed in terms of possible mechanisms of opiate-alcohol interaction.

Conditioned taste aversion induced by self-administered drugs: paradox revisited

In this paper we have reviewed the literature on Conditioned Taste Aversion (CTA) with specific attention to the "apparent paradox" in this literature. This paradox refers to the fact that drugs which are self-administered (SA) by animals and are therefore presumed to possess positive reinforcing properties are also endowed with the capacity to induce a CTA. We have argued that the CTA literature contains evidence of the existence of two qualitatively distinct types of CTA, one which is mediated by emetic agents and the other induced by SA drugs. We first provided evidence to support the notion that the traditional explanation of CTA as a function of "drug toxicity" and its resultant gastrointestinal distress does not fit the data on the nature of CTA induced by SA drugs. We proposed instead that "drug shyness" or the novelty of the drug state of these psychoactive SA drugs constitutes a better explanation of the CTA of SA drugs. We provided further evidence suggesting a functional relationship between the positive reinforcing and aversive properties of SA drugs. We have based this contention on a review of the behavioral, physiological and neurochemical data concerning the nature of CTA of SA drugs. The examination of these data reveals that the neural mechanisms underlying both the positive and aversive properties of SA drugs are the same and at the same time different from the neural mechanisms underlying the induction of CTA by emetic agents. Finally, we discussed the relevance of this interaction between the positive and aversive properties of SA drugs in the context of their abuse liability and the control they exert on drug-oriented behavior.

Ethanol-induced hypothermia in rats: possible involvement of opiate kappa receptors

The effect of the opiate antagonists naloxone and MR2266 on ethanol-induced hypothermia and changes in Ca2+-stimulated Mg2+-ATPase activity in brain regions were investigated in the present study. Administration of different doses of ethanol (0.5-2 g/kg, IP) produced a dose-dependent hypothermia. Ca2+/Mg2+-ATPase activity in the hypothalamus was stimulated at 30 min and 2 hr after ethanol (2 g/kg, IP) treatment. In cortex, enzyme activity was inhibited by ethanol at 30 min with no change seen at 2 hr. Naloxone (7.5 mg/kg, SC) at a dose which did not affect body temperature or enzyme activity, partially inhibited ethanol-induced hypothermia and enzyme activity at the earliest time (30 min) but not at 2 hours. The opiate Kappa antagonist MR2266 (5 mg/kg, SC), however, significantly protected against ethanol hypothermia and enzyme activation measured at 30-120 min. This evidence suggests that ethanol-induced hypothermia and subsequent activation changes of Ca2+/Mg2+-ATPase in the hypothalamus may be regulated by opiate Kappa receptors, and that Ca2+ ions play an important role in mediating the effects of ethanol.

Acetaldehyde and morphine interaction in the preexposure conditioned taste aversion paradigm in the rat

The interaction between acetaldehyde, the primary metabolite of ethanol, and morphine was examined in the preexposure conditioned taste aversion (CTA) paradigm. Rats that were preexposed to acetaldehyde did not display a CTA to morphine. While acetaldehyde at the dose tested did not induce a strong CTA, morphine preexposure, however, reversed the decrease in saccharin associated with acetaldehyde injection thus reflecting a symmetrical interaction between acetaldehyde and morphine. Implications for a mechanism of ethanol-opiates interaction are discussed.

Endogenous opiates: 1984

This paper is the seventh in an annual series of reviews of research involving the endogenous opiate peptides, each installment being restricted to work published during the previous year. As in the past three years, the review this year is limited to non-analgesic and behavioral studies of the opiate peptides. The specific topics this year include: stress, tolerance and dependence, consummatory responses, gastric and renal activity, alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, seizures and neurological disorders, activity, and miscellaneous other topics.

Stereospecific interaction between alcohol and opiates mediated by the mu high-affinity binding receptor

Experiment 1 revealed that preexposure to morphine blocked ethanol-induced CTA, and that conversely, preexposure to ethanol blocked morphine-induced CTA. In experiment 2, naloxazone injected 4.5 hr before the preexposure drug significantly reversed the interaction between morphine and ethanol. The results of Experiment 3 showed that preexposure to levorphanol blocked both morphine- and ethanol-induced CTAs while dextrorphan at the same dose did not affect the CTAs. These findings are discussed in terms of the involvement of the opiate receptors and their ligands in opiate-ethanol interaction.

Receptor stereospecificity in opiate-ethanol interaction using the preexposure-conditioned taste aversion (CTA) paradigm

In the first experiment, rats were conditioned with different doses of levorphanol or dextrorphan to a novel tasting saccharin solution. In the second experiment, rats were either preexposed to levorphanol or dextrorphan and conditioned with either morphine or ethanol to the saccharin solution. The results showed that levorphanol, but not dextrorphan, at 1, 5 and 10 mg/kg doses effectively induced a CTA. Preexposure to the 5 mg/kg dose of levorphanol blocked both morphine- and ethanol-induced CTAs. Dextrorphan at the same dose did not affect the CTAs. These findings are discussed in terms of the involvement of the opiate receptors in opiate-ethanol interaction.