Reward In Mice Research Articles

Protein receptor for activated C kinase 1 is involved in morphine reward in mice

Opiate addiction is associated with upregulation of cAMP signaling in the brain. cAMP-responsive element binding protein (CREB), a nuclear transcription factor, is a downstream component of the extracellular signal-regulated protein kinase (ERK) pathway, which has been shown to regulate different physiological and psychological responses of drug addiction. RACK1, the protein receptor for activated C kinase 1, is a multifunctional scaffolding protein known to be a key regulator of various signaling cascades in the CNS. RACK1 functions specifically in integrin mediated activation of ERK cascade and targets active ERK. We examined if RACK1 is involved in the mechanism of drug addiction by regulating CREB in mouse hippocampus and prefrontal cortex. Several expressions were observed. Chronic administration of morphine made the expression of RACK1 and CREB mRNA increase in hippocampus and prefrontal cortex. The expression of RACK1 and CREB protein was strongly positive in CA1, CA3 and dentate gyrus (DG) of the hippocampus of morphine-treated mice brain, especially the pyramidal neurons in the DG of the hippocampus. Using the small interfering RNA technology, we determined that the expression of CREB mRNA was decreased in hippocampus and prefrontal cortex of morphine-treated mice. The expression of RACK1 and CREB protein was negative in CA1, CA3 and DG of hippocampus. These findings suggest that morphine reward can influence the expression of RACK1 in mouse hippocampus and prefrontal cortex through regulating CREB transcription.

Cocaine does not produce reward in absence of dopamine transporter inhibition

We have previously reported that knockin mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) do not experience cocaine reward, as measured by conditioned place preference. This conclusion has come under scrutiny because some genetically modified mice show cocaine-induced conditioned place preference in a narrow dose range, that is, responding at doses around 10 mg/kg, but not at 5 and 20 mg/kg, the doses we tested in DAT-CI mice. These results raise the possibility that we have missed the optimal dose for cocaine response. Here we report that cocaine does not produce reward in DAT-CI mice at low, moderate, and high doses, including 10 mg/kg. This study strengthens our conclusion that DAT inhibition is required for cocaine reward in mice with a functional dopaminergic system.

Melatonin reverses the expression of morphine-induced conditioned place preference through its receptors within central nervous system in mice

A growing body of evidence indicates the prominent actions of melatonin on the opioidergic system. Nevertheless, effect of melatonin on rewarding properties of morphine is still obscure. In particular, effect of melatonin on the expression of morphine reward is unknown. We evaluated the effect of exogenous administration of melatonin on the expression of morphine reward in mice using a conditioned place preference (CPP) paradigm. The conditioned place preference was induced by morphine (s.c., 3 mg/kg, once each day for 5 consecutive days) in mice. Our data showed that the intraperitoneal (i.p.) administration of melatonin (12.5–50 mg/kg) reversed the expression of morphine-induced conditioned place preference in a dose-dependent manner. Furthermore, the intracerebroventricular (i.c.v.) administration of melatonin (0.125–0.5 mg/kg) also resulted in dose-dependent reversal effect on the expression of morphine-induced conditioned place preference. We further investigated which of melatonin receptor subtypes within the central nervous system was mediating this reversal action in mice using luzindole (2-benzyl- N-acetyltryptamine, a non-selective antagonist for melatonin MT 1 and MT 2 receptors) and K185 { N-butanoyl-2-(5,6,7-trihydro-11-methoxybenzo[3,4]cyclohept[2,1-α]indol-13-yl)ethanamine, a selective antagonist for melatonin MT 2 receptor}. It was shown that the i.c.v. administration of either K185 (5, 20 μg) or luzindole (6.25, 12.5 μg) significantly antagonized the reversal effect of melatonin (50 mg/kg, i.p) on the expression of morphine-induced conditioned place preference, while the i.c.v. administration of 20 μg of K185 or 12.5 μg of luzindole by itself did not alter the expression of morphine-induced conditioned place preference. These results suggest that melatonin reverses the expression of morphine-induced rewarding effect, and this action is mediated by the activation of melatonin MT 2 receptor subtype within the central nervous system.

Increased amphetamine-induced hyperactivity and reward in mice overexpressing the dopamine transporter

The dopamine transporter (DAT) plays a key role in the regulation of dopaminergic signaling wherein it controls both the spatial and temporal actions of dopamine. Here we evaluated the behavioral and neurochemical consequences of increased DAT function by generating DAT transgenic mice (DAT-tg) that overexpress the transporter. These mice were generated by pronuclear injection of a bacterial artificial chromosome containing the mouse DAT locus, yielding an anatomical expression pattern of DAT-tg identical to WT. In DAT-tg mice there is a 3-fold increase in the levels of total and membrane-expressed DAT, but synaptic plasma membrane fractions of DAT-tg mice show only a 30% increase in transporter levels. Functional studies reveal that in the DAT-tg animals there is a 50% increase in the rate of dopamine (DA) uptake resulting in extracellular levels of DA that are decreased by approximately 40%. Behaviorally, DAT-tg animals display similar locomotor stimulation when treated with DAT blockers such as GBR12909, methylphenidate, and cocaine. However, these mice demonstrate markedly increased locomotor responses to amphetamine compared with WT animals. Furthermore, compared with controls, there is a 3-fold greater increase in the amount of DA released by amphetamine in DAT-tg mice that correlates with the 3-fold increase in protein expression. Finally, DAT-tg animals show reduced operant responding for natural reward while displaying preference for amphetamine at much lower doses (0.2 and 0.5 mg/kg) than WT mice (2 mg/kg). These results suggest that overexpression of DAT leads to a marked increase in sensitivity to psychomotor and rewarding properties of amphetamine.

Increased amphetamine-induced hyperactivity and reward in mice overexpressing the dopamine transporter

Increased amphetamine-induced hyperactivity and reward in mice overexpressing the dopamine transporter

Melatonin enhances the rewarding properties of morphine: involvement of the nitric oxidergic pathway

Melatonin has different interactions with opioids including the enhancement of the analgesic effects of morphine and also reversal of tolerance and dependence to morphine. The present study assessed the effect of melatonin on morphine reward in mice using a conditioned place preference (CPP) paradigm. Our data showed that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal (i.p.) administration of melatonin (1-40 mg/kg) alone did not induce either CPP or conditioned place aversion (CPA), while the combination of melatonin (5-20 mg/kg) and sub-effective dose of morphine (0.5 mg/kg) led to rewarding effect. We further investigated the involvement of the nitric oxidergic pathway in the enhancing effect of melatonin on morphine CPP, by a general nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME). L-NAME (1 and 5 mg/kg, i.p.) alone or in combination with morphine (0.5 mg/kg) did not show any significant CPP or CPA. Co-administration of L-NAME (5 mg/kg) with an ineffective combination of melatonin (1 mg/kg) plus morphine (0.5 mg/kg) produced significant CPP that may imply the similarity of action of melatonin and L-NAME and involvement of the nitric oxidergic pathway in this regard. Our results indicate that pretreatment of animals with melatonin enhances the rewarding properties of morphine via a mechanism which may involve the nitric oxidergic pathway.

Effects of enriched environment on morphine-induced reward in mice

Drug addiction and abuse have been extremely serious problems in our society. The effect of different environmental conditions on the susceptibility of human to drug addiction and abuse is still not fully understood. It was recently reported that enriched environment can trigger long-term modification in neural functions and might prevent the occurrence of pathogenic behaviors. Here we investigated the effects of enriched environment on morphine-induced reward in mice. We found that the enriched environment attenuated the acute morphine (10 mg/kg)-induced hyperlocomotion and repeated morphine (10 mg/kg)-induced behavioral sensitization. Moreover, the environmental condition blocked the conditioned place preference (CPP) of the mice induced by morphine (5 mg/kg). Associated with behavioral alterations, the expression of FosB-like proteins in the nucleus accumbens of the brain was down-regulated in the mice housed in the enriched environmental condition, but not in the standard environment. Taken together, these results indicate that enriched environmental condition leads to decrease in sensitivity of the mice to morphine-induced reward.

An antisense oligodeoxynucleotide to the mu opioid receptor attenuates cocaine-induced behavioral sensitization and reward in mice

Numerous studies support a role for the endogenous opioid system in cocaine-influenced behavior. Few of these studies, however, selectively delineate a role for the mu opioid receptor (MOR) in this regard. This investigation examined if the MOR modulates cocaine-induced behavior in mice using a 17-base antisense oligodeoxynucleotide (AS ODN) directed against the MOR coding sequence 16–32. Specifically, cocaine-induced behavioral sensitization and conditioned reward were investigated. For the sensitization study, C57BL/6J mice received eight intermittent i.c.v. infusions of saline, mismatch oligodeoxynucleotide (ODN) (20 μg/4 μl) or AS ODN (20 μg/4 μl) over 20 days. Mice also received concomitant once daily i.p. injections of saline (4 ml/kg) or cocaine (15 mg/kg) for 10 days. There was a 7-day withdrawal period, after which all mice were challenged with cocaine (15 mg/kg) to test for behavioral sensitization. For the conditioned place preference (CPP) study, mice received five i.c.v. infusions of mismatch ODN or MOR AS ODN (days 1–5). An unbiased counterbalanced conditioning procedure was used where mice were conditioned with saline (4 ml/kg, i.p.) and cocaine (15 mg/kg, i.p.) on alternate days for four sessions (days 3–6). Mice were tested on day 7 for CPP. Immediately following testing, [ 3H]DAMGO ( d-Ala 2, N-Me-Phe 4, Gly-ol 5-enkephalin) receptor binding to brain homogenates was conducted. MOR AS attenuated cocaine-induced behavioral sensitization and conditioned reward. MOR AS ODN also reduced [ 3H]DAMGO binding. Collectively, these findings implicate the MOR as playing an important neuromodulatory role in the behavioral effects of cocaine in mice.

Cocaine: It's All About the Dopamine

Cocaine has complex effects on neurophysiology and behavior, in part because it has at least three major targets, the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepiniphrine transporter (NET). These transporters remove neurotransmitters that are released by neurons into the extracellular space at synapses. Thus, inhibition of the transporters prolongs neuronal stimulation. There is much evidence that dopaminergic signals are important for the "reward" effect on behavior that contributes to the addictive properties of cocaine. However, DAT-knockout mice still respond to cocaine and will administer it to themselves if the drug is made available. Although this argues against the DAT as a primary target for the addictive properties of cocaine, adaptive changes in knockout animals are known to mask important physiological mechanisms. Chen et al. therefore created mice in which the DAT was replaced with a mutant that had relatively normal dopamine transport function but required nearly 100 times greater concentration of cocaine to be inhibited than did the normal transporter. The reward effect of cocaine was measured in a behavioral test in which mice are placed in a box with three easily recognized, distinct chambers. Animals experiencing a reward effect will spend more time in the chamber that they associate with the effects of a drug, in this case cocaine. (The association is formed earlier by keeping the animal in a particular chamber after the drug is administered.) In the mutant animals, cocaine suppressed locomotion (which it normally stimulates) and did not produce a reward effect in the behavioral test. Thus, the authors conclude that, in the presence of functional DAT, the reward effect of cocaine requires its inhibition of DAT. This confirms the DAT as the primary target of efforts to understand addiction and to develop therapeutic therapies to treat addicted individuals. R. Chen, M. R. Tilley, H. Wei, F. Zhou, F. M. Zhou, S. Ching, N. Quan, R. L. Stephens, E. R. Hill, T. Nottoli, D. D. Han, H. H. Gu, Abolished cocaine reward in mice with a cocaine-insensitive dopamine transporter. Proc. Natl. Acad. Sci. U.S.A. 103 , 9333-9338 (2006). [Abstract] [Full Text]

Abolished cocaine reward in mice with a cocaine-insensitive dopamine transporter

There are three known high-affinity targets for cocaine: the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepinephrine transporter (NET). Decades of studies support the dopamine (DA) hypothesis that the blockade of DAT and the subsequent increase in extracellular DA primarily mediate cocaine reward and reinforcement. Contrary to expectations, DAT knockout (DAT-KO) mice and SERT or NET knockout mice still self-administer cocaine and/or display conditioned place preference (CPP) to cocaine, which led to the reevaluation of the DA hypothesis and the proposal of redundant reward pathways. To study the role of DAT in cocaine reward, we have generated a knockin mouse line carrying a functional DAT that is insensitive to cocaine. In these mice, cocaine suppressed locomotor activity, did not elevate extracellular DA in the nucleus accumbens, and did not produce reward as measured by CPP. This result suggests that blockade of DAT is necessary for cocaine reward in mice with a functional DAT. This mouse model is unique in that it is specifically designed to differentiate the role of DAT from the roles of NET and SERT in cocaine-induced biochemical and behavioral effects.

Methamphetamine Reward in Mice as Assessed by Conditioned Place Preference Test with Supermex® Sensors: Effect of Subchronic Clorgyline Pretreatment

Recent studies in our laboratory have shown that methamphetamine (METH)-induced hyperlocomotion and behavioral sensitization in mice were inhibited by clorgyline, an irreversible monoamine oxidase inhibitor. In this study, the effect of clorgyline pretreatment on METH-induced rewarding effect was assessed by a conditioned place preference (CPP) test, using an apparatus developed with Supermex sensors (infrared pyroelectric sensors). Although intact male ICR mice showed significant CPP for METH (0.5 mg/kg, i.p.), pretreatment with subchronic clorgyline (0.1 and 10 mg/kg, s.c.) did not affect the magnitude of CPP. At a dose of 1 mg/kg, pretreatment of the mice with clorgyline showed a similar CPP index in both saline/saline and METH/saline pairing groups. During the conditioning session, the mice did not express behavioral sensitization to METH. Pretreatment with clorgyline (0.1, 1, and 10 mg/kg) decreased striatal apparent monoamine turnover in a dose-dependent manner. These results indicated that clorgyline pretreatment (0.1 and 10 mg/kg) did not influence the METH-induced rewarding effect in mice, although pretreatment of the mice with clorgyline at a dose of 1 mg/kg appeared to influence the CPP for METH.

Adolescent and adult responsiveness to the incentive value of cocaine reward in mice: Role of neuronal nitric oxide synthase (nNOS) gene

A major concern in adolescent psychostimulant abuse is the long-term consequence of this practice, because early drug exposure may cause long-term adaptations, which render the organism more susceptible to drug abuse later in life. The incentive value of drug and natural reward in rodents is commonly assessed by the conditioned place preference (CPP) paradigm, which involves Pavlovian learning. The aims of the present study were to investigate: a) the acquisition, expression, maintenance and reinstatement of cocaine CPP from periadolescence (PD24–45) through adulthood (PD70); b) potential sexual dimorphism in adolescence and adulthood in response to cocaine-induced CPP; and c) the role of the neuronal nitric oxide synthase (nNOS) gene in long-term neural plasticity underlying responsiveness to cocaine and cocaine-associated cues. Adolescent wild type (WT) mice acquired significant cocaine (20 mg/kg) CPP that was maintained from PD24 through PD43. Upon extinction, CPP was reinstated in adulthood (PD70) following a priming injection of cocaine (5 mg/kg). In contrast, cocaine CPP acquired between PD26 and PD31 in adolescent nNOS knockout (KO) mice, was neither maintained nor reinstated by cocaine. There was no sexual dimorphism in adolescent WT and KO mice. Genotype differences and sexual dimorphism were observed in adult mice. Cocaine CPP in adult WT males (PD89–94) was maintained for 4 weeks post training, and subsequently reinstated by cocaine priming; the magnitude of CPP in adult WT males was lower than in female counterparts. CPP in adult KO males (PD88–93) was neither maintained nor reinstated by cocaine priming; in contrast, CPP in adult KO females was not significantly different from adult WT females. Results suggest that the nNOS gene is essential during adolescence of both sexes for the development of long-term neural plasticity underlying responsiveness to the incentive value of cocaine reward. Sexual dimorphism in response to cocaine CPP emerges in adulthood; nNOS contribution to long-term plasticity is therefore sexually dimorphic and age-dependent in female but not in male subjects.

Deletion of dopamine D1 and D3 receptors differentially affects spontaneous behaviour and cocaine‐induced locomotor activity, reward and CREB phosphorylation

Co-localization of dopamine D1 and D3 receptors in striatal neurons suggests that these two receptors interact at a cellular level in mediating dopaminergic function including psychostimulant-induced behaviour. To study D1 and D3 receptor interactions in cocaine-mediated effects, cocaine-induced locomotion and reward in mice lacking either D1, D3 or both receptors were analysed. Spontaneous locomotor activity was increased in D1-/- and D1-/-D3-/- mice and D1-/-D3-/- mice did not exhibit habituation of spontaneous rearing activity. Cocaine (20 mg/kg) increased locomotor activity in wild-type and D3-/- mice, failed to stimulate activity in D1-/- mice and reduced activity in D1-/-D3-/- mice. In the conditioned place preference, all groups exhibited reward at 5, 10 and 20 mg/kg of cocaine. D1-/-D3-/- mice did not demonstrate preference at 2.5 mg/kg of cocaine although preference was observed in wild-type, D1-/- and D3-/- mice. The transcription factor cAMP-responsive element binding protein (CREB) is activated by phosphorylation in striatal regions following dopamine receptor activation. Striatal pCREB levels following acute cocaine were increased in wild-type and D3-/- mice and decreased in D1-/- and D1-/-D3-/- mice. After repeated administration of 2.5 mg/kg of cocaine, D1-/- mice had lower pCREB levels in caudate-putamen and nucleus accumbens. Our findings suggest that, although spontaneous and cocaine-induced horizontal activity depended mainly on the presence of the D1 receptor, there may be crosstalk between D1 and D3 receptors in rearing habituation and the perception of cocaine reward at low doses of the drug. Furthermore, alterations in pCREB levels were associated with changes in cocaine-induced locomotor activity but not reward.

Involvement of the pineal gland in diurnal cocaine reward in mice

Contribution of circadian mechanisms to the psychostimulant-induced behaviors has been suggested. The pineal gland is important component of circadian mechanisms. Using pinealectomized mice and sham-operated controls, we tested the contribution of pineal gland to the rewarding effects of cocaine in conditioned place preference test. Experiments were performed both during the day and at night. Controls with intact pineal glands demonstrated significant decrease in cocaine-induced conditioned place preference at night compared to daytime, whereas pinealectomized mice did not show any diurnal differences. Circadian mechanisms regulated by the pineal gland thus appear critically involved in cocaine-induced reward.

Morphine conditioned reward is inhibited by MPEP, the mGluR5 antagonist

In the present study we examined the effect of MPEP [2-methyl-6-(phenylethynyl)-pyridine] a potent, selective and systemically active metabotropic glutamate receptor (mGluR) type I (subtype mGluR5) antagonist on conditioned morphine reward in mice. In an unbiased version of conditioned place preference (CPP) paradigm, single conditioning with 10 mg/kg of morphine produced reliable place preference. MPEP at 30, but not 10 mg/kg significantly inhibited the acquisition as well as expression of morphine-induced CPP, but it neither produced place preference or aversion, nor affected locomotor activity of mice. Effects of MPEP on learning and memory were studied in the elevated plus maze model of spatial learning. In contrast to 0.1 mg/kg of MK-801, which inhibited the acquisition of this task, 30 mg/kg of MPEP affected neither learning nor memory retrieval. These data suggest that mGluR5 may be involved in conditioned morphine reward.

Unconditioned and conditioned factors contribute to the ‘reinstatement’ of cocaine place conditioning following extinction in C57BL/6 mice

Relapse to drug use following prolonged periods of abstinence results, in part, from the ability of contextual cues paired previously with self-administered drug to elicit drug craving and -seeking behavior. Given the popularity of the mouse for the genetic analysis of drug-induced behaviors, a place conditioning model of drug-seeking behavior was used to examine the ability of cocaine (COC) to reinstate extinguished conditioned reward in mice. In a series of experiments, COC place conditioning was produced in male C57BL/6 (B6) mice by four pairings of COC (15 or 25 mg/kg, IP) with the non-preferred compartment of a two-compartment place conditioning apparatus. Following a post-conditioning test (Post-Test), place conditioning was extinguished by repeated testing. The mice were then challenged with one of five COC doses (0, 5, 10, 15 or 25 mg/kg, IP) and allowed free access to both environments. Following extinction, COC injections reinstated place conditioning to 100% or greater, relative to the Post-Test. In a control experiment, mice received either COC or SAL paired with non-preferred compartment and were then challenged with either COC (15 mg/kg, IP) or SAL on the Post-Test. COC-conditioned, but not SAL-conditioned, mice exhibited place conditioning when tested in a COC-free state. Interestingly, COC injection on the Post-Test elicited an increase in approach behavior in both SAL- and COC-conditioned mice and this increase was equivalent to that produced by COC conditioning alone. No direct relationships were observed between the magnitude of place conditioning and either COC-induced or -conditioned locomotor hyperactivity in the non-preferred compartment. Thus, at least two independent processes appear to underlie the ability of a COC injection to elicit approach behavior towards the non-preferred compartment of a biased place conditioning apparatus in mice—reactivation of the conditioned incentive motivational properties of COC-paired cues and elicitation of unconditioned behavioral disinhibition. One or both of these processes sensitizes with the passage of time, increasing the propensity of B6 mice to approach non-preferred environments upon COC re-administration.

Ethanol-conditioned place preference is reduced in dopamine D2 receptor-deficient mice

Pharmacological blockade studies have supported a role of the dopamine system in ethanol reward for many years, but receptor subtype specificity has been difficult to establish. Recently, genetically engineered mice lacking functional dopamine D2 receptors have been shown to drink less ethanol in a two-bottle choice task. To determine whether reduced ethanol intake reflects a reduction in ethanol reward, D2 receptor-deficient [knockout (KO)] mice were compared to heterozygous (HET) and wild-type (WT; C57BL/6×DBA/2 F2 hybrid) mice in a place conditioning task. Under conditions that produced reliable place preference in both WT and HET mice, KO mice showed no evidence of place conditioning, suggesting that D2 receptor gene inactivation reduced ethanol reward or the ability to learn about ethanol reward. Consistent with previous findings, this mutation also produced a gene dose-related reduction in basal activity levels. Moreover, KO and HET mice showed enhancement of ethanol-stimulated activity relative to WT mice. However, differences in basal and ethanol-stimulated activity did not explain the differences in place conditioning. Overall, this study strongly supports the conclusion that dopamine D2 receptors normally influence ethanol reward in mice.

Ethanol Reinforcement in Nondeprived Mice: Effects of Abstinence and Naltrexone

Background: Operant experiments which indicate that ethanol can serve as a reinforcer to maintain lever responding during limited periods of access have not been conducted on non-food-deprived mice, as they have for rats and monkeys. Furthermore, there are no reports of the effects of chronic ethanol and subsequent abstinence on ethanol reward in mice. Finally, although naltrexone reduces responding for ethanol in food-deprived mice, the effects of the drug on ethanol reward for non-food-deprived mice have not been reported. Methods: In three experiments, lever responding for ethanol (10–12%) was established in C57BL/6 (B6) mice by using either sucrose or saccharin fading procedures commonly used for rats. Experiment 1 examined both appetitive and consummatory responses while sucrose was faded from the ethanol solutions. Experiment 2 examined lever responding and ethanol intake (1) during saccharin fading; (2) when reinforcement schedules, reward availability, and primary conditioned reinforcers were manipulated; and (3) when mice were allowed chronic ethanol consumption followed by forced abstinence. Experiment 3 examined the effects of low doses of naltrexone on ethanol reward. Results: Lever responding for ethanol can be established in non-food-deprived mice with the sucrose and saccharin fading procedures commonly used for rats. Lever responses increased with decreases in the reinforcer and increases in schedule demand, which indicated the reward value of the ethanol solution. Removal of ethanol from the solution reduced consumption with no change in the appetitive, instrumental response, which indicated that the two responses were under control of different stimuli, perhaps mediated by different neural mechanisms. Forced abstinence after chronic ethanol exposure increased responding for the drug, which suggested increased reward value. Naltrexone reduced responding as previously reported for food-deprived B6 mice. Conclusions: Ethanol appears to serve as a reinforcer for non-food-deprived or non-water-deprived B6 mice. Its reinforcing effects are increased by forced abstinence after chronic exposure and are decreased by naltrexone.

Ethanol Reinforcement in Nondeprived Mice: Effects of Abstinence and Naltrexone

Operant experiments which indicate that ethanol can serve as a reinforcer to maintain lever responding during limited periods of access have not been conducted on non-food-deprived mice, as they have for rats and monkeys. Furthermore, there are no reports of the effects of chronic ethanol and subsequent abstinence on ethanol reward in mice. Finally, although naltrexone reduces responding for ethanol in food-deprived mice, the effects of the drug on ethanol reward for non-food-deprived mice have not been reported. In three experiments, lever responding for ethanol (10-12%) was established in C57BL/6 (B6) mice by using either sucrose or saccharin fading procedures commonly used for rats. Experiment 1 examined both appetitive and consummatory responses while sucrose was faded from the ethanol solutions. Experiment 2 examined lever responding and ethanol intake (1) during saccharin fading; (2) when reinforcement schedules, reward availability, and primary conditioned reinforcers were manipulated; and (3) when mice were allowed chronic ethanol consumption followed by forced abstinence. Experiment 3 examined the effects of low doses of naltrexone on ethanol reward. Lever responding for ethanol can be established in non-food-deprived mice with the sucrose and saccharin fading procedures commonly used for rats. Lever responses increased with decreases in the reinforcer and increases in schedule demand, which indicated the reward value of the ethanol solution. Removal of ethanol from the solution reduced consumption with no change in the appetitive, instrumental response, which indicated that the two responses were under control of different stimuli, perhaps mediated by different neural mechanisms. Forced abstinence after chronic ethanol exposure increased responding for the drug, which suggested increased reward value. Naltrexone reduced responding as previously reported for food-deprived B6 mice. Ethanol appears to serve as a reinforcer for non-food-deprived or non-water-deprived B6 mice. Its reinforcing effects are increased by forced abstinence after chronic exposure and are decreased by naltrexone.