Behavioral sensitization and tolerance to cocaine and the occupation of dopamine receptors by dopamine

Locomotor sensitization modulates voluntary self-administration of methamphetamine in Drosophila melanogaster

As complexities of addictive behaviors cannot be fully captured in laboratory studies, scientists use simple addiction-associated phenotypes and measure them in laboratory animals. Locomotor sensitization, characterized by an increased behavioral response to the same dose of the drug, has been extensively used to elucidate the genetic basis and molecular mechanisms of neuronal plasticity. However, to what extent it contributes to the development of addiction is not completely clear. We tested if the development of locomotor sensitization to methamphetamine affects voluntary self-administration, and vice versa, in order to investigate how two drug-associated phenotypes influence one another. In our study, we used the genetically tractable model organism, Drosophila melanogaster, and quantified locomotor sensitization and voluntary self-administration to methamphetamine using behavioral tests that were developed and adapted in our laboratory. We show that flies express robust locomotor sensitization to the second dose of volatilized methamphetamine, which significantly lowers preferential self-administration of methamphetamine. Naive flies preferentially self-administer food with methamphetamine over plain food. Exposing flies to volatilized methamphetamine after voluntary self-administration abolishes locomotor sensitization. We tested period null (per⁰¹ ) mutant flies and showed that they do not develop locomotor sensitization, nor do they show preferential self-administration of methamphetamine. Our results suggest that there may be partially overlapping neural circuitry that regulates the expression of locomotor sensitization and preferential self-administration to methamphetamine and that this circuitry requires a functional per gene.

Epigenetic mechanisms associated with addiction-related behavioural effects of nicotine and/or cocaine: implication of the endocannabinoid system

The addictive use of nicotine (NC) and cocaine (COC) continues to be a major public health problem, and their combined use has been reported, particularly during adolescence. In neural plasticity, commonly induced by NC and COC, as well as behavioural plasticity related to the use of these two drugs, the involvement of epigenetic mechanisms, in which the reversible regulation of gene expression occurs independently of the DNA sequence, has recently been reported. Furthermore, on the basis of intense interactions with the target neurotransmitter systems, the endocannabinoid (ECB) system has been considered pivotal for eliciting the effects of NC or COC. The combined use of marijuana with NC and/or COC has also been reported. This article presents the addiction-related behavioural effects of NC and/or COC, based on the common behavioural/neural plasticity and combined use of NC/COC, and reviews the interacting role of the ECB system. The epigenetic processes inseparable from the effects of NC and/or COC (i.e. DNA methylation, histone modifications and alterations in microRNAs) and the putative therapeutic involvement of the ECB system at the epigenetic level are also discussed.

Appropriate experimental approaches for predicting abuse potential and addictive qualities in preclinical drug discovery

INTRODUCTION

Drug abuse is an increasing social and public health issue, putting the onus on drug developers and regulatory agencies to ensure that the abuse potential of novel drugs is adequately assessed prior to product launch.

AREAS COVERED

This review summarizes the core preclinical data that frequently contribute to building an understanding of abuse potential for a new molecular entity, in addition to highlighting models that can provide increased resolution regarding the level of risk. Second, an important distinction between abuse potential and addiction potential is drawn, with comments on how preclinical models can inform on each.

EXPERT OPINION

While the currently adopted preclinical models possess strong predictive validity, there are areas for future refinement and research. These areas include a more refined use of self-administration models to assess relative reinforcement; and the need for open innovation in pursuing improvements. There is also the need for careful scientifically driven application of models rather than a standardization of methodologies, and the need to explore the opportunities that may exist for enhancing the value of physical dependence and withdrawal studies by focusing on withdrawal-induced drug seeking, rather than broad symptomology.

Effect of food restriction on cocaine locomotor sensitization in Sprague-Dawley rats: role of kappa opioid receptors

RATIONALE

The interaction between repeated cocaine exposure and food restriction on sensitization to the stimulatory effects of cocaine has not been characterized.

OBJECTIVES

To compare cocaine sensitization in rats free fed and food restricted, and begin to explore the role of the stress-responsive dynorphin/kappa opioid system.

METHODS

Male rats were maintained for 10 days on two feeding conditions: free fed or food restricted (85 % of free fed weight). Test 1 of locomotor reactivity to cocaine (3, 9, or 15 mg/kg, IP) was followed by a sensitizing regimen of cocaine exposure (0 or 30 mg/kg/day × 5 days, IP), by a 10-day drug-free period, and by Test 2 of reactivity to the same cocaine dose. In a second experiment, rats received an injection of norbinaltorphimine (nor-BNI; 0, 5 or 20 mg/kg, SC) 10 days prior to each locomotion test, and plasma corticosterone (CORT) was assessed after Test 2.

RESULTS

On Test 1, it was found that food restriction enhanced locomotor responses to all doses of cocaine. On Test 2, it was found that free fed and food restricted animals displayed similar sensitized responses to cocaine. This, however, was not observed in nor-BNI-treated rats. Furthermore, 20 mg/kg nor-BNI reduced both the locomotor response to cocaine on Test 2 and the effect of cocaine and food restriction on CORT plasma levels.

CONCLUSIONS

These results indicate that the interaction between cocaine sensitization and food restriction is not synergistic, and that it involves activation of kappa-opioid receptors.

Chronic psychostimulant exposure to adult, but not periadolescent rats reduces subsequent morphine antinociception

Preweanling methylphenidate (MPH) exposure produces a long lasting enhanced sensitivity to opioids. Two important questions are whether this enhancement is specific to the age of psychostimulant exposure and the type of psychostimulant. To answer these questions periadolescent (PD 35) and adult (PD 55) rats received daily injections of saline, MPH, or methamphetamine (METH) for 10 consecutive days. Two weeks later, acute morphine antinociception was assessed on the hot plate using a cumulative dose response procedure. Following acute antinociceptive testing, morphine tolerance was induced in half the animals by administering morphine twice a day over 2 days. Rats pretreated with MPH and METH during the periadolescent period of ontogeny showed no change in acute morphine antinociception, but rats exposed to a relatively high METH dose (3 mg/kg) displayed enhanced morphine tolerance compared to saline pretreated controls. MPH and METH pretreatment during adulthood led to a reduction in morphine antinociceptive potency and an apparent reduction in morphine tolerance. When combined with our previously published findings, these data indicate that the developmental stage during which MPH and METH exposure occurs differentially alters adult morphine responsiveness. That is, psychostimulant exposure to preweanling rats enhances morphine antinociception and facilitates the development of tolerance, whereas psychostimulant exposure to adult rats reduces subsequent morphine antinociception and tolerance. These alterations indicate that it could be important for physicians to know about prior psychostimulant use when prescribing opioids for pain relief.

Assessing locomotor-stimulating effects of cocaine in rodents

Locomotor activity procedures are useful for characterizing the behavioral effects of a drug, the influence of pharmacological, neurobiological, and environmental manipulations on drug sensitivity, and changes in activity following repeated administration (e.g., tolerance or sensitization) are thought to be related to the development of an addiction-like behavioral phenotype. The effects of cocaine on locomotor activity have been relatively extensively characterized. Many of the published studies use between-subject experimental designs, even though changes in sensitivity within a particular individual due to experimental manipulations, or behavioral and pharmacological histories is potentially the most important outcome as these changes may relate to differential development of an addiction-like phenotype in some, but not all, animals (including humans). The two behavioral protocols described herein allow extensive within-subject analyses. The first protocol uses daily locomotor activity levels as a stable baseline to assess the effects of experimental manipulations, and the second uses a pre- versus post-session experimental design to demonstrate the importance of drug-environment interactions in determining the behavioral effects of cocaine.

Cocaine addiction and personality: a mathematical model

The existence of a close relation between personality and drug consumption is recognized, but the corresponding causal connection is not well known. Neither is it well known whether personality exercises an influence predominantly at the beginning and development of addiction, nor whether drug consumption produces changes in personality. This paper presents a dynamic mathematical model of personality and addiction based on the unique personality trait theory (UPTT) and the general modelling methodology. This model attempts to integrate personality, the acute effect of drugs, and addiction. The UPTT states the existence of a unique trait of personality called extraversion, understood as a dimension that ranges from impulsive behaviour and sensation-seeking (extravert pole) to fearful and anxious behaviour (introvert pole). As a consequence of drug consumption, the model provides the main patterns of extraversion dynamics through a system of five coupled differential equations. It combines genetic extraversion, as a steady state, and dynamic extraversion in a unique variable measured on the hedonic scale. The dynamics of this variable describes the effects of stimulant drugs on a short-term time scale (typical of the acute effect); while its mean time value describes the effects of stimulant drugs on a long-term time scale (typical of the addiction effect). This understanding may help to develop programmes of prevention and intervention in drug misuse.

Effects of ethanol and 3,4-methylenedioxymethamphetamine (MDMA) alone or in combination on spontaneous and evoked overflow of dopamine, serotonin and acetylcholine in striatal slices of the rat brain

Ethanol (EtOH) potentiates the locomotor effects of 3,4-methylenedioxymetamphetamine (MDMA) in rats. This potentiation might involve pharmacokinetic and/or pharmacodynamic mechanisms. We explored whether the latter could be local. Using a slice superfusion approach, we assessed the effects of MDMA (0.3, 3microm) and/or EtOH (2mm) on the spontaneous outflow and electrically evoked release of serotonin (5-HT), dopamine (DA) and acetylcholine (ACh) in the striatum, and for comparison, on 5-HT release in hippocampal and neocortical tissue. MDMA and less effectively EtOH, augmented the outflow of 5-HT in all regions. The electrically evoked 5-HT release was increased by MDMA at 3microm in striatal slices only. With nomifensine throughout, EtOH significantly potentiated the 0.3microm MDMA-induced outflow of 5-HT, but only in striatal slices. EtOH or MDMA also enhanced the spontaneous outflow of DA, but MDMA reduced the electrically evoked DA release. With fluvoxamine throughout superfusion, EtOH potentiated the effect of MDMA on the spontaneous outflow of DA. Finally, 3microm MDMA diminished the electrically evoked release of ACh, an effect involving several receptors (D2, 5-HT2, NMDA, nicotinic, NK1), with some interactions with EtOH. Among other results, we show for the first time a local synergistic interaction of EtOH and MDMA on the spontaneous outflow of striatal DA and 5-HT, which could be relevant to the EtOH-induced potentiation of hyperlocomotion in MDMA-treated rats. These data do not preclude the contribution of other pharmacodynamic and/or pharmacokinetic mechanisms in vivo but support the hypothesis that EtOH may affect the abuse liability of MDMA.

Less is more: antipsychotic drug effects are greater with transient rather than continuous delivery

BACKGROUND

Most studies on the effects of antipsychotics focus on achieving threshold levels of the drug. The speed and frequency with which drug concentrations reach threshold levels and rise and fall within the day are generally ignored. Based on prior data, we predicted that variations in the within-day kinetics of antipsychotic drug delivery would produce different outcomes, even if we held achieved dose, route, and total duration of treatment constant.

METHODS

We compared the effects of within-day continuous (via minipump) versus transient (via subcutaneous injection) haloperidol treatment (n = 4-9/condition/experiment) at doses that yield equivalent peak levels of striatal D2 receptor occupancy (approximately 74%).

RESULTS

Over time, transient haloperidol gained efficacy, while continuous haloperidol lost efficacy in two animal models of antipsychotic-like effects (the suppression of amphetamine-induced locomotion and conditioned avoidance responding). This was related to the fact that continuous treatment led to a greater increase in striatal D2 receptor numbers--particularly D2 receptors in a high-affinity state for dopamine--relative to transient treatment and produced behavioral dopamine supersensitivity (as indicated by an enhanced locomotor response to amphetamine following antipsychotic treatment cessation). Treatment kinetics also influenced the postsynaptic response to haloperidol. Transient treatment increased striatal c-fos messenger RNA (mRNA) expression, while continuous treatment did not.

CONCLUSIONS

Relative to continuous antipsychotic exposure, within-day transient exposure is more efficacious behaviorally and is associated with a distinct molecular and gene expression profile. Thus, differences in the within-day kinetics of antipsychotic treatment can have different efficacy, and the potential clinical implications of this should be explored further.

Modulation of the endocannabinoid system: therapeutic potential against cocaine dependence

Dependence on cocaine is still a main unresolved medical and social concern, and in spite of research efforts, no pharmacological therapy against cocaine dependence is yet available. Recent studies have shown that the endocannabinoid system participates in specific stages and aspects of drug dependence in general, and some of this evidence suggests an involvement of the cannabinoid system in cocaine effects. For example, cocaine administration has been shown to alter brain endocannabinoid levels, and the endocannabinoid system has been involved in long-term modifications of brain processes that might play a role in neuro/behavioral effects of psychostimulant drugs like cocaine. Human studies show that marijuana dependence is frequently associated with cocaine dependence, and that the cannabinoid receptor CNR1 gene polymorphism might be related to cocaine addiction. This article will review the main papers in the field showing how a modulation of different components of the cannabinoid system might interact with some of the neurobiological/behavioral effects of cocaine related to its reinforcing effects, evaluated in preclinical models or in clinical settings. The goal of this review will be to provide insights into the complex picture of cocaine abuse and addiction, and to extrapolate from such endocannabinoid-cocaine interactions useful information to test the therapeutic potential of cannabinoid ligands and endocannabinoid-level enhancers against cocaine dependence for future preclinical/clinical trials.

Augmented behavioral response and enhanced synaptosomal calcium transport induced by repeated cocaine administration are decreased by calcium channel blockers

Recent studies suggest that calcium influx via L-type calcium channels is necessary for psychostimulant-induced behavioral sensitization. In addition, chronic amphetamine upregulates subtype Cav1.2-containing L-type calcium channels. In the present studies, we assessed the effect of calcium channel blockers (CCBs) on cocaine-induced behavioral sensitization and determined whether the functional activity of L-type calcium channels is altered after repeated cocaine administration. Rats were administered daily intraperitoneal injections of either flunarizine (40 mg/kg), diltiazem (40 mg/kg) or cocaine (20 mg/kg) and the combination of the CCBs and cocaine for 30 days. Motor activities were monitored on Day 1, and every 6th day during the 30-day treatment period. Daily cocaine administration produced increased locomotor activity. Maximal augmentation of behavioral response to repeated cocaine administration was observed on Day 18. Flunarizine pretreatment abolished the augmented behavioral response to repeated cocaine administration while diltiazem was less effective. Measurement of tissue monoamine levels on Day 18 revealed cocaine-induced increases in DA and 5-HT in the nucleus accumbens. By contrast to behavioral response, diltiazem was more effective in attenuating increases in monoamine levels than flunarizine. Cocaine administration for 18 days produced increases in calcium uptake in synaptosomes prepared from the nucleus accumbens and frontal cortex. Increases in calcium uptake were abolished by flunarizine and diltiazem pretreatment. Taken together, the augmented cocaine-induced behavioral response on Day 18 may be due to increased calcium uptake in the nucleus accumbens leading to increased dopamine (DA) and serotonin (5-HT) release. Flunarizine and diltiazem attenuated the behavioral response by decreasing calcium uptake and decreasing neurochemical release.

The effect of ‘binge’ cocaine administration on the expression of cyclin-dependent kinase 5 and its activator p35 in various regions of rat brain

The present study was aimed at determining whether the administration of cocaine in 'binge' pattern regimen that evoked tolerance to the locomotor stimulant effects of cocaine also influenced the expression of cyclin-dependent kinase 5 (Cdk5) and its activator p35 in the amygdala, medial prefrontal cortex, nucleus accumbens septi and caudate-putamen. Western blot techniques revealed that acute and repeated 'binge' cocaine decreased expression of the Cdk5 protein in the amygdala. In the medial prefrontal cortex, only exposure to repeated 'binge' cocaine decreased the content of the Cdk5 protein. 'Binge' cocaine administration also altered the expression of Cdk5 activator p35 protein. In the amygdala, only repeated 'binge' cocaine decreased the expression of p35, while in the medial prefrontal cortex, a decrease was observed after acute and repeated 'binge' cocaine exposure. In neither the nucleus accumbens septi nor the caudate-putamen acute or repeated 'binge' cocaine modified the expression of Cdk5 and p35. The above data indicate that in contrast to sensitizing doses of cocaine, a single and repeated binge of cocaine, which evoked tolerance to its locomotor stimulant effects, decreases expression of Cdk5 and p35 and possibly decreases the efficacy of neurotransmission or induces brain plastic changes regulated by Cdk5 and its activator p35.

Neuroadaptations of total levels of adenylate cyclase, protein kinase A, tyrosine hydroxylase, cdk5 and neurofilaments in the nucleus accumbens and ventral tegmental area do not correlate with expression of sensitized or tolerant locomotor responses to cocaine

Neuroadaptations induced by high-dose cocaine treatment have been hypothesized to persist after the cessation of drug treatment and mediate the expression of sensitization and tolerance to cocaine. We looked for evidence of these neuroadaptations in rats receiving more modest behaviorally effective cocaine treatments. Rats were exposed to either a sensitizing regimen of seven once-daily injections of 15 mg/kg cocaine or a tolerance-producing regimen involving a continuous infusion of the same daily dose. We assessed enzyme activity levels of protein kinase A and adenylate cyclase, and protein levels of tyrosine hydroxylase, cdk5 and neurofilaments in the nucleus accumbens and ventral tegmental area. Only protein kinase A activity levels were altered by cocaine treatment, but this alteration persisted for only 7 days, whereas a sensitized locomotor response was still evident at 21 days. Although behavioral tolerance to cocaine was seen the day after the termination of treatment, none of the molecular measures was altered on this or any other day. Thus, although increased protein kinase A activity can temporarily modulate sensitized responses to cocaine, alterations in total levels of the molecules assessed in our study do not correlate with the expression of sensitized or tolerant locomotor responses to cocaine.

Ontogenesis of behavioral sensitization and conditioned place preference induced by psychostimulants in laboratory rodents

The present review deals mainly with the ontogenesis of two important phenomena involved in vulnerability to several neuropsychiatric disorders, namely with drug-induced sensitization (both contextual and non-contextual) and with conditioned place preference. The term 'infancy' covers the first three postnatal weeks during development in rats and mice. Conversely, the term 'adolescence' may cover the whole postnatal period ranging from weaning (PND 21) to adulthood (at least PND 60) or specifically the period around the onset of puberty (animals aged 33-44 days). Recent studies in rats demonstrated that the establishment of a context-dependent sensitization appears during the first (for repeated drug administration) or during the second (for a single drug administration) postnatal week. However, the memory of drug-context association is transient in developing pups (lasting one or two days following the drug pretreatment). The long-term retention of drug-context associations matures progressively, and is complete by the third week of postnatal life. Finally, those mechanisms responsible for an adult-like profile of context-independent pharmacological sensitization appear later during ontogenesis, being mature by the fourth week of postnatal life. Another set of experiments extended this ontogenetic characterization by comparing adolescent and adult mice. When compared to the latter, the former subjects exhibit a greater amphetamine-induced locomotor sensitization, almost no sensitization of aversive stereotyped behaviors, and a less marked place conditioning. The strength of the drug-induced place conditioning was also directly compared with the unconditioned novelty-seeking drive. In conclusion, neonatal rats are able to show a relatively short-lasting retention of sensitized drug effects (short-term sensitization), whereas the ability to exhibit relatively long-lasting sensitized effects matures progressively during infancy (long-term sensitization). On the other hand, adolescent mice show a reduced sensitization of drug-induced psychotic symptoms, together with a more marked sensitization of arousing and euphorigenic properties of the drug and a reduced incentive memory of its hedonic effects. These age-related changes do imply very different degrees of vulnerability to drug addiction and several other neuropsychiatric disorders.

Sensitivity of binding of high-affinity dopamine receptor radioligands to increased synaptic dopamine

PET and SPECT studies have documented that D2 radioligands of moderate affinity, but not radioligands of high affinity, are sensitive to pharmacological challenges that alter synaptic dopamine levels. The objective of this work was to determine whether the brain kinetics of high-affinity radioligands for dopamine D1 ([(3)H]SCH 23390) and D2 ([(123)I]epidepride) receptors were altered by a prolonged elevation of synaptic dopamine induced by the potent cocaine analog RTI-55. Mice were injected intravenously with radioligands either 30 min after or 4 h before intraperitoneal administration of RTI-55 (2 mg/kg). In separate experiments, the pharmacological effects of RTI-55 were assessed biochemically by measuring uptake of dopamine in synaptosomes prepared from RTI-treated mice and behaviorally by monitoring locomotor activity. Consistent with the expected elevation of synaptic dopamine, RTI-55 induced a long-lasting decrement in dopamine uptake measured ex vivo, and a prolonged increase in locomotor activity. RTI-55 injected prior to the radioligands induced a significant (P < 0.05) increase in striatal concentration of [(123)I]epidepride at 15 min, relative to saline-treated controls, but there were no differences between the two groups at later time-points. For [(3)H]SCH 23390, both initial striatal uptake and subsequent clearance were slightly increased by preadministration of RTI-55. Administration of RTI-55 4 h after the radioligands (i.e., when it was presumed that a state of near equilibrium binding of the radioligands had been reached), was associated with a significant reduction of striatal radioactivity for both radiotracers. Our results are consistent with increased competition between dopamine and radioligand for binding to both D1 and D2 receptors after treatment with RTI-55. We suggest that the magnitude of the competition is reduced by failure of the receptor binding of high-affinity radioligands to rapidly attain equilibrium.

Does locomotor response to novelty in rats predict susceptibility to develop sensitization to cocaine and PHNO?

It has been suggested that the locomotor response of rats to novelty is positively correlated with motor stimulant effects of acute injections with psychomotor stimulants, and liability to self-administer these drugs. In addition, response to novelty appears to be inversely correlated with an individual's susceptibility to develop behavioural sensitization (an increase in the behavioural response to a given dose of stimulant after repeated treatments). To test some of these putative relationships, 96 rats were allocated to one of two subgroups based on a median split of locomotor responses to novelty. Animals then received 10 successive injections of either vehicle, cocaine (10 mg/kg), or the direct D2 agonist, (+)-4-propyl-9-hydroxynaphthoxazine (PHNO: 15 microg/kg), and locomotor activity was monitored. Conditioning tests and additional sensitization and cross-sensitization tests were conducted. Results showed that locomotor responses to novelty are not significantly correlated with locomotor effects of either acute injection with cocaine or PHNO, or rate of development of behavioural sensitization to these drugs. However, locomotor responses to novelty did predict level of locomotor and stereotypy responses to cocaine, and to a lessor extent to PHNO. Cocaine-treated, but not PHNO-treated, rats exhibited drug-conditioned-like effects. Cross-sensitization between cocaine and PHNO was not observed, indicating independent mechanisms for sensitization. It is concluded that the locomotor response to novelty can predict level of locomotion and stereotypy produced by cocaine and PHNO, but does not predict the degree or rate of behavioural sensitization to either of these drugs.

Effects of cocaine context on NAcc dopamine and behavioral activity after repeated intravenous cocaine administration

In two conditioning experiments, identical procedures (previously shown to produce place preferences for a cocaine-paired environment) were used to assess dopaminergic and behavioral activity correlates of cocaine reward conditioning and sensitization. In these experiments, animals received repeated injections of intravenous cocaine (4.2 mg/kgx6) or saline (0.2 mlx6) on alternating days. One group in each of these experiments ('Cocaine Cues') occupied a consistent distinctive environment during cocaine treatments and testing sessions. For the other conditioned group ('Novel'), all procedures were the same, except that the last cocaine injection was administered while animals were occupying a novel environment. During day 1 and day 6 of the cocaine treatment, behavioral activity was assessed in experiment 1 and in vivo microdialysis procedures were conducted in experiment 2. Over the course of the conditioning sessions, cocaine-induced behavioral activity (locomotion and rearing) increased significantly in the Cocaine Cues group, but not in the Novel group. In addition, cocaine-induced increases in NAcc dopamine levels were significantly greater when cocaine-experienced animals were tested in a cocaine-paired environment compared to equally experienced and cocaine-naive animals tested in a novel environment. Context-dependent behavioral sensitization is a well-documented phenomenon. The observation of a corresponding enhancement of dopamine efflux in lieu of a lengthy withdrawal period is uncommon, but can be attributed to methodological differences across studies. The present study uniquely demonstrates concurrent context-dependent potentiation of behavioral and dopaminergic responses to cocaine occurring in conjunction with cocaine reward.

Dopamine receptor-mediated regulation of striatal cholinergic activity: positron emission tomography studies with norchloro[18F]fluoroepibatidine

Large numbers of in vitro studies and microdialysis studies suggest that dopaminergic regulation of striatal acetylcholine (ACh) output is via inhibitory dopamine D2 receptors and stimulatory dopamine D1 receptors. Questions remain as to the relative predominance of dopamine D2 versus D1 receptor modulation of striatal ACh output under physiological conditions. Using positron emission tomography, we first demonstrate that norchloro[18F]fluoroepibatidine ([18F]NFEP), a selective nicotinic ACh receptor (nAChR) ligand, was sensitive to changes of striatal ACh concentration. We then examined the effect of quinpirole (D2 agonist), raclopride (D2 antagonist), SKF38393 (D1 agonist), and SCH23390 (D1 antagonist) on striatal binding of [18F]NFEP in the baboon. Pretreatment with quinpirole increased the striatum (ST) to cerebellum (CB) ratio by 26+/-6%, whereas pretreatment with raclopride decreased the ST/CB ratio by 22+/-2%. The ratio of the distribution volume of [18F]NFEP in striatum to that in cerebellum, which corresponds to (Bmax/K(D)) + 1 (index for nAChR availability), also showed a significant increase (29 and 20%; n = 2) and decrease (20+/-3%; n = 3) after pretreatment with quinpirole and raclopride, respectively. However, both the D1 agonist and antagonist had no significant effect. This suggests that under physiological conditions the predominant influence of endogenous dopamine on striatal ACh output is dopamine D2, not D1, receptor-mediated.