The effect of morphine preexposure on the acquisition of morphine-induced taste aversions: A nonassociative effect

Preexposure to running attenuates rats' running-based flavour avoidance: Testing associative blocking with a cover cues or context change

Voluntary running in activity wheels by rats leads to a Pavlovian conditioned aversion to the flavour consumed immediately before the running, causing the rats to avoid that flavour. This learning process, known as running-based flavour avoidance learning (FAL), is weakened when the rats have had repeated exposure to the wheels before. According to the associative account, the association between the background context and running established during the preexposure phase blocks the conditioning of the target flavour because the running is highly predictable by the background context from the outset of the FAL phase. Experiments 1 and 2 examined this account by introducing another flavour as a cue signalling wheel access during the preexposure phase. In the framework of the associative account, the introduction of this cue should impede the formation of the context-running association during the preexposure phase, thereby hindering the contextual blocking of aversive conditioning for the target flavour in the FAL phase. This would result in unweakened FAL. Although the results of Experiment 1 align with this prediction, in Experiment 2, when highly distinct flavours were used as the target and second cues, the preexposure effect was not eliminated. This contradicts the predictions of the associative account, indicating that Experiment 1 may have been influenced by stimulus generalisation. In Experiment 3, changing background contexts between the preexposure and FAL phases had no impact on the preexposure effect, contrary to the predictions of the associative account. In general, the associative account was not supported.

The role of the aversive effects of drugs in self-administration: assessing the balance of reward and aversion in drug-taking behavior

Since the first experimental demonstration that a drug of abuse supports instrumental behavior, drugs have been discussed in the context of their rewarding effects, which are assumed to drive and maintain drug-taking behavior. Indeed, drug reward has been fundamental in the formulation of most models of drug use, abuse, and addiction. Over the last several decades, however, drugs of abuse have been increasingly recognized as complex pharmacological compounds producing multiple stimulus effects, not all of which are rewarding. The aversive effects of such drugs, for example, have been described by a number of researchers working in the field, although few attempts have been made to investigate the role of these aversive effects in drug taking. The present paper offers a historical perspective on the view that drugs of abuse are complex pharmacological compounds with multiple stimulus effects. In doing so, we argue that the discussion of drug reward only may be insufficient in accounting for drug taking and we present evidence for the theoretical position that both the rewarding and the aversive effects of drugs should be taken into consideration in ongoing attempts to model drug-taking behavior. The present review summarizes several decades of research characterizing the aversive effects of major drugs of abuse, as well as more recent studies seeking to assess directly the role of drug aversion in drug taking.

Rats preexposed to MDMA display attenuated responses to its aversive effects in the absence of persistent monoamine depletions

RATIONALE

The abuse potential of a given drug may be mediated by both its rewarding and aversive effects, the latter of which are often far less characterized.

OBJECTIVES

Using the conditioned taste-aversion (CTA) preparation, the present experiments examined changes in the aversive effects of the commonly used recreational drug MDMA following repeated drug exposures.

METHODS

Experiment 1 used three varying doses of MDMA (1.0, 1.8, and 3.2 mg/kg) to determine a dose that produced taste aversions of intermediate strength. Experiments 2 and 3 characterized the effects of repeated preexposures to MDMA (1.8 or 3.2 mg/kg) on taste aversions induced by MDMA (1.8 mg/kg). Additionally, levels of several monoamines and metabolites were analyzed in frontal cortex and caudate-putamen from subjects in Experiment 3 to assess for persistent monoamine depletions.

RESULTS

MDMA induced dose-dependent taste aversions. Preexposure to MDMA (at both doses) resulted in an attenuation of MDMA-induced taste aversions. These effects were not likely due to persistent monoamine depletions, as subjects preexposed to the higher MDMA dose did not differ from controls in levels of monoamines or metabolites in either brain region examined.

CONCLUSIONS

Prior MDMA experience weakened the ability of MDMA to induce taste aversions. This attenuation of MDMA's aversive effects may occur with low doses that do not persistently alter monoamine levels.

Naloxone, not proglumide or MK-801, alters effects of morphine preexposure on morphine-induced taste aversions

Both cholecystokinin (CCK) antagonists and N-methyl-D-aspartate (NMDA) antagonists block or reduce the development of morphine tolerance in several analgesic assays. The present experiments were performed to assess the ability of the CCK antagonist proglumide and the NMDA antagonist MK-801 to affect tolerance to the aversive properties of morphine as indexed by conditioned taste aversion (CTA) learning. Specifically, male Sprague-Dawley rats were exposed to either vehicle or morphine (5 mg/kg) in combination with either proglumide (5 mg/kg; Experiment 1), MK-801 (0.1 mg/kg; Experiment 2) or naloxone (1, 3.2 mg/kg; Experiment 3). Saccharin was then presented and was followed by an injection of either vehicle or morphine (10 mg/kg). Animals preexposed to and conditioned with morphine acquired an attenuated morphine-induced aversion to saccharin. While neither proglumide nor MK-801 had an effect on this attenuation, naloxone blocked the effects of morphine preexposure, suggesting that neither CCK nor NMDA may be involved in the aversive effects of morphine (or their modulation by drug exposure). That the attenuating effects of morphine preexposure on a morphine-induced CTA can be blocked suggests that the weakening of the aversive effects of morphine with chronic use can be prevented, an effect that may have implications for overall drug acceptability.

Morphine preexposure facilitates morphine place preference and attenuates morphine taste aversion

Repeated morphine preexposure has been reported to enhance measures of morphine reward (conditioned place preference; CPP) and attenuate measures of morphine aversion (conditioned taste aversion; CTA). These effects are generally independently assessed, limiting the ability to determine if the enhancing and attenuating effects of morphine exposure are mediated by a common factor. To assess any potential relationship between these two effects, the present study examined the impact of morphine preexposure on these motivational properties of morphine using a combined CTA/CPP procedure in which the same animals receive concurrent taste and place conditioning. Specifically, male Sprague-Dawley rats were preexposed to morphine [5 mg/kg; subcutaneously (sc)] or equivolume drug vehicle. Following preexposure, animals were given saccharin to drink and injected with morphine sulfate (1 or 5 mg/kg sc) or drug vehicle (CTA). Immediately thereafter, they were placed on one side of a two-compartment chamber (CPP). On the next day, they were given water followed by injections of the drug's vehicle and then placed in the other compartment. There were four such conditioning cycles after each of which a CTA and CPP test were given. While preexposure to morphine attenuated morphine-induced CTAs, morphine-induced CPPs were enhanced within the same animals. These effects of morphine preexposure were dose- and time-dependent and parallel. These data indicate that the attenuating and sensitizing effects of morphine preexposure on taste aversions and place preferences, respectively, could be mediated by a common mechanism, although other possibilities for these effects of morphine preexposure remain.

Cocaine preexposure fails to sensitize the acquisition of cocaine-induced taste aversions

In two separate experiments, rats were given either an intraperitoneal (IP) injection of 10 mg/kg cocaine once a day for 10 consecutive days (Experiment 1) or a single IP injection of 40 mg/kg of cocaine (Experiment 2) prior to receiving repeated pairings of a novel saccharin solution with cocaine (32 mg/ kg; subcutaneous; SC). Although vehicle-preexposed subjects given saccharin-cocaine pairings readily acquired an aversion to the cocaine-associated saccharin solution, subjects preexposed to cocaine (whether 10 times or only once) displayed a retarded acquisition of the aversion. That is, cocaine preexposure attenuated the acquisition of cocaine-induced taste aversions. There was no difference in the degree of attenuation between the two preexposure conditions. Thus, under conditions that are effective in inducing sensitization within other behavioral preparations there was no evidence of sensitized cocaine-induced taste aversions. The results from the present investigation are similar to reports from this laboratory and others demonstrating that preexposure to cocaine, as with a range of other psychoactive drugs, results in weaker taste aversions. The basis for the attenuating effects of cocaine preexposure was discussed in terms of an adaptation to the aversive effects of cocaine.

Blockage of the effects of testosterone on extinction of a conditioned taste aversion by estradiol: time of action

Testosterone (T) prolongs the extinction of a conditioned taste aversion only if it is present during extinction. Experiments were conducted to determine whether estradiol (E) blocks the effects of T by acting during acquisition or extinction. In the first experiment, gonadectomized male and female rats injected with estradiol dipropionate (EP) and testosterone propionate (TP) during extinction had significantly faster extinction rates than those only injected with TP. Treating gonadectomized rats with TP prior to as well as during extinction did not prevent EP from blocking the effects of T. In Experiment 2, E was equally effective in preventing T from prolonging extinction when it was implanted in gonadectomized males during acquisition, extinction, or both acquisition and extinction. Thus, E does not have to be present concurrently with T during extinction to be effective. This suggests that E does not act on a T-related mechanism but rather acts independently of T.

Peripheral injections of vasopressin control behavior by way of interoceptive signals for hypertension

The stimulus properties of peripheral injections of vasopressin were assessed using conditioned taste aversion techniques. Conditioned taste aversion induced by vasopressin was blocked by prior exposure to vasopressin but not to another aversive agent, apomorphine. Prior exposure to behaviorally equivalent doses of another hypertensive agent, angiotensin II, blocked also conditioned taste aversion induced by vasopressin and this effect was fully reciprocal, since prior exposure to AVP blocked the aversive effect of angiotensin II. The protection offered by prior exposure to angiotensin II was not due to an endogenous release of AVP since the aversive properties of angiotensin II were not blocked by administration of a specific antagonist of the vasopressor effects of vasopressin. These data suggest that the interoceptive cues which are responsible for the conditioned taste aversion induced by vasopressin are related to the hypertensive action of this peptide.