Ethanol deprivation and central 5-HT deficiency differentially affect the mRNA editing of the 5-HT2C receptor in the mouse brain

BACKGROUND

Serotonin (5-HT) 5-HT2C receptor mRNA editing (at five sites, A-E), implicated in neuropsychiatric disorders, including clinical depression, remains unexplored during alcohol abstinence-often accompanied by depressive symptoms.

METHODS

We used deep sequencing to investigate 5-HT2C receptor editing in mice during early ethanol deprivation following prolonged alcohol exposure and mice lacking tryptophan hydroxylase (TPH)2, a key enzyme in central 5-HT production. We also examined Tph2 expression in ethanol-deprived animals using quantitative real-time PCR (qPCR).

RESULTS

Cessation from chronic 10% ethanol exposure in a two-bottle choice paradigm enhanced immobility time and decreased latency in the forced swim test (FST), indicating a depression-like phenotype. In the hippocampus, ethanol-deprived "high ethanol-drinking" mice displayed reduced Tph2 expression, elevated 5-HT2C receptor editing efficiency, and decreased frequency of the D mRNA variant, encoding the less-edited INV protein isoform. Tph2-/- mice showed attenuated receptor editing in the hippocampus and elevated frequency of non-edited None and D variants. In the prefrontal cortex, Tph2 deficiency increased receptor mRNA editing at site D and reduced the frequency of AB transcript, predicting a reduction in the corresponding partially edited VNI isoform.

CONCLUSIONS

Our findings reveal differential effects of 5-HT depletion and ethanol cessation on 5-HT2C receptor editing. Central 5-HT depletion attenuated editing in the prefrontal cortex and the hippocampus, whereas ethanol deprivation, coinciding with reduced Tph2 expression in the hippocampus, enhanced receptor editing efficiency specifically in this brain region. This study highlights the interplay between 5-HT synthesis, ethanol cessation, and 5-HT2C receptor editing, providing potential mechanism underlying increased ethanol consumption and deprivation.

Novel GPR18 Ligands in Rodent Pharmacological Tests: Effects on Mood, Pain, and Eating Disorders

The lack of selective pharmacological tools has limited the full unraveling of G protein-coupled receptor 18 (GPR18) functions. The present study was aimed at discovering the activities of three novel preferential or selective GPR18 ligands, one agonist (PSB-KK-1415) and two antagonists (PSB-CB-5 and PSB-CB-27). We investigated these ligands in several screening tests, considering the relationship between GPR18 and the cannabinoid (CB) receptor system, and the control of endoCB signaling over emotions, food intake, pain sensation, and thermoregulation. We also assessed whether the novel compounds could modulate the subjective effects evoked by Δ⁹-tetrahydrocannabinol (THC). Male mice or rats were pretreated with the GPR18 ligands, and locomotor activity, depression- and anxiety-like symptoms, pain threshold, core temperature, food intake, and THC-vehicle discrimination were measured. Our screening analyses indicated that GPR18 activation partly results in effects that are similar to those of CB receptor activation, considering the impact on emotional behavior, food intake, and pain activity. Thus, the orphan GPR18 may provide a novel therapeutic target for mood, pain, and/or eating disorders, and further investigation is warranted to better discern its function.

Tph2 Gene Expression Defines Ethanol Drinking Behavior in Mice

Indirect evidence supports a link between disrupted serotonin (5-hydroxytryptamine; 5-HT) signaling in the brain and addictive behaviors. However, the effects of hyposerotonergia on ethanol drinking behavior are contradictory. In this study, mice deficient in tryptophan hydroxylase 2 (Tph2^−/−), the rate-limiting enzyme of 5-HT synthesis in the brain, were used to assess the role of central 5-HT in alcohol drinking behavior. Life-long 5-HT depletion in these mice led to an increased ethanol consumption in comparison to wild-type animals in a two-bottle choice test. Water consumption was increased in naïve 5-HT-depleted mice. However, exposure of Tph2^−/− animals to ethanol resulted in the normalization of water intake to the level of wild-type mice. Tph2 deficiency in mice did not interfere with ethanol-evoked antidepressant response in the forced swim test. Gene expression analysis in wild-type animals revealed no change in Tph2 expression in the brain of mice consuming ethanol compared to control mice drinking water. However, within the alcohol-drinking group, inter-individual differences in chronic ethanol intake correlated with Tph2 transcript levels. Taken together, central 5-HT is an important modulator of drinking behavior in mice but is not required for the antidepressant effects of ethanol.

Adaptive mechanisms following antidepressant drugs: Focus on serotonin 5-HT2A receptors

BACKGROUND

There is a strong support for the role of serotonin (5-HT) neurotransmission in depression and in the mechanism of action of antidepressants. Among 5-HT receptors, 5-HT2A receptor subtype seems to be an important target implicated in the above disorder.

METHODS

The aim of the study was to investigate the effects of antidepressants, such as imipramine (15 mg/kg), escitalopram (10 mg/kg) and tianeptine (10 mg/kg) as well as drugs with antidepressant activity, including N-acetylcysteine (100 mg/kg) and URB597 (a fatty acid amide hydrolase inhibitor, 0.3 mg/kg) on the 5-HT2A receptor labeling pattern in selected rat brain regions. Following acute or chronic (14 days) drug administration, rat brains were analyzed by using autoradiography with the 5-HT2A receptor antagonist [3H]ketanserin.

RESULTS

Single dose or chronic administration of imipramine decreased the radioligand binding in the claustrum and cortical subregions. The [3H]ketanserin binding either increased or decreased in cortical areas after acute N-acetylcysteine and URB597 administration, respectively. A similar shift towards reduction of the [3H]ketanserin binding was detected in the nucleus accumbens shell following either acute treatment with imipramine, escitalopram, N-acetylcysteine and URB597 or repeated administration of imipramine, tianeptine and URB597.

CONCLUSIONS

In conclusion, the present result indicate different sensitivity of brain 5-HT2A receptors to antidepressant drugs depending on schedule of drug administration and rat brain regions. The decrease of accumbal shell 5-HT2A receptor labeling by antidepressant drugs exhibiting different primary mechanism of action seems to be a common targeting mechanism associated with the outcome of depression treatment.

Chronic methamphetamine self-administration dysregulates 5-HT2A and mGlu2 receptor expression in the rat prefrontal and perirhinal cortex: Comparison to chronic phencyclidine and MK-801

Chronic methamphetamine (meth) abuse often turns into a compulsive drug-taking disorder accompanied by persistent cognitive deficits and re-occurring psychosis. Possible common neurobiological substrates underlying meth-induced deficits and schizophrenia remain poorly understood. Serotonin 2A (5-HT2A) and metabotropic glutamate 2 (mGlu2) receptors co-regulate psychosis-like behaviors and cognitive function in animals. Therefore, in the present study we examined the effects of chronic exposure to three different drugs known to produce persistent deficits in sensorimotor gating and cognition [meth, phencyclidine (PCP) and MK-801] on the expression of 5-HT2A and mGlu2 within the rat medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC) and perirhinal cortex (PRh). Adult male rats underwent 14 days of: (a) meth self-administration (6 h/day), (b) phencyclidine (PCP; 5 mg/kg, twice/day) administration, or (c) MK-801 (0.3 mg/kg, twice/day) administration. Seven days after the discontinuation of drug administration, tissues of interest were collected for protein expression analysis. We found that despite different pharmacological mechanism of action, chronic meth, PCP, and MK-801 similarly dysregulated 5-HT2A and mGlu2, as indicated by an increase in the 5-HT2A/mGlu2 expression ratio in the mPFC (all three tested drugs), PRh (meth and PCP), and dHPC (MK-801 only). Complementary changes in G-protein expression (increase in Gαq and decrease in Gαi) were also observed in the mPFC of meth animals. Finally, we found that 5-HT2A/mGlu2 cooperation can be mediated in part by the formation of the receptor heteromer in some, but not all cortical regions. In summary, these data suggest that a shift towards increased availability (and G-protein coupling) of cortical 5-HT2A vs. mGlu2 receptors may represent a common neurobiological mechanism underlying the emergence of psychosis and cognitive deficits observed in subjects with meth use disorder and schizophrenia.

Changes in the Brain Endocannabinoid System in Rat Models of Depression

A growing body of evidence implicates the endocannabinoid (eCB) system in the pathophysiology of depression. The aim of this study was to investigate the influence of changes in the eCB system, such as levels of neuromodulators, eCB synthesizing and degrading enzymes, and cannabinoid (CB) receptors, in different brain structures in animal models of depression using behavioral and biochemical analyses. Both models used, i.e., bulbectomized (OBX) and Wistar Kyoto (WKY) rats, were characterized at the behavioral level by increased immobility time. In the OBX rats, anandamide (AEA) levels were decreased in the prefrontal cortex, hippocampus, and striatum and increased in the nucleus accumbens, while 2-arachidonoylglycerol (2-AG) levels were increased in the prefrontal cortex and decreased in the nucleus accumbens with parallel changes in the expression of eCB metabolizing enzymes in several structures. It was also observed that CB1 receptor expression decreased in the hippocampus, dorsal striatum, and nucleus accumbens, and CB2 receptor expression decreased in the prefrontal cortex and hippocampus. In WKY rats, the levels of eCBs were reduced in the prefrontal cortex (2-AG) and dorsal striatum (AEA) and increased in the prefrontal cortex (AEA) with different changes in the expression of eCB metabolizing enzymes, while the CB1 receptor density was increased in several brain regions. These findings suggest that dysregulation in the eCB system is implicated in the pathogenesis of depression, although neurochemical changes were linked to the particular brain structure and the factor inducing depression (surgical removal of the olfactory bulbs vs. genetic modulation).

The Involvement of Norepinephrine in Behaviors Related to Psychostimulant Addiction

Although it is generally accepted that the abuse-related effects of amphetamines and cocaine result from the activation of the brain dopaminergic (DA) system, the psychostimulants also alter other neurotransmitter systems. In particular, they increase extracellular levels of norepinephrine (NE) and serotonin by inhibiting respective plasma membrane transporters and/or inducing release. The present review will discuss the preclinical findings on the effects of the NE system modulation (lesions, pharmacological and genetic approaches) on behaviors (locomotor hyperactivity, behavioral sensitization, modification of intracranial self-stimulation, conditioned place preference, drug self-administration, extinction/reinstatement of drug seeking behavior) related to the psychostimulant addiction.

Cocaine self-administration differentially affects allosteric A2A-D2 receptor-receptor interactions in the striatum. Relevance for cocaine use disorder

In the current study behavioral and biochemical experiments were performed to study changes in the allosteric A2AR-D2R interactions in the ventral and dorsal striatum after cocaine self-administration versus corresponding yoked saline control. By using ex vivo [(3)H]-raclopride/quinpirole competition experiments, the effects of the A2AR agonist CGS 21680 (100 nM) on the KiH and KiL values of the D2-like receptor (D2-likeR) were determined. One major result was a significant reduction in the D2-likeR agonist high affinity state observed with CGS 21680 after cocaine self-administration in the ventral striatum compared with the yoked saline group. The results therefore support the hypothesis that A2AR agonists can at least in part counteract the motivational actions of cocaine. This action is mediated via the D2-likeR by targeting the A2AR protomer of A2AR-D2-like R heteroreceptor complexes in the ventral striatum, which leads to the reduction of D2-likeR protomer recognition through the allosteric receptor-receptor interaction. In contrast, in the dorsal striatum the CGS 21680-induced antagonistic modulation in the D2-likeR agonist high affinity state was abolished after cocaine self-administration versus the yoked saline group probably due to a local dysfunction/disruption of the A2AR-D2-like R heteroreceptor complexes. Such a change in the dorsal striatum in cocaine self-administration can contribute to the development of either locomotor sensitization, habit-forming learning and/or the compulsive drug seeking by enhanced D2-likeR protomer signaling. Potential differences in the composition and stoichiometry of the A2AR-D2R heteroreceptor complexes, including differential recruitment of sigma 1 receptor, in the ventral and dorsal striatum may explain the differential regional changes observed in the A2A-D2-likeR interactions after cocaine self-administration.

Discovering the mechanisms underlying serotonin (5-HT)2A and 5-HT2C receptor regulation following nicotine withdrawal in rats

We have previously demonstrated that nicotine withdrawal produces depression-like behavior and that serotonin (5-HT)2A/2C receptor ligands modulate that mood-like state. In the present study we aimed to identify the mechanisms (changes in radioligand binding, transcription or RNA-editing) related to such a behavioral outcome. Rats received vehicle or nicotine (0.4 mg/kg, s.c.) for 5 days in home cages. Brain 5-HT2A/2C receptors were analyzed on day 3 of nicotine withdrawal. Nicotine withdrawal increased [(3)H]ketanserin binding to 5-HT2A receptors in the ventral tegmental area and ventral dentate gyrus, yet decreased binding in the nucleus accumbens shell. Reduction in [(3)H]mesulergine binding to 5-HT2C receptors was seen in the ventral dentate gyrus. Profound decrease in the 5-HT2A receptor transcript level was noted in the hippocampus and ventral tegmental area. Out of five 5-HT2C receptor mRNA editing sites, deep sequencing data showed a reduction in editing at the E site and a trend toward reduction at the C site in the hippocampus. In the ventral tegmental area, a reduction for the frequency of CD 5-HT2C receptor transcript was seen. These results show that the reduction in the 5-HT2A receptor transcript level may be an auto-regulatory response to the increased receptor density in the hippocampus and ventral tegmental area during nicotine withdrawal, while decreased 5-HT2C receptor mRNA editing may explain the reduction in receptor labeling in the hippocampus. Serotonin (5-HT)2A/2C receptor ligands alleviate depression-like state in nicotine-withdrawn rats. Here, we show that the reduction in 5-HT2A receptor transcript level may be an auto-regulatory response to the increased receptor number in the hippocampus and ventral tegmental area during nicotine withdrawal, while attenuated 5-HT2C receptor mRNA editing in the hippocampus might explain reduced inverse agonist binding to 5-HT2C receptor and suggest a shift toward a population of more active receptors. 5-HT, serotonin; 5-HT2A R, 5-HT2A receptor; 5-HT2C R, 5-HT2C receptor.

Context-controlled nicotine-induced changes in the labeling of serotonin (5-HT)2A and 5-HT2C receptors in the rat brain

BACKGROUND

We have previously demonstrated that serotonin (5-HT)2A and 5-HT2C receptor ligands modulate the sensitizing effects of nicotine. In the present study we used male rats to verify the hypothesis that the binding pattern of 5-HT2A and 5-HT2C receptors in the brain is altered by chronic nicotine treatment in different environments.

METHODS

Rats were given repeatedly vehicle or nicotine in different sensitizing regimens (home or experimental cages). On day 10, animals were challenged with nicotine (expression of nicotine sensitization) or vehicle in either home or experimental cages, and were sacrificed immediately after the experiment.

RESULTS

Repeated treatment with nicotine in home cages evoked significant increases in [(3)H]ketanserin binding to 5-HT2A receptors in the prefrontal cortex, striatal subregions and ventral tegmental area as well as reductions in [(3)H]mesulergine binding to 5-HT2C receptors in subregions of the prefrontal cortex. In contrast, nicotine paired with environmental context produced robust increases in 5-HT2A receptor labeling in the infralimbic cortex and decreased [(3)H]ketanserin binding in striatal subregions and ventral tegmental area; 5-HT2C receptor labeling in the prefrontal cortex fell.

CONCLUSIONS

The present data indicate that chronic nicotine administration in home cages induces bi-directional neuroplastic changes within 5-HT2A and 5-HT2C receptors in the prefrontal cortex. Pairing the nicotine with environmental context potentiates the neuroplastic response in the latter region and evokes opposite changes in 5-HT2A receptor binding in striatal and tegmental regions compared with nicotine administered in the absence of the context, indicating a modulatory role of environmental context in the expression of nicotine-induced sensitization.

Inhibitory actions of mGlu4 receptor ligands on cocaine-, but not nicotine-, induced sensitizing and conditioning locomotor responses in rats

BACKGROUND

Male Wistar rats were used to verify the hypothesis that metabotropic glutamate 4 (mGlu4) receptor ligands may modulate the locomotor effects evoked by cocaine or nicotine.

METHODS

The preferential mGlu4 receptor orthosteric agonist (2S)-2-amino-4-[hydroxy[hydroxy(4-hydroxy-3-methoxy-5-nitrophenyl)methyl]phosphoryl]butanoic acid (LSP1-2111) and the mGlu4 receptor positive allosteric modulator (+)-cis-N(1)-(3,4-dichlorophenyl)cyclohexane-1,2-dicarboxamide (Lu AF21934) were used in the study. Rats were given repeated pairings of a test environment with cocaine (10mg/kg), nicotine (0.4 mg/kg) or the respective vehicles for 5 days. On day 10, animals were challenged with cocaine (10mg/kg, cocaine sensitization), nicotine (0.4 mg/kg, nicotine sensitization) or vehicle (conditioned hyperlocomotion) in experimental cages.

RESULTS

Given on day 10, LSP1-2111 (3mg/kg) as well as Lu AF21934 (2.5-5mg/kg) decreased the expression of cocaine sensitization. In another set of experiments, LSP1-2111 (3mg/kg) and Lu AF21934 (5mg/kg) administered on day 10 attenuated the conditioned hyperlocomotion in rats treated repeatedly with cocaine. Neither LSP1-2111 (1-3mg/kg) nor Lu AF21934 (2.5-5mg/kg) changed the expression of nicotine sensitization and conditioned hyperlocomotion in rats treated repeatedly with nicotine. None of the mGlu4 receptor agonist/modulator altered the basal locomotor activity or acute hyperactivity to cocaine or nicotine.

CONCLUSIONS

The present data indicate that pharmacological stimulation of mGlu4 receptors reduces the cocaine-induced expression of sensitization as well as conditioned hyperactivity. In contrast, mGlu4 receptor activation seems to be devoid of any effect on the locomotor effects of nicotine.

LC/MS/MS evaluation of cocaine and its metabolites in different brain areas, peripheral organs and plasma in cocaine self-administering rats

BACKGROUND

We employed a cocaine intravenous self-administration model based on positive reinforcement of animals' instrumental reactions (i.e., lever pressing) rewarded by a dose of the drug. We also carried out simultaneous characterization of the pharmacokinetics of cocaine and its metabolites in rats during withdrawal; in this part of the experiments, we investigated the cocaine (2 mg/kg, iv)-induced changes in the distribution, rate constant, clearance and t₁/₂ of the parent drug and its metabolites in different structures of the brain and in peripheral tissues.

METHODS

By using liquid chromatography-tandem mass spectrometry (LC/MS/MS) we measured the levels of cocaine and its major metabolites.

RESULTS

Our results demonstrate differences in the levels of cocaine after cocaine self-administration in the rat, with the highest concentration seen in the striatum and the lowest in the cerebellum. Cocaine metabolites determined in the rat brain remained at very low levels (benzoylecgonine), irrespectively of the brain area, whereas the norcocaine concentration varied from 1.56 μg/g (the nucleus accumbens) to 2.73 μg/g (the striatum).

CONCLUSION

A tandem LC/MS/MS is a valid method for evaluation of brain and peripheral levels of cocaine and its metabolites. Our results demonstrate brain area-dependent differences in the levels of cocaine after its self-administration in the rat. There were also differences in pharmacokinetic parameters among the brain areas and peripheral tissues following a bolus iv injection of cocaine to rats withdrawn from cocaine; among brain structures the slowest metabolic rate was detected for the striatum.

Accumbal and pallidal dopamine, glutamate and GABA overflow during cocaine self-administration and its extinction in rats

We investigated the changes in dopamine (DA), glutamate and γ-aminobutyric acid (GABA) during cocaine self-administration in rats implanted with guide cannulae into the nucleus accumbens and ventral pallidum. After stabilized cocaine self-administration, separate groups of rats underwent extinction (10 days) procedure in which cocaine infusion was replaced by saline injections. With using a 'yoked' procedure, the effects of cocaine or its withdrawal on the level of neurotransmitters were evaluated by dual-probe microdialysis. Repeated cocaine administration reduced basal glutamate levels in the nucleus accumbens and ventral pallidum, whereas it did not affect basal accumbal DA levels. Only rats that self-administered cocaine had increased basal GABA overflow in both examined brain structures. Active or passive cocaine administration elevated extracellular accumbal DA, however, the extent of cocaine-evoked DA level was significantly higher in rats that self-administered cocaine while both groups of animals showed also an attenuation of GABA level in the nucleus accumbens. On day 10 of extinction training, rats previously given cocaine revealed decreases in the basal accumbal concentration of glutamate while the basal GABA levels were significantly enhanced as compared with baseline of saline-yoked controls. Potassium depolarization delayed the reduction of the accumbal and pallidal extracellular glutamate levels in the active and passive cocaine groups. The present data indicate that changes in DA and GABA neurotransmission during maintenance phase mirror the motivational aspects of cocaine intake. Depending on acute (24 hours) or late (10 days) cocaine withdrawal, different neurotransmitter systems (i.e. glutamate or GABA) seem to be involved.

A novel mechanism of cocaine to enhance dopamine d2-like receptor mediated neurochemical and behavioral effects. An in vivo and in vitro study

Recent in vitro results suggest that cocaine may exert direct and/or indirect allosteric enhancing actions at dopamine (DA) D(2) receptors (D(2)Rs). In the present paper we tested the hypothesis that cocaine in vivo can enhance the effects of the D(2)-likeR agonist quinpirole in rats by using microdialysis and pharmacological behavioral studies. Furthermore, in vitro D(2)-likeR binding characteristics and Gα(i/o)-protein coupling, in the absence and in the presence of cocaine, have been investigated in rat striatal membranes. Intra-nucleus accumbens perfusion of the D(2)-likeR agonist quinpirole (10 μM) reduced local dialysate glutamate levels, whereas cocaine (10 and 100 nM) was ineffective. At a low concentration (100 nM), cocaine significantly enhanced quinpirole-induced reduction of accumbal extracellular glutamate levels. The behavioral experiments showed that cocaine (0.625 mg/kg), but not the DA uptake blocker GBR 12783 (1.25 mg/kg), enhanced quinpirole (1 mg/kg)-induced hyperlocomotion. Finally, cocaine (100 nM), but not GBR 12783 (200 nM), produced a small, but significant increase in the efficacy of DA to stimulate binding of GTPγS to striatal D(2)-likeRs, whereas the D(2)-likeR binding characteristics were unchanged in striatal membranes by cocaine in the nM range. The significant increase in the maximal response to DA-stimulated GTPγS binding to D(2)-likeRs by 100 nM cocaine remained in the presence of GBR 12783. The observed cocaine-induced enhancement of the Gα(i/o)-protein coupling of D(2)Rs may be in part because of allosteric direct and/or indirect enhancing effects of cocaine at these receptors. These novel actions of cocaine may have relevance for understanding the actions of cocaine upon accumbal DA, and/or glutamate transmission and thus its rewarding as well as relapsing effects.

Differential effects of serotonin (5-HT)2 receptor-targeting ligands on locomotor responses to nicotine-repeated treatment

We verified the hypothesis that serotonin (5-HT)(2) receptors control the locomotor effects of nicotine (0.4 mg kg(-1)) in rats by using the 5-HT(2A) receptor antagonist M100907, the preferential 5-HT(2A) receptor agonist DOI, the 5-HT(2C) receptor antagonist SB 242084, and the 5-HT(2C) receptor agonists Ro 60-0175 and WAY 163909. Repeated pairings of a test environment with nicotine for 5 days, on Day 10 significantly augmented the locomotor activity following nicotine administration. Of the investigated 5-HT(2) receptor ligands, M100907 (2 mg kg(-1)) or DOI (1 mg kg(-1)) administered during the first 5 days in combination with nicotine attenuated or enhanced, respectively, the development of nicotine sensitization. Given acutely on Day 10, M100907 (2 mg kg(-1)), Ro 60-0175 (1 mg kg(-1)), and WAY 163909 (1.5 mg kg(-1)) decreased the expression of nicotine sensitization. In another set of experiments, where the nicotine challenge test was performed on Day 15 in animals treated repeatedly (Days: 1-5, 10) with nicotine, none of 5-HT(2) receptor ligands administered during the second withdrawal period (Days: 11-14) to nicotine-treated rats altered the sensitizing effect of nicotine given on Day 15. Our data indicate that 5-HT(2A) receptors (but not 5-HT(2C) receptors) play a permissive role in the sensitizing effects of nicotine, while stimulation of 5-HT(2A) receptors enhances the development of nicotine sensitization and activation of 5-HT(2C) receptors is essential for the expression of nicotine sensitization. Repeated treatment with the 5-HT(2) receptor ligands within the second nicotine withdrawal does not inhibit previously established sensitization.

Effects of serotonin (5-HT)2 receptor ligands on depression-like behavior during nicotine withdrawal

A pronounced withdrawal syndrome including depressed mood prevents cigarette smoking cessation. We tested if blockade or activation of serotonin (5-HT)(2) receptors affected the time of immobility (as an indirect measure of depression-like behavior) in naïve animals and in those withdrawn from chronic nicotine in the forced swim test (FST). The antidepressant imipramine was used as a control. In the FST, the selective 5-HT(2A) receptor antagonist M100,907 (1-2 mg/kg, but not 0.5 mg/kg), the selective 5-HT(2C) receptor antagonist SB 242,084 (0.3-1 mg/kg, but not 0.1 mg/kg), the 5-HT(2C) receptor agonists Ro 60-0175 (10 mg/kg, but not 3 mg/kg) and WAY 163,909 (1.5-10 mg/kg, but not 0.75 mg/kg) as well as imipramine (30 mg/kg, but not 15 mg/kg) decreased the immobility time while the non-selective 5-HT(2) receptor agonist DOI (0.1-1 mg/kg) was inactive in naïve rats. We found an increase in immobility time in rats that were withdrawn from nicotine exposure after 5 days of chronic nicotine treatment. This effect increased from day 1 until day 10 following withdrawal of nicotine, with maximal withdrawal effects on day 3. M100,907 (1 mg/kg), SB 242,084 (0.3 mg/kg), Ro 60-0175 (3 mg/kg), WAY 163,909 (0.75-1.5 mg/kg) and imipramine (15-30 mg/kg) shortened the immobility time in rats that had been removed from nicotine exposure for 3 days. Locomotor activity studies indicated that the effects of SB 242,084 might have been non-specific, as we noticed enhanced basal locomotion in naïve rats. This data set demonstrates that 5-HT(2A) receptor antagonist and 5-HT(2C) receptor agonists exhibited effects similar to antidepressant drugs and abolished the depression-like effects in nicotine-withdrawn rats. These drugs should be considered as adjuncts to smoking cessation therapy, to ameliorate abstinence-induced depressive symptoms.

Interactions of serotonin (5-HT)2 receptor-targeting ligands and nicotine: locomotor activity studies in rats

Male Wistar rats were used to verify the hypothesis that serotonin (5-HT)(2A) or 5-HT(2C) receptors may control the locomotor effects evoked by nicotine (0.4 mg/kg). The 5-HT(2A) receptor antagonist (M100,907), the 5-HT(2A) receptor agonist (DOI), the 5-HT(2C) receptor antagonist (SB 242,084), and the 5-HT(2C) receptor agonists (Ro 60-0175 and WAY 163,909) were used. M100,907 (0.5-2mg/kg) did not alter, while DOI (1 mg/kg) enhanced the nicotine-induced hyperlocomotion. The effect of DOI was antagonized by M100,907 (1 mg/kg). SB 242,084 (0.25-1 mg/kg) augmented, while Ro 60-0175 (1 and 3 mg/kg) and WAY 163,909 (1.5 mg/kg) decreased the overall effect of acute nicotine; effects of Ro 60-0175 and WAY 163,909 were attenuated by SB 242,084 (0.125 mg/kg). In another set of experiments, M100,907 (2 mg/kg) on Day 10 attenuated, while DOI (0.1-1 mg/kg) enhanced the nicotine-evoked conditioned hyperlocomotion in rats repeatedly (Days 1-5) treated with nicotine in experimental chambers. SB 242,084 (0.125 or 1 mg/kg) did not change, while Ro 60-0175 (1 mg/kg) or WAY 163,909 (1.5 mg/kg) decreased the expression of nicotine-induced conditioned hyperactivity. Only DOI (0.3 and 1 mg/kg) and SB 242,084 (1 mg/kg) enhanced the basal locomotion. The present data indicate that 5-HT(2A) receptors are significant for the expression of nicotine-evoked conditioned hyperactivity. Conversely, 5-HT(2C) receptors play a pivotal role in the acute effects of nicotine. Pharmacological stimulation of 5-HT(2A) receptors enhances the conditioned hyperlocomotion, while activation of 5-HT(2C) receptors decreases both the response to acute nicotine and conditioned hyperactivity.

Effect of varenicline on the acute and repeated locomotor responses to nicotine in rats

The objective of this study was to evaluate the efficacy of varenicline, a novel partial agonist at alpha 4 beta 2 and full agonist at alpha 7 nicotinic acetylcholine receptor (nAChR) subtypes, in blocking the locomotor effects of acute or repeated treatments with nicotine (0.4 mg/kg, s.c.) in rats. Varenicline (0.3-3 mg/kg, s.c.) by itself enhanced the basal locomotor activity in naive rats while it had an inhibitory effect on acute nicotine-induced hyperlocomotion. Varenicline (0.3-3 mg/kg) did not change the nicotine-evoked conditioned locomotion, but when administered to nicotine-sensitized rats (0.1 and 1 mg/kg), reduced the expression of nicotine sensitization. In another set of experiments, varenicline (1 mg/kg) administered during the second withdrawal period (days 11-14) to nicotine-treated rats, attenuated the reestablishment of the expression of nicotine sensitization. Our pharmacological analyses further support the hypothesis that varenicline might be a useful treatment for smoking cessation considering its actions on the locomotor and reinforcing effects of nicotine without inhibition of conditioned locomotion.

Alterations in gamma-aminobutyric acid(B) receptor binding in the rat brain after reinstatement of cocaine-seeking behavior

We examined neuroadaptive changes in GABA(B) receptor binding following reinstatement of cocaine-seeking behavior in rat brain structures using a "yoked" procedure and quantitative autoradiographic analysis. To estimate the distribution of GABA(B) receptors in several brain areas, we used [(3)H]CGP 54626, a GABA(B) receptor antagonist. The binding of [(3)H]CGP 54626 in the nucleus accumbens and the amygdaloid complex was decreased by about 20% in rats that actively administered cocaine. Similar decreases were seen in the animals that were passively administered cocaine; these rats also demonstrated decreased GABA(B) receptor binding in the prefrontal and frontal cortices, septum and dorsal striatum. The binding of [(3)H]CGP 54626 in several rat brain areas was decreased during 10-day withdrawal from self-administered cocaine. The cocaine-priming dose (10 mg/kg, ip) induced a significant increase of [(3)H]CGP 54626 binding in the core of the nucleus accumbens, substantia nigra (reticular part), prefrontal and frontal cortices and septum in rats withdrawn from cocaine self-administration. The presentation of the conditioned stimulus (tone + light) associated with previous cocaine self-administration induced a significant decrease of [(3)H]CGP 54626 binding in the mediodorsal thalamic nucleus and amygdaloid complex in the rats withdrawn from cocaine self-administration. Increases in GABA(B) receptor binding in limbic regions during cocaine-induced reinstatement likely reflect motivational states that were present during active drug self-administration.

Effects of agmatine on nicotine-evoked behavioral responses in rats

Agmatine [2-(4-aminobutyl)guanidine], is a novel endogenous ligand at alpha2-adrenoceptors, imidazoline and N-methyl-D-asparate receptors, as well as a nitric oxide synthase inhibitor. The present study tested whether agmatine (5-40 mg/kg, sc) modulated the locomotor, sensitizing, and discriminative stimulus effects of nicotine in male Wistar rats. Agmatine (10-40 mg/kg) affected neither the basal locomotor activity, nor the nicotine (0.4 mg/kg, sc)-evoked hyperactivation. A challenge with saline or nicotine (0.4 mg/kg, sc) on day 10 to rats treated repeatedly (for 5 days) with nicotine (0.4 mg/kg, sc; and exposed to experimental chambers), resulted in the expression of nicotine-evoked conditioned hyperlocomotor response or behavioral sensitization. Given on day 10, agmatine at a dose of 40 mg/kg (but not 20 mg/kg) attenuated nicotine-induced conditioned hyperactivity. However, when this dose was administered to the nicotine-sensitized rats, agmatine failed to alter the effect of the challenge dose of nicotine. In rats trained to discriminate nicotine (0.4 mg/kg, sc) from saline in a two-lever water-reinforced fixed ratio 10 task, agmatine (20 or 40 mg/kg) did not substitute for the training dose of nicotine. In combination studies, pretreatment with agmatine (5-40 mg/kg) did not affect the nicotine (0.4 mg/kg) discrimination and the fixed dose of agmatine (20 mg/kg) did not change the effects of the lower doses of nicotine (0.05-0.2 mg/kg). Our pharmacological analyses indicate that agmatine does not affect the locomotor, sensitizing, or subjective effects of nicotine. However, these data do show an inhibitory effect of agmatine over the expression of nicotine-induced conditioned hyperlocomotion.

Effects of 1-methyl-1,2,3,4-tetrahydroisoquinoline on the behavioral effects of cocaine in rats

The efficacy of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), a member of endogenous tetrahydroisoquinolines, in cocaine- and food-maintained responding in self-administration procedures under a fixed ratio 5 schedule of reinforcement as well as in cocaine and food seeking behaviors in male Wistar rats was examined. The effects of 1MeTIQ on cocaine discrimination and on basal locomotor activity were also assessed. In rats trained to self-administered either cocaine (0.5 mg/kg/injection) paired with the cue (light+tone) or food under a fixed ratio 5 schedule of reinforcement, 1MeTIQ (25 - 50 mg/kg) dose-dependently decreased the cocaine-maintained responding, but did not alter the food-maintained responding. 1MeTIQ (25 - 50 mg/kg) decreased the cocaine seeking behavior reinstated by a noncontingent presentation of cocaine (10 mg/kg, i.p.), but altered neither behavior reinstated by a discrete cue (tone+light) nor food-induced reinstatement. In rats trained to discriminate cocaine (10 mg/kg) from saline in water-reinforced fixed ratio 20 task, pretreatment with 1MeTIQ resulted in neither substitution nor significant alterations in the cocaine (1.25 - 10 mg/kg)-induced discriminative stimulus effects. 1MeTIQ (25 - 50 mg/kg) did not produce also a significant changes in basal horizontal activity. In conclusion, our present results outline a significance of exogenously applied 1MeTIQ in attenuating drug-evoked relapses to cocaine as well as the direct rewarding properties of cocaine (that model the cocaine-induced "high"), but not cocaine subjective effects. Moreover, a dissociation between effects of 1MeTIQ on cocaine vs. food-maintained responding was demonstrated.

Effects of the serotonin 5-HT2A and 5-HT2C receptor ligands on the discriminative stimulus effects of nicotine in rats

The present study tested the hypothesis that serotonergic (5-HT) 5-HT2A or 5-HT2C receptors or their pharmacological stimulation modulated the discriminative stimulus effects of nicotine in male Wistar rats. To this end the selective 5-HT2A receptor antagonist R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol (M100,907; 0.5-1 mg/kg, i.p.), the functional 5-HT2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI; 0.1-1 mg/kg, s.c.), the selective 5-HT2C receptor antagonist 6-chloro-5-methyl-1-{[2-(2-methylpyrid-3-yloxy)pyrid-5-yl]carbamoyl}indoline (SB 242,084; 0.25-1 mg/kg, i.p.) and the 5-HT2C receptor agonists (S)-2-chloro-5-fluoro-indol-1-yl)-1-methylethylamine fumarate (Ro 60-0175; 0.3-1 mg/kg, s.c.) and (7bR, 10aR)-1,2,3,4,8,9,10,10a-octahydro-7bH-cyclopenta-[b][1,4]diazepino[6,7,1hi]indole (WAY 163,909; 0.75-1.5 mg/kg, i.p.) were used. Additionally, the effects of the selective alpha4beta2 nicotinic acetylcholine receptor subtype agonist 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-IA; 0.01 mg/kg, s.c.) were investigated. In rats trained to discriminate (-)-nicotine (0.4 mg/kg, s.c.) from saline in a two-lever, water-reinforced fixed ratio 10 task, substitutions were not observed with 5-HT2 receptor ligands (<32% nicotine-lever responding), conversely 5-IA induced a full substitution (100% nicotine-lever responding). In combination studies, fixed doses of M100,907 (0.5-1 mg/kg) or SB 242,084 (0.25-1 mg/kg) did not alter the dose-response curve of nicotine, while DOI (0.3 mg/kg), Ro 60-0175 (1 mg/kg) and WAY 163,909 (1 and 1.5 mg/kg) attenuated the discriminative stimulus effects of nicotine. The decrease in the expression of the discriminative stimulus effects of nicotine produced by DOI was blocked by M100,907 (1 mg/kg), but not by SB 242,084 (1 mg/kg), while that evoked by Ro 60-0175 or WAY 163,909 was blocked by SB 242,084 (1 mg/kg), but not by M100,907 (1 mg/kg). Further studies showed that DOI (0.3 mg/kg) and Ro 60-0175 (1 mg/kg), but not WAY 163,909 (1.5 mg/kg) blocked full substitution of 5-IA (0.01 mg/kg) for nicotine. Our pharmacological analyses indicate that tonic activation of 5-HT2A or 5-HT2C receptors is not required for subjective effects of nicotine, however these receptors appear to have inhibitory influence on nicotine cue, since pharmacological stimulation of either receptor attenuates the discriminative stimulus effects of nicotine.

Diverse effects of GABA-mimetic drugs on cocaine-evoked self-administration and discriminative stimulus effects in rats

RATIONALE

Recent data indicate that gamma-aminobutyric acid (GABA) is a modulator of behavioral responses to cocaine.

OBJECTIVE

The efficacy of gabapentin (a cyclic GABA analogue), tiagabine (a GABA reuptake inhibitor), or vigabatrin (an inhibitor of GABA transaminase and reuptake) to alter cocaine-seeking behavior and discriminative effects was examined in rats.

MATERIALS AND METHODS

Rats were trained to press a lever for cocaine (0.5 mg/kg per infusion) paired with a cue (light + tone) using a fixed ratio (FR) 5 schedule of reinforcement. After extinction, the cocaine-seeking behavior was reinstated by cocaine priming (10 mg/kg). Another group of rats was trained to discriminate cocaine (10 mg/kg) from saline in a two-lever FR 20 task.

RESULTS

Vigabatrin (150-250 mg/kg) decreased cocaine-maintained responding, whereas tiagabine (10 mg/kg) significantly reduced responses on the "active" lever. Vigabatrin (150-250 mg/kg) significantly decreased responding to the cocaine-priming dose and a nonsignificant attenuation of cocaine-induced reinstatement was seen after tiagabine (5-10 mg/kg). Gabapentin (10-30 mg/kg) failed to alter maintenance of cocaine self-administration or drug-induced reinstatement. Pretreatment with either gabapentin, tiagabine, or vigabatrin resulted in neither reinstatement of cocaine seeking nor alterations in cocaine discrimination.

CONCLUSIONS

Our study demonstrates that vigabatrin (only at the 150 mg/kg dose) exerted inhibitory actions on cocaine-maintained responding and attenuated the reinstatement of extinguishing responding more effectively than gabapentin or tiagabine and with less evidence of motor impairment than the latter drugs. Present findings do not support a role for gabapentin or tiagabine for the possible treatment of cocaine relapse, whereas albeit limited effects of vigabatrin may be seen.

Effects of serotonin 5-HT1B receptor ligands on the cocaine- and food-maintained self-administration in rats

In order to substantiate the concept that cocaine behavioral effects may be influenced by serotonin (5-HT)1B receptors, male Wistar rats were trained to self-administer cocaine intravenously (0.5 mg/kg/injection), and were systemically pretreated with the selective 5-HT1B receptor antagonist N-[3-[3-(dimethylamine)ethoxy]-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1'-biphenyl]-4-carboxamide hydrochloride (SB 216641), or with the agonist 5-propoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine hydrochloride (CP 94253) before test session during the maintenance phase. The effects of the 5-HT1B receptor ligands on a control reinforcer (food)-induced self-administration and on basal locomotor activity were also assessed. SB 216641 (2.5-7.5 mg/kg) was inactive in altering the cocaine (0.5 mg/kg/injection)-maintained responding and at the highest dose (7.5 mg/kg) it did not alter the self-administration of a cocaine dose on the descending limb of the cocaine (0.125-0.5 mg/kg/injection) dose-effect function. On the other hand, CP 94253 (2.5-7.5 mg/kg) attenuated the cocaine (0.5 mg/kg/injection)-maintained responding with a significant inhibitory effect seen at 7.5 mg/kg, while its doses of 2.5-5 mg/kg potently reduced the self-administration of cocaine (0.125-0.25 mg/kg/injection), in a manner similar to the effect produced by increasing the unit dose of cocaine. The inhibitory effects of CP 94253 (5 mg/kg) on the cocaine (0.125 or 0.25 mg/kg/injection) self-administration were blocked by SB 216641 (7.5 mg/kg). Food reinforcing potential was not altered when either SB 216641 or CP 94253 was given in a dose range between 2.5-7.5 mg/kg. Moreover, none of the 5-HT1B receptor ligands altered horizontal locomotor activity while CP 9253 significantly reduced vertical activity. Our present findings extend previous observations that tonic activation of 5-HT1B receptors is not required for cocaine reinforcement while pharmacological stimulation of 5-HT1B receptors enhances such a property of the psychostimulant. Furthermore, we demonstrated that 5-HT1B receptor agonist-induced enhancement of cocaine reward was independent of an alteration in natural reinforcement.

Involvement of cannabinoid CB1 receptors in drug addiction: effects of rimonabant on behavioral responses induced by cocaine

A lot of evidence indicate that endocannabinoids and cannabinoid CB(1) receptors are implicated in drug addiction. In the present study, we investigated the effect of the cannabinoid CB(1) receptor antagonist/partial agonist rimonabant on the cocaine-maintained reinforcement and relapse to cocaine seeking as well as on the cocaine challenge-induced hyperactivity in sensitized rats and on discriminative stimulus effects of cocaine in rats. We found that endocannabinoids were not involved in maintenance of cocaine reinforcement and its subjective effects since pharmacological blockade of cannabinoid CB(1) receptors altered neither self-administration nor discriminative stimulus effects of cocaine. On the other hand, withdrawal from repeated access or exposure to cocaine and then a reinstatement of cocaine-seeking behavior or a sensitized locomotor response to a single cocaine challenge, respectively, was potently reduced by pretreatment with rimonabant. The latter observations may show that repeated cocaine treatment and the drug withdrawal produce--apart from behavioral effects--also different neural consequences in the endocannabinoid systems in rats.

Various GABA-mimetic drugs differently affect cocaine-evoked hyperlocomotion and sensitization

To substantiate the notion that cocaine behavioral effects may be influenced by gamma-aminobutyric acid (GABA) neurotransmission male Wistar rats were injected with gabapentin (a cyclic GABA analogue), tiagabine (a GABA reuptake inhibitor), or vigabatrin (a GABA transaminase inhibitor) before acute or repeated treatment with cocaine evoking either locomotor hyperactivation or sensitization. Gabapentin (1-30 mg/kg), tiagabine (2.5-10 mg/kg) or vigabatrin (75-250 mg/kg) attenuated the cocaine (10 mg/kg)-induced hyperactivation and in the highest doses they also decreased basal locomotor activation. Vigabatrin (75-250 mg/kg) dose-dependently reduced the development of cocaine sensitization in rats treated repeatedly (days 1-5) with cocaine (10 mg/kg) and then challenged with cocaine (10 mg/kg) following 5-day withdrawal; the remaining drugs were ineffective. When injected acutely with a cocaine challenge dose, gabapentin (3-10 mg/kg) or vigabatrin (150 mg/kg), but not tiagabine, significantly attenuated the expression of cocaine sensitization. The present results show that enhanced GABA-ergic neurotransmission exerted inhibitory actions on acute responses to cocaine, however, only in a case of vigabatrin the inhibition seems to be unrelated to the inhibitory effect of the drugs on basal locomotor activity. The finding that vigabatrin protected against the development and the expression of cocaine sensitization further supports its therapeutic potential in the treatment of cocaine dependence.

Evaluation of the role of nicotinic acetylcholine receptor subtypes and cannabinoid system in the discriminative stimulus effects of nicotine in rats

Male Wistar rats were trained to discriminate (-)-nicotine (0.4 mg/kg) from saline under a two-lever, fixed-ratio 10 schedule of water reinforcement. During test sessions the following drugs were coadministered with saline (substitution studies) or nicotine (0.025-0.4 mg/kg; combination studies): the alpha4beta2 nicotinic acetylcholine receptor subtype antagonist dihydro-beta-erythroidine (DHbetaE), the non-selective nicotinic acetylcholine receptor subtype antagonist mecamylamine, the alpha7 nicotinic acetylcholine receptor subtype antagonist methyllycaconitine (MLA), the alpha4beta2 nicotinic acetylcholine receptor subtype agonist 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-IA), the cannabinoid CB1 receptor antagonist/partial agonist rimonabant, the cannabinoid CB2 receptor antagonist N-[(1S)-endo-1,3,3-trimethylbicyclo-[2.2.1]heptan-2-yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methybenzyl)pyrazole-3-carboxamide (SR 144528), the cannabinoid CB1/2 receptor agonists (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol (CP 55,940) or R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl)-methanone mesylate (WIN 55,212-2), the endogenous cannabinoid agonist and non-competitive alpha7 nicotinic acetylcholine receptor subtype antagonist anandamide, the anandamide uptake and fatty acid amide hydrolase inhibitor N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM-404), the fatty acid amide hydrolase inhibitor cyclohexylcarbamic acid 3'-carbamoyl-biphenyl-3-yl ester (URB 597), AM-404+anandamide or URB 597+anandamide. 5-IA (0.01 mg/kg) fully substituted for nicotine, while other drugs were inactive. In combination studies, DHbetaE and mecamylamine dose-dependently attenuated the discriminative stimulus effects of nicotine and the full substitution of 5-IA, while MLA, rimonabant, SR 144528, CP 55,940, WIN 55,212-2, and URB 597 did not alter the nicotine cue. Pretreatment with AM-404+anandamide or URB 597+anandamide weakly enhanced nicotine-lever responding. Our pharmacological analyses demonstrates that the expression of nicotine discrimination is under the control of nicotinic acetylcholine receptor subtypes composed of alpha4beta2 (but not of alpha7) subunits. Furthermore, we excluded the involvement of either cannabinoid CB1 and CB2 receptors or increases in the endocannabinoid tone in the nicotine discrimination.

Involvement of adenosine A2A and dopamine receptors in the locomotor and sensitizing effects of cocaine

Recent data indicate that cocaine locomotor responses may be influenced by dopamine (DA) neurotransmission and adenosine neuromodulation involving the A2A receptor (A2AR). Male Wistar rats were injected with MSX-3 (1-25 mg/kg; an antagonist of A2AR), CGS 21680 (0.05-0.2 mg/kg; an agonist of A2AR), SCH 23390 (0.125-0.25 mg/kg; an antagonist of DA D1/5R), raclopride (0.1-0.8 mg/kg; an antagonist of DA D2/3R), nafadotride (0.2-0.4 mg/kg; an antagonist of DA D3R) or 7-OH-PIPAT (0.01-1 mg/kg; an agonist of DA D3R) to verify the hypothesis that adenosine A2AR and DA receptors and their antagonistic interactions may control locomotor and sensitizing effects of cocaine. In well-habituated animals, MSX-3 (5 mg/kg) increased, while raclopride (0.4-0.8 mg/kg) decreased basal locomotor activation; the other drugs were inactive. The locomotor hyperactivation induced by acute cocaine (10 mg/kg) was enhanced by MSX-3 (5-25 mg/kg) or nafadotride (0.4 mg/kg), while CGS 21680 (0.2 mg/kg), SCH 23390 (0.25 mg/kg), raclopride (0.2-0.8 mg/kg) or 7-OH-PIPAT (0.1 mg/kg) decreased this effect of cocaine. Given during the development of sensitization (in combination with 5-daily cocaine, 10 mg/kg, injections), MSX-3 (5-25 mg/kg) increased, but CGS 21680 (0.2 mg/kg) and raclopride (0.8 mg/kg) reduced the locomotor response to a cocaine challenge dose (10 mg/kg) on day 10. When injected acutely with a cocaine challenge dose (on day 10), CGS 21680 (0.2 mg/kg), raclopride (0.2-0.8 mg/kg) or 7-OH-PIPAT (1 mg/kg) reduced, while MSX-3 (5 mg/kg) or nafadotride (0.4 mg/kg) enhanced the expression of cocaine sensitization. The present results show that adenosine A2ARs and DA D3Rs exert inhibitory actions on acute locomotor responses to cocaine and on the expression of cocaine sensitization, while DA D2Rs had an opposing role in such effects. Pharmacological stimulation of adenosine A2ARs protected against both the development and expression of cocaine sensitization, which may offer a therapeutic potential of A2AR agonists in the treatment of cocaine dependence. The results suggest an antagonistic role of A2ARs in D2R-mediated cocaine actions based at least in part on the existence of A2A/D2 heteromeric receptor complexes.

The serotonergic system and its role in cocaine addiction

Cocaine is an alkaloid with psychostimulant action and high addictive potential. It possesses high affinity for the transporters of dopamine, serotonin (5-HT) and noradrenaline, and blocks reuptake of the above-mentioned monoamines. The present review summarized the contribution of 5-HT neurotransmitter system to rewarding and aversive properties of cocaine, to cocaine withdrawal and its long-term abuse. The present state of knowledge of 5-HT neurotransmission justifies the opinion that pharmacological manipulation in the 5-HT system may efficiently counteract the effects of cocaine withdrawal and prevent reinstatement of its abuse.

Effects of CP 154,526, a CRF1 receptor antagonist, on behavioral responses to cocaine in rats

We examined the influence of CP 154,526, a selective antagonist of corticotropin-releasing factor (CRF)1 receptors, in the locomotor, sensitizing, discriminative stimulus and rewarding effects of cocaine, as well as on the cocaine-induced reinstatement of cocaine-seeking behavior in male Wistar rats. CP 154,526 in doses of 5, 10 and 20 mg/kg, which did not affect basal locomotor activity, dose-dependently reduced the hyperactivation evoked by cocaine. To assess the effects of CP 154,526 on the expression of cocaine sensitization, the rats were injected with either saline or cocaine (10 mg/kg) for 5 days, and were then challenged with cocaine (10 mg/kg) after pretreatment with saline or CP 154,526 on day 5 of withdrawal. The cocaine-induced hyperactivity in sensitized rats was reduced by CP 154,526 (10 and 20 mg/kg). In rats trained to discriminate cocaine (10 mg/kg) from saline, pretreatment with CP 154,526 (5-20 mg/kg) did not affect the cocaine (1.25-10 mg/kg)-induced discriminative stimulus effects. In a self-administration model, the rats were trained to self-administer cocaine (0.5 mg/kg/infusion) in the FR 5 schedule of reinforcement. Administration of CP 154,526 (10-20 mg/kg) did not alter the rewarding effects of cocaine, assessed as the number of active-lever presses and infusions; however, following a 10-day extinction phase, CP 154,526 (5-20 mg/kg) significantly decreased in a dose-dependent manner the cocaine (10 mg/kg) priming-induced reinstatement of cocaine-seeking behavior. The present study implies that CRF1 receptors control the expression of cocaine hyperactivation and sensitization as well as the cocaine-induced relapse behavior, but do not play any role in cocaine discrimination and self-administration. These findings may suggest that CRF1 receptor antagonists should be considered as possible medications in the treatment of cocaine addiction.