Chronic Lorcaserin Treatment Reverses the Nicotine Withdrawal-Induced Disruptions to Behavior and Maturation in Developing Neurons in the Hippocampus of Rats.

Preclinical data have shown that treatment with serotonin (5-HT)2C receptor agonists inhibits the behavioral effects of nicotine, including self-administration, reinstatement, and locomotor responses to nicotine. Since the data on the effects of 5-HT2C receptor agonism on nicotine withdrawal signs are limited, we aimed to investigate whether 5-HT2C receptor agonism alleviated the behavioral and neurobiochemical (hippocampal neurogenesis) consequences of nicotine withdrawal in Sprague-Dawley rats. Our data indicate that withdrawal from nicotine self-administration induced locomotor hyperactivity, lengthened immobility time (the forced swim test), induced 'drug-seeking' behavior and deficits in cognition-like behavior (the novel object recognition task). A two-week exposure to the 5-HT2C receptor agonist lorcaserin attenuated locomotor hyperactivity and induced recovery from depression-like behavior. Analyses of brain slices from nicotine-withdrawn animals revealed that lorcaserin treatment recovered the reduced number of doublecortin (DCX)-positive cells, but it did not affect the number of Ki-67- or 5-bromo-2'-deoxyuridine (BrdU)-positive cells or the maturation of proliferating neurons in drug-weaned rats. To summarize, we show that lorcaserin alleviated locomotor responses and depression-like state during nicotine withdrawal. We propose that the modulatory effect of lorcaserin on the 'affective' aspects of nicotine cessation may be linked to the positive changes caused by the compound in hippocampal neurogenesis during nicotine withdrawal.

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.

Increased Ethanol Consumption and Locomotion Develop upon Ethanol Deprivation in Rats Overexpressing the Adenosine (A)2A Receptor.

Preclinical data indicate that ethanol produces behavioral effects that can be regulated by many neurotransmitters and neuromodulators like adenosine (A). The most important receptors with respect to the rewarding effects of ethanol seem to be the A2A receptors. This study used a transgenic strategy, specifically rats overexpressing the A2A receptor, to characterize the neurobiological mechanisms of ethanol consumption as measured by intermittent access to 20% ethanol in a two-bottle choice paradigm. In this model, no change in ethanol consumption was observed in transgenic animals compared to wild type controls during the acquisition/maintenance phase. Following alcohol deprivation, only transgenic rats overexpressing the A2A receptor exhibited escalation of ethanol consumption and drank more (by ca. 90%), but not significantly, ethanol than did the wild type rats. During ethanol withdrawal, the immobility time of rats overexpressing the A2A receptor in the forced swim test was lower than that of wild type rats. Moreover, transgenic rats withdrawn from ethanol, compared to the drug-naive transgenic animals, exhibited an increase above 70% in locomotion. The results indicated that the overexpression of A2A receptors may be a risk factor for the escalation of ethanol consumption despite the reduction in depression-like signs of ethanol withdrawal.

Understanding the Functional Plasticity in Neural Networks of the Basal Ganglia in Cocaine Use Disorder: A Role for Allosteric Receptor-Receptor Interactions in A2A-D2 Heteroreceptor Complexes.

Our hypothesis is that allosteric receptor-receptor interactions in homo- and heteroreceptor complexes may form the molecular basis of learning and memory. This principle is illustrated by showing how cocaine abuse can alter the adenosine A2AR-dopamine D2R heterocomplexes and their receptor-receptor interactions and hereby induce neural plasticity in the basal ganglia. Studies with A2AR ligands using cocaine self-administration procedures indicate that antagonistic allosteric A2AR-D2R heterocomplexes of the ventral striatopallidal GABA antireward pathway play a significant role in reducing cocaine induced reward, motivation, and cocaine seeking. Anticocaine actions of A2AR agonists can also be produced at A2AR homocomplexes in these antireward neurons, actions in which are independent of D2R signaling. At the A2AR-D2R heterocomplex, they are dependent on the strength of the antagonistic allosteric A2AR-D2R interaction and the number of A2AR-D2R and A2AR-D2R-sigma1R heterocomplexes present in the ventral striatopallidal GABA neurons. It involves a differential cocaine-induced increase in sigma1Rs in the ventral versus the dorsal striatum. In contrast, the allosteric brake on the D2R protomer signaling in the A2AR-D2R heterocomplex of the dorsal striatopallidal GABA neurons is lost upon cocaine self-administration. This is potentially due to differences in composition and allosteric plasticity of these complexes versus those in the ventral striatopallidal neurons.

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.

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.

Nicotine dependence - human and animal studies, current pharmacotherapies and future perspectives.

Nicotine dependence is a disease of constantly growing importance. This mini-review describes the effects of nicotine in humans and focuses on the various laboratory animal models developed to study the dependence-related behavioral effects of nicotine. In addition, we outline the current therapeutic approaches designed to substitute nicotine from cigarette smoke with safer compounds or to relieve symptoms of nicotine withdrawal during smoking abstinence. Since several pharmacotherapies have failed to be efficacious in all patients, we also assess the clinical effectiveness of newer agents in relation to existing drugs.

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.

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.

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.

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.