Motivational effects of compounding discriminative stimuli associated with food and cocaine

Self-administration of agonists selective for dopamine D2, D3, and D4 receptors by rhesus monkeys

Dopamine receptor mechanisms are believed to play a role in the reinforcing effects of cocaine and other drugs of abuse. The lack of receptor-selective agonists has made it difficult to determine the role of the individual dopamine receptors in mediating these reinforcing effects. In this study, rhesus monkeys with a history of intravenous cocaine self-administration were tested for the reinforcing effects of several D(3)-preferring agonists, a D(2)-preferring agonist, and a D(4) agonist. The D(2)-preferring agonist did not maintain responding in any monkeys, and the D(4) agonist was self-administered at low rates, just above those maintained by saline, in one monkey. The D(3)-preferring agonists were self-administered by approximately half of the animals, although at lower rates than cocaine. These results indicate that the apparent limited reinforcing effectiveness of D(2)-like agonists requires activity at D(3) receptors. Previous data from this laboratory and others also suggest that these drugs may not serve as reinforcers directly; the behavior may be maintained by response-contingent delivery of stimuli previously paired with cocaine. The ability of drug-related stimuli to maintain responding apparently differs among monkeys and other organisms, and may be related to individual differences in drug-taking behavior in humans.

A stimulus-control account of regulated drug intake in rats

RATIONALE

Patterns of drug self-administration are often highly regular, with a consistent pause after each self-injection. This pausing might occur because the animal has learned that additional injections are not reinforcing once the drug effect has reached a certain level, possibly due to the reinforcement system reaching full capacity. Thus, interoceptive effects of the drug might function as a discriminative stimulus, signaling when additional drug will be reinforcing and when it will not.

OBJECTIVE

This hypothetical stimulus control aspect of drug self-administration was emulated using a schedule of food reinforcement.

MATERIALS AND METHODS

Rats' nose-poke responses produced food only when a cue light was present. No drug was administered at any time. However, the state of the light stimulus was determined by calculating what the whole-body drug level would have been if each response in the session had produced a drug injection. The light was only presented while this virtual drug level was below a specific threshold. A range of doses of cocaine and remifentanil were emulated using parameters based on previous self-administration experiments.

RESULTS

Response patterns were highly regular, dose-dependent, and remarkably similar to actual drug self-administration.

CONCLUSION

This similarity suggests that the emulation schedule may provide a reasonable model of the contingencies inherent in drug reinforcement. Thus, these results support a stimulus control account of regulated drug intake in which rats learn to discriminate when the level of drug effect has fallen to a point where another self-injection will be reinforcing.

Stimulus control and compounding with ambient odor as a discriminative stimulus on a free-operant baseline

Previous experiments have demonstrated that the simultaneous presentation of independently established discriminative stimuli can control rates of operant responding substantially higher than the rates occasioned by the individual stimuli. This "additive summation" phenomenon has been shown with a variety of different reinforcers (e.g., food, water, shock avoidance, cocaine, and heroin). Discriminative stimuli previously used in such studies have been limited to the visual and auditory sensory modalities. The present experiment sought to (1) establish stimulus control on a free-operant baseline with an ambient olfactory discriminative stimulus, (2) compare olfactory control to that produced with an auditory discriminative stimulus, and (3) determine whether compounding independently established olfactory and auditory discriminative stimuli produces additive summation. Rats lever pressed for food on a variable-interval schedule in the presence of either a tone or an odor, with comparable control developed to each stimulus. In the absence of these stimuli responding was not reinforced. During stimulus compounding tests, the tone-plus-odor compound occasioned more than double the responses occasioned by either the tone or odor presented individually. Thus, the current study (1) established stimulus control with an ambient olfactory discriminative stimulus in a traditional free-operant setting and (2) extended the generality of stimulus-compounding effects by demonstrating additive summation when olfactory and auditory discriminative stimuli were presented simultaneously.

Cocaine sensitization and dopamine mediation of cue effects in rodents, monkeys, and humans: areas of agreement, disagreement, and implications for addiction

BACKGROUND

Sensitization of mesocorticolimbic dopamine projections has been a valuable model of neurobiological adaptation to chronic exposure to cocaine and other psychostimulants.

DISCUSSIONS

In addition to providing an explanation of exaggerated responses to drugs that might explain their increased ability to serve as reinforcers, sensitization has also been incorporated into influential theories of how drug associated cues can acquire increased salience and incentive motivation. However, almost all of the work exploring behavioral and neurochemical sensitization has been conducted in rodents. Importantly, the relatively small amount of work conducted in human and nonhuman primates differs from the rodent work in some important regards. This review will examine areas of convergence and divergence between the rodent and primate literature on sensitization and the ability of drug associated environmental cues to elicit dopamine release. The implications of this comparison for expanding addiction research beyond dopaminergic mechanisms in the striatum/nucleus accumbens will be considered.

The dissociation of heroin-seeking patterns induced by contextual, discriminative, or discrete conditioned cues in a model of relapse to heroin in rats

RATIONALE

The role of heroin-related stimuli in motivating the resumption of heroin use is not fully understood.

OBJECTIVES

The objective was to characterize the relative importance of drug-related contextual stimuli, discriminative stimuli (DS), or discrete conditioned stimuli (CSs) on drug seeking when rats were reintroduced into the operant context after withdrawal.

METHODS

Nose-poke responding by male rats was reinforced with intravenous heroin (0.05 mg/kg per infusion, 4-h session daily) under a progressive ratio schedule of reinforcement for 14 days. Each session began with the illumination of a green light in the active hole that served as DS. Each earned heroin injection was paired with a 5-s compound cue light and the sound of the infusion pump that served as the discrete CSs.

RESULTS

Response rates of heroin seeking induced by the contextual stimuli were comparable to the average rates of responding during self-administration training, but rates induced by either DS or CSs were greater than those induced by the contextual stimuli alone (P<0.05). The responding induced by contingent presentations of CSs was higher than that of DS after extinction of instrumental behavior. The drug seeking induced by CSs can be maintained after 3 days extinction with DS in the original context, although the responding elicited by DS cannot be recovered after 3 days of extinction with CSs.

CONCLUSIONS

The relapse to drug seeking can be elicited separately by environmental cues, heroin-predictive DS, or discrete CSs in the same rat after withdrawal.

Conditioned inhibition of cocaine seeking in rats

Despite its potential relevance to the treatment of drug abuse, conditioned inhibition of drug seeking has not been systematically investigated before. In this study, rats could self-administer cocaine by lever pressing whenever a click or tone was present. Responding was not reinforced when a light was present. The light was presented simultaneously with the click (i.e., in an excitatory context) in 1 group, but the light was always presented alone in another group. When it was later presented in compound with the tone, the light was a highly effective conditioned inhibitor, suppressing cocaine seeking by 92% in the former group and by 74% in the latter. These results suggest ways to improve cue-oriented behavioral treatments for drug abuse.

Ethanol-maintained responding of rats is more resistant to change in a context with added non-drug reinforcement

Alternative non-drug reinforcers reliably decrease drug-maintained responding in self-administration procedures. Studies of the resistance to change of food-maintained behavior, however, have found that responding in the presence of a stimulus associated with an alternative reinforcer is more resistant to disruption. This increase in persistence occurs despite lower response rates when the alternative reinforcer is present. The present experiment examined if, in addition to decreasing response rates, an alternative non-drug reinforcer also increases the persistence of drug-maintained responding. Rats self-administered oral ethanol in a multiple schedule of reinforcement in which responding was reinforced in two components signaled by different stimuli. In one component, response-independent food was delivered in addition to the earned ethanol. The effects of the alternative food reinforcer on response rates and resistance to extinction in the two components were examined. As in previous experiments on the resistance to change of food-maintained operant behavior, response rates were lower, but more resistant to extinction in the presence of the stimulus associated with the alternative reinforcer. These findings suggest that all the reinforcers obtained in a context in which drugs are consumed may contribute to the persistence of drug seeking in that context. This increase in persistence may occur even if the alternative reinforcers interfere with drug seeking.

Phasic alterations in dopamine and serotonin release in striatum and prefrontal cortex in response to cocaine predictive cues in behaving rhesus macaques

The ability of environmental cues associated with cocaine availability to cause relapse may result from conditioned activation of dopamine (DA) release. We examined this hypothesis in macaque monkeys by conducting microdialysis studies in animals during exposure to a cocaine predictive compound cue. In addition to studying DA release in mesolimbic and sensorimotor striatum, both DA and serotonin levels were determined in the prefrontal cortex (medial orbitofrontal and anterior cingulate). The compound cue employed visual, auditory, and olfactory components, and was salient to the animals as demonstrated by anticipatory lever pressing in the absence of cocaine. During a 10-min period of exposure prior to cocaine availability, there was no significant increase in striatal or cortical DA. The addition of a DA uptake inhibitor to the striatal perfusate to reduce the potential interference of neuronal uptake did not alter the results. In contrast to the lack of any change in striatal DA, a significant decrease in extracellular serotonin in the prefrontal cortex during the 10 min of cue exposure was observed.

Influence of cue-conditioning on acquisition, maintenance and relapse of cocaine intravenous self-administration

Conditioning theories propose that, through a Pavlovian associative process, discrete stimuli acquire the ability to elicit neural states involved in the maintenance and relapse of a drug-taking behaviour. Experimental evidence indicates that drug-related cues play a role in relapse, however, their influence on the development and maintenance of drug self-administration has been poorly investigated. In this report, we analysed the effects of a drug-associated cue light on acquisition, maintenance and reinstatement of intravenous cocaine self-administration. The results show that a cocaine-associated cue light can act as an incentive in absence of the drug, but does not directly modify drug-reinforcing effects. Contingent and non-contingent presentations of a cocaine-associated cue light reinstated an extinguished self-administration behaviour. However, regardless of whether or not a cue light was associated with cocaine infusions, rats acquire cocaine intravenous self-administration reaching the same levels of intake. Furthermore, after self-administration has been acquired in presence of the cue light, the omission of the cue light or its non-contingent presentation did not modify rat behaviour. In conclusion, our work shows that cocaine-associated explicit cues do not directly interfere with the reinforcing effects of the drug.

Effects of compounding drug-related stimuli: escalation of heroin self-administration

Previous experiments have demonstrated that presenting independently established discriminative stimuli in compound can substantially increase operant responding maintained by food reinforcement or shock avoidance. Recently, this phenomenon was also shown to occur with cocaine self-administration. The present study further assessed the generality of these stimulus-compounding effects by systematically replicating them with heroin self-administration. Rats' nose-poke responses produced intravenous heroin (0.025 mg/kg per infusion) on a variable-ratio schedule when either a tone or a light was present. In the absence of these stimuli, responding was not reinforced. Once discriminative control by the tone and light had been established, the stimuli were presented in compound under extinction (with heroin discontinued) or maintenance conditions (with heroin available during test-stimulus presentations). In extinction, the tone-light compound increased responding approximately threefold compared to tone or light alone. Under maintenance conditions, compounding increased heroin intake approximately twofold. These effects closely matched those obtained earlier with cocaine. This consistency across pharmacological classes and across drug and nondrug reinforcers further confirms that (a) self-administered drugs support conditioning and learning in a manner similar to that supported by other reinforcers; and (b) multiple drug-related cues interact in lawful and predictable ways to affect drug seeking and consumption.