The effects of pipradrol on the acquisitionof responding with conditioned reinforcement: a role for sensory preconditioning

α4-Containing GABAA Receptors on DRD2 Neurons of the Nucleus Accumbens Mediate Instrumental Responding for Conditioned Reinforcers and Its Potentiation by Cocaine

Extrasynaptic GABAA receptors (GABAARs) composed of α4, β, and δ subunits mediate GABAergic tonic inhibition and are potential molecular targets in the modulation of behavioral responses to natural and drug rewards. These GABAARs are highly expressed within the nucleus accumbens (NAc), where they influence the excitability of the medium spiny neurons. Here, we explore their role in modulating behavioral responses to food-conditioned cues and the behavior-potentiating effects of cocaine. α4-Subunit constitutive knock-out mice (α4-/-) showed higher rates of instrumental responding for reward-paired stimuli in a test of conditioned reinforcement (CRf). A similar effect was seen following viral knockdown of GABAAR α4 subunits within the NAc. Local infusion of the α4βδ-GABAAR-preferring agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; Gaboxadol) into the NAc had no effect on responding when given alone but reduced cocaine potentiation of responding for conditioned reinforcers in wild-type, but not α4-/- mice. Finally, specific deletion of α4-subunits from dopamine D2, but not D1, receptor-expressing neurons (DRD2 and DRD1 neurons), mimicked the phenotype of the constitutive knockout, potentiating CRf responding, and blocking intra-accumbal THIP attenuation of cocaine-potentiated CRf responding. These data demonstrate that α4-GABAAR-mediated inhibition of DRD2 neurons reduces instrumental responding for a conditioned reinforcer and its potentiation by cocaine and emphasize the importance of GABAergic signaling within the NAc in mediating the effects of cocaine.

Examination of the effects of varenicline, bupropion, lorcaserin, or naltrexone on responding for conditioned reinforcement in nicotine-exposed rats

Smoking tobacco remains one of the leading causes of preventable deaths in North America. Nicotine reinforces smoking behavior, in part, by enhancing the reinforcing properties of reward-related stimuli, or conditioned stimuli (CSs), associated with tobacco intake. To investigate how pharmaceutical interventions may affect this property of nicotine, we examined the effect of four US Food and Drug Administration (FDA) approved drugs on the ability of nicotine to enhance operant responding for a CS as a conditioned reinforcer. Thirsty rats were exposed to 13 Pavlovian sessions where a CS was paired with water delivery. Nicotine (0.4 mg/kg) injections were administered before each Pavlovian session. Then, in separate groups of rats, the effects of varenicline (1 mg/kg), bupropion (10 and 30 mg/kg), lorcaserin (0.6 mg/kg), and naltrexone (2 mg/kg), and their interaction with nicotine on responding for conditioned reinforcement were examined. Varenicline and lorcaserin each reduced nicotine-enhanced responding for conditioned reinforcement, whereas naltrexone had a modest effect of reducing response enhancements by nicotine. In contrast, bupropion enhanced the effect of nicotine on this measure. The results of these studies may inform how pharmaceutical interventions can affect smoking cessation attempts and relapse through diverse mechanisms, either substituting for, or interacting with, the reinforcement-enhancing properties of nicotine.

Examining the reinforcement-enhancement effects of phencyclidine and its interactions with nicotine on lever-pressing for a visual stimulus

Nicotine is a widely-abused drug, yet its primary reinforcing effect does not seem potent as other stimulants such as cocaine. Recent research on the contributing factors toward chronic use of nicotine-containing products has implicated the role of reinforcement-enhancing effects of nicotine. The present study investigates whether phencyclidine (PCP) may also possess a reinforcement-enhancement effect and how this may interact with the reinforcement-enhancement effect of nicotine. PCP was tested for two reasons: (1) it produces discrepant results on overall reward, similar to that seen with nicotine and (2) it may elucidate how other compounds may interact with the reinforcement-enhancement of nicotine. Adult male Sprague-Dawley rats were trained to lever press for brief visual stimulus presentations under fixed-ratio (FR) schedules of reinforcement and then were tested with nicotine (0.2 or 0.4 mg/kg) and/or PCP (2.0mg/kg) over six increasing FR values. A selective increase in active lever-pressing for the visual stimulus with drug treatment was considered evidence of a reinforcement-enhancement effect. PCP and nicotine separately increased active lever pressing for a visual stimulus in a dose-dependent manner and across the different FR schedules. The addition of PCP to nicotine did not increase lever-pressing for the visual stimulus, possibly due to a ceiling effect. The effect of PCP may be driven largely by its locomotor stimulant effects, whereas the effect of nicotine was independent of locomotor stimulation. This dissociation emphasizes that distinct pharmacological properties contribute to the reinforcement-enhancement effects of substances.

Responding for conditioned reinforcement in C57BL/6 and CD-1 mice, and Sprague-Dawley rats: Effects of methylphenidate and amphetamine

RATIONALE

Characterization of responding for conditioned reinforcement in mice is important to implement genetic tools in examining the neurobiological mechanisms underlying reward-related learning and incentive motivation.

METHODS

Inbred C57BL/6 mice, outbred CD-1 mice, and outbred Sprague-Dawley rats underwent Pavlovian conditioning in which a conditioned stimulus (CS) was paired with saccharin. Subsequently, subjects were allowed to respond for that CS in tests of responding for conditioned reinforcement. Experiments measured the effects of methylphenidate (MPH) and amphetamine (AMPH) on lever pressing for conditioned reinforcement in mice and rats. We further examined the stability of responding for conditioned reinforcement in mice after repeated testing and the extinction of this behaviour following omission of the reinforcer. We also determined whether the CS exhibited reinforcing properties if it was not paired with saccharin.

RESULTS

C57BL/6 and CD-1 mice learned to respond for a conditioned reinforcer similarly to rats, and the behaviour was stable over time. MPH increased responding in CD-1 mice and rats, but not in C57BL/6 mice. AMPH only increased responding in rats. Responding was reduced following omission of the conditioned reinforcer, and responding was only established when the CS was paired with saccharin.

CONCLUSIONS

These experiments characterize a conditioned reinforcement test which produces stable responding in two different mouse backgrounds. These findings also show that dopaminergic psychomotor stimulants can differently affect rats and mice in tests of responding for conditioned reinforcement.

Responding for a conditioned reinforcer, and its enhancement by nicotine, is blocked by dopamine receptor antagonists and a 5-HT(2C) receptor agonist but not by a 5-HT(2A) receptor antagonist

An aspect of nicotine reinforcement that may contribute to tobacco addiction is the effect of nicotine to enhance the motivational properties of reward-associated cues, or conditioned stimuli (CSs). Several studies have now shown that nicotine enhances responding for a stimulus that has been paired with a natural reinforcer. This effect of nicotine to enhance responding for a conditioned reinforcer is likely due to nicotine-induced enhancements in mesolimbic dopaminergic activity, but this has not been directly assessed. In this study, we assessed roles for dopamine (DA) D1 or D2 receptors, and two serotonin (5-HT) receptor subtypes known to modulate DA activity, the 5-HT2C or 5-HT2A subtypes, on nicotine-enhanced responding for a conditioned reinforcer. Water-restricted rats were exposed to Pavlovian conditioning sessions, where a CS was paired with water delivery. Then, in a second phase, animals were required to perform a novel, lever-pressing response for presentations of the CS as a conditioned reinforcer. Nicotine (0.4 mg/kg) enhanced responding for the conditioned reinforcer. To examine potential roles for dopamine (DA) and serotonin (5-HT) receptors in this effect, separate groups of animals were used to assess the impact of administering the D1 receptor antagonist SCH 23390, D2 receptor antagonist eticlopride, 5-HT2C receptor agonist Ro 60-0175, or 5-HT2A receptor antagonist M100907 on nicotine-enhanced responding for conditioned reinforcement. SCH 23390, eticlopride, and Ro 60-0175 all reduced responding for conditioned reinforcement, and the ability of nicotine to enhance this effect. M100907 did not alter this behavior. Together, these studies indicate that DA D1 and D2 receptors, but not 5-HT2A receptors, contribute to the effect of nicotine to enhance responding for a conditioned reinforcer. This effect can also be modulated by 5-HT2C receptor activation.

Differential effects of nucleus accumbens core, shell, or dorsal striatal inactivations on the persistence, reacquisition, or reinstatement of responding for a drug-paired conditioned reinforcer

Drug-paired conditioned reinforcers can maintain persistent instrumental responding, thus providing a model of some aspects of long-term drug addiction. The purpose of the present study was to investigate the effects of inactivating the dorsal striatum (DStr), nucleus accumbens (NAcc) core, or NAcc shell on different types of responding, each maintained by drug-paired conditioned reinforcers. Inactivations were achieved by infusing a combination of baclofen and muscimol prior to (1) persistent responding for a drug-paired conditioned reinforcer, (2) reacquisition of this instrumental response after extinction by omission of the contingent conditioned stimulus (CS), or (3) CS (cue)-induced reinstatement of the original (and different) instrumental response that had previously delivered cocaine. Inactivation of the DStr attenuated persistent responding for a cocaine-paired conditioned reinforcer, as well as its reacquisition after extinction of this response, while the only effect of inactivation of the NAcc shell was to increase CS (cue)-induced reinstatement of the extinguished instrumental response that had previously delivered cocaine. Inactivation of the NAcc core affected all measures of responding maintained by drug-paired conditioned reinforcers. These results are discussed with reference to the neural systems involved in different aspects of responding maintained by drug-paired conditioned reinforcers.

Complex interactions between nicotine and nonpharmacological stimuli reveal multiple roles for nicotine in reinforcement

RATIONALE

Although considerable progress has been made, we do not yet fully understand the behavioral and neurobiological basis of nicotine reinforcement, and without this knowledge, treatment strategies aimed at reducing smoking remain deficient.

OBJECTIVES

This review describes an original perspective on nicotine reinforcement, which arises from substantial evidence of complex interactions between nicotine and nonpharmacological stimuli. We hypothesize that nicotine reinforcement derives from at least two sources: (1) primary reinforcement, an action that requires response-dependent drug administration and is capable of conveying secondary reinforcing effects on associated stimuli, and (2) the reinforcement-enhancing effect of nicotine, which directly enhances behavior maintained by salient nonnicotine stimuli and does not require a contingent relationship between drug administration and reinforced operant responding. Although novel for nicotine, this hypothesis has origins in an extensive literature on the reinforcing effects of psychostimulants. Empirical support for this hypothesis, based largely on animal models of reinforcement, will be presented.

CONCLUSIONS

Animal models of drug reinforcement have evolved to reflect our growing awareness of the multidimensional nature of drug dependence in humans. Investigating the interaction between nicotine and nonpharmacological stimuli within the context of the drug self-administration paradigm in rats has generated new insights into the paradox of how nicotine, an apparently weak primary reinforcer, can sustain the robust behavior observed in self-administration and in smoking. The hypothesis presented in this paper--that nicotine acts as both a primary reinforcer and an enhancer of other nonnicotine reinforcers--provides important direction for future investigations into the neurobiology of nicotine reinforcement and treatments for smoking cessation.

Changes in dopamine efflux associated with extinction, CS-induced and d-amphetamine-induced reinstatement of drug-seeking behavior by rats

The present experiment employed chronoamperometry with stearate-graphite paste electrodes to monitor dopamine efflux in the nucleus accumbens during extinction and subsequent reinstatement of bar-pressing for a conditioned stimulus (CS) following presentation of a CS or following a systemic injection of d-amphetamine. Rats self-administered d-amphetamine (0.25 mg/kg per infusion) for 3 h a day on 6 consecutive days. Each infusion was paired with a flashing light CS. On the 7th day, rats self-administered d-amphetamine for 1 h, followed by 10 h of extinction. Presentation of the CS 2 days following extinction induced small and transient increases in responding for the CS, with no significant associated increases in DA efflux. Lower rates of responding were observed in rats that had received random presentations of the CS during d-amphetamine self-administration, and in an experimentally-naïve control group. A subsequent systemic injection of d-amphetamine increased dopamine efflux in the nucleus accumbens in all groups and was most effective in reinstating bar-pressing in the CS-d-amphetamine paired group. This is consistent with the hypothesis that exposure to psychostimulant drugs, and a drug-paired CS, can reinstate drug-seeking behavior. Together, these findings suggest that enhanced DA efflux may contribute to the reinstatement of drug-seeking behavior induced by the single administration of a psychostimulant drug, but not transient reinstatement induced by presentation of a drug-paired CS alone following extinction.

Bidirectional effects of dopamine D2 receptor antagonists on responding for a conditioned reinforcer

In general, the administration of dopamine (DA) antagonists has been shown to result in the attenuation of reward processes. Recently, however, it has been suggested that low doses of DA antagonists can enhance the incentive value of a primary reinforcer. The present study examined the effect of DA receptor antagonists on responding for a conditioned stimulus (CS) and compared their effects to that produced by d-amphetamine. For 12 days, food-deprived rats were trained to associate a CS with a food reward. In the test phase, the CS was presented following a response on one of two levers (CR), whereas responding on the other lever (NCR) had no consequence. Low doses of d-amphetamine (0.5 mg/kg), sulpiride (4 mg/kg), pimozide (0.025 mg/kg), and raclopride (0.05 mg/kg) selectively enhanced responding on CR. A low dose of haloperidol (0.01 mg/kg) led to a nonspecific increase in lever responding. Treatment with larger doses of these compounds as well as with the D1 antagonist SCH23390 attenuated responding on CR. Both CR and NCR responding were reduced following administration of higher doses of d-amphetamine. Results indicate that responding for a conditioned reinforcer is potentiated following administration of low doses of D2 receptor antagonists, suggesting that D2 receptor blockade can facilitate incentive motivation.

Enhanced reward-related responding following cholera toxin infusion into the nucleus accumbens

In recent years, considerable focus has been directed to understanding how drugs of abuse affect neuronal function at the molecular level. For example, repeated administration of stimulants or opiates can induce long-lasting alterations in gene expression, transcription factors, and signal transduction pathways. Our laboratory previously showed that intraaccumbens infusion of cholera toxin (CTX), which alters the Gs protein such that production of cyclic Adenosine Monophosphate (AMP) is upregulated, causes pronounced, long-lasting motor activation and sensitization to stimulants. In the present experiments, the effect of intraaccumbens infusion of cholera toxin on reward-related responding was investigated. The conditioned reinforcement (CR) paradigm was employed, which measures an animal's instrumental response to obtain presentation of a stimulus previously paired with a primary reward. When this stimulus supports acquisition of a new operant response (lever-pressing), it is termed a conditioned reinforcer (CR). In the first experiment, the effects of bilateral intraaccumbens infusion of CTX (100 ng/1 microliter) were examined on previously-established responding. CTX treatment resulted in enhanced responding for the CR. This enhancement developed over several days and reached its peak 3 days following infusion. In the second experiment, the influence of CTX was examined on acquisition of responding for the CR. The group treated with CTX (100 ng) discriminated between the CR and control (NCR) lever earlier than the vehicle-infused group, and showed greater levels of responding on the CR lever. In the third experiment, it was determined that infusion of CTX (300 ng bilaterally) into the anterior dorsal striatum did not affect levels of responding, although a later test with cocaine in these animals (25 mg/kg, intraperitoneally) (i.p.) indicated that they were capable of potentiated responding. These data are interpreted as evidence that the G(S) protein-cyclic AMP second messenger system within the nucleus accumbens is directly involved in reward-related behavior.

Effects of cocaine on briefly signaled versus completely signaled delays to reinforcement

Key pecking by 4 pigeons was maintained by a multiple schedule consisting of two variable-interval 60-s schedules wherein each food presentation followed a nonresetting 27-s delay that was either briefly signaled at its outset or completely signaled. Brief-signal duration was adjusted so that response rates maintained by the briefly and completely signaled delays of reinforcement were similar. In general, acute administration of small to intermediate doses (0.3 to 3.0 mg/kg) of cocaine produced either small increases in response rates in both components or no change, and larger doses (5.6 to 13.0 mg/kg) decreased response rates. Chronic (i.e., daily) cocaine administration (10.0 mg/kg) resulted in tolerance to the rate-decreasing effects in both components. Cocaine's effects were generally similar whether delays were completely or briefly signaled. Discontinuation of cocaine administration and subsequent removal of the delay signals also had similar effects in both components of the multiple schedule. Taken together, these results are consistent with the view that the two types of delay signals were equally effective in maintaining responding during the variable-interval schedules.

Locating reward cue at response manipulandum (CAM) induces symptoms of drug abuse

Appetitive instrumental discrimination learning procedures provide for CAM (cue and manipulandum) when the reward cue (discriminative stimulus positively correlated with positive reinforcement) is located at the response manipulandum (object that when contacted or manipulated defines the performance of the instrumental response). Evidence reviewed shows that CAM induces excessive and compulsive instrumental responding relative to otherwise comparable non-CAM control procedures. In humans, symptoms of drug abuse are particularly likely when the drug-taking implement (response manipulandum at which instrumental drug-taking is directed) is also predictive of the drug's rewarding effects (reward cue). Evidence that the predictive relationship between a drug-taking implement and drug reward relates to drug abuse is reviewed, and implications for treatment and prevention are considered. CAM is related to neurobiological models of drug abuse that emphasize the role of the neurotransmitter dopamine (DA). CAM produces convergence of DA-mediated responding for conditioned reinforcement with DA mediation of psychomotor activation and incentive-motivational processes to yield reflexive cue-directed responding not observed in non-CAM controls.

The D1 agonist SKF 38393 attenuates amphetamine-produced enhancement of responding for conditioned reward in rats

The present study investigated the hypothesis that the D1 subtype of DA receptors is critically involved in reward-related learning. The effects of SKF 38393, a D1-specific agonist, on amphetamine-produced enhancement of responding for conditioned reward were tested. We exposed 69 male Wistar rats to an experimental design consisting of three phases. The preexposure phase consisted of five sessions during which the rats were exposed to an operant chamber containing two levers. One lever produced a lights-off stimulus (3 s) and the other a tone stimulus (3 s). This was followed by four conditioning sessions during which the levers were removed and the rats were exposed to pairings of the lights-off stimulus with food. This phase was followed by two test sessions during which the levers were present and the number of responses made on each lever was calculated as a ratio of the number of responses made during the preexposure phase. A group receiving saline during the test sessions showed a higher ratio of responding for the lights off stimulus than the tone stimulus, demonstrating that the lights-off stimulus had become a conditioned reward. Amphetamine [2.0 mg/kg, intraperitoneally (i.p.), 5 min before the test] enhanced responding specifically on the lever producing the conditioned reward. Groups receiving SKF 38393 (5.0, 10.0, and 20.0 mg/kg, i.p., 5 min before the test) failed to show significantly greater responding for the lights-off stimulus than the tone, indicating a reduction or elimination of the conditioned reward effect. Moreover, SKF 38393 dose dependently reduced the amphetamine-produced enhancement of responding for conditioned reward.(ABSTRACT TRUNCATED AT 250 WORDS)

Mesoaccumbens dopamine-opiate interactions in the control over behaviour by a conditioned reinforcer

These experiments examined the role of dopamine-opiate interactions in the ventral tegmental area (VTA) and nucleus accumbens in the mediation of reinforcement-related behaviour. It has been shown previously that opiates induce a dopamine-dependent increase in locomotor activity in rats when infused into the VTA, and a dopamine-independent hyperactivity when infused into the nucleus accumbens. The present study investigated the generality and significance of these two findings, by examining dopamine-opiate interactions in the control over behaviour exerted by a conditioned reinforcer (CR), an arbitrary stimulus which gains control by association with primary reinforcement. Rats were trained to associate a light/noise stimulus with sucrose reinforcement, and the efficacy of the CR in controlling behaviour was assessed by measuring its ability to support a new lever pressing response. Responding on one lever (CR lever) produced the CR, responding on the other lever had no programmed consequences. In experiment 1, intra-accumbens infusions of d-amphetamine (10 micrograms), the D1 dopamine receptor agonist SKF-38393 (0.1 microgram), the D2 dopamine receptor agonist LY-171555 (quinpirole; 0.1 microgram) or the opiate receptor agonist [D-Ala2]-methionine enkephalinamide (DALA; 1 microgram) selectively increased responding on the CR lever. Infusion with DALA intra-VTA had no effect. However, pretreatment with DALA intra-VTA (10 x 1 microgram/day) subsequently reduced the selectivity of the response to infusions intra-accumbens with d-amphetamine or SKF-38393, and blocked the response to LY-171555 or DALA. Pretreatment also shifted to the right the dose-response function for DALA intra-accumbens. In experiment 2, intra-accumbens infusions of d-amphetamine, SKF-38393, LY-171555 or DALA again increased responding on the CR lever only. Pretreatment with intra-accumbens d-amphetamine (5 x 1 microgram/day) reduced the selectivity of the response subsequently to d-amphetamine, and blocked the response to SKF-38393, LY-171555 or DALA. In experiment 3, intra-accumbens infusions of the mu-opiate receptor agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (0.003-0.1 microgram), or the delta-opiate receptor agonist [D-Pen2,5]-enkephalin (0.03-1 microgram) enhanced selectively responding on the CR lever. Thus, the dopamine-dependent locomotor-stimulant properties of intra-VTA infusions of opiates are associated with impaired conditioned reinforcer efficacy. Finally, repeated stimulation of the mesoaccumbens dopamine pathway may compromise the dopamine-independence of the opiate system within the nucleus accumbens.

Dopamine D1 and D2 antagonists attenuate amphetamine-produced enhancement of responding for conditioned reward in rats

It has been suggested that the dopamine D1 receptor may play an important role in reward. The present study was undertaken to investigate the roles of dopamine D1 and D2 receptor subtypes in responding for conditioned reward. This was done by examining the effects of the D1 antagonist SCH 23390 and the D2 antagonists pimozide and metoclopramide on amphetamine-produced enhancement of responding for conditioned reward. The procedure consisted of three distinct phases. During the pre-exposure phase the rats were exposed to an operant chamber containing two levers. One lever produced a lights-off stimulus (3 s) and the other a tone stimulus (3 s). This was followed by four conditioning sessions during which the levers were removed and the rats were exposed to pairings of the lights-off stimulus with food. This phase was followed by two test sessions during which the levers were present and the number of responses made on each was calculated as a ratio of the number of responses made during the pre-exposure phase. A group receiving the vehicle during the test sessions showed a greater ratio of responding for the lights-off stimulus than the tone stimulus, indicating that the lights-off stimulus had become a conditioned reward. Amphetamine (0.1, 1.0, 2.0 and 5.0 mg/kg, IP, 5 min prior to test) specifically enhanced responding on the lever producing conditioned reward. SCH 23390 (5.0 and 10.0 micrograms/kg, SC, 2 h before test) and pimozide (0.1 and 0.2 mg/kg, IP, 4 h before test) dose-dependently shifted the peak in the amphetamine dose-response function to the right, indicating an attenuation of conditioned reward.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine functions in appetitive and defensive behaviours

The data reviewed here are compatible with the hypothesis that telencephalic dopamine activity is elicited by motivationally significant stimuli which in turn creates a neural state in which animals are more prepared to respond to significant stimuli in the environment. This analysis may be viewed as extensions of both the sensorimotor hypothesis, which depicts dopamine as potentiating the ability of stimuli to elicit responses (Clody and Carlton, 1980; Marshall et al., 1974; White, 1986) and of the incentive motivational hypothesis, which emphasizes the importance of dopamine in responding to stimuli that serve as signals of biologically significant events (Blackburn et al., 1989a; Crow, 1973; Mogenson and Phillips, 1976). In addition, we have sought to emphasize that not all responses are equally dependent upon the integrity of forebrain dopamine activity. Some responses, such as ingestion of standard foods by hungry animals, copulation, and escape, are relatively impervious to dopamine disruption. Further, once other behaviours, such as avoidance or appetitive operant responses, have been acquired, they can be maintained at an initially high rate despite perturbation of dopamine systems, although performance deteriorates with repeated testing. This analysis has emerged from the joint consideration of how both appetitive and defensive behaviours are influenced by dopamine antagonists, along with an examination of dopamine release during sequences of behaviour. The data reviewed suggest that dopamine is involved in fundamental psychological processes through which environmental stimuli come to exert control over certain aspects of behaviour. In the future, as knowledge in this field advances, there will have to be an integration of the literature on dopamine and motivation with the literature on dopamine and motor systems. We expect that dopamine release will be seen as a mechanism by which important environmental cues, of innate or learned significance, lead to a general enhancement of motor skeletal responses directed towards distal cues. We conclude with a caveat: Caution must be exercised when attempting to infer a general role of any neurotransmitter in motivated behaviour based on the study of a limited number of motivational systems. Although neurotransmitter pathways may figure prominently in the control of certain behaviours, it is incorrect to think of neurotransmitters as having a single role in behaviour. However, when comparative analyses reveal a common thread among different motivational systems, as is becoming apparent for the general role of mesotelencephalic dopamine pathways in behaviour, then the goal of generating coherent and comprehensive theory concerning a neurotransmitter's function in behaviour will begin to be realised.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of drug reward mechanisms by intracranial injections

Intracranial drug injections are useful in localizing brain areas where drugs of abuse initiate their habit-forming actions. However, serious methodological problems accompany such studies. Pharmacological controls are necessary to assess non-receptor-mediated local actions of the drug, anatomical controls are necessary to rule out drug efflux to distal sites of action, and behavioral controls are necessary to separate rewarding from general activating effects of drugs. Five brain sites have been advanced as sites of rewarding opiate actions: the ventral tegmental area (VTA), nucleus accumbens septi (NAS), lateral hypothalamus, periaqueductal gray, and hippocampus. Current evidence appears to confirm two of these--VTA and NAS; evidence is currently incomplete in the case of the hippocampus and is conflicting in the case of the lateral hypothalamus and periaqueductal gray. Two sites have been advanced as sites of rewarding psychomotor stimulant actions: NAS and the frontal cortex; each site seems implicated, but puzzling differences between amphetamine and cocaine findings remain to be resolved. Each of the clearly implicated sites is local to dopamine cell bodies or dopamine terminals that have been implicated in the rewarding effects of brain stimulation, food, and sex.

Food-paired stimuli as conditioned reinforcers: effects of d-amphetamine

Seven pigeons were studied in two experiments in which key pecks were reinforced under a second-order schedule wherein satisfaction of variable-interval schedule requirements produced food or a brief stimulus. In the second part of each session, responses produced only the brief stimulus according to a variable-interval schedule (food extinction). For the 4 pigeons in Experiment 1, the response key was red throughout the session. In separate phases, the brief stimulus was either paired with food, not paired with food, or not presented during extinction. d-Amphetamine (0.3 to 10.0 mg/kg) dose-dependently reduced food-maintained responding during the first part of the session and, at intermediate dosages, increased responding during the extinction portion of the session. The magnitude of these increases, however, did not consistently depend on whether the brief stimulus was paired, not paired, or not presented. It was also true that under nondrug conditions, response rates during extinction did not differ reliably depending on pairing operations for the brief stimulus. In Experiment 2, 3 different pigeons responded under a procedure wherein the key was red in the component with food presentations and blue in the extinction component (i.e., multiple schedule). Again, d-amphetamine produced dose-related decreases in responding during the first part of a session and increases in responding in the second part of the session. These increases, however, were related to the pairing operations; larger increases were observed when the brief stimulus was paired with food than when it was not or when it was not presented at all. Under nondrug conditions, the paired brief stimulus controlled higher response rates during extinction than did a nonpaired stimulus or no stimulus. These findings suggest that d-amphetamine can enhance the efficacy of conditioned reinforcers, and that this effect may be more robust if conditioned reinforcers occur in the context of a signaled period of extinction.

Potentiation of the effects of reward-related stimuli by dopaminergic-dependent mechanisms in the nucleus accumbens

Three experiments examined the behavioural, pharmacological and neural specificity of the previously reported potentiation of responding with conditioned reinforcement following intra-accumbens d-amphetamine, by studying the effects of intra-accumbens dopamine (DA) and noradrenaline, using an acquisition of a new response procedure. In experiment 1, the effects of intra-cerebral DA infusions (5, 20, 50 micrograms/2 microliters) were compared in four conditions: (i) intra-accumbens DA following positive pairing of the conditioned stimulus (CS) and water during training; (ii) as (i) but also following a systemic dose of the DA receptor antagonist alpha-flupenthixol; (iii) intra-accumbens DA following random pairing of the CS and water during training; and (iv) as (i) but with intra-caudate rather than intra-accumbens DA. The results showed that only with intra-accumbens DA in the positive pairing condition was there a significant dose-dependent increase in responding. In experiment 2, the effects of a higher range of doses (20, 100, 200 micrograms) and smaller infusion volume (5, 25, 50 micrograms/l microliters) of intra-accumbens DA were studied, in comparison with a similar range of doses (5, 25, 50 micrograms/l microliters) of intra-accumbens noradrenaline (NA). Only DA produced a selective, dose-dependent increase in responding with conditioned reinforcement. In experiment 3 neurotoxic lesions of the dorsal noradrenergic bundle (DNAB) using 6-hydroxydopamine producing profound (about 90%) depletion of cortical and nucleus accumbens NA levels had no effect on the increased responding with conditioned reinforcement produced by intra-accumbens d-amphetamine (3, 10, 30 micrograms/l microliters).(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine and conditioned reinforcement. I. Differential effects of amphetamine microinjections into striatal subregions

In the conditioned reinforcement paradigm, animals learn a new instrumental response reinforced solely by conditioned reward (a stimulus that has previously been associated with primary reward). It has been shown that psychostimulants potentiate responding for conditioned reward and there is evidence that the nucleus accumbens is involved in this effect. The present experiments extend this work and examine the roles of various striatal subregions in the enhancement of responding for conditioned reward. In the conditioning phase, hungry rats were trained to associate a light/click stimulus with food delivery, with no levers present in the operant chamber. In the test phase, two levers were present and responding on one provided conditioned reward (presentation of the compound stimulus but no food). During this phase, microinjections of d-amphetamine (0, 0.2, 2.0, 20.0 micrograms/0.5 microliters) were made into seven striatal subregions in separate groups of rats. Injection of amphetamine into the nucleus accumbens elicited a dose-dependent, selective increase in responding for CR. Injections into posterior regions of the striatum had no effect. Significant and selective increases in CR responding were noted after injections into two regions neighboring the nucleus accumbens, the anterior dorsal and the ventromedial striatum, although the magnitude of these effects was considerably less than that following accumbens injections. Injections into ventrolateral regions increased responding in some rats, but this effect was very variable and not selective for the CR lever. These results are interpreted as evidence for functional heterogeneity of the striatum with regard to enhancement of conditioned reinforcement. The findings are discussed in relation to the theory that increased dopaminergic activity in the nucleus accumbens results in amplification of the response to a previously learned reward-related signal.

Limbic-striatal interactions in reward-related processes

Stimuli with formerly motivationally neutral properties gain incentive properties from their predictive association with primary reinforcement, and are termed conditioned reinforcers. Infusions of the indirect dopamine (DA) agonist d-amphetamine into the ventral striatum selectively enhanced responding for a light that was positively correlated with water. These selective stimulatory effects were blocked by 6-OHDA lesions of the ventral, but not dorsal striatum, and resembled those produced by DA itself when infused into the ventral striatum. However, the choice of the lever producing the conditioned reinforcer was not affected by ventral striatal DA depletion. This choice was, however, attenuated by excitotoxic lesions of the basolateral nucleus of the amygdala, suggesting an interaction between this structure and DA-dependent processes of the ventral striatum in the processes by which reward-related stimuli come to affect action. Parallel findings were obtained using a second order schedule of sexual reinforcement. Other observations indicated that the amygdala lesion did not impair the discriminative property of the reward-related stimuli, or affect primary motivation. Experiments using explicit conditional visuo-spatial discrimination tasks suggest that similar discriminative properties are preserved following DA depletion from the ventral, but not the dorsal, striatum. These results possibly represent a dissociation between stimulus-response (or habit) and stimulus-reward associative mechanisms, modulated respectively by the dorsal and ventral striatum.

Pipradrol-induced hyperthermia in the rabbit

Intravenous administration of the stimulant drug pipradrol evoked a dose-dependent hyperthermic response in rabbits. This drug effect was significantly reduced by pretreatment with chlorpromazine, phenoxybenzamine, intravenous BE-2254 or intracerebroventricular BE-2254. In animals pretreated with cyproheptadine or atropine, the hyperthermic effect was significantly potentiated. However, haloperidol, pimozide, diphenhydramine and pentobarbital failed to appreciably influence the drug response. These findings suggest that pipradrol-induced hyperthermia in the rabbit involves activation of central alpha-adrenergic receptors and is apparently enhanced by blockade of serotonergic or cholinergic receptors.

Enhanced behavioural control by conditioned reinforcers following microinjections of d-amphetamine into the nucleus accumbens

Stimulant drugs have been shown to enhance the control over behaviour exerted by stimuli previously correlated with primary reinforcers, termed conditioned reinforcers (CR). Experiment 1 examined the possible neuroanatomical specificity of the enhancement of conditioned reinforcement following intracerebral injections of d-amphetamine. Thirsty rats were trained to associate a light with water. In the test phase, water was no longer presented but the light (CR) was intermittently produced by responding on one of two novel levers. Rats with bilateral guide cannulae aimed at the nucleus accumbens, posterior caudate nucleus, or medio-dorsal nucleus of the thalamus received four counterbalanced microinfusions of d-amphetamine (10, 20, 30 micrograms/2 microliters) or vehicle (control) over 4 test days. There was a dose-dependent selective increase in responding on the lever that produced the light (CR) with intra-accumbens d-amphetamine infusions. Quantitatively similar, but much more variable effects were found with intra-caudate infusions and no effects following intra-thalamic d-amphetamine. Experiment 2 provided evidence that the enhanced control over responding by a CR with intra-accumbens d-amphetamine is behaviourally specific. Three groups of rats received a compound tone--plus--light stimulus that was positively, negatively or randomly correlated with water during training. Intra-accumbens d-amphetamine produced selective increases in responding only if the contingent stimulus had been positively correlated. The results suggest that the nucleus accumbens may play an important role in d-amphetamine's enhanced control over behaviour exerted by conditioned reinforcers.

The role of dopamine in locomotor activity and learning

The discovery that the brain contains neurons utilizing dopamine (DA) as their transmitter has led to studies of the behavioral function of these neurons. Changes in overall level of activity of DA neurons appear to produce parallel changes in locomotor activity. Additionally, DA neurons seem to mediate in part the effects of biologically significant (reinforcing) stimuli on learning. One way in which reinforcing stimuli produce learning is to increase the incentive motivational (response-eliciting) properties of neutral stimuli associated with them; also, reinforcing stimuli maintain the incentive motivational properties of previously conditioned incentive stimuli. Normal DA functioning appears to be required for the establishment and maintenance of incentive learning in naive animals. Previous incentive learning in trained animals can influence behavior for a time even when the function of DA neurons is disrupted; however, with continued testing in the absence of normal DA functioning, previously established conditioned incentive stimuli cease to influence behavior. From these observations and recent physiological, anatomical and biochemical studies of DA systems it is suggested that the biological substrate of DA-mediated incentive learning is a heterosynaptic facilitation of muscarinic cholinergic synapses. This model has important clinical implications since it has been suggested that DA hyperfunctioning underlies the development of schizophrenia.

Contrasting interactions of pipradrol, d-amphetamine, cocaine, cocaine analogues, apomorphine and other drugs with conditioned reinforcement

The effects of various psychomotor stimulant drugs and drugs outside this class were examined on the efficacy of stimuli previously paired with reinforcement or reward (conditioned reinforcers, CR) in controlling responding. Pipradrol (5-45 mumol/kg), d-amphetamine (1.25-15.0 mumol/kg), and the cocaine analogues WIN 35,428 (0.1-30.0 mumol/kg) and in one of two determinations WIN 35,065-2 (0.1-29.0 mumol/kg) all generally increased responding on a lever providing CR, but did not change or decreased responding on a lever providing no CR (NCR). Cocaine (5-125 mumol/kg) and chlordiazepoxide (3.75-60.0 mumol/kg) had no significant effects. Morphine (3.2-32.0 mumol/kg) and alpha-flupenthixol (0.02-2.0 mumol/kg) generally reduced responding on both levers. Apomorphine (0.1-1.0 mumol/kg) generally increased responding on both levers. Neurochemical data showed that d-amphetamine was generally more potent than pipradrol in its effects on in vitro monoamine uptake and release.

The acquisition of responding with conditioned reinforcement: effects of cocaine, (+)-amphetamine and pipradrol

1 A procedure for examining the acquisition of a lever-pressing operant with conditioned reinforcement was used to compare the effects of three psychomotor stimulants. 2 Hungry rats were trained to associated and auditory tone (i.e., conditioned reinforcer) with food. Preference for the tone was then measured after treatment with pipradol (5, 10, 15 mg/kg), cocaine (1, 5, 10 mg/kg) or (+)-amphetamine (0.5, 1.5, 5.0 mg/kg). 3 In agreement with previous data, 10 mg/kg of pipradrol enhanced the effect of conditioned reinforcement whereas animals treated with any of the doses of (+)-amphetamine showed no effect. 4 Rats treated with cocaine (1 or 5 mg/kg) showed an effect of conditioned reinforcement but the effect was not significantly greater than in controls. 5 The present data suggest important differences in enhancement of responding for conditioned reinforcement by various drugs in the psychomotor stimulant class. These differences in turn may be related to the pharmacological actions of these compounds on release of catecholamines from different storage pools.

Perseverative behaviour after amphetamine; dissociation of response tendency from reward association

Low doses of amphetamine were found to alter the ability of marmosets to take account of changes in reward values of object stimuli in a visual discrimination task. Under amphetamine, animals changed their motor responses and stimulus choice in order to preserve the acquired reward value or meaning of certain stimuli. These results suggest that the perseverative effect of amphetamine on behaviour is due to impaired cognitive flexibility rather than to an enhancement of motor habit.