Discriminative stimulus properties of 1.25mg/kg clozapine in rats: Mediation by serotonin 5-HT2 and dopamine D4 receptors.

The atypical antipsychotic drug clozapine remains one of most effective treatments for schizophrenia, given a lack of extrapyramidal side effects, improvements in negative symptoms, cognitive impairment, and in symptoms in treatment-resistant schizophrenia. The adverse effects of clozapine, including agranulocytosis, make finding a safe clozapine-like a drug a goal for drug developers. The drug discrimination paradigm is a model of interoceptive stimulus that has been used in an effort to screen experimental drugs for clozapine-like atypical antipsychotic effects. The present study was conducted to elucidate the receptor-mediated stimulus properties that form this clozapine discriminative cue by testing selective receptor ligands in rats trained to discriminate a 1.25mg/kg dose of clozapine from vehicle in a two choice drug discrimination task. Full substitution occurred with the 5-HT2A inverse agonist M100907 and the two preferential D4/5-HT2/α1 receptor antagonists Lu 37-114 ((S)-1-(3-(2-(4-(1H-indol-5-yl)piperazin-1-yl)ethyl)indolin-1-yl)ethan-1-one) and Lu 37-254 (1-(3-(4-(1H-indol-5-yl)piperazin-1-yl)propyl)-3,4-dihydroquinolin-2(1H)-one). Partial substitution occurred with the D4 receptor antagonist Lu 38-012 and the α1 adrenoceptor antagonist prazosin. Drugs selective for 5-HT2C, 5-HT6 muscarinic, histamine H1, and benzodiazepine receptors did not substitute for clozapine. The present findings suggest that 5-HT2A inverse agonism and D4 receptor antagonism mediate the discriminative stimulus properties of 1.25mg/kg clozapine in rats, and further confirm that clozapine produces a complex compound discriminative stimulus.

Ethanol withdrawal-induced motor impairment in mice.

RATIONALE: Human ethanol withdrawal manifests as multiple behavioral deficits with distinct time courses. Most studies with mice index ethanol withdrawal severity with the handling-induced convulsion (HIC). Using the accelerating rotarod (ARR), we recently showed that ethanol withdrawal produced motor impairment. OBJECTIVES: This study aimed (a) to characterize further the ARR withdrawal trait, (b) to assess generalizability across additional behavioral assays, and (c) to test the genetic correlation between ethanol withdrawal ARR impairment and HICs. RESULTS: The severity of the ARR performance deficit depends on ethanol vapor dose and exposure duration, and lasts 1-4 days. Fatigue could not explain the deficits, which were also evident after intermittent exposure to ethanol vapor. Withdrawing mice were also impaired on a balance beam, but not on a static dowel or in foot slip errors per distance traveled in the parallel rod floor test, where they showed reduced locomotor activity. To assess genetic influences, we compared Withdrawal Seizure-Prone and -Resistant mice, genetically selected to express severe vs. mild withdrawal HICs, respectively. The ARR scores were approximately equivalent in all groups treated with ethanol vapor, though Withdrawal Seizure-Prone (WSP) mice may have displayed a slightly more severe deficit as control-treated WSP mice performed better than control-treated Withdrawal Seizure-Resistant mice. CONCLUSIONS: These studies show that ethanol withdrawal motor impairment is sensitive to a range of ethanol doses and lasts for several days. Multiple assays of behavioral impairment are affected, but the effects depend on the assay employed. Genetic contributions to withdrawal-induced ARR impairment appear largely distinct from those leading to severe or mild HICs.

Striatal signal transduction and drug addiction.

Drug addiction is a severe neuropsychiatric disorder characterized by loss of control over motivated behavior. The need for effective treatments mandates a greater understanding of the causes and identification of new therapeutic targets for drug development. Drugs of abuse subjugate normal reward-related behavior to uncontrollable drug-seeking and -taking. Contributions of brain reward circuitry are being mapped with increasing precision. The role of synaptic plasticity in addiction and underlying molecular mechanisms contributing to the formation of the addicted state are being delineated. Thus we may now consider the role of striatal signal transduction in addiction from a more integrative neurobiological perspective. Drugs of abuse alter dopaminergic and glutamatergic neurotransmission in medium spiny neurons of the striatum. Dopamine receptors important for reward serve as principle targets of drugs abuse, which interact with glutamate receptor signaling critical for reward learning. Complex networks of intracellular signal transduction mechanisms underlying these receptors are strongly stimulated by addictive drugs. Through these mechanisms, repeated drug exposure alters functional and structural neuroplasticity, resulting in transition to the addicted biological state and behavioral outcomes that typify addiction. Ca(2+) and cAMP represent key second messengers that initiate signaling cascades, which regulate synaptic strength and neuronal excitability. Protein phosphorylation and dephosphorylation are fundamental mechanisms underlying synaptic plasticity that are dysregulated by drugs of abuse. Increased understanding of the regulatory mechanisms by which protein kinases and phosphatases exert their effects during normal reward learning and the addiction process may lead to novel targets and pharmacotherapeutics with increased efficacy in promoting abstinence and decreased side effects, such as interference with natural reward, for drug addiction.

Route of administration influences substitution patterns in rats trained to discriminate methadone vs. vehicle.

Replacement therapy with the synthetic mu-opioid agonist methadone is an efficacious treatment for opioid abuse. While much is known about methadone's pharmacology, its discriminative stimulus properties remain largely unexplored. The present study sought to establish methadone discrimination in rats. Moreover, some research suggests that route of administration alters the discriminative stimulus of methadone. Thus, the present study also compared intraperitoneal (i.p.) and subcutaneous (s.c.) routes of administration. Male Sprague-Dawley rats were trained to discriminate 3.0mg/kg methadone (i.p.) from vehicle in a two-lever discrimination procedure. Generalization tests were conducted with a variety of compounds administered i.p. and s.c. Methadone fully substituted for itself, yielding ED(50)s of 1.5mg/kg (i.p.) and 0.2mg/kg (s.c.). Naltrexone (i.p.), an opioid antagonist produced a dose-dependent reduction in methadone-appropriate responding. The methadone stereoisomers fully substituted for methadone when given s.c.; however, when administered i.p., (+) and (-) methadone produced partial and no substitution, respectively. Heroin fully generalized to methadone regardless of administration route, while morphine fully substituted when given s.c., but not i.p. The kappa-agonist U50-488 failed to generalize to methadone with either route of administration. These results demonstrated that methadone's discriminative stimulus is mediated through mu-opioid receptor activity and is similar to that of commonly abused opioids (heroin, morphine). Additionally, route of administration produced differential results for many of the drugs tested, suggesting decreased drug bioavailability following i.p. administration due to hepatic first pass metabolism. Taken together, these results suggest that methadone's shared subjective effects with abused opioids, as well as its unique metabolic properties contribute to its efficacy in opioid maintenance therapy.

The role of M1 muscarinic cholinergic receptors in the discriminative stimulus properties of N-desmethylclozapine and the atypical antipsychotic drug clozapine in rats.

RATIONALE: The discriminative stimulus properties of clozapine (CLZ) have been studied for decades because it remains the prototype for atypical antipsychotic drug effects and yet is unique in many ways, including increased efficacy in treatment-resistant schizophrenia and in reducing suicidality. Recent studies have indicated that the active CLZ metabolite N-desmethylclozapine (NDMC) may play a role in mediating the cognitive efficacy of CLZ and may also have atypical antipsychotic properties. OBJECTIVES: The present study sought to determine if NDMC has discriminative stimulus properties similar to that of its parent drug CLZ. MATERIALS AND METHODS: Rats were trained to discriminate 1.25 mg/kg CLZ from vehicle in a two-choice drug discrimination task. RESULTS: Although NDMC (2.5-20.0 mg/kg) failed to substitute for CLZ, the combination of NDMC (5.0 and 10.0 mg/kg) with a low dose (0.3125 mg/kg) of CLZ produced full substitution (>80% CLZ-appropriate responding) for the 1.25 mg/kg CLZ training dose. Co-administration of the M1-preferring receptor antagonist trihexyphenidyl (6.0 mg/kg) with a 5.0 mg/kg dose of NDMC produced partial substitution (>60% to <80% CLZ-appropriate responding) for CLZ, while administration of trihexyphenidyl alone (0.3-12.0 mg/kg) failed to substitute for CLZ. CONCLUSIONS: These findings suggest that NDMC produces discriminative stimulus effects that are different from those elicited by its parent drug CLZ. This difference may be due to the agonist properties of NDMC at M(1) muscarinic cholinergic receptors.

Further characterization of the discriminative stimulus properties of the atypical antipsychotic drug clozapine in C57BL/6 mice: role of 5-HT(2A) serotonergic and alpha (1) adrenergic antagonism.

RATIONALE: The discriminative stimulus properties of the atypical antipsychotic drug (APD) clozapine (CLZ) have recently been studied in C57BL/6 mice, a common background strain for genetic alterations. However, further evaluation is needed to fully characterize CLZ's discriminative cue in this strain of mice. OBJECTIVES: The objectives of the study were to confirm the previous findings using a shorter pretreatment time and to further characterize the receptor mechanisms mediating the discriminative stimulus properties of CLZ by testing APDs, selective ligands, and N-desmethylclozapine (CLZ's major metabolite) in C57BL/6 mice. MATERIALS AND METHODS: C57BL/6 male mice were trained to discriminate 2.5 mg/kg CLZ (s.c.) from vehicle in a two-lever drug discrimination task. RESULTS: Generalization testing with CLZ yielded an ED(50) = 1.19 mg/kg. Substitution testing with APDs showed that the atypical APDs quetiapine, sertindole, zotepine, iloperidone, and melperone fully substituted for CLZ (> or =80% CLZ-appropriate responding), but aripiprazole did not. The typical APDs chlorpromazine and thioridazine substituted for CLZ (fluphenazine and perphenazine did not). The serotonin (5-HT) (2A) antagonist M100907 and the alpha(1)-adrenoceptor antagonist prazosin fully substituted for CLZ. The H(1) histaminergic antagonist pyrilamine, dopamine agonist amphetamine, and the selective serotonin reuptake inhibitor fluoxetine did not substitute for CLZ. While N-desmethylclozapine did not substitute for CLZ when tested alone, N-desmethylclozapine plus a low dose of CLZ combined in an additive manner produced full substitution. CONCLUSIONS: CLZ's discriminative cue in C57BL/6 mice is a "compound" cue mediated in part by antagonism of 5-HT(2A) and alpha(1) receptors.

A comparison of the discriminative stimulus properties of the atypical antipsychotic drug clozapine in DBA/2 and C57BL/6 inbred mice.

Inbred mouse strain comparisons are an important aspect of pharmacogenetic research, especially in strains known to differ in regard to specific neurotransmitter systems. DBA/2 mice differ from C57BL/6 mice in terms of both functional and anatomical characteristics of dopamine systems. Given the importance of D2 antagonism in the action of antipsychotic drugs and in theories regarding schizophrenia (i.e. the dopamine hypothesis), this study compared the discriminative stimulus properties of the atypical antipsychotic drug clozapine (CLZ) in C57BL/6 and DBA/2 inbred mice. DBA/2 and C57BL/6 mice were trained to discriminate 2.5 mg/kg of CLZ from vehicle in a two-lever drug discrimination procedure and tested with a variety of antipsychotic drugs and selective ligands. Both strains of mice readily acquired the CLZ discrimination. The atypical antipsychotic drugs olanzapine and risperidone fully substituted for CLZ in both DBA/2 and C57BL/6 mice, but ziprasidone fully substituted only in the C57BL/6 mice. The typical antipsychotic drug haloperidol produced partial substitution for CLZ in the DBA/2 mice, and the dopamine agonist amphetamine required a higher dose to reduce response rates significantly in DBA/2 mice as compared with C57BL/6 mice. Antagonism of serotonergic (5-HT2A/2B/2C) receptors with ritanserin and alpha1-adrenergic receptors with prazosin engendered CLZ-appropriate responding only in the C57BL/6 mice. Thus, while serotonergic and alpha-adrenergic antagonism were shown to be important for CLZ's discriminative cue in C57BL/6 mice, none of the selective ligands produced CLZ-appropriate responding in DBA/2 mice. Differences in dopamine-mediated functions between the two strains of mice may explain some of the findings in this study.

Motor impairment: a new ethanol withdrawal phenotype in mice.

Alcoholism is a complex disorder with genetic and environmental risk factors. The presence of withdrawal symptoms is one criterion for alcohol dependence. Genetic animal models have followed a reductionist approach by quantifying various effects of ethanol withdrawal separately. Different ethanol withdrawal symptoms may have distinct genetic etiologies, and therefore differentiating distinct neurobiological mechanisms related to separate signs of withdrawal would increase our understanding of various aspects of the complex phenotype. This study establishes motor incoordination as a new phenotype of alcohol withdrawal in mice. Mice were made physically dependent on ethanol by exposure to ethanol vapor for 72 h. The effects of ethanol withdrawal in mice from different genetic backgrounds were measured on the accelerating rotarod, a simple motor task. Ethanol withdrawal disrupted accelerating rotarod behavior in mice. The disruptive effects of withdrawal suggest a performance rather than a learning deficit. Inbred strain comparisons suggest genetic differences in magnitude of this withdrawal phenotype. The withdrawal-induced deficits were not correlated with the selection response difference in handling convulsion severity in selectively bred Withdrawal Seizure-Prone and Withdrawal Seizure-Resistant lines. The accelerating rotarod seems to be a simple behavioral measure of ethanol withdrawal that is suitable for comparing genotypes.

Acute nicotine reduces and repeated nicotine increases spontaneous activity in male and female Lewis rats.

The Lewis (LEW) strain of rat appears more sensitive to nicotine than other strains in self-administration, conditioned place preference, and drug discrimination behavioral studies. The present study sought to further evaluate the behavioral effects of chronic nicotine treatment in the LEW strain by assessing spontaneous activity, which has consistently revealed sensitization to chronic nicotine administration in Sprague Dawley (SD) rats. High active and low active male and female LEW rats (N=8 per group) were treated twice daily with either nicotine (0.4 mg/kg, sc) or vehicle for 14 consecutive days. Regardless of baseline activity level or sex, spontaneous activity was significantly decreased, compared to saline-treated rats, after a single nicotine injection. However, spontaneous activity increased in both low- and high-activity rats (both sexes) over the two weeks of nicotine administration to levels that were significantly higher than saline-treated rats. Based on these findings, acute and chronic nicotine administration had greater suppressive and enhancing effects on spontaneous activity in LEW rats compared to other strains of rats previously studied. These results further clarify the behavioral sensitivity of the LEW strain of rat to nicotine exposure and lend credence to the role of genetics in the individual susceptibility to nicotine dependence.

The effects of acute and repeated nicotine doses on spontaneous activity in male and female Sprague Dawley rats: analysis of brain area epibatidine binding and cotinine levels.

Previous research in this laboratory has shown that nicotine's effects on spontaneous activity are contingent on individual differences, attenuating activity in high active rats and increasing it in low active rats. This study was designed to further evaluate this phenomenon, and to compare it with nicotine's effects on nicotinic acetylcholine receptor (nAChR) expression in several brain regions. Male and female Sprague-Dawley rats selected for differences in baseline activity were administered nicotine twice daily for 14 days, and its effects on spontaneous activity were evaluated following 1, 13 and 27 doses. Furthermore, [(3)H] epibatidine binding and plasma cotinine levels were evaluated 24 h after the 28th dose. Contrary to previous findings, the effects of repeated nicotine on spontaneous activity were minimally contingent on baseline activity levels. Following an initial attenuation, males, but not females, exhibited sensitization to nicotine's effects on spontaneous activity. [(3)H] epibatidine was significantly increased in several brain regions in both male and female nicotine-treated animals, and in females selected for high activity at baseline. However, a clear relationship between these effects and spontaneous activity was not found, due to the lack of consistent effects of nicotine administration and baseline activity on spontaneous activity. Interestingly, significant correlations suggest that rats exhibiting higher spontaneous activity on the final test day were differentially marked by higher [(3)H] epibatidine. Cotinine levels were higher in low activity males than in high activity males, but no differences were observed between high and low activity females. Thus, no clear relationship between this variable and spontaneous activity could be discerned. Based on these data, no simple relationships between the effects of nicotine administration or baseline activity on [(3)H] epibatidine binding, nicotine metabolism, or spontaneous activity were observed. However, a relationship between [(3)H] epibatidine and spontaneous activity on the final test day is suggested.

Neurochemical and behavioral effects of bupropion and mecamylamine in the presence of nicotine.

The primary mechanism of action of bupropion, a smoking cessation drug, is commonly believed to involve the dopaminergic system although evidence exists that bupropion also has effects at nicotinic acetylcholine receptors (nAChRs). This study evaluated the disruptive effects of nicotine on response rates in the presence of bupropion and the nAChR antagonist, mecamylamine, as well as the ability of these drugs to alter nicotine-stimulated nAChR function in various brain areas. Rats were trained to respond on a single lever under a variable interval 15 (VI15) schedule for food reinforcement. Initially, dose effect curves were generated for nicotine, bupropion and mecamylamine. Upon determining the dose of nicotine (1.2 mg/kg) effective in completely disrupting rates of responding, it was established that both mecamylamine and bupropion block nicotine's rate-reducing effects. This result suggests that bupropion shares behavioral effects with mecamylamine when administered in the presence of nicotine. To explore this relationship further, the effect of in vivo administration of bupropion or mecamylamine on nicotine-stimulated (86)Rb(+) efflux was studied in synaptosomes prepared from the frontal cortex, hippocampus, striatum and thalamus. Nicotine-stimulated (86)Rb(+) efflux from all brain regions was significantly reduced in rats administered 3.0 mg/kg mecamylamine (s.c.) 15 min prior to dissection compared to control rats. In contrast, a significant increase in nicotine-stimulated (86)Rb(+) efflux was observed in all brain regions from rats administered 30.0 mg/kg bupropion (s.c.) 15 min prior to dissection compared to control rats. Taken together these results demonstrate that when administered in the presence of nicotine, bupropion elicits unique pharmacological differences such that it exhibits both nAChR agonist- and antagonistic-like effects.

Discriminative stimulus properties of the atypical antipsychotic drug clozapine in rats trained to discriminate 1.25 mg/kg clozapine vs. 5.0 mg/kg clozapine vs. vehicle.

Clozapine, the prototype for atypical antipsychotic drugs, is used in the drug discrimination paradigm as a model for screening atypical from typical antipsychotic drugs. Previous drug discrimination studies in rats have shown that a 1.25 mg/kg clozapine training dose provides full stimulus generalization (i.e.) >or=80% condition-appropriate responding) to most atypical antipsychotic drugs, although a 5.0 mg/kg clozapine training dose appears necessary to provide stimulus generalization to other atypical antipsychotic drugs. The present study sought to characterize the pharmacological mechanisms that mediate these clozapine training doses. In rats trained to discriminate 1.25 vs. 5.0 mg/kg clozapine vs. vehicle in a three-choice drug discrimination task, various receptor-selective compounds were tested for stimulus generalization. The antidepressant mianserin was also tested. Full stimulus generalization from the 1.25 mg/kg clozapine training dose occurred only to mianserin (98.8%). Partial substitution (i.e. >or=60% and <80% condition-appropriate responding) to the 5.0 mg/kg clozapine training dose occurred for the muscarinic receptor antagonist scopolamine. The combined total percentage of responding on the 1.25 and 5.0 mg/kg clozapine levers, however, was well above the full substitution criteria at the 0.25, 0.5, and 1.0 mg/kg scopolamine doses. The M1 agonist N-desmethylclozapine, the nicotinic antagonist mecamylamine, the D1 antagonist SCH 23390, the D4 antagonist LU 38-012, the 5-HT1A agonist (+)-8-OH-DPAT, the 5-HT1A antagonist WAY 100 635, the 5-HT2A/2B/2C antagonist ritanserin, the 5-HT6 antagonist RO4368554, the alpha1 antagonist prazosin, the alpha2 antagonist yohimbine, and the histamine H1 antagonist pyrilamine all failed to substitute for either the 1.25 or the 5.0 mg/kg clozapine training doses. These results are consistent with previous evidence that antidepressant drugs have a tendency to substitute for clozapine and that muscarinic receptor antagonism may mediate the discriminative stimulus properties of 5.0 mg/kg clozapine. The lack of stimulus generalization from either clozapine training dose to other receptor-selective compounds, however, fails to explain how this model screens atypical from typical antipsychotic drugs and suggests that the discriminative stimulus properties of clozapine consist of a compound cue.

Evidence of cellular nicotinic receptor desensitization in rats exhibiting nicotine-induced acute tolerance.

RATIONALE: Individuals vary in their susceptibility to nicotine addiction. However, there is little evidence that behavioral sensitivity to nicotine is dependent upon the functional state of nicotinic cholinergic receptors (nAChRs). OBJECTIVE: To determine the relationship between in vivo pharmacological desensitization (in other words, acute tolerance) and brain regional nAChR function. METHODS: Male Sprague-Dawley rats, trained to discriminate nicotine (0.4 mg/kg free base) from saline in a two-lever drug discrimination task, were tested for the development of acute tolerance. Rats were injected with 0.4 mg/kg nicotine, tested for nicotine discrimination for 2 min, then injected with the same dose of nicotine 90 min, 180 min, and 270 min after the first injection and tested for nicotine discrimination after each injection. These subjects were separated into two groups, desensitizers (DZ) and nondesensitizers (NDZ), based upon performance in the repetitive dosing drug discrimination paradigm. The sensitivity of nAChRs in specific brain regions of these two groups was assessed by the use of an 86Rb+ efflux assay using synaptosomes prepared from the frontal cortex, hippocampus, striatum, and "thalamus," which included the midbrain and hypothalamus as well as the thalamus. RESULTS: The nicotine-induced increase in 86Rb+ efflux was significantly greater in NDZ as compared to DZ in the "thalamus." There was no statistically significant difference in the effects of nicotine in the frontal cortex, hippocampus, and striatum of these two groups. A significant correlation was observed between thalamic 86Rb+ efflux and the rate of behavioral desensitization of individual rats. CONCLUSION: These findings are consistent with the concept that the production of acute tolerance by nicotine in vivo correlates directly with its ability to induce nAChR desensitization at the cellular level.

Serotonin receptor mechanisms mediate the discriminative stimulus properties of the atypical antipsychotic clozapine in C57BL/6 mice.

RATIONALE: The atypical antipsychotic drug (APD) clozapine (CLZ) has been shown to have a robust discriminative cue in rats, pigeons, and monkeys in two-choice drug discrimination procedures. OBJECTIVES: The present study determined whether a two-choice drug discrimination procedure with CLZ could be established in C57BL/6 mice and whether this procedure could distinguish between atypical and typical APDs. METHODS: C57BL/6 male mice were trained to discriminate 2.5 mg/kg CLZ from vehicle in a two-lever drug discrimination procedure. RESULTS: Generalization testing with CLZ produced full substitution at the 2.5- and 5.0-mg/kg doses with an ED50 of 1.14 mg/kg. The atypical APDs olanzapine (ED50=0.24 mg/kg), risperidone (ED50=0.072 mg/kg), and ziprasidone (ED50=0.33 mg/kg) fully substituted for CLZ's discriminative cue, while the typical APD haloperidol failed to substitute for CLZ. Generalization testing with selective ligands showed that the serotonin (5-HT)2A/2B/2C antagonist ritanserin fully substituted for CLZ (ED50=2.08 mg/kg) and that the 5-HT receptor agonist quipazine significantly attenuated CLZ's discriminative cue without disrupting response rates. The muscarinic receptor antagonist scopolamine, the dopamine agonist amphetamine, and the 5-HT agonist quipazine failed to substitute for CLZ. CONCLUSIONS: These results demonstrated that antagonism of 5-HT receptors plays an important role in mediating the discriminative stimulus properties of the atypical APD CLZ in C57BL/6 mice. The atypical APDs olanzapine, risperidone, and ziprasidone fully substituted for CLZ, while the typical APD haloperidol did not. These results suggest that CLZ drug discrimination in C57BL/6 mice may be an effective preclinical behavioral assay for screening atypical from typical antipsychotic drugs.

Differential behavioral responses to nicotine in Lewis and Fischer-344 rats.

Individual and strain variability in the effects of nicotine suggests the involvement of a genetic component in nicotinic cholinergic receptor (nAChR) function, which may help explain nicotine's variable behavioral and pharmacological effects in different individuals. The present study evaluated differential responses to the discriminative stimulus (DS) and rewarding properties of nicotine in Lewis (LEW) and Fischer-344 (F-344) rats. Drug discrimination (DD) data suggest that the LEW rat is more sensitive to nicotine as LEW rats acquired the nicotine discrimination at a dose of 0.4 mg/kg, whereas F-344 rats acquired the dose of 0.9 mg/kg (all nicotine doses expressed as free base). Similarly, LEW rats exhibited nicotine-conditioned place preference (CPP) at 0.6 mg/kg, whereas the F-344 rats did not. Subsequent testing with a higher dose (0.9 mg/kg) failed to maintain the nicotine-CPP in the LEW rats. Conversely, nicotine-place preference in the F-344 rats was not changed at the higher dose. Taken together, these results suggest potential differences of sensitivities in LEW and F-344 rats to the rewarding and discriminative stimulus (DS) properties of nicotine. These findings support previous research by demonstrating that the F-344 rat is less sensitive to nicotine compared to the LEW rat.