Neuroleptics block the positive reinforcing effects of amphetamine but not of morphine as measured by place conditioning

The effects of opioid peptides on dopamine release in the nucleus accumbens: an in vivo microdialysis study

An involvement of the mesolimbic dopamine (DA) system in mediating the motivational effects of opioids has been suggested. Accordingly, the present study employed the technique of in vivo microdialysis to examine the effects of selective mu-, delta-, and kappa- opioids on DA release in the nucleus accumbens (NAC) of anesthetized rats. Microdialysis probes were inserted into the NAC and perfusates were analyzed for DA and its metabolites, dihydroxyphenylacetic acid (DO-PAC) and homovanillic acid (HVA), using a reverse-phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (i.c.v.) administration of selective mu-opioid [D-Ala2, N-methyl-Phe4, Gly5-ol]-enkephalin (DAMGO) or delta-opioid [D-Pen2, D-Pen5]-enkephalin (DPDPE) agonists, at doses that function as positive reinforcers in rats, resulted in an immediate and significant increase in extracellular DA. DOPAC and HVA levels were also significantly increased. The effects of DAMGO were blocked by the selective mu-antagonist D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) whereas those of DPDPE were blocked by the delta-antagonist allyl2-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864). In contrast to mu- and delta-agonists, the kappa-agonist N-CH3-Tyr-Gly-Gly-Phe-Leu-Arg-N-CH3-Arg-D-Leu-NHC2H5 (E-2078), a dynorphin analog that produces aversive states, decreased DA release in a biphasic manner. Norbinaltorphimine, a selective kappa-antagonist, could block this effect. These results demonstrate that mu-, delta-, and kappa-opioid agonists differentially affect DA release in the NAC and this action is centrally mediated.

Haloperidol prevents the reinstatement of amphetamine-rewarded runway responding in rats

Animals were trained to traverse a straight-alley once each day for a reward of 1.0 mg/kg SC d-amphetamine sulfate. After 14 days of acquisition, extinction trials were initiated in which the amphetamine reward was replaced by injections of physiological saline. After running speeds had decreased to less than one third those of preextinction values, rats received a single amphetamine-rewarded trial either in the absence or presence of haloperidol (0.075, 0.15 or 0.3 mg/kg IP). Twenty-four hours later, animals were tested for reinstatement of operant running in a single drug-free Test trial. Animals that were nondrugged during the amphetamine-rewarded trial demonstrated a statistically reliable increase in running speed on the Test trial relative to extinction baseline speeds. In contrast, animals that were under the influence of medium or high doses of haloperidol during the amphetamine-rewarded trial failed to show Test day increases in running speed. This result did not stem from some residual sedative or performance impairing quality of the drug since a "motor control group" administered a high dose of haloperidol shortly after a rewarded trial, was able to demonstrate unimpaired reinstatement of operant running on Test day (i.e., 24 hr later). These findings support the view that dopamine systems play a role in the neural substrates underlying the incentive motivational properties of amphetamine reinforcement.

Postconditioning effects of magnesium on cocaine conditioned place preference in mice

Magnesium chloride (MgCl2) has recently been shown to have stimulant-like properties. Because stimulants are known to induce conditioned place preference (CPP), the CPP procedure was used to test the hypothesis that cocaine and MgCl2 share similar stimulus properties. This would be shown if cocaine-induced CPP could be enhanced in a postconditioning preference test by MgCl2 and other stimulants. Mice were conditioned with 5.0 mg/kg cocaine to the nonpreferred end of a three-compartment straight shuttle box. All groups showed significant shifts in preference from the preconditioning test to the postconditioning test. There were no changes in place preference over test days in mice that were injected only with saline and therefore not conditioned. When animals were given acute injections of either saline, 5.0 mg/kg cocaine, 1.0 mg/kg amphetamine, 30 mg/kg MgCl2, 10 mg/kg pentobarbital, or 0.25 mg/kg haloperidol following conditioning with cocaine, amphetamine and MgCl2 elevated the conditioned cocaine effect, and pentobarbital and haloperidol decreased the conditioned cocaine effect compared to saline. In addition, there was a dose-dependent influence of MgCl2, with 30 mg/kg producing the maximum effect on the conditioned cocaine effect.

The reserpine-sensitive dopamine pool mediates (+)-amphetamine-conditioned reward in the place preference paradigm

The neural basis of amphetamine-conditioned reward was investigated in the conditioned place preference paradigm. When bilaterally injected into the nucleus accumbens before the test session, a dopamine receptor blocker, alpha-flupenthixol, blocked the expression of the amphetamine-conditioned place preference. alpha-Flupenthixol had no significant effect on spontaneous locomotor activity. Bilateral microinjections of a tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine (alpha-MPT), decreased (+)-amphetamine locomotor stimulation in a dose-dependent fashion. Two doses of alpha-MPT that completely blocked (+)-amphetamine locomotor stimulation had no effect on the expression of the conditioned place preference when injected into the nucleus accumbens before testing. Reserpine administered subcutaneously before testing blocked the expression of the conditioned place preference. A dose of reserpine (4.0 mg/kg), which completely blocked the conditioned place preference, did not attenuate (+)-amphetamine-induced locomotor stimulation. This dose of reserpine depleted dopamine in the nucleus accumbens to 4% of its normal value. These data show that (+)-amphetamine-conditioned reward, expressed as a conditioned place preference, is mediated by dopamine release in the nucleus accumbens. Moreover, the dopamine is released from the reserpine sensitive pool, and probably not from the newly synthesized alpha-MPT-sensitive pool.

Selective D1 and D2 dopamine antagonists decrease response rates of food-maintained behavior and reduce the discriminative stimulus produced by heroin

Animals were trained to discriminate heroin from saline in a two-lever food-reinforced paradigm. Tests with the heroin metabolites O6-monoacetylmorphine and morphine suggest that the heroin discriminative stimulus was mediated by monoacetylmorphine. The heroin discriminative stimulus was not blocked by pretreatment with low doses of the D1 dopamine antagonist SCH23390 or the D2 antagonist spiperone; higher doses of the antagonists produced decreases both in selection of the drug-appropriate lever after heroin, and in food-maintained responding. The data suggest that dopamine may mediate the heroin discriminative stimulus. When administered in the absence of opioids, the D2 antagonist spiperone did not have rate-decreasing effects, whereas SCH23390 did. Heroin partially reversed the rate-decreasing effects of SCH23390, possibly as a result of the ability of opioids to release dopamine.

Conditioned aversion after delay place conditioning with amphetamine

Male, Sprague-Dawley rats received subcutaneous injections of either dextroamphetamine sulfate (AMP; 3.0 mg/kg) or vehicle [VEH (phosphate buffer); 1 ml/kg] immediately before (standard conditioning) or after (delay conditioning) conditioning sessions in a place-conditioning paradigm. AMP was paired for 4 conditioning sessions with one compartment of a three-compartment place-conditioning apparatus; VEH was paired for 4 conditioning sessions with another compartment. Animals were then tested for place preference or aversion by determining the proportion of time spent in each compartment during a 15-minute test session. Standard conditioning with AMP produced a place preference while delay conditioning produced a place aversion. Similar findings had earlier been reported from studies involving conditioned place-preferences and aversions with nicotine. These studies demonstrated that the time of drug administration can be as strong a determinant of place-conditioning effects as the drug itself.

Conditioned place preference in the corral: a procedure for measuring reinforcing properties of drugs

A novel conditioned place preference (CPP) method is described. The behavioral testing apparatus is a circular open field consisting of 4 uniform quadrants that are equally preferred by the rats prior to drug treatment. In an illustrative experiment, rats received an i.p. injection of either morphine (10 mg/kg), substance P (50 micrograms/kg) or vehicle (phosphate-buffered 0.01 M acetic acid in saline) on 3 consecutive days and were placed into their assigned treatment quadrant. Four animals were simultaneously treated with the aid of barriers, which restricted each rat to its treatment quadrant. On the test for CPPs, when provided a choice between the 4 quadrants, rats treated with morphine and substance P exhibited preferences for the quadrant which had been paired with the drugs, illustrating the usefulness of this procedure for assessing the reinforcing properties of the two drugs. Gross locomotor activity was not influenced by either treatment. The advantages of this version of the CPP method over the conventional shuttle-box procedures are discussed.

The use of place conditioning in studying the neuropharmacology of drug reinforcement

The place conditioning paradigm has proven successful in identifying the neural mechanisms of drug reinforcement. Two classes of drugs, opiates and psychomotor stimulants, have received the most study, and in each case an important role for DA neurons of the mesolimbic system has been established. Moreover, both receptor subtypes, D1 and D2, appear to be involved. Despite this progress, the substrates of drug reward are not completely understood. First, a role for DA has not been established for all stimulants: DA receptor blockade failed to affect conditioned place preferences produced by the stimulants methylphenidate, nomifensine, or bupropion. Second, preliminary evidence suggests that intact serotonergic transmission is important in morphine place conditioning, but a similar consistent finding has not been observed with amphetamine place conditioning. Further study may reveal an interesting dissociation of serotonin's role in the rewarding effects of psychomotor stimulants and opiates. Finally, the role of the opiate receptor subtype kappa is not known; also, the significance of the several anatomical sites that support opiate place conditioning remains unclear.

Opioid mediation of cocaine-induced hyperactivity and reinforcement

The mechanisms by which cocaine produces hyperactivity and reinforcement remain poorly understood. Since reinforcement is also a property of other drugs of abuse including opiates, we examined the possible mediation of these cocaine-induced behaviors by endogenous opioid peptides. In this study, we have confirmed reports that cocaine increases locomotor activity and conditioned place preference in rats. We have also demonstrated that opioid receptor blockade with naloxone antagonizes completely the locomotor-activating effect of cocaine and attenuates the strength of the place preference conditioning produced by cocaine. These data support the thesis that endogenous opioids are involved in mediation of cocaine-induced behavior.

Opiate reward: sites and substrates

Opiates appear to have rewarding actions at more than one locus in the brain. Studies of the effects of dopaminergic lesions and dopamine receptor blockade indicate that intravenous heroin self-administration depends importantly on a dopaminergic substrate. Mapping of effective injection sites for morphine-conditioned place preference establishes one site of rewarding action near the dopamine cell bodies of the ventral tegmental area (VTA). Studies of the complex interactions of opiates, neuroleptics, and brain stimulation reward confirm that reward-related VTA opioid actions are dopamine-dependent. Opioid injections into the nucleus accumbens (NAS) also facilitate brain stimulation reward and serve as rewards in their own right, though these actions have not yet been localized by identification of negative sites in surrounding regions. The relation of this putative reward site to the dopamine system is not yet clear. Suggestions that the lateral hypothalamus or periaqueductal gray contain opioid reward sites remain to be confirmed. While opioid injections into these sites can be rewarding, these rewarding effects have not been localized to these sites, and opiate injections into each of these areas are reported not to facilitate brain stimulation reward. Intravenous heroin self-administration is not disrupted by kainic acid lesions of the bed nucleus of the lateral hypothalamus. Thus only the VTA and the NAS are firmly established as sites of opiate rewarding actions. Recent reports suggest that the kappa-opioid dynorphin may also have central rewarding actions and central and peripheral aversive actions; the CA3 region of the hippocampus is a possible site of the rewarding action.

Differential mechanisms in the acquisition and expression of heroin-induced place preference

These experiments examined the neurochemical mechanisms involved in the development and expression of place conditioning produced by heroin. Conditioned place preferences (CPP) lasting up to 8 weeks were obtained with doses of 50-1000 micrograms/kg heroin, using a regimen shown not to produce physical dependence. Naloxone pretreatment (50 micrograms/kg) during conditioning prevented the acquisition of heroin-induced CPP, but when given only on the test day, naloxone (50 or 1000 micrograms/kg) did not prevent the expression of heroin CPP. Clonidine disrupted the establishment of heroin CPP at 20 micrograms/kg, but disrupted its expression only at debilitating doses (100 and 200 micrograms/kg). Pimozide attenuated the acquisition (100 micrograms/kg) and expression (250 micrograms/kg) of heroin CPP. Together, these results support a role for opioid and catecholamine systems in the acquisition of heroin reinforcement, but they suggest that once heroin CPP is established, its expression in opiate-free subjects is not opiate receptor mediated and is relatively refractory to pharmacological treatments which disrupt acquisition. The data challenge the notion that the conditioned effects of opiates in drug-free animals are related to the release of endogenous opioids, and they also may help to explain why naloxone and clonidine are ineffective in the treatment of opiate addiction.

SCH 23390 blocks drug-conditioned place-preference and place-aversion: anhedonia (lack of reward) or apathy (lack of motivation) after dopamine-receptor blockade?

The influence of the D1 antagonist SCH 23390 on the motivational properties of rewarding (morphine, nicotine and diazepam) and aversive (naloxone, phencyclidine and picrotoxin) drugs was studied in the rat in a two-compartment place-conditioning paradigm, which included a pre-conditioning test for spontaneous place-preference. The specific D1 dopamine-receptor antagonist SCH 23390 (0.05 mg/kg SC), paired with both compartments or, separately, with the preferred or with the non-preferred compartment, failed to affect the spontaneous unconditioned preference of the animal. Pairing of morphine (1.0 mg/kg SC), nicotine (0.6 mg/kg SC) or diazepam (1.0 mg/kg IP) with the less preferred compartment induced significant preference for that compartment. Pairing of SCH 23390 (0.05 mg/kg SC) with both compartments completely blocked the place-preference induced by morphine, nicotine and diazepam. Naloxone (0.8 mg/kg SC), phencyclidine (2.5 mg/kg SC) or picrotoxin (2.0 mg/kg IP) paired with the preferred compartment elicited place-aversion. Pairing of SCH 23390 (0.05 mg/kg SC) with both compartments abolished also the place-aversion induced by naloxone, phencyclidine and picrotoxin. The results indicate that blockade of dopamine transmission blocks the motivational properties of rewarding as well as aversive stimuli. It is suggested that neuroleptics rather than simply blocking the rewarding impact of positive reinforcers (anhedonia, lack of pleasure) exert a more general influence on conditioned behaviour by blocking the affective impact of negative as well as positive reinforcers (apathy, lack of motivation).

The influence of brain catecholamines on 'drug taking behaviour' relative to oral self-administration of d-amphetamine by rats

Oral administration of 5 mg/kg of d-amphetamine to adult Wistar rats caused brain NE to decrease to approx. 80% of the control level during 4-24 h after acute treatment and slowly further to 65% after 24 days of self-administration via drinking water. The norepinephrine (NE)-reducing effect was first recognized at 1 mg/kg and appeared to peak at 5 mg/kg of d-amphetamine. Brain dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were only shortly affected. Neither iprindole nor desipramine altered the effect of amphetamine on brain NE. DA was decreased by both inhibitors depending on the duration of pre-treatment. Iproniazid and alpha-methyl-p-tyrosine antagonized and potentiated respectively the amphetamine effect on NE- and DA-concentration after 4 days of simultaneous treatment. In the free choice experiment (water vs. 0.005% d-amphetamine solution) rats developed an aversion to amphetamine. The number of rats taking the drug and the consumption by rats still drinking it declined gradually from 100% and approx. 3 mg/kg/day to 50% and approx. 1.5 mg/kg/day, respectively, during 18 days. The time course of the developing aversive reaction to oral amphetamine ran approximately parallel to that of NE-depletion. Iprindole and desipramine intensified, iproniazid and propranolol weakened, while alpha-methyl-p-tyrosine and haloperidol hardly influenced the aversive effect of amphetamine. It is concluded that the development of aversive behaviour in response to oral d-amphetamine is mediated not only through the depleting effect of amphetamine on NE stores but also by its direct stimulation at beta-adrenergic receptors in the CNS.

Selective D1 and D2 dopamine agonists produce opposing effects in place conditioning but not in conditioned taste aversion learning

The neurotransmitter, dopamine (DA), has been implicated in place conditioning but the role of D1 and D2 receptors has not been investigated. In Experiment 1, the effects of SKF 38393 (0, 0.01, 0.1, 1.0, 10.0 mg/kg) and quinpirole (0, 0.01, 0.1, 1.0, 2.0, 4.0 mg/kg), preferential D1 and D2 receptor agonists, respectively, were evaluated and compared to (+)-amphetamine (0, 0.01, 0.1, 1.0, 2.0, 4.0 mg/kg). The experiment consisted of three phases. During the preexposure phase, rats explored two distinctive end compartments adjoined by a small tunnel. The time spent in each compartment was recorded. During the 8-day conditioning phase, rats were treated with drug and confined to one compartment for 30 min. On alternate days, rats received saline and were placed in the opposite compartment. Test days occurred over the remaining three days during which drug-free animals explored both compartments. Rats conditioned with (+)-amphetamine demonstrated a dose-dependent increase in time spent in the drug-paired environment from preexposure to test indicating the establishment of a conditioned place preference. Treatment with quinpirole also resulted in a conditioned place preference, however, only an intermediate dose was effective. In contrast, SKF 38393 produced a dose-dependent decrease in time spent on the drug-paired side suggesting the establishment of a place aversion. The idea that D1 receptors may be exclusively involved in mediating the aversive properties of psychomotor stimulants was tested in Experiment 2 employing a conditioned taste aversion paradigm. The results did not support this notion; it was found that both quinpirole and SKF 38393 produced a conditioned taste aversion.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioids and sexual behavior in the male rat

Naloxone in the doses of 4 or 16 mg/kg failed to affect copulatory behavior of testosterone-treated castrated male rats. Morphine 10 mg/kg, administered 60 min before behavioral observation, reduced the proportion of animals displaying sexual behavior. Doses of 2.5 or 5 mg/kg reduced the latency to the second ejaculation, whereas the few animals still copulating after morphine 10 mg/kg showed a reduced latency to the first ejaculation. The same doses of morphine administered 5 min before behavioral observation produced a dose-dependent reduction of mount, intromission and ejaculation percentages. However, those animals that did copulate showed a normal copulatory behavior. D-Ala2-Met5 enkephalinamide (DALA) infused into the left cerebral ventricle in a dose of 5 micrograms 5 or 60 min before tests had no effect. When the peptide was infused 30 sec after the first intromission, the number of intromissions as well as the latency to ejaculation were reduced. Opioids may facilitate ejaculatory mechanisms, perhaps as a consequence of their rewarding properties. Moreover, in animals treated with DALA after the first intromission, the number of intromissions and the latency to ejaculation were similar for the first and second copulatory series, while these parameters were much reduced upon the second ejaculation for control animals. It is possible that liberation of endogenous opioids is the cause of ejaculation-induced facilitation of subsequent sexual behavior.

Motivational effects of opioids: influence of D-1 versus D-2 receptor antagonists

An unbiased place preference conditioning procedure was used to: (i) characterize the motivational effects of dopamine (DA)-receptor antagonists and (ii) examine the role of D-1 versus D-2 DA receptors in mediating the reinforcing and aversive properties of opioids. Acute administration of the D-1 antagonist, SCH 23390 (0.001-0.5 mg/kg), produced conditioned place aversions. In contrast, the D-2 antagonists, (-)-sulpiride and spiperone, were motivationally neutral, lacking reinforcing or aversive effects. Chronic infusion of SCH 23390 (1.0 mg/kg per day) during the conditioning sessions abolished the reinforcing effect of the mu-opioid agonist, morphine, and the place aversions produced by the kappa-opioid agonist, U-69593, and the opioid antagonist, naloxone. D-2 antagonists were ineffective in modifying the motivational properties of opioid agonists and naloxone. These data demonstrate the involvement of D-1 but not D-2 receptors in the motivational properties of opioids and suggest that the D-1 receptor is critical for the expression of reinforcing and aversive motivational states.

The dopamine innervation of the visceral cortex mediates the aversive effects of opiates

We have previously reported that opiates acting on peripheral receptors produce aversive effects whereas opiates acting on central brain receptors produce rewarding effects. The neurotransmitter dopamine (DA) has previously been implicated in both opiate reinforcing (positive) and aversive (negative) effects. We, therefore, chose to investigate the effects of disruption of DA systems on these two motivational properties of the opiate, morphine. Moreover, we sought to determine the brain site where dopamine might act as a mediator of these motivational effects. One group of rats received 6-hydroxydopamine (6-OHDA) lesions of the visceral (agranular insular) cortex to destroy dopaminergic innervation to this area. A separate group of animals were pretreated with intraperitoneal (IP) injections of the DA receptor blocker, alpha-flupenthixol (0.8 mg/kg), followed in both groups by 15 mg/kg (IP) morphine. Both 6-OHDA-lesioned and alpha-flupenthixol-pretreated subjects failed to develop the normal aversion to saccharin seen in control groups following conditioned taste aversion training with morphine. In a place conditioning paradigm, the aversive effects produced by low IP injections of morphine (acting on peripheral receptors) were blocked by 6-OHDA lesions of the visceral cortex. However, DA depletion of the visceral cortex did not disrupt the ability of animals to acquire a morphine place preference. Taken together, these results indicate that DA innervation of the visceral cortex mediates the aversive, but not the rewarding, properties of opiates.

Drugs abused by humans preferentially increase synaptic dopamine concentrations in the mesolimbic system of freely moving rats

The effect of various drugs on the extracellular concentration of dopamine in two terminal dopaminergic areas, the nucleus accumbens septi (a limbic area) and the dorsal caudate nucleus (a subcortical motor area), was studied in freely moving rats by using brain dialysis. Drugs abused by humans (e.g., opiates, ethanol, nicotine, amphetamine, and cocaine) increased extracellular dopamine concentrations in both areas, but especially in the accumbens, and elicited hypermotility at low doses. On the other hand, drugs with aversive properties (e.g., agonists of kappa opioid receptors, U-50,488, tifluadom, and bremazocine) reduced dopamine release in the accumbens and in the caudate and elicited hypomotility. Haloperidol, a neuroleptic drug, increased extracellular dopamine concentrations, but this effect was not preferential for the accumbens and was associated with hypomotility and sedation. Drugs not abused by humans [e.g., imipramine (an antidepressant), atropine (an antimuscarinic drug), and diphenhydramine (an antihistamine)] failed to modify synaptic dopamine concentrations. These results provide biochemical evidence for the hypothesis that stimulation of dopamine transmission in the limbic system might be a fundamental property of drugs that are abused.

Antagonist treatment in nucleus accumbens or periaqueductal grey affects heroin self-administration

The role of opiate receptors in the periaqueductal grey and nucleus accumbens in maintenance of intravenous heroin self-administration was examined by means of intracranial microinjections of the quaternary opiate antagonist methyl naltrexone. Over a dose range of 0-3.0 micrograms, pre-session infusions of methyl naltrexone in either brain site produced dose-related increases in responding for heroin (0.06 mg/kg/infusion) on a CRF schedule, without causing significant changes in responding on a second activity control lever. Involvement of the periaqueductal grey was also examined in animals administering a lower heroin dose (0.03 mg/kg/infusion) in shorter sessions in order to minimize drug exposure prior to treatment. In this experiment, infusion of methyl naltrexone produced selective increases in responding for heroin, whereas treatment with the identical dose of methyl naltrexone had no effect on cocaine self-administration (1.0 mg/kg/infusion) in the same animals. With respect to the nucleus accumbens, these data confirm its involvement in opiate self-administration. Data for the periaqueductal grey provide the first evidence that opiate receptors in the vicinity of this brain region may play a role in intravenous opiate self-administration.

Cocaine-induced place conditioning: importance of route of administration and other procedural variables

It has been shown that pretreatment with dopamine (DA) receptor blockers disrupts the effect of intravenously (IV) and intracerebrally (ICV), but not intraperitoneally (IP) administered cocaine on place preference conditioning (PPC). The present study was undertaken to further evaluate possible differences between IV and IP cocaine PPC. To this end, several factors which may differentially influence IV and IP cocaine PPC were examined. Firstly, dose-response effects were studied. Intravenous cocaine produced PPC within a narrow dose range (0.5-2.5 mg/kg). Animals receiving IV injections of 5 and 10 mg/kg cocaine experienced convulsions and did not show PPC. For IP cocaine a 10-fold increase in dose (10 mg/kg) and twice the number of training trials was required in order to obtain PPC equal in magnitude to that with IV cocaine (0.5 mg/kg; two trials). Cocaine PPC was retained at least 1 month. Following IV cocaine preference developed for the side associated with the drug regardless of whether the conditioning was to the least or most preferred side. After IP cocaine, preference developed for the drug side only when the drug was paired with the least preferred side. Rats trained with IV, but not IP, cocaine significantly preferred the drug familiar side to a novel compartment. Preference for the IV or IP cocaine side developed regardless of whether testing was carried out in the drugged or undrugged state, excluding possible state-dependent effects as an explanation of the cocaine PPC. The results show PPC procedure to be a valid test for evaluating rewarding properties of IV cocaine. However, they fail to show rewarding effects of IP cocaine.

Fentanyl-induced conditional place preference: lack of associated conditional neurochemical events

Three experiments were performed to determine if stimuli previously paired with the reinforcing effects of fentanyl elicit changes in the activity of dopaminergic neurons that are similar to the unconditional effects of the drug. Experiment 1 characterized the unconditional effects of fentanyl (0.04 mg/kg SC) on neurochemical indices of dopaminergic activity in rats. Both acute and repeated fentanyl injections (five injections administered at 48-h intervals) increased the concentrations of the dopamine metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) within the striatum (STR), nucleus accumbens (NAS), and olfactory tubercle (OT). Acute injections elicited a greater increase in metabolite concentrations in the NAS than in the STR, suggesting that there are regional differences in the sensitivity of dopaminergic neurons to fentanyl. In experiments 2 and 3, fentanyl (five injections; 0.04 mg/kg SC) was paired with environmental stimuli using a place preference conditioning paradigm. The fentanyl-paired stimuli failed to elicit conditional changes in DOPAC or HVA concentrations within the STR, NAS, or OT even though rats exhibited a preference for the drug-paired compartment of the shuttle box. These results indicate that the secondary reinforcing effects of stimuli previously paired with fentanyl may not reflect the ability of these stimuli to elicit measurable changes in the activity of mesolimbic or nigrostriatal dopaminergic neurons.

Place preference conditioning reveals the involvement of D1-dopamine receptors in the motivational properties of mu- and kappa-opioid agonists

The role of D1 dopamine receptors in mediating the motivational properties of opioid agonists was investigated by use of place preference conditioning. Administration of the D1 receptor antagonist SCH 23390 (0.001-0.5 mg/kg) or the kappa-opioid receptor agonist U-69593 (0.16 mg/kg) produced conditioned place aversions. In contrast, the mu-opioid agonist, morphine (3.0 mg/kg) was appetitively reinforcing. Chronic infusion of SCH 23390 (1.0 mg/kg/day) during conditioning abolished the effects of both opioid agonists. These data demonstrate the specific involvement of D1 receptors in the motivational properties of mu- and kappa-opioid agonists and suggest that dopaminergic systems are crucial for the expression of both reinforcing and aversive motivational states.

Intravenous cocaine-induced place preference: attenuation by haloperidol

Cocaine reward was demonstrated by establishing a conditioned place preference (CPP) to a distinctive location paired with cocaine administered either intravenously (i.v., 0.5 mg/kg) or intraperitoneally (i.p., 10 mg/kg). Significant i.p. or i.v. cocaine CPP was observed following the second conditioning trial. Haloperidol (0.2 mg/kg) pretreatment disrupted CPP induced by i.v., but not i.p., cocaine. The haloperidol effect built up over successive trials. The involvement of dopaminergic transmission in i.v. cocaine-induced CPP is discussed.

Blockade of D-1 receptors by SCH 23390 antagonizes morphine- and amphetamine-induced place preference conditioning

Morphine (0.5 mg/kg s.c.) and amphetamine (1 mg/kg s.c.) used in a two-compartment place preference test induced strong place preference when paired to the non-preferred environments. They did not modify preference under basal conditions but completely reversed morphine- and amphetamine-induced place preference. Pairing of haloperidol (0.2 mg/kg s.c.) to both environments also abolished the morphine-induced place preference.

Facilitation of the expression but not the acquisition of latent inhibition by haloperidol in rats

In the latent inhibition (LI) paradigm, nonreinforced preexposure to a stimulus retards subsequent conditioning to that stimulus. The administration of haloperidol in both the preexposure and the conditioning stages was found to enhance LI in the conditioned emotional response (CER) procedure (Weiner and Feldon, 1986). The present experiments investigated the effects of 0.1 mg/kg haloperidol administration on LI in a two-way avoidance procedure, consisting of two stages: preexposure, in which the to-be-conditioned stimulus, tone, was repeatedly presented without reinforcement; and conditioning, in which the animals acquired a two-way avoidance response with the tone serving as the warning signal. Experiments 1 and 2 tested whether the administration of haloperidol confirmed to the preexposure stage, where learning to ignore the nonreinforced stimulus takes place, would suffice to enhance the LI effect. In Experiment 1, preexposure and conditioning were conducted 24 hr apart. LI was obtained in both the placebo and haloperidol conditions, but the effect was not more pronounced under the drug. In addition, haloperidol-treated animals exhibited impaired avoidance performance. In Experiment 2, preexposure and conditioning were given 72 hr apart. With this interval, haloperidol did not affect avoidance performance. However, also under these conditions, the magnitude of the LI effect was not larger in the haloperidol-treated groups, indicating that the administration of the drug in the preexposure stage alone did not suffice to enhance LI.(ABSTRACT TRUNCATED AT 250 WORDS)

A potential role of saline trials in morphine-induced place-preference conditioning

The necessary conditions to alter rats' initial preferences for two sides of a shuttlebox were investigated, using procedures that are often used in the study of drug reinforcement. In Experiment 1, pairings of morphine sulfate (15 mg/kg, intraperitoneally) and either the nonpreferred side or a holding box was factorially combined with alternate-day pairings of saline and either the preferred side or a holding box. Pairings of saline and the preferred side were necessary and sufficient to increase preferences for the initially nonpreferred side. In Experiment 2, pairings of saline and the nonpreferred side, but not the holding box, strengthened the initial preference, regardless of whether morphine or saline injections preceded alternate-day holding-box placements. In Experiment 3, injection and placement in the preferred side in an unpaired manner, or placement only, decreased preferences for that side more than saline injections alone or no treatment. Paired saline injections and placement produced a greater change in preference than no treatment.

Haloperidol induces a partial reinforcement extinction effect in rats: implications for a dopamine involvement in food reward

The hypothesis that dopamine antagonist drugs attenuate the reinforcing properties of food was investigated in hungry rats trained to traverse a straight runway for food reward. Testing consisted of a single trial per day during which latencies to leave the start box and to traverse the alley were recorded. In each experiment, a reinforcement phase lasting 30 consecutive days was immediately followed by a 21 day extinction phase. The runway responses of animals that experienced intermittent food reward during the reinforcement phase of the experiments, was later found to be more resistant to extinction than those of continuously reinforced animals. This "partial reinforcement extinction effect" (PREE) was also observed in animals that experienced periodic reductions in the quantity, but not quality, of food reward. Intermittent pretreatment with 0.15 mg/kg of haloperidol during the reinforcement phase produced a PREE that was indistinguishable from that produced by reward omission on those same trials. Control groups for motor debilitation and for non-associative drug effects did not demonstrate a PREE. These results are consistent with the view that central dopamine substrates play a role in the neural basis of food reward.

Visceral cortex lesions block conditioned taste aversions induced by morphine

Rats with bilateral ibotenic acid or sham lesions of the visceral (agranular insular) cortex were tested for a conditioned taste aversion (CTA) to saccharin after five pairings of morphine sulphate injections (15 mg/kg IP) with consumption of a novel solution (0.1% saccharin). Lesioned animals demonstrated no evidence of the morphine-induced CTA that was seen in the sham operated animals. A third group of rats received ibotenic acid lesions but had saccharin consumption paired with saline vehicle injections. This group had the normal preference (seen in naive rats) for saccharin on testing, showing that the visceral cortex lesion had no effect on the ability of the rats to discriminate saccharin from water. In order to test if visceral cortex lesions abolish specifically the CTA induced by morphine, we ran a similar set of CTA experiments using two new novel flavours and either 15 or 75 mg/kg IP lithium chloride (LiCl) as the unconditioned stimuli. Dose dependent CTA's to the LiCl were established in all groups indicating that the visceral cortex plays no role in mediating the aversive effect of LiCl. Using the condition place preference paradigm we investigated the role of the visceral cortex in the expression of morphine's rewarding aspects. Identical place preferences were found in groups of rats with or without visceral cortex lesions suggesting that this cortical region plays no role in either the perception of morphine's rewarding effects or the association of morphine's rewarding properties with sensory stimuli. Visceral cortex lesions also had no effect on the establishment of a conditioned place aversion to a high dose of LiCl (75 mg/kg IP). Thus, visceral cortex appears critical for the establishment of a morphine-induced CTA, but is not crucial for mediating gross taste discrimination, the aversive aspects of LiCl nor the rewarding properties of morphine.