Nicotine withdrawal disrupts new contextual learning

Interactions between nicotine and learning could contribute to nicotine addiction. Although previous research indicates that nicotine withdrawal disrupts contextual learning, the effects of nicotine withdrawal on contextual memories acquired before withdrawal are unknown. The present study investigated whether nicotine withdrawal disrupted recall of prior contextual memories by examining the effects of nicotine withdrawal on recall of nicotine conditioned place preference (CPP) and contextual fear conditioning. C57BL/6J mice trained in CPP exhibited a significant preference for an initially non-preferred chamber that was paired with 0.35 mg/kg nicotine. Following CPP, mice were implanted with mini-osmotic pumps containing 6.3 mg/kg/d nicotine or saline. Pumps were removed twelve days later and nicotine CPP was retested 24 h later. Mice withdrawn from chronic nicotine exhibited CPP, suggesting that older drug-context associations are not disrupted by nicotine withdrawal. One hour later, the same mice were trained in contextual and cued fear conditioning; nicotine withdrawal disrupted contextual but not cued fear conditioning. A subsequent experiment demonstrated that nicotine withdrawal did not disrupt recall of contextual or cued fear conditioning when acquisition occurred before nicotine withdrawal. These data suggest that nicotine withdrawal disrupts new contextual learning, but does not alter contextual learning that occurred before withdrawal.     

Effects of acute alcohol administration on object recognition learning in C57BL/6J mice.

The goal of the present study was to investigate effects of alcohol intoxication on the object recognition learning task. Male C57BL/6J mice habituated to saline injections and exploratory arena received different doses of ethanol (0, 1.6 or 2.4 g/kg) before or after a 10-min training session. During training, animals were exposed to a small object (a marble or a die). On the next day, during a 10-min testing session, animals were exposed to two objects: the familiar object from the previous day and a novel object. Analysis of behavior during testing showed that mice injected with 0 and 1.6 g/kg of ethanol before training spent more time exploring a novel than a familiar object during testing. In contrast, mice injected with 2.4 g/kg ethanol spent equal amounts of time exploring the novel and the familiar object. Mice injected with this dose of ethanol after training did not show a decreased ratio of object exploration during testing. Analysis of behavior during training showed that mice injected with this dose of ethanol spent less time exploring the object, although their locomotor activity was not decreased. Our results show that in C57BL/6J mice, ethanol intoxication interferes with exploratory activity during object exploration, but not with consolidation of memory.     

Interactive effects of ethanol and nicotine on learning in C57BL/6J mice depend on both dose and duration of treatment

Objective and rationale Alcohol and nicotine are commonly co-abused; one possible explanation for co-abuse is that each drug ameliorates the aversive effects of the other. Both drugs have dose-dependent effects on learning and memory. Thus, this study examined the interactive effects of acute ethanol and acute, chronic, or withdrawal from chronic nicotine on fear conditioning in C57BL/6J mice. Materials and methods Conditioning consisted of auditory conditioned stimulus-foot-shock unconditioned stimulus pairings. For acute studies, saline or ethanol, then saline or nicotine was administered before training, and saline or nicotine was also administered before testing. For chronic and withdrawal studies, saline or nicotine was administered chronically, and ethanol or saline was administered before training. Results Acute nicotine (0.09 mg/kg) reversed ethanol-induced deficits (1.0 and 1.5 g/kg) in contextual and cued fear conditioning, whereas a low dose of ethanol (0.25 g/kg) reversed nicotine (6.3 mg kg⁻¹ day⁻¹) withdrawal-induced deficits in contextual conditioning. Tolerance developed for the effects of nicotine on ethanol-induced deficits in conditioning and cross-tolerance between chronic nicotine and acute ethanol was seen for the enhancing effects of ethanol on conditioning. Conclusions The complex and sometimes polar actions of ethanol and nicotine on behavior may contribute to co-abuse of these drugs. Specifically, smoking may initially reduce the aversive effects of ethanol, but tolerance develops for this effect. In addition, low doses of alcohol may lessen nicotine withdrawal symptoms.     

Genetic background influences the effects of withdrawal from chronic nicotine on learning and high-affinity nicotinic acetylcholine receptor binding in the dorsal and ventral hippocampus

Rationale

The effects of nicotine on cognitive processes may play an important role in nicotine addiction. Nicotine withdrawal impairs hippocampus-dependent learning and genetic factors influence this effect. However, the neural changes that contribute to these impairments are unknown. Chronic nicotine upregulates hippocampal nicotinic acetycholine receptors (nAChRs), which may contribute to cognitive deficits when nicotine administration ceases. If nAChR upregulation underlies withdrawal deficits in learning, then strains of mice exhibiting withdrawal deficits in hippocampus-dependent learning should also show upregulation of hippocampal nAChRs.

Objectives

Here, we examined the effects of nicotine withdrawal on fear conditioning and [³H]epibatidine binding in the dorsal and ventral hippocampus in two inbred mouse strains and their F1 hybrids.

Methods

Male C57BL/6NTac, 129S6/SvEvTac, and B6129SF1/Tac mice were administered chronic nicotine (18 mg/kg/day) for 12 days through osmotic pumps and then were trained and tested in fear conditioning 24 h after cessation of nicotine treatment.

Results

Nicotine withdrawal impaired hippocampus-dependent contextual conditioning in C57BL/6NTac mice but not 129S6/SvEvTac or B6129SF1/Tac mice; no changes were observed in hippocampus-independent cued fear conditioning. Upregulated [³H]epibatidine binding was found in the dorsal, but not ventral, hippocampus of C57BL/6NTac mice and in the ventral hippocampus of B6129SF1/Tac mice after chronic nicotine.

Conclusions

Upregulation of high-affinity binding sites in the dorsal hippocampus of C57BL/6NTac mice, the only strain that exhibited nAChR upregulation in this region and withdrawal deficits in contextual conditioning, suggests that upregulation of high-affinity binding sites in the dorsal hippocampus mediates, in part, nicotine withdrawal deficits in contextual conditioning and genetic background modulates these effects.     

The interactive effects of nicotinic and muscarinic cholinergic receptor inhibition on fear conditioning in young and aged C57BL/6 mice

Both normal aging and age-related disease, such as Alzheimer's disease, have diverse effects on forebrain-dependent cognitive tasks as well as the underlying neurobiological substrates. The purpose of the current study was to investigate if age-related alterations in the function of the cholinergic system are associated with memory impairments in auditory-cued and contextual fear conditioning. Young (2-3 months) and aged (19-20 months) C57BL/6 mice were administered scopolamine (0.1, 0.3, 0.5, or 1.0 mg/kg), a muscarinic cholinergic receptor antagonist, mecamylamine (1.0 and 2.0 mg/kg), a nicotinic cholinergic receptor antagonist, both scopolamine and mecamylamine (0.1 and 1.0 mg/kg, respectively), or saline prior to training. Training consisted of two white-noise CS (85 dB, 30 s)-footshock US (0.57 mA, 2 s) presentations. Testing occurred 48 h post-training. Scopolamine administration impaired contextual and cued fear conditioning in young and aged mice, although the aged mice were less sensitive to disruption by scopolamine. Mecamylamine did not disrupt conditioned fear in the young or aged mice. Scopolamine and mecamylamine co-administration, at doses sub-threshold for disrupting fear conditioning with separate administration, disrupted contextual and auditory-cued fear conditioning in the young mice, indicating that in the young mice the muscarinic and nicotinic cholinergic processes interact in the formation and maintenance of long-term memories for conditioned fear. Co-administration of both antagonists did not disrupt fear conditioning in the aged mice, indicating that age-related alterations in the cholinergic receptor subtypes may occur.     

Dopamine D3 receptor knockout mice and the motivational effects of ethanol

Dopamine D3 receptors have been implicated in the behavioral effects of abused drugs including ethanol. The present experiments characterized the acquisition of ethanol-induced place conditioning and ethanol self-administration in D3 knockout (D3 KO) mice compared with C57BL/6J (C57) mice. For place conditioning, D3 KO and C57 mice received six pairings of a tactile stimulus with ethanol (3 g/kg i.p.). D3 KO mice showed higher basal locomotor activity levels in comparison with the C57 mice during conditioning. Ethanol produced similar magnitudes of conditioned place preference in both genotypes. In a two-bottle drinking procedure, mice of each genotype received 24 h access to water and either 3% or 10% v/v ethanol. No difference was noted between D3 KO and C57 mice in either consumption or preference. In an operant self-administration procedure using 23 h sessions, D3 KO and C57 mice received access to 10% v/v ethanol on an FR4 schedule of reinforcement, food on an FR1 schedule of reinforcement and water from a sipper tube. D3 KO and C57 mice had similar response rates of ethanol and food as well as similar water intakes. Overall, these results indicate that elimination of D3 receptor function has little influence on ethanol reward or intake.     

Fear memory impairing effects of systemic treatment with the NMDA NR2B subunit antagonist, Ro 25-6981, in mice: attenuation with ageing

N-methyl-D-aspartate receptors (NMDARs) are mediators of synaptic plasticity and learning and are implicated in the pathophysiology of neuropsychiatric disease and age-related cognitive dysfunction. NMDARs are heteromers, but the relative contribution of specific subunits to NMDAR-mediated learning is not fully understood. We characterized pre-conditioning systemic treatment of the NR2B subunit-selective antagonist Ro 25-6981 for effects on multi-trial, one-trial and low-shock Pavlovian fear conditioning in C57BL/6J mice. Ro 25-6981 was also profiled for effects on novel open field exploration, elevated plus-maze anxiety-like behavior, startle reactivity, prepulse inhibition of startle, and nociception. Three-month (adult) and 12-month old C57BL/6Tac mice were compared for Ro 25-6981 effects on multi-trial fear conditioning, and corticolimbic NR2B protein levels. Ro 25-6981 moderately impaired fear learning in the multi-trial and one-trial (but not low-shock) conditioning paradigms, but did not affect exploratory or anxiety-related behaviors or sensory functions. Memory impairing effects of Ro 25-6981 were absent in 12-month old mice, although NR2B protein levels were not significantly altered. Present data provide further evidence of the memory impairing effects of selective blockade of NR2B-containing NMDARs, and show loss of these effects with ageing. This work could ultimately have implications for elucidating the pathophysiology of learning dysfunction in neuropsychiatric disorders and ageing.     

小鼠条件性恐惧模型的建立和品系敏感性研究

目的在敏感品系上建立稳定可靠的小鼠条件性恐惧模型。方法采用给予30 s声音刺激(85 dB,5000 Hz),后2 s伴随给予不可逃避的足底电击(0.6 mA,持续2 s),声音-电击共配对5次,每次间隔2 min的方法建立小鼠条件性恐惧模型,在ICR、DBA/2(简称DBA)和C57BL/6J(简称C57)3种品系小鼠上评价场景恐惧和声音线索恐惧获得和表达的行为特点,以确定敏感品系鼠;分别于条件性恐惧训练第2,7,14,21和28天对C57小鼠场景恐惧和声音线索恐惧进行检测,以评价该品系小鼠条件性恐惧的自然消退特点;采用30 s声音+30 s间隔+10次消退训练的模式进行声音线索恐惧消退训练,并于24 h后进行消退保持测试,以评价该品系小鼠消退训练和保持的特点。结果声音-电击配对5次可成功诱导C57小鼠形成场景恐惧和声音线索恐惧,DBA小鼠可诱导形成声音线索恐惧但无法形成场景恐惧,ICR小鼠场景恐惧和声音线索恐惧均未诱导成功,确定C57小鼠为敏感品系鼠。C57小鼠呈现时间依赖的恐惧反应自然消退,其中场景恐惧在训练后第7天僵住百分率消退至(25±12)%,声音线索恐惧训练后第28天僵住百分率维持在(48±22)%;C57小鼠经10轮30 s声音消退训练,僵住时间百分率由(55±30)%消退至(32±27)%,但24 h后消退保持测试时僵住时间百分率又回升至(47±35)%。结论以敏感品系C57小鼠为研究对象,建立了恐惧获得、表达、自然消退、消退训练和消退保持各阶段恐惧行为检测的实验方法,成功建立小鼠条件性恐惧模型。     

乙醇对嗜酒和非嗜酒小鼠体重和肝脏组织形态的影响

目的比较乙醇对嗜酒和非嗜酒小鼠躯体损害的易感性。方法以近交系C57BL/6J小鼠(嗜酒者)和远交系ICR小鼠(非嗜酒者)为实验动物,采用强制性饮酒程序:饮酒组小鼠饮用乙醇溶液8d,乙醇浓度从3%递增至20%。每天检测小鼠体重、摄食量、饮液量和乙醇摄入量。饮用乙醇期间取小鼠肝脏用组织化学方法观察肝脏组织病理学变化。结果乙醇对嗜酒小鼠体重增长的抑制作用明显强于非嗜酒小鼠。乙醇对两组小鼠的摄食量均有抑制作用,抑制程度无显著差异。乙醇抑制非嗜酒小鼠的饮液量,但对嗜酒小鼠无影响。实验期间两组小鼠的饮酒量相当,但嗜酒小鼠的肝脏损伤更为严重。结论嗜酒小鼠对乙醇毒性作用的耐受性弱于非嗜酒小鼠。     

Effect of pattern and number of chronic ethanol exposures on subsequent voluntary ethanol intake in C57BL/6J mice

Rationale We previously demonstrated that chronic ethanol exposure and withdrawal experience significantly increased subsequent voluntary ethanol intake in C57BL/6J mice. This study was conducted to examine chronic ethanol conditions that optimize this enhanced ethanol-drinking behavior. Objectives The purpose of this study was to examine whether the pattern and/or number of chronic ethanol exposures influence subsequent ethanol intake. Methods C57BL/6J mice were trained to drink ethanol (15% v/v) in a limited access situation (2 h/day) until stable intake was achieved. In experiment 1, mice received two cycles of chronic ethanol exposure delivered either in an intermittent [multiple withdrawal (MW)] or continuous [continuous exposure (CE)] pattern. One week of daily drinking sessions followed each exposure. In experiment 2, mice received either two or four cycles of chronic intermittent ethanol exposure (MW), each followed by a week of testing sessions. Three additional weeks of ethanol intake testing followed the last ethanol (or air) exposure. Results Experiment 1: Only the MW group evidenced a significant increase in ethanol intake compared to controls after the first chronic ethanol exposure. Both MW and CE groups consumed more ethanol than controls after the second ethanol-exposure period. Experiment 2: Ethanol intake in MW mice compared to controls significantly increased after two or four cycles of chronic ethanol exposure/withdrawal, and this heightened ethanol intake lasted longer in mice that received four cycles of chronic intermittent ethanol exposure. Conclusions Increased voluntary ethanol intake after chronic ethanol exposure and withdrawal experience may be accelerated by intermittent (as opposed to continuous) ethanol exposure, and the effect may last longer with increased number of such experiences.     

Scheduled access to ethanol results in motor impairment and tolerance in female C57BL/6J mice

We recently reported a method where water-restricted mice were given scheduled access to ethanol followed by access to water. C57BL/6J mice would repeatedly self-administer ethanol in amounts that produced high and stable blood ethanol concentrations (BEC) [Finn DA, Belknap JK, Cronise K, Yoneyama N, Murillo A, Crabbe JC. A procedure to produce high alcohol intake in mice. Psychopharmacol 2005;178:471-480]. The studies reported here demonstrate that behavioral signs of motor impairment result from these high alcohol intakes, and that there was some evidence of tolerance development across repeated sessions. Female C57BL/6J mice were allowed 30 min access to ethanol (5% v/v) followed by 2.5 h access to water either: every 3rd day for 12 days; every 2nd day for 28 days; or every 2nd day for 9 days. On intervening days, mice had 3 h access to water. A control group had daily access to water only. Mice consumed 2-2.5 g/kg ethanol in 30 min, resulting in BECs of 1.4-1.5 mg/ml. Motor impairment was assessed using the accelerating or fixed speed rotarod, balance beam or screen test. In all studies, mice were tested for motor impairment immediately after 30 min access to ethanol or water. In Experiment 1, ethanol-exposed mice had shorter latencies to fall from the fixed speed rotarod and more foot slips on the balance beam than the control group, indicating motor impairment. After drinking ethanol, mice also fell from a screen more quickly than during sober pretraining. In Experiment 2, mice tested (without prior training) for motor impairment and tolerance on the fixed speed rotarod at 6.5 and 10 RPM showed repeated motor impairment in the ethanol group, but did not develop tolerance. In Experiment 3, mice were first given rotarod training at 10 RPM. Following each fluid access period, performance was impaired in mice self-administering ethanol at 10, but not 15 RPM, when compared to control mice. There was no evidence of tolerance across days. However, on the last day, all mice were tested at both RPM following an i.p. injection of 2 g/kg ethanol. Ethanol-experienced mice were less impaired at both RPM than the ethanol-na?ve mice, indicating tolerance development according to this between-groups index. These results suggest that C57BL/6J mice will repeatedly consume alcohol in amounts that produce motor impairment under these scheduled fluid access conditions, and that a modest degree of tolerance can be detected using appropriate tests.     

Co-learning facilitates memory in mice: A new avenue in social neuroscience

Social context affects brain function but our understanding of its neurobiology is at an early stage. The mere presence of one individual can alter the cognitive capacities of another and social learning has been demonstrated in many species, including the mouse. We asked several questions: 1. How can active engagement of two familiar mice in the same learning activity (co-learning) alter their memory? 2. Under which environmental conditions (aversive vs non-aversive) can we expect the memory to be enhanced, impaired, or not affected? 3. Can a genetic factor modify the co-learning effect on memory? More specifically, can co-learning correct memory deficits in autistic-like BTBR inbred mice with deficient sociability? We demonstrated that pairs of familiar inbred mice of the same or different genotypes (C57BL/6J and BTBR) that were habituated to new objects and their spatial location, had enhanced episodic memory in the spatial object recognition test, whereas individually-trained animals failed to solve this task. Notably, the co-learning effect was genotype-dependent. BTBR mice paired with BTBR cage-mates in the habituation session modestly ameliorated their performance in the object recognition test but co-learning with a familiar C57BL/6J mouse completely normalized episodic memory deficit. Next, we explored the co-learning effect on fear memory in these inbred strains. Interestingly, mice of both genotypes displayed significantly enhanced contextual fear memory once they had been conditioned together with BTBR animals. The same influence of BTBR presence was observed on cued fear memory in C57BL/6J mice, whereas a modest co-learning effect was found on cued fear conditioning in the BTBR strain. Taken together, we demonstrated for the first time the co-learning effect on cognitive capacities in mice, which can be modified by genetic background and environmental conditions. The possible implications of this methodological approach in social neuroscience are discussed. This article is part of a Special Issue entitled 'Cognitive Enhancers'.     

Hippocampal nAChRs mediate nicotine withdrawal-related learning deficits

Nicotine modulation of learning may contribute to its abuse liability. The role of hippocampal nicotinic acetylcholine receptors (nAChRs) in the effects of acute, chronic and withdrawal from chronic nicotine on learning was assessed via intrahippocampal drug infusion in mice. Acute dorsal hippocampal nicotine infusion enhanced contextual fear conditioning. Conversely, chronic intrahippocampal infusion of a matched dose had no effect, and withdrawal from chronic infusion impaired learning. Thus, hippocampal functional adaptation, evidenced by learning deficits during abstinence, occurs with the transition from acute to chronic nicotine exposure. To investigate which hippocampal nAChRs mediate these adaptations, C57BL/6, β2 nAChR subunit knockout (KO), and wildtype (WT) mice treated chronically with systemic nicotine received intrahippocampal dihydro-β-erythroidine (a high affinity nAChR antagonist). Intrahippocampal dihydro-β-erythroidine precipitated learning deficits in all but the KO mice. Therefore, the action of nicotine at hippocampal β2⁎ nAChRs mediates adaptations in hippocampal function that underlie withdrawal deficits in contextual fear conditioning.     

Antagonism of the behavioral effects of ethanol by naltrexone in BALB/c,C57BL/6, and DBA/2 mice

The effects of naltrexone on the increase in locomotor activity induced by a low dose (1.35 g/kg IP) of ethanol and on the duration of loss of righting reflex after a high dose (3.5 g/kg) of ethanol were studied in BALB/c, DBA/2, and C57BL/6 mice. Ethanol increased locomotor activity in DBA and BALB mice, but not in C57BL mice. Naltrexone, at a dose of 0.1 mg/kg, antagonized the ethanol-induced increase in locomotion similarly in DBA and BALB mice. The duration of loss of righting reflex was, however, differentially affected in all three strains by naltrexone. The BALB mice affected in all three strains by naltrexone. The BALB mice were the most sensitive strain (1 mg/kg naltrexone significantly counteracted ethanol hypnosis), the C57BL mice were intermediate (8 mg/kg naltrexone required to antagonize this effect of ethanol), and the DBA mice were least sensitive (no effect evident even at the highest dose of 8 mg/kg) to naltrexone. Thus, naltrexone could antagonize the behavioral effects of a low and high dose of ethanol, but the three strains, which differ in their behavioral response to ethanol, also were differentially sensitive to the effect of naltrexone in reversing ethanol-induced hypnosis and ethanol-induced changes in locomotor activity.     

The duration of nicotine withdrawal-associated deficits in contextual fear conditioning parallels changes in hippocampal high affinity nicotinic acetylcholine receptor upregulation

A predominant symptom of nicotine withdrawal is cognitive deficits, yet understanding of the neural basis for these deficits is limited. Withdrawal from chronic nicotine disrupts contextual learning in mice and this deficit is mediated by direct effects of nicotine in the hippocampus. Chronic nicotine treatment upregulates nicotinic acetylcholine receptors (nAChR); however, it is unknown whether upregulation is related to the observed withdrawal-induced cognitive deficits. If a relationship between altered learning and nAChR levels exists, changes in nAChR levels after cessation of nicotine treatment should match the duration of learning deficits. To test this hypothesis, mice were chronically administered 6.3mg/kg/day (freebase) nicotine for 12 days and trained in contextual fear conditioning on day 11 or between 1 to 16 days after withdrawal of treatment. Changes in [(125)I]-epibatidine binding at cytisine-sensitive and cytisine-resistant nAChRs and chronic nicotine-related changes in α4, α7, and β2 nAChR subunit mRNA expression were assessed. Chronic nicotine had no behavioral effect but withdrawal produced deficits in contextual fear conditioning that lasted 4 days. Nicotine withdrawal did not disrupt cued fear conditioning. Chronic nicotine upregulated hippocampal cytisine-sensitive nAChR binding; upregulation continued after cessation of nicotine administration and the duration of upregulation during withdrawal paralleled the duration of behavioral changes. Changes in binding in cortex and cerebellum did not match behavioral changes. No changes in α4, α7, and β2 subunit mRNA expression were seen with chronic nicotine. Thus, nicotine withdrawal-related deficits in contextual learning are time-limited changes that are associated with temporal changes in upregulation of high-affinity nAChR binding.     

Atomoxetine reverses nicotine withdrawal-associated deficits in contextual fear conditioning.

Recent evidence suggests that the cognitive symptoms of nicotine withdrawal and the cognitive symptoms of attention deficit hyperactivity disorder (ADHD) may share neural correlates. Thus, therapeutics that ameliorate ADHD symptoms may also ameliorate nicotine-withdrawal symptoms. The present research tested this hypothesis in an animal model of nicotine withdrawal-associated cognitive deficits using atomoxetine, a norepinephrine reuptake inhibitor that is approved by the FDA to treat the symptoms of ADHD. C57BL/6 mice were prepared with osmotic minipumps that administered 6.3 mg/kg/day of nicotine or saline, and the minipumps were removed after 12 days of continuous treatment. Twenty-four hours later, mice were trained in delay fear conditioning using two paired presentations of an auditory conditioned stimulus (CS) with a footshock unconditioned stimulus. Testing for freezing in response to the training context and for freezing in response to the CS occurred the next day. Nicotine-withdrawn mice and their saline-treated counterparts received either saline or atomoxetine before training and the context test. Consistent with previous research, the results indicate that mice withdrawn from chronic nicotine demonstrated lower levels of contextual fear conditioning than mice that were not withdrawn from chronic nicotine. Atomoxetine dose-dependently reversed the deficit, suggesting that nicotine withdrawal may be associated with changes in noradrenergic function, acetylcholinergic function, and/or with changes in cell signaling cascades that are activated by both nicotine and norepinephrine. These data suggest that atomoxetine may be efficacious for treating nicotine withdrawal-associated cognitive deficits that promote relapse in abstinent smokers.     

Genetic differences in the rewarding and activating effects of morphine and ethanol

The influence of genotype on the rewarding and locomotor activating effects of morphine and ethanol was examined in the place conditioning paradigm. Two inbred mouse strains (C57BL/6J and DBA/2J) were exposed to a differential conditioning procedure in which each mouse received four pairings of a distinctive floor stimulus with IP injection of morphine (0, 2.5, 5 or 10 mg/kg) or ethanol (0, 1, 2, 3 or 4 g/kg). A different floor stimulus was paired with saline. Conditioning trials lasted 30 min and each experiment concluded with a floor preference test in the absence of drug. In accord with previous studies, morphine evoked a dose-dependent increase in activity during conditioning that was greater in C57BL/6J mice than in DBA/2J mice. In contrast, ethanol produced a dose-dependent increase in activity that was greater in DBA/2J than in C57BL/6J mice. Both strains showed conditioned place preference with morphine, but only the DBA/2J strain showed conditioned place preference with ethanol. No conditioned place aversion was seen. With both drugs, stronger place preference conditioning was obtained in DBA/2J mice, supporting the general conclusion that sensitivity to drug reward is influenced by genotype. The fact that the same genotype is more sensitive to the rewarding effects of two different drugs supports theories postulating commonality in the biological mechanisms of drug reward. Although the outcome of the ethanol study supports predictions of the psychomotor stimulant theory of addiction concerning the relationship between drug-induced activation and reward, the outcome of the morphine study does not. The direction of the strain difference in conditioned place preference is opposite to what might be predicted on the basis of strain differences previously reported in drug consumption and preference studies, suggesting that genetic differences in drug consumption may not accurately reflect postabsorptive motivational effects of drug.     

Influence of genetic background on alcohol drinking and behavioral phenotypes of 5-HT3 receptor over-expressing mice

Behavioral effects of genetic manipulations are influenced by the background genetics of mouse strains used for the creation of transgenic mice. One strategy to address whether background genes may compromise interpretation of phenotype is the production of congenics. 5-HT3 receptor over-expressing mice have been behaviorally characterized on a B6SJL/F2 background (B6SJL/F2-OE mice), and were found to consume less ethanol failed to develop conditioned place preference to moderate doses of cocaine and demonstrate improved hippocampal-dependent learning. To assess the contribution of parental strain genetics to these behaviors, we bred the transgene onto two well-defined backgrounds that differ in ethanol consumption and contextual fear conditioning, C57Bl/6J (B6) and DBA/2J (D2) strains. The behavioral phenotype of B6SJL/F2-OE was recapitulated in C57Bl/6J-OE mice. However, the effect of transgene over-expression on behavior was only apparent for one aspect of the novelty test using DBA/2J-OE mice. Results underscore the need to consider the genetic environment conferred by strain selection on the effects of genetic manipulation in mice.     

Fixed-ratio schedules of oral ethanol self-administration in inbred mouse strains

Previous studies of ethanol reinforcement in BALB/cJ and C57BL/6J mice have shown that over a range of concentrations oral ethanol appeared to serve as a reinforcer only for the C57BL/6J mice. In the previous studies BALB/cJ mice maintained rates of responding for ethanol that only slightly exceeded the rates maintained by the vehicle, water. However, the quantity of ethanol consumed with the continuous reinforcement schedule (fixed ratio one) may have led to pharmacologically significant effects, given the high sensitivity to ethanol of this genotype. The present study tested whether and to what extent ethanol would maintain responding under increasing fixed ratio size in these two strains of mice at ethanol concentrations of 0%, 8%, and 16% (w/v). For the C57BL/6J mice, as fixed-ratio size increased from 1 to 2, 4, and 8, there were almost directly proportional increases in response rate at ethanol concentrations of 8% and 16% (w/v), but not at 0%. Post-session blood ethanol levels confirmed intake of pharmacologically significant quantities. The volume consumed per unit of body weight decreased as fixed-ratio size increased. For the BALB/cJ mice, at no condition did ethanol maintain responding at levels that significantly exceeded vehicle maintained responding. BALB/cJ mice did not differ from C57BL/6J mice as fixed-ratio size was increased during vehicle conditions. These results, along with earlier findings, demonstrate that ethanol can serve as a reinforcer for C57BL/6J mice but not in BALB/cJ mice over a range of schedule conditions. They further support the conclusion that genotype is an important determinant of ethanol reinforced behavior.