Discriminative stimulus properties of cocaine: modulation by dopamine D1 receptors in the nucleus accumbens

Effects of the Synthetic Cathinone α-Pyrrolidinobutiothiophenone (α-PBT) on Discriminative Stimulus Effects and Intracranial Self-Stimulation Thresholds in Male Rats

Recently, the abuse of synthetic cathinones is increasing among young people. α-Pyrrolidinobutiothiophenone (α-PBT), a synthetic cathinone, is a designer drug that is freely traded online with no legal restrictions. Moreover, there is currently no scientific basis for legal regulation. Here, we examined the addictive properties of α-PBT using a drug discrimination (DD) task. We also investigated the role of α-PBT in brain stimulation reward (BSR) using an intracranial self-stimulation (ICSS) paradigm in rats. Initially, the rats were trained to discriminate between cocaine and saline. After the discrimination training criteria were met, we determined the dose-effect curves of cocaine and conducted generalization tests with α-PBT and α-pyrrolidinopentiothiophenone (α-PVT) using a cumulative dosing protocol. In a separate set of studies, we examined the dopaminergic mechanisms underlying the function of α-PBT as an interoceptive stimulus (17.8 mg/kg) by intraperitoneally injecting either the dopamine (DA) D1 antagonist SCH23390 (0.06 and 0.12 mg/kg) or the D2 antagonist eticlopride (0.05 and 0.1 mg/kg) 15 min before DD testing. Brain reward function was measured using an ICSS procedure to examine the effects of α-PBT on ICSS threshold under the frequency-rate procedure. Our results showed that α-PBT functioned as a discriminative cue similar to cocaine in rats. More importantly, SCH23390 abolished the effects of α-PBT as an interoceptive stimulus in a dose-dependent manner in rats trained to press a lever to receive cocaine. Similarly, eticlopride dose-dependently attenuated the effect of α-PBT used as a discriminative cue. Additionally, cumulative α-PBT administration dose-dependently lowered ICSS thresholds compared with those in saline-treated rats. Furthermore, α-PBT-induced potentiation of BSR was abolished by pretreatment with both SCH23390 and eticlopride. Taken together, our results suggest that α-PBT can function as a cocaine-like discriminative cue via the activation of D1 and D2 receptors. α-PBT also appears to influence BSR by reducing the brain reward threshold via changes in D1 and D2 receptors. The present study suggests that α-PBT could have addictive properties through DA D1 and D2 receptors and thus poses a threat to humans.

Effects of muscarinic receptor antagonists on cocaine discrimination in wild-type mice and in muscarinic receptor M1, M2, and M4 receptor knockout mice

Muscarinic M₁/M₄ receptor stimulation can reduce abuse-related effects of cocaine and may represent avenues for treating cocaine addiction. Muscarinic antagonists can mimic and enhance effects of cocaine, including discriminative stimulus (S^D) effects, but the receptor subtypes mediating those effects are not known. A better understanding of the complex cocaine/muscarinic interactions is needed to evaluate and develop potential muscarinic-based medications. Here, knockout mice lacking M₁, M₂, or M₄ receptors (M₁^-/-, M₂^-/-, M₄^-/-), as well as control wild-type mice and outbred Swiss-Webster mice, were trained to discriminate 10mg/kg cocaine from saline. Muscarinic receptor antagonists with no subtype selectivity (scopolamine), or preferential affinity at the M₁, M₂, or M₄ subtype (telenzepine, trihexyphenidyl; methoctramine, AQ-RA 741; tropicamide) were tested alone and in combination with cocaine. In intact animals, antagonists with high affinity at M₁/M₄ receptors partially substituted for cocaine and increased the S^D effect of cocaine, while M₂-preferring antagonists did not substitute, and reduced the S^D effect of cocaine. The cocaine-like effects of scopolamine were absent in M₁^-/- mice. The cocaine S^D attenuating effects of methoctramine were absent in M₂^-/- mice and almost absent in M₁^-/- mice. The findings indicate that the cocaine-like S^D effects of muscarinic antagonists are primarily mediated through M₁ receptors, with a minor contribution of M₄ receptors. The data also support our previous findings that stimulation of M₁ receptors and M₄ receptors can each attenuate the S^D effect of cocaine, and show that this can also be achieved by blocking M₂ autoreceptors, likely via increased acetylcholine release.

Effects of dopamine D1-like and D2-like antagonists on cocaine discrimination in muscarinic receptor knockout mice

Muscarinic and dopamine brain systems interact intimately, and muscarinic receptor ligands, like dopamine ligands, can modulate the reinforcing and discriminative stimulus (S(D)) effects of cocaine. To enlighten the dopamine/muscarinic interactions as they pertain to the S(D) effects of cocaine, we evaluated whether muscarinic M1, M2 or M4 receptors are necessary for dopamine D1 and/or D2 antagonist mediated modulation of the S(D) effects of cocaine. Knockout mice lacking M1, M2, or M4 receptors, as well as control wild-type mice and outbred Swiss-Webster mice, were trained to discriminate 10mg/kg cocaine from saline in a food-reinforced drug discrimination procedure. Effects of pretreatments with the dopamine D1 antagonist SCH 23390 and the dopamine D2 antagonist eticlopride were evaluated. In intact mice, both SCH 23390 and eticlopride attenuated the cocaine discriminative stimulus effect, as expected. SCH 23390 similarly attenuated the cocaine discriminative stimulus effect in M1 knockout mice, but not in mice lacking M2 or M4 receptors. The effects of eticlopride were comparable in each knockout strain. These findings demonstrate differences in the way that D1 and D2 antagonists modulate the S(D) effects of cocaine, D1 modulation being at least partially dependent upon activity at the inhibitory M2/M4 muscarinic subtypes, while D2 modulation appeared independent of these systems.

Elevated Expression of Serotonin 5-HT(2A) Receptors in the Rat Ventral Tegmental Area Enhances Vulnerability to the Behavioral Effects of Cocaine

The dopamine mesocorticoaccumbens pathway which originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens and prefrontal cortex is a circuit important in mediating the actions of psychostimulants. The function of this circuit is modulated by the actions of serotonin (5-HT) at 5-HT(2A) receptors (5-HT(2A)R) localized to the VTA. In the present study, we tested the hypothesis that virally mediated overexpression of 5-HT(2A)R in the VTA would increase cocaine-evoked locomotor activity in the absence of alterations in basal locomotor activity. A plasmid containing the gene for the 5-HT(2A)R linked to a synthetic marker peptide (Flag) was created and the construct was packaged in an adeno-associated virus vector (rAAV-5-HT(2A)R-Flag). This viral vector (2 μl; 10(9-10) transducing units/ml) was unilaterally infused into the VTA of male rats, while control animals received an intra-VTA infusion of Ringer's solution. Virus-pretreated rats exhibited normal spontaneous locomotor activity measured in a modified open-field apparatus at 7, 14, and 21 days following infusion. After an injection of cocaine (15 mg/kg, ip), both horizontal hyperactivity and rearing were significantly enhanced in virus-treated rats (p < 0.05). Immunohistochemical analysis confirmed expression of Flag and overexpression of the 5-HT(2A)R protein. These data indicate that the vulnerability of adult male rats to hyperactivity induced by cocaine is enhanced following increased levels of expression of the 5-HT(2A)R in the VTA and suggest that the 5-HT(2A)R receptor in the VTA plays a role in regulation of responsiveness to cocaine.

The discriminative stimulus properties of methylphenidate in C57BL/6J mice

Methylphenidate (MPH) therapy for attention-deficit/hyperactivity disorder is common in children and adults. Concerns regarding abuse of MPH prompted studies to better understand its pharmacology. We used an established drug discrimination task to determine whether MPH could be discriminated by C57BL/6J (B6) mice. B6 mice learned to discriminate cues produced by racemic MPH (dl-MPH 5.0 mg/kg) or half the dose of pure d-isomer (2.5 mg/kg), and dose-response tests established appropriate reductions in discrimination with declining dose. Importantly, the two drug forms generalized to each other completely in substitution tests; consistent with reports that the l-isomer is pharmacodynamically inactive. An additional experiment indicated that lower doses (1 and 2 mg/kg) of dl-MPH did not support acquisition of MPH discrimination despite extensive training. Mice acquired discrimination of dl-MPH only when the dose was increased to 4 mg/kg. Thus, although these lower doses increased drug lever responding in mice trained on the higher dose, their stimuli were not sufficient to support acquisition of the discrimination task. These findings correspond to earlier studies conducted in our laboratory on threshold doses needed to produce stimulatory effects of motor activity in B6 mice. These preclinical findings provide insight into the relative potency, and by extension, efficacy of dl-MPH versus d-MPH doses.

Reduced sensitivity to the locomotor-stimulant effects of cocaine is associated with increased sensitivity to its discriminative stimulus properties

Outbred Long-Evans rats exhibit wide variation in their locomotor response to cocaine. Here, we investigated the relationship between these individual differences and interoceptive effects of cocaine in low cocaine responder (LCR) and high cocaine responder (HCR) phenotypes. Rats were trained to discriminate cocaine (10.0 mg/kg, intraperitoneally) from saline by repeated pairings of injections with one of two response levers. In subsequent tests for stimulus generalization to other cocaine doses (1.25-15.0 mg/kg), LCRs exhibited partial-to-full generalization at 1.85 and 2.5 mg/kg cocaine, respectively, whereas HCRs did not. When the selective 5-HT reuptake inhibitor fluoxetine (5.0 mg/kg) was coadministered with saline or different cocaine doses, we observed similar upward shifts in dose-response in both phenotypes. In contrast, coadministration of the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 0.3 mg/kg) led to partial substitution of DOI for cocaine and enhancement of the stimulus properties of 1.25 mg/kg cocaine in LCRs only. Finally, a retest of cocaine-induced locomotion after discrimination testing revealed marked behavioral sensitization in LCRs and modest changes in behavior in HCRs. Taken together, these results suggest that initial sensitivity to the locomotor-stimulant effects of cocaine is inversely related to its interoceptive properties and that differences in 5-HT systems may contribute to the phenotypic differences observed.

Augmented cocaine-induced accumbal dopamine efflux, motor activity and place preference in rats fed with a tryptophan-deficient diet

In the present study we demonstrate that consumption of a tryptophan-deficient diet for a period of 14 days decreased the striatal serotonin and 5-hydroxyindolacetic acid tissue content in rats, whereas the level of dopamine remained unchanged. Under this condition of diminished serotonergic tone, a challenge dose of cocaine (10mg/kg, i.p.) significantly increased motor activity and dopamine extracellular content in the nucleus accumbens compared to rats fed with a balanced diet. We moreover found that pretreatment with cocaine (7 and 10mg/kg, i.p.) produced a significant increase in preference for a cocaine-associated environment in the tryptophan-deficient group compared to control rats. Our experiments show that a low tone of serotonergic system, augments the behavioural reinforcing effect of cocaine and that this effect may be due to a increased cocaine-induced accumbal dopamine release. These data indicate that a tryptophan-deficient diet alters the behavioural and neurochemical effect of psychostimulants, such as cocaine, and suggest an important role of serotonin in modulation of these effects.

Contribution of serotonin (5-HT) 5-HT2 receptor subtypes to the discriminative stimulus effects of cocaine in rats

RATIONALE

The serotonin (5-hydroxytryptamine; 5-HT) 5-HT2 receptor (5-HT2R) family is an important regulator of the behavioral responsiveness to cocaine.

OBJECTIVE

The present study is an analysis of the role of the 5-HT2R subtypes (5-HT2AR, 5-HT2BR, and 5-HT2CR) in the discriminative stimulus effects of cocaine.

METHODS

Male Wistar rats were trained to discriminate cocaine (10 mg/kg) from saline in a two-lever, water-reinforced FR 20 task, and we investigated the ability of the 5-HT2AR antagonist 1(Z)-[2-(dimethylamino)ethoxyimino]-1(2-fluorophenyl)-3-(4-hydroxyphenyl)-2(E)-propene (SR 46349B), the 5-HT2BR antagonist N-(1-methyl-5-indolyl)-N'-(3-methyl-5-isothiazolyl) urea (SB 204741), and the 5-HT2CR antagonist [(+)-cis-4,5,7a,8,9,10,11,11a-octahydro-7H-10-methylindolo(1,7-bC)(2,6)naphthyridine (SDZ SER-082) to substitute for or to modulate the stimulus effects of cocaine.

RESULTS

Pretreatment with SR 46349B (0.5-1 mg/kg) resulted in a rightward shift of the cocaine dose-response curve, while SDZ SER-082 (1 mg/kg) shifted the dose-response for cocaine to the left; SB 204741 (1-3 mg/kg) was inactive.

CONCLUSIONS

Our pharmacological analyses of selective antagonists of 5-HT2AR, 5-HT2BR, and 5-HT2CR indicate oppositional influence of 5-HT2AR and 5-HT2CR on the stimulus effects of cocaine and exclude a role for the 5-HT2BR. These data suggest that 5-HT2AR and 5-HT2CR may be important in modulating the subjective effects of cocaine in humans.

m-Chlorophenylpiperazine (mCPP) Modulates the Discriminative Stimulus Effects of Cocaine Through Actions at the 5-HT₂C Receptor

Agonists acting at the serotonin-1B receptor (5-HT1BR) and 5-HT2CR have been reported to potentiate and block, respectively, the discriminative stimulus effects of cocaine. The present investigation reassessed the antagonistic effects of the mixed 5-H2C/1BR agonist m-chlorophenylpiperazine (mCPP) on the discriminative stimulus effects of cocaine in the presence or absence of selective antagonism of the 5-HT1BR or 5-HT2CR. The stimulus effects of cocaine were attenuated by mCPP at doses that reduced response rates. The selective 5-HT2CR antagonist SB 242084, but not the selective 5-HT1BR antagonist GR 127935, reversed the mCPP-evoked attenuation of the cocaine cue and the suppression of response rates. These results demonstrate that the suppressive effects of mCPP on cocaine discrimination are related to stimulation of the HT2CR.

Acute and subacute effects of risperidone and cocaine on accumbens dopamine and serotonin release using in vivo microvoltammetry on line with open-field behavior

In vivo microvoltammetry was used to detect dopamine (DA) and serotonin (5-HT) release from nucleus accumbens (NAcc) of freely moving, male, Sprague-Dawley laboratory rats, while animals' locomotor (forward ambulations) and stereotypic behavior (fine movements of sniffing and grooming) were monitored at the same time with infrared photobeams. Monoamine release mechanisms were determined by using a depolarization blocker (gamma-butyrolactone, gamma BL). Miniature carbon sensors (BRODERICK PROBES microelectrodes) smaller than a human hair were used in conjunction with a semidifferential electrochemical circuit to detect release of each monoamine in separate signals and within seconds. The purpose was to evaluate the neuropharmacology of the 5-HT(2)/DA(2) antagonist risperidone in its current therapeutic role as an atypical antipsychotic medication as well as in its potential role as pharmacotherapy for cocaine psychosis and withdrawal symptoms. Acute (single drug dose) and subacute (24-h follow-up studies in the same animal, no drug administration) studies were performed for each treatment group. The hypothesis for the present studies is derived from a growing body of evidence that cocaine-induced psychosis and schizophrenic psychosis share similar neurochemical and behavioral manifestations. Results showed that (1) Acute administration of risperidone (2 mg/kg sc) significantly increased DA and 5-HT release in NAcc above baseline (habituation) values (P<.001) while locomotion and stereotypy were virtually unaffected. In subacute studies, DA release did not differ from baseline (P>.05), whereas 5-HT release was significantly increased above baseline (P<.001). Locomotion increased over baseline but not to a significant degree, while stereotypy was significantly increased above baseline (P<.05). (2) Acute administration of cocaine (10 mg/kg ip) significantly increased both DA and 5-HT release above baseline (P<.001), while locomotion and stereotypy were significantly increased over baseline (P<.001). In subacute studies, DA decreased significantly below baseline (P<.001) and significant decreases in 5-HT release occurred at 15, 20, 50 and 55 min (P<.05). Behavior increased above baseline but did not reach a statistically significant degree. (3) Acute administration of risperidone/cocaine (2 mg/kg sc and 10 mg/kg ip, respectively) showed a significant block of the cocaine-induced increase in DA release in the first hour (P<.001) and 5-HT release in both hours of study (P<.001). Cocaine-induced locomotion and stereotypy were blocked simultaneously with the monoamines (P<.001). In subacute studies, DA and 5-HT release returned to baseline while locomotion and stereotypy increased insignificantly above baseline. Thus, (a) these studies were able to tease out pharmacologically the critical differences between presynaptic and postsynaptic responses to drug treatment(s) and these differences may lead to more effective therapies for schizophrenic and/or cocaine psychosis. (b) Taken together with other data, these acute studies suggest that risperidone may possibly act via inhibition of presynaptic autoreceptors to produce the observed increases in accumbens DA and 5-HT release, whereas cocaine may be acting at least in part via serotoninergic modulation of DA postsynaptically. The subacute data suggest that pharmacokinetics may play a role in risperidone's action and that neuroadaptation may play a role in the mechanism of action of cocaine. Finally, the ability of risperidone to block cocaine-induced psychostimulant neurochemistry and behavior during acute studies while diminishing the withdrawal symptoms of cocaine during subacute studies suggests that risperidone may be a viable pharmacotherapy for cocaine psychosis and withdrawal.

Hyperlocomotive and discriminative stimulus effects of cocaine are under the control of serotonin(2C) (5-HT(2C)) receptors in rat prefrontal cortex

The serotonin2C (5-hydroxytryptamine2C; 5-HT2C) receptor (5-HT2CR) is found in abundance in dopamine (DA) mesocorticolimbic pathways and is one of the important target proteins that modulates the behavioral effects of cocaine. In the present study, the hypothesis was tested that the 5-HT2CR in the prefrontal cortex (PFC) may control either spontaneous or cocaine-evoked locomotor activity as well as the discriminative stimulus properties of cocaine. In male Sprague-Dawley rats implanted with bilateral cannulae aimed at the PFC, local microinjections of the preferential 5-HT2CR agonist 6-chloro-2-(1-piperazinyl)pyrazine hydrochloride (MK 212) (0.05-0.5 microg/side) did not alter spontaneous activity, but dose-dependently decreased horizontal hyperactivity evoked by cocaine (10 mg/kg i.p.). Given alone, the selective 5-HT2CR antagonist 8-[5-(2,4-dimethoxy-5-(4-trifluorophenylsulfonamido)phenyl-5-oxopentyl]-1,3,8-triazo-spiro[4.5]decane-2,4-dione hydrochloride (RS 102221) (5 microg/side) increased basal locomotor activity of rats expressed in the vertical plane. Microinjections of RS 102221 (5 microg/side, but not 0.15-1.5 microg/side) significantly enhanced the horizontal activity induced by cocaine (10 mg/kg). In rats trained to discriminate cocaine (10 mg/kg i.p.) from saline (i.p.) in a two-lever, water-reinforced fixed ratio 20 task, intra-PFC microinjections of MK 212 (0.05 and 0.5 microg/side) did not substitute for cocaine, but attenuated the stimulus effects of cocaine. However, intra-PFC microinjections of RS 102221 (1.5 and 5 microg/side) evoked 13 and 40% cocaine-lever responding when tested alone and enhanced the recognition of cocaine. These data indicate that the PFC is a brain site at which the 5-HT2CR exerts an inhibitory control over the hyperactive and discriminative stimulus effects of cocaine known to be dependent upon activation of the DA mesoaccumbens circuit.

Cocaine-like subjective effects of nicotine are not blocked by the D1 selective antagonist ecopipam (SCH 39166)

The effects of ecopipam (a D(1) selective antagonist) or triazolam pretreatment on the subjective and physiological effects of intravenously administered nicotine were examined in 10 cigarette-smoking cocaine abusers. Under double-blind, randomized conditions, subjects received oral capsule pretreatment (0, 30 or 100 mg ecopipam, or 0.25 mg/70 kg triazolam), followed 120 min later by an intravenous injection of 2 mg/70 kg nicotine or saline. Subjective ratings, heart rate and blood pressure were assessed before and repeatedly after each intravenous injection. Compared to oral placebo pretreatment, both ecopipam and triazolam pretreatment produced significant elevations in subject-reported capsule effect and observer ratings of sleepiness/sedation. Nicotine increased ratings of 'drug effect', 'rush', 'high', 'stimulated', 'liking', 'good effects' and 'bad effects', and produced modest increases in heart rate and blood pressure. Following both intra-venous saline and nicotine injection, ecopipam tended to reduce heart rate and blood pressure. Although both ecopipam and triazolam lowered several subjective ratings following intravenous saline injection, neither ecopipam nor triazolam affected nicotine subjective effects. In contrast to Romach et al. (Arch Gen Psychiatry 56: 1101-1106, 1999), who showed that pretreatment with ecopipam blocked cocaine subjective effects, the current study found no attenuation of the subjective effects of nicotine, and thus, provides no support for the hypothesis that D(1) receptors mediate the cocaine-like subjective effects of nicotine.

Effects of 5-HT1B receptor ligands microinjected into the ventral tegmental area on cocaine discrimination in rats

Some recent data indicate a significant interaction between serotonin (5-hydroxytryptamine; 5-HT) and dopamine in mesolimbic brain structures (e.g. the ventral tegmental area) which modulate the behavioral effects of cocaine in rats. The present study investigated the role of 5-HT(1B) receptors in the ventral tegmental area in the discriminative stimulus effects of cocaine in rats. Male Wistar rats were trained to discriminate cocaine (10 mg/kg, intraperitoneally (i.p.)) from saline (i.p.) in a two-choice, water-reinforced fixed-ratio 20 procedure. After reaching the cocaine-saline discrimination criterion, the rats were stereotaxically implanted with bilateral cannulae in the ventral tegmental area and were then microinjected with selective 5-HT(1B) receptor ligands. In substitution studies, microinjections of the 5-HT(1B) receptor antagonist, 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR 55562; 0.1-1 microg/side), did not evoke cocaine-lever responding, whereas the 5-HT(1B) receptor agonist, 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one (CP 93129; 0.3-1 microg/side), induced partial substitution for cocaine. Intra-tegmental microinjections with the 5-HT(1B) receptor antagonist, GR 55562 (0.1-1 microg/side), before cocaine (5 mg/kg), which alone produced 98% cocaine-lever responses, decreased in a dose-dependent manner the discriminative stimulus effects of the psychostimulant. On the other hand, combination tests using a fixed dose of CP 93129 (0.3 or 1 microg/side), given into the ventral tegmental area prior to low systemic doses of cocaine (1.25-2.5 mg/kg), increased cocaine discrimination. These results seem to indicate that tegmental 5-HT(1B) receptors are necessary to express the discriminative stimulus effects of cocaine.

Sigma(1) (sigma(1)) receptor antagonists represent a new strategy against cocaine addiction and toxicity

Cocaine is a highly addictive substance abused worldwide. Its mechanism of action involves initially inhibition of neuronal monoamine transporters in precise brain structures and primarily the dopamine reuptake system located on mesolimbic neurons. Cocaine rapidly increases the dopaminergic neurotransmission and triggers adaptive changes in numerous neuronal circuits underlying reinforcement, reward, sensitization and the high addictive potential of cocaine. Current therapeutic strategies focus on counteracting the cocaine effects directly on the dopamine transporter, through post-synaptic D(1), D(2) or D(3) receptors or through the glutamatergic, serotoninergic, opioid or corticotropin-releasing hormone systems. However, cocaine administration also results in the activation of numerous particular targets. Among them, the sigma(1) (sigma(1)) receptor is involved in several acute or chronic effects of cocaine. The present review will first bring concise overviews of the present strategies followed to alleviate cocaine addiction and animal models developed to analyze the pharmacology of cocaine addiction. Evidence involving activation of the sigma(1) receptor in the different aspects of cocaine abuse, will then be detailed, following acute, repeated, or overdose administration. The therapeutic potentials and neuropharmacological perspectives opened by the use of selective sigma(1) receptor antagonists in cocaine addiction will finally be discussed.

Serotonin 5-HT(2C) receptors in nucleus accumbens regulate expression of the hyperlocomotive and discriminative stimulus effects of cocaine

The serotonin 5-HT(2C) receptor (5-HT(2C)R) is abundant in the nucleus accumbens (NAc) shell and is considered an important target for 5-HT to modulate the dopamine (DA) mesoaccumbens circuit, which plays a prominent role in the behavioral effects of cocaine. The present study analyzed the ability of intra-NAc shell infusions of the 5-HT(2C)R agonists, MK 212 and RO 60-0175, or the 5-HT(2C)R antagonist, RS 102221, to alter either spontaneous or cocaine-evoked activity as well as the discriminative stimulus properties of cocaine. In male Sprague--Dawley rats implanted with bilateral cannulae aimed at the NAc shell, locally injected MK 212 (0.05--0.5 microg/side) or RO 60-0175 (0.5--5 microg/side) did not alter spontaneous activity, but dose-dependently enhanced hyperactivity evoked by cocaine (10 mg/kg ip). In rats trained to discriminate cocaine (10 mg/kg ip) from saline (ip) in a two-lever, water-reinforced FR 20 task, intra-NAc microinfusion of MK 212 (0.05 microg/side) or RO 60-0175 (0.5 microg/side) evoked 37% or 48% cocaine lever responding, respectively. Both MK 212 (0.05 microg/side) and RO 60-0175 (0.5 microg/side) enhanced the discriminability of submaximal doses of cocaine (0.625--2.5 mg/kg). Moreover, intra-NAc infusion of RS 102221 (0.05--1.5 microg/side) dose-dependently attenuated the stimulus effects of cocaine. These data reinforce the hypothesis that 5-HT(2C)R plays a role in the regulatory neurochemistry of the NAc shell that is important to the full expression of the behaviors evoked by cocaine.

Differential regulation of the mesoaccumbens circuit by serotonin 5-hydroxytryptamine (5-HT)2A and 5-HT2C receptors

Serotonin [5-hydroxytryptamine (5-HT)] 5-HT(2A) and 5-HT(2C) receptors (5-HT(2A)Rs and 5-HT(2C)Rs), which innervate the dopamine mesoaccumbens pathway, may play an important role in the behavioral effects of cocaine. To test this hypothesis, the present study measured cocaine-evoked locomotor activity after bilateral microinjection of selective 5-HT(2A)R and 5-HT(2C)R antagonists into the ventral tegmental area (VTA) or the nucleus accumbens (NAc) shell. Locomotor activity was measured after intracranial microinjection of saline (0.2 microl/side), the selective 5-HT(2A)R antagonist R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine methanol (M100907) (0.1 or 0.3 microg. 0.2 microl(-1). side(-1)), or the selective 5-HT(2C)R antagonist 8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulfon-amido)phenyl-5-oxopentyl)]-1,3,8-triazaspiro[4.5]decane-2,4-dione hydrochloride (RS 102221) (0.05-0.5 microg. 0.2 microl(-1). side(-1)) followed by an injection of saline (1 ml/kg, i.p.) or cocaine (10 mg/kg, i.p.). Microinjection of M100907 (0.1-0.3 microg/side) into the VTA or RS 102221 (0.15-0.5 microg/side) into the NAc shell attenuated cocaine-induced hyperactivity in a dose-related manner. However, hyperactivity evoked by cocaine was not altered by microinjection of RS 102221 into the VTA or M100907 into the NAc shell. No changes in basal activity were observed after microinjection of M100907 or RS 102221 into either brain region. These findings are the first to demonstrate that the behavioral effects of cocaine are generated in part by activation of 5-HT(2A)Rs in the VTA and by activation of 5-HT(2C)Rs in the NAc shell. The selective regulation of the mesoaccumbens circuit by 5-HT(2A)Rs and 5-HT(2C)Rs implicates these 5-HT receptors as important in the behavioral outcomes of systemic cocaine administration.

Electroretinogram and cue-elicited craving in withdrawn cocaine-dependent patients: a replication

BACKGROUND

We previously reported that cocaine-dependent patients with a reduced blue cone b wave electroretinogram (ERG) responses had significantly more cue-elicited craving.

METHODS

A new series of 21 recently withdrawn cocaine-dependent patients completed a craving questionnaire at baseline and following cue exposure; an ERG was also performed.

RESULTS

Cocaine-dependent patients with a blunted ERG blue cone response (<0.5 microV) showed greater increases in craving following cue exposure. When subjects were included from our preliminary study (N = 14), these differences became highly significant.

CONCLUSIONS

Patients with a reduced ERG response may represent a subgroup more vulnerable to cocaine craving and future relapse.

In vivo autoradiography of [3H]SCH 39166 in rat brain: selective displacement by D1/D5 antagonists

The purpose of this study was to examine the receptor occupancy of D1/D5 antagonists for D1-like dopamine receptors in rat brain using [3H]SCH 39166, a highly selective D1/D5 antagonist with low affinity for 5HT2 receptors. A single concentration of triated SCH 39166 was administered to rats, with or without competing doses of the Dl/D5 antagonist SCH 23390 and unlabeled SCH 39166. the D2-like antagonists haloperidol or the 5-HT, antagonist ketanserin. The bound radioactivity in the cortex, striatum, nucleus accumbens and olfactory tubercle was then quantified using an in vivo autoradiographic procedure. The results indicated that [3H]SCH 39166 was dose dependently displaced by the Dl/D5 antagonists in regions associated with both the nigro-striatal pathway and the mesolimbic dopamine pathway, particularly the nucleus accumbens. Neither haloperidol nor ketanserin displaced [3H]SCH 39166 in any of the regions examined. The data were compared with previously published data examining the in vivo binding of [3H]SCH 39166 in rat brain homogenates. The relative values obtained were comparable to values detected in rat brain homogenates after in vivo binding of [3H]SCH 39166.

The dopamine D(1) receptor agonist SKF-82958 serves as a discriminative stimulus in the rat

We examined the discriminative stimulus effects of the high-efficacy dopamine D(1) receptor agonist (+/-)6-chloro-7, 8-dihydroxy-3-ally1-phenyl-2,3,4,5-tetrahydro-1H-3benzazepine++ + hydrobromide (SKF-82958) in rats trained to discriminate SKF-82958 (0.03 mg/kg) from vehicle in a two-lever food-reinforced drug discrimination task. SKF-82958 produced dose-related increases in responding to the SKF-82958 appropriate lever with full substitution occurring at the training dose. Pretreatment with the dopamine D(1)/D(5) receptor antagonist (-)-trans-6,7,7a,8,9, 13b-hexahydro-3-chloro-2hydroxy-N-methyl-5H-benzo-[d]naphtho -¿2, 1-b¿azepine (SCH-39166) (0.01 mg/kg) attenuated the discriminative stimulus effects of SKF-82958. Pretreatment with the dopamine D(2) receptor antagonist raclopride (0.03 mg/kg) had no effect. The high-efficacy dopamine D(1) receptor agonist R(+)6chloro-7, 8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF-81297) fully substituted for SKF-82958, whereas the low-efficacy dopamine D(1) receptor agonist (+/-)1-phenyl-2,3,4, 5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF-38393) produced only partial substitution. The dopamine D(2) receptor agonist trans-(+/-)-4,4a,5,6,7,8,8a, 9-octahydro-5-propyl-1H-propyl-1H-pyrazolo[3,4-g]quinoline dihydrochloride (quinpirole) and the indirect dopamine agonist cocaine did not substitute fully for the SKF-82958 discriminative stimulus cue. These results demonstrate that the high-efficacy dopamine D(1) receptor agonist SKF-82958 can serve as an effective discriminative stimulus in the rat, and that these effects are mediated by a dopamine D(1)-like receptor mechanism.

Structure and function of dopamine receptors

Dopamine (DA) is the most abundant catecholamine in the brain. The involvement and importance of DA as a neurotransmitter in the regulation of different physiological functions in the central nervous system (CNS) is well known. Deregulation of the dopaminergic system has been linked with Parkinson's disease, Tourette's syndrome, schizophrenia, attention deficit hyperactive disorder (ADHD) and generation of pituitary tumours. This review focuses on the pharmacological and biochemical features shared by the dopamine receptors. We address their coupling to secondary messenger pathways and their physiological function based upon studies using pharmacological tools, specific brain lesions and, more recently, genetically modified animal models.

Non-amine dopamine transporter probe [(3)H]tropoxene distributes to dopamine-rich regions of monkey brain

Drug development in psychopharmacology has adhered to the unwritten precept that compounds targeting monoamine transporters must contain an amine nitrogen in the molecular structure. A series of non-amine-bearing aryloxatropanes that are potent inhibitors of the dopamine transporter (DAT) challenged this precept. In the present study, we investigated the brain distribution of a selective, high-affinity DAT non-amine, [(3)H]tropoxene (2-carbomethoxy-3, 4dichloro-3-aryl-8-oxabicyclo[3.2.1] octene), which binds to the DAT in monkey striatum. The autoradiographic distribution of [(3)H]tropoxene was conducted in tissue sections of rhesus (Macaca mulatta) monkey brain. Highest accumulation of the radioligand was detected in the putamen and caudate nucleus, with significant levels also observed in the nucleus accumbens and substantia nigra. Moderate to low levels of [(3)H]tropoxene binding were noted in the hypothalamus, amygdala, ventral tegmental area, and thalamus. The distribution of [(3)H]tropoxene was restricted to brain regions previously identified as expressing DAT, and the relative densities of [(3)H]tropoxene binding sites in various brain regions corresponded to those observed with other selective monoamine radioligands for the DAT. This is the first report to demonstrate that transporter-selective compounds that bear no amine nitrogen in their structure bind selectively to brain regions rich in the transporter. The results support our conclusion that an amine nitrogen is not necessary for compounds to bind to monoamine transporters and distribute in brain according to the known distribution of transporters. The findings provide further incentives to investigate the pharmacological potential of transport inhibitors lacking an amine nitrogen in the molecular structure.

Specific reductions of striatal prodynorphin and D1 dopamine receptor messenger RNAs during cocaine abstinence

It is well established that the opioid neuropeptide and dopamine systems are altered following the use of cocaine. However very little information is available about their possible involvement during cocaine abstinence. In the present study, the mRNA expression of the dopamine receptors, D1 and D2, and the opioid peptides, prodynorphin and proenkephalin, were analyzed in the rat striatum using in situ hybridization histochemistry. Saline or cocaine (30 mg/kg, i.p.) were administered to rats once daily for 1 or 10 days. To examine cocaine abstinence, animals were treated for 10 days as described followed by a 10-day drug free period. Acute and intermittent cocaine administration elevated the prodynorphin mRNA expression in the dorsal striatum, consistent with previous reports, while the abstinent phase resulted in a significant reduction of prodynorphin mRNA levels in the ventrorostral striatum. The D1-receptor mRNA was decreased in the caudorostral striatum during cocaine withdrawal, a finding opposite to the increase observed following a single injection of the drug. Proenkephalin and the D2-receptor mRNAs were not altered during cocaine abstinence, though proenkephalin was elevated following acute but not repeated cocaine administration. These results show long-term suppression on prodynorphin and D1-receptor systems in specific striatal populations localized mainly in rostral areas during withdrawal from cocaine.

The role of the nitric oxide (NO) pathway in the discriminative stimuli of amphetamine and cocaine

To examine the role of the nitric oxide (NO) pathway in the stimulus effects induced by some psychostimulants, separate groups of rats were trained to discriminate between amphetamine (AMPH; 0.5 mg/kg) and saline, or cocaine (COC; 5 mg/kg) and saline using a standard two-lever operant procedure. Substitution studies showed that AMPH and COC generalized for the training drugs in a dose-dependent manner, their ED50, values being 0.1 mg/kg and 1.2 mg/kg, respectively. The dose-response function of both those psychostimulants did not change in the course of the experiment. Moreover, AMPH and COC induced cross-substitution effects towards each other. Successive combination tests demonstrated that injection of a fixed dose of the NO synthase (NOS) inhibitor 7-nitro indazole (7-NI; 25 mg/kg) plus different doses of AMPH or COC resulted in a leftward shift in the dose-response curves of those psychostimulants and a decrease in their ED50 values. On the other hand, pretreatment with the NO donor molsidomine (MOL), injected in a fixed dose of 100 mg/kg before AMPH and COC, shifted the dose-response curves of the psychostimulants to the right and increased their ED50 values. Our results indicate that NO plays an inhibitory role in the dopamine (DA)-evoked discrimination effects of AMPH and COC in rats.

Effect of cocaine self-administration on striatal dopamine D1 receptors in rhesus monkeys

An array of evidence indicates that long-term exposure to cocaine alters several components of the brain dopamine system. Because the release of dopamine in the nucleus accumbens (NAc) has been implicated in mediating the reinforcing effects of cocaine, changes in dopamine function can have profound effects on drug-seeking and drug-taking behavior. The present study examined the effects of the chronic self-administration of cocaine on the D1 family of dopamine receptors in the rhesus monkey. The brains of three rhesus monkeys that had intravenously self-administered an average of 1.35 mg/kg cocaine per day for 18-22 months were compared to the brains of three cocaine-naive controls. The in vitro quantitative autoradiographic technique was used to quantify binding densities of the D1 ligand [3H]SCH-23390 on cryostat-cut sections of fresh frozen tissue. In animals that self-administered cocaine, the density of D1 binding was significantly lower in the regions of the striatum at the level where the nucleus accumbens is most fully developed. The shell of the NAc showed the largest difference with significantly lower D1 binding also detected in adjacent regions of the caudate nucleus and the putamen. No differences were found in the rostral pole of the NAc or the dorsal striatum at that level. These findings suggest that chronic self-administration of cocaine can modulate the density of dopamine D1 receptors in specific portions of the primate striatum. Such changes might underlie some of the behavioral consequences, like drug dependence and craving, of long-term cocaine use.

Investigations into the nature of a 7-OH-DPAT discriminative cue: comparison with D-amphetamine

In the present study, separate squads of rats were trained to discriminate either the dopamine D3 receptor preferring ligand 7-hydroxy-2-(di-N-propylamino)-tetralin (7-OH-DPAT) (0.03 mg/kg) from saline, or D-amphetamine (0.3 mg/kg) from saline using a standard operant schedule (FR10 schedule reinforcement). Following stable acquisition of responding, tests of generalisation and antagonism were conducted. A number of dopamine agonists having high dopamine D2-like receptor (D2, D3 or D4) affinity generalised to the 7-OH-DPAT, but not amphetamine, cue. The dopamine D2/3 receptor agonist SKF38393 showed no generalisation to either drug cue. Subsequent correlational analysis suggested that this effect was most likely mediated through the dopamine D3 receptor. The dopamine D2/3 receptor antagonist raclopride significantly attenuated both cues. The failure of these drugs to generalise to amphetamine, suggest that there is little involvement of the dopamine D3 receptor subtype in mediating its discriminative stimulus properties.

Mediation of the discriminative stimulus properties of cocaine by mesocorticolimbic dopamine systems

This paper provides a brief review of the scientific evidence implicating the mesocorticolimbic dopamine (DA) system in modulating the discriminative stimulus properties of cocaine in rats. Briefly, systemic administration of DA releasers, reuptake inhibitors, and DA D1, D2, and putative D3 receptor agonists engendered partial to full substitution for the discriminative stimulus effects of cocaine. Dopamine D1 and D2 receptor antagonists attenuate this behavioral property of cocaine. Intracranial microinjection studies have indicated certain key limbic nuclei us loci of action for DA in mediating the discriminative stimulus effects of cocaine. Microinjections of cocaine into either DA cell body (i.e., ventral tegmental area, substantia nigra) or DA terminal regions (i.e., prefrontal cortex, central amygdala, caudate putamen) have failed to reproduce the systemic cocaine discriminative stimulus. Only infusion of cocaine into the nucleus accumbens has been demonstrated to substitute fully for the systemic effects of this psychostimulant. Interestingly, microinjections of the DA D1 receptor antagonist SCH 23390 into either the prefrontal cortex, nucleus accumbens, or central or basolateral amygdala have been demonstrated to block the discriminative stimulus properties of cocaine. Although a determination of the antagonism of the cocaine discriminative stimulus following intra-accumbens microinjection of DA D2 receptor antagonists has not been made, intra-accumbens administration of the DA D2 receptor antagonist sulpiride blocked the discriminative stimulus effects of another psychostimulant, amphetamine. 6-Hydroxydopamine lesions of DA terminals in the nucleus accumbens also attenuated the dose-effect curve for systemic administration of cocaine. Taken together, this intracranial evidence suggests that DA D1 and D2 receptors in the mesocorticolimbic system are involved in modulating the discriminative stimulus properties of psychostimulants and that the nigrostriatal DA system is not primarily involved.

Differential effects of mu-, delta- and kappa-opioid receptor agonists on the discriminative stimulus properties of cocaine in rats

The effects of selective mu-, delta- and kappa-opioid receptor agonists on the discriminative stimulus properties of cocaine were examined in rats trained to discriminate between cocaine (10 mg/kg) and saline. Cocaine produced a dose-related increase in cocaine-appropriate responses in all of the rats. In generalization tests, neither morphine (mu-opioid receptor agonist) nor N-methyl-N-7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8-11-4-benzofu ranacetamide (U50,488H: kappa-opioid receptor agonist) generalized to the discriminative stimulus properties of cocaine. On the other hand, the newly synthesized non-peptide selective delta-opioid receptor agonist 2-methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha-octahydro-quinolino(2,3,3,-g)isoquinoline (TAN-67) partially generalized (56.7% cocaine-appropriate responses) to the discriminative stimulus properties of cocaine. Intracerebroventricular (i.c.v.) administration of [D-Ala2]deltorphin II (peptide delta 2-opioid receptor agonist) completely generalized, while neither [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO: mu-opioid receptor agonist) nor [D-Pen2,D-Pen5]enkephalin (DPDPE; delta 1-opioid receptor agonist) generalized to the discriminative stimulus properties of cocaine. These results suggest that the discriminative stimulus properties of cocaine may be partially mediated by delta-opioid (especially delta 2-opioid) receptors. In combination tests, pretreatment with morphine (3.0 mg/kg) and TAN-67 (3.0 and 10 mg/kg) significantly potentiated the discriminative stimulus properties cocaine. In contrast, pretreatment with U50,488H (2.0 and 4.0 mg/kg) scarcely shifted the discriminative stimulus properties of cocaine. Furthermore, the potentiating effect of 3.0 mg/kg morphine on the discriminative stimulus properties of cocaine was attenuated by 2.0 mg/kg U50,488H. In contrast, the potentiating effect of 10 mg/kg TAN-67 on the discriminative stimulus properties of cocaine was not reversed by either 2.0 or 4.0 mg/kg U50,488H. These results suggest that mu-, delta- and kappa-opioid receptor agonists modulate the discriminative stimulus properties of cocaine through different mechanisms, perhaps through different effects on the dopaminergic system.

Modulation of the discriminative stimulus properties of cocaine: comparison of the effects of fluoxetine with 5-HT1A and 5-HT1B receptor agonists

The present investigation examined the ability of serotonin (5-HT) agonists to substitute for, or alter (i.e. enhance or antagonize), the discriminative stimulus properties of a moderately low dose of cocaine (5 mg/kg) utilizing a two-lever, water-reinforced FR 20 drug discrimination procedure in rats. In substitution tests, the 5-HT1A receptor partial agonists buspirone and gepirone, the 5-HT1A/B receptor agonist RU 24969 and the 5-HT1B/2C receptor agonist m-trifluoromethyl-phenylpiperazine (TFMPP) failed to substitute for the cocaine stimulus, although RU 24969 did engender a maximum of 72% cocaine-lever responding. Fluoxetine (4 mg/kg) engendered primarily saline-appropriate responding. In combination tests, a fixed dose of either fluoxetine (4 mg/kg), RU 24969 (0.5 mg/kg) or TFMPP (0.5 mg/kg) produced a leftward shift in the cocaine dose-response curve (0.313-5 mg/kg). In contrast, buspirone (2.5-20 mg/kg) resulted in a dose-dependent attenuation (approximately 60% reduction) of the cocaine stimulus. Moreover, a dose of 10 mg/kg of buspirone co-administered with various doses of cocaine (1.25-10 mg/kg) engendered a rightward shift in the cocaine dose-response curve. Gepirone in combination with cocaine neither enhanced nor antagonized the cocaine discriminative stimulus. Whereas 5-HT agonists do not fully substitute for cocaine, the present results demonstrate that 5-HT1B, but not 5-HT1A, receptor agonists can modulate the discriminative stimulus properties of cocaine in a manner similar to that observed following administration of the 5-HT reuptake inhibitor fluoxetine. The ability of buspirone, but not gepirone, to attenuate the cocaine stimulus probably reflects its dopamine (DA) D2 receptor antagonist properties and not its efficacy at 5-HT1A receptors.

A genetic approach to study mechanisms of cocaine action

The brain dopamine system is thought to be the major target for the neuropharmacological actions of psychomotor stimulants such as cocaine. To investigate the mechanisms of cocaine action, we used a genetic approach, the gene-targeting technique, and generated D1 dopamine receptor mutant mice. Locomotor activity analysis in response to cocaine indicates that, in contrast to control mice which showed a dose-dependent increase in locomotion, D1 receptor mutant mice exhibited a dose-dependent decrease, suggesting that D1 receptors play an essential role in mediating such effects. Extracellular single unit recording of dopamine sensitive nucleus accumbens neurons in the D1 receptor mutant mice and control mice revealed a marked reduction in the inhibitory effects of cocaine and dopamine on the generation of action potentials, suggesting that D1 receptors play a fundamental role in cocaine- and dopamine-mediated neurophysiological effects within the nucleus accumbens. From these analyses, we conclude that the D1 dopamine receptor plays essential roles in mediating these effects of cocaine. In the future, the use of this powerful genetic approach will be essential for elucidating the molecular components of the signal transduction pathway leading to anatomical, cellular and behavioral changes upon cocaine administration and dopamine neurotransmission.

Discriminative stimulus effects of cocaine: antagonism by dopamine D1 receptor blockade in the amygdala

Mesolimbic dopamine (DA) D1 and D2 receptors appear to be involved in mediating the discriminative stimulus effects of cocaine. The purpose of the present study was to investigate the role of the amygdala DA D1 receptors, in modulating the stimulus effects of cocaine. Thus, rats were trained to discriminate cocaine (10 mg/kg, IP) from saline using a two-lever, water-reinforced FR 20 drug discrimination task. In substitution tests, systemic (IP) administration of cocaine (0.625-20 mg/kg) produced a dose-related increase in cocaine-lever responding. Intracranial bilateral injections of cocaine (20-200 micrograms, total dose) into the central amygdala engendered, at best, a partial substitution (< 60% drug-lever responding) for the systemic cocaine cue. Central amygdala microinjections of artificial cerebrospinal fluid (ACSF; 1 microliter/side) or SCH 23390 (0.5-2 microgram, total dose) resulted in primarily saline-appropriate responding. In antagonism tests, bilateral injections of the DA D1 receptor antagonist SCH 23390 (0.5-2 microgram, total dose) into the central amygdala produced a dose-related blockade of a systemic dose of cocaine (5 mg/kg) that engendered > 85% cocaine-lever responding when given alone. Additionally, bilateral injection of a fixed dose of SCH 23390 (2 micrograms) into the central amygdala resulted in a rightward shift in the cocaine dose-response curve (2.5-20 mg/kg). Although administration of cocaine into the central amygdala does not mimic the systemic cocaine cue, the present results demonstrate that DA D1 receptors located within the central amygdala appear to have a modulatory role upon the discriminative stimulus properties of cocaine.

Elimination of cocaine-induced hyperactivity and dopamine-mediated neurophysiological effects in dopamine D1 receptor mutant mice

The brain mesoaccumbens dopamine system is intricately involved in the psychomotor stimulant activities of cocaine. However, the extent to which different dopamine receptors mediate these effects has not yet been firmly established. The present study used dopamine D1 receptor mutant mice produced by gene targeting to investigate the role of this receptor in the effects induced by cocaine. In contrast with wild-type mice, which showed a dose-dependent increase in locomotion, D1 mutant mice exhibited a dose-dependent decrease. Electrophysiological studies of dopamine-sensitive nucleus accumbens neurons demonstrated a marked reduction in the inhibitory effects of cocaine on the generation of action potentials. In addition, the inhibitory effects of dopamine as well as D1 and D2 agonists were almost completely abolished, whereas those of serotonin were unaffected. D2-like dopamine receptor binding was also normal. These results demonstrate the essential role of the D1 receptor in the locomotor stimulant effects of cocaine and in dopamine-mediated neurophysiological effects within the nucleus accumbens.