Mice genetically selected for differences in open-field activity after ethanol

Dopamine D2 receptors in the bed nucleus of the stria terminalis modulate alcohol-related behaviors

Dysregulation of the dopamine (DA) system is a hallmark of substance abuse disorders, including alcohol use disorder (AUD). Of the DA receptor subtypes, the DA D2 receptors (D2Rs) play a key role in the reinforcing effects of alcohol. D2Rs are expressed in numerous brain regions associated with the regulation of appetitive behaviors. One such region is the bed nucleus of the stria terminalis (BNST), which has been linked to the development and maintenance of AUD. Recently, we identified alcohol withdrawal-related neuroadaptations in the periaqueductal gray/dorsal raphe to BNST DA circuit in male mice. However, the role of D2R-expressing BNST neurons in voluntary alcohol consumption is not well characterized. In this study, we used a CRISPR-Cas9-based viral approach, to selectively reduce the expression of D2Rs in BNST VGAT neurons and interrogated the impact of BNST D2Rs in alcohol-related behaviors. In male mice, reduced D2R expression potentiated the stimulatory effects of alcohol and increased voluntary consumption of 20% w/v alcohol in a two-bottle choice intermittent access paradigm. This effect was not specific to alcohol, as D2R deletion also increased sucrose intake in male mice. Interestingly, cell-specific deletion of BNST D2Rs in female mice did not alter alcohol-related behaviors but lowered the threshold for mechanical pain sensitivity. Collectively, our findings suggest a role for postsynaptic BNST D2Rs in the modulation of sex-specific behavioral responses to alcohol and sucrose.

Alcohol Sensitivity as an Endophenotype of Alcohol Use Disorder: Exploring Its Translational Utility between Rodents and Humans

Alcohol use disorder (AUD) is a complex, chronic, relapsing disorder with multiple interacting genetic and environmental influences. Numerous studies have verified the influence of genetics on AUD, yet the underlying biological pathways remain unknown. One strategy to interrogate complex diseases is the use of endophenotypes, which deconstruct current diagnostic categories into component traits that may be more amenable to genetic research. In this review, we explore how an endophenotype such as sensitivity to alcohol can be used in conjunction with rodent models to provide mechanistic insights into AUD. We evaluate three alcohol sensitivity endophenotypes (stimulation, intoxication, and aversion) for their translatability across human and rodent research by examining the underlying neurobiology and its relationship to consumption and AUD. We show examples in which results gleaned from rodents are successfully integrated with information from human studies to gain insight in the genetic underpinnings of AUD and AUD-related endophenotypes. Finally, we identify areas for future translational research that could greatly expand our knowledge of the biological and molecular aspects of the transition to AUD with the broad hope of finding better ways to treat this devastating disorder.

A Mechanism Linking Two Known Vulnerability Factors for Alcohol Abuse: Heightened Alcohol Stimulation and Low Striatal Dopamine D2 Receptors

Alcohol produces both stimulant and sedative effects in humans and rodents. In humans, alcohol abuse disorder is associated with a higher stimulant and lower sedative responses to alcohol. Here, we show that this association is conserved in mice and demonstrate a causal link with another liability factor: low expression of striatal dopamine D2 receptors (D2Rs). Using transgenic mouse lines, we find that the selective loss of D2Rs on striatal medium spiny neurons enhances sensitivity to ethanol stimulation and generates resilience to ethanol sedation. These mice also display higher preference and escalation of ethanol drinking, which continues despite adverse outcomes. We find that striatal D1R activation is required for ethanol stimulation and that this signaling is enhanced in mice with low striatal D2Rs. These data demonstrate a link between two vulnerability factors for alcohol abuse and offer evidence for a mechanism in which low striatal D2Rs trigger D1R hypersensitivity, ultimately leading to compulsive-like drinking.

A Prospective 5-Year Re-examination of Alcohol Response in Heavy Drinkers Progressing in Alcohol Use Disorder

BACKGROUND

The main neurobiological theories of the development of addiction, including tolerance, sensitization, incentive-sensitization, and allostasis, have not been tested in longitudinal human alcohol response research. To address this issue, we conducted the first controlled prospective investigation of subjective and neuroendocrine responses to alcohol measured over a 5-year interval in at-risk young adult heavy drinkers (HD) and light drinker control subjects.

METHODS

Participants were 156 individuals, 86 heavy drinkers and 70 light drinkers, undergoing an initial oral alcohol challenge testing (.8 g/kg alcohol vs. placebo) and an identical re-examination testing 5 to 6 years later. Alcohol use disorder (AUD) symptoms and drinking behaviors were assessed in the interim follow-up period.

RESULTS

At re-examination, HD continued to exhibit higher sensitivity on alcohol's stimulating and rewarding effects with lower sensitivity to sedative effects and cortisol reactivity, relative to light drinkers. In HD with high AUD symptom trajectories over follow-up, heightened alcohol stimulation and reward persisted at re-examination. HD with low AUD symptoms showed reduced alcohol stimulation over time and lower reward throughout compared with the HD with high and intermediate AUD symptoms.

CONCLUSIONS

Results support the early stage phase of the allostasis model, with persistently heightened reward sensitivity and stimulation in heavy drinkers exhibiting AUD progression in early mid-adulthood. While there are multiple pathways to development of a disorder as complex as AUD, maintenance of alcohol stimulatory and rewarding effects may play an important role in the continuation and progression of alcohol addiction.

Mice bred for severity of acute alcohol withdrawal respond differently in a go/no-go task

BACKGROUND

Mice selectively bred for high or low withdrawal to acute alcohol differ on a number of traits, including consumption of alcohol, conditioned place preference for alcohol, and sensitivity to alcohol-induced locomotor activity. One trait that has not been examined in these mice is behavioral inhibition.

METHODS

High and low alcohol withdrawal mice (second replicate: high and low acute alcohol withdrawal [HAW-2/LAW-2]) were trained and tested in a Go/No-go task. Mice were administered 0.0, 0.5, 1.0, and 1.5 g/kg ethanol (EtOH) on 3 occasions according to an incomplete Latin Square. A separate cohort of C57BL/6J (B6) and DBA/2J (D2) mice (the progenitor strains for HAW-2/LAW-2 mice) underwent the same protocol, using the same EtOH doses.

RESULTS

HAW-2 and LAW-2 mice did not differ in behavioral inhibition at baseline, although LAW-2 mice did have higher overall levels of responding in the task. EtOH did not alter behavioral inhibition in these mice. However, it did decrease responses to the Go cue, and this effect was greater in HAW-2 mice than in LAW-2 mice. D2 mice had lower behavioral inhibition than B6 mice at baseline, and EtOH slightly decreased behavioral inhibition in both strains.

CONCLUSIONS

The findings with D2 and B6 mice generally fit with the existing literature. However, the lack of a difference in behavioral inhibition between HAW-2 and LAW-2 mice was unexpected, as well as the absence of any effect of these doses of EtOH on behavioral inhibition in these mice. Nonetheless, the findings do suggest that selectively breeding for high or low withdrawal to acute alcohol can lead to differences in operant behavior in the Go/No-go task.

Accentuating effects of nicotine on ethanol response in mice with high genetic predisposition to ethanol-induced locomotor stimulation

BACKGROUND

Co-morbid use of nicotine-containing tobacco products and alcohol is prevalent in alcohol dependent individuals. Common genetic factors could influence initial sensitivity to the independent or interactive effects of these drugs and play a role in their co-abuse.

METHODS

Locomotor sensitivity to nicotine and ethanol, alone and in combination, was assessed in mice bred for high (FAST) and low (SLOW) sensitivity to the locomotor stimulant effects of ethanol and in an inbred strain of mouse (DBA/2J) that has been shown to have extreme sensitivity to ethanol-induced stimulation in comparison to other strains.

RESULTS

The effects of nicotine and ethanol, alone and in combination, were dependent on genotype. In FAST and DBA/2J mice that show high sensitivity to ethanol-induced stimulation, nicotine accentuated the locomotor stimulant response to ethanol. This effect was not found in SLOW mice that are not stimulated by ethanol alone.

CONCLUSIONS

These data indicate that genes underlying differential sensitivity to the stimulant effects of ethanol alone also influence sensitivity to nicotine in combination with ethanol. Sensitivity to the stimulant effects of nicotine alone does not appear to predict the response to the drug combination, as FAST mice are sensitive to nicotine-induced stimulation, whereas SLOW and DBA/2J mice are not. The combination of nicotine and ethanol may have genotype-dependent effects that could impact co-abuse liability.

GABAB receptor activation attenuates the stimulant but not mesolimbic dopamine response to ethanol in FAST mice

Neural processes influenced by γ-aminobutyric acid B (GABA(B)) receptors appear to contribute to acute ethanol sensitivity, including the difference between lines of mice bred for extreme sensitivity (FAST) or insensitivity (SLOW) to the locomotor stimulant effect of ethanol. One goal of the current study was to determine whether selection of the FAST and SLOW lines resulted in changes in GABA(B) receptor function, since the lines differ in sensitivity to the GABA(B) receptor agonist baclofen and baclofen attenuates the stimulant response to ethanol in FAST mice. A second goal was to determine whether the baclofen-induced reduction in ethanol stimulation in FAST mice is associated with an attenuation of the mesolimbic dopamine response to ethanol. In Experiment 1, the FAST and SLOW lines were found to not differ in GABA(B) receptor function (measured by baclofen-stimulated [(35)S]GTPγS binding) in whole brain or in several regional preparations, except in the striatum in one of the two replicate sets of selected lines. In Experiment 2, baclofen-induced attenuation of the locomotor stimulant response to ethanol in FAST mice was not accompanied by a reduction in dopamine levels in the nucleus accumbens, as measured by microdialysis. These data suggest that, overall, GABA(B) receptor function does not play an integral role in the genetic difference in ethanol sensitivity between the FAST and SLOW lines. Further, although GABA(B) receptors do modulate the locomotor stimulant response to ethanol in FAST mice, this effect does not appear to be due to a reduction in tonic dopamine signaling in the nucleus accumbens.

Do initial responses to drugs predict future use or abuse?

Individuals vary in their initial reactions to drugs of abuse in ways that may contribute to the likelihood of subsequent drug use. In humans, most drugs of abuse produce positive subjective states such as euphoria and feelings of well-being, which may facilitate repeated use. In nonhumans, many drugs initially increase locomotor activity and produce discriminative stimulus effects, both of which have been considered to be models of human stimulant and subjective states. Both humans and nonhumans vary in their sensitivity to early acute drug effects in ways that may predict future use or self-administration, and some of these variations appear to be genetic in origin. However, it is not known exactly how the initial responses to drugs in either humans or nonhumans relate to subsequent use or abuse. In humans, positive effects of drugs facilitate continued use of a drug while negative effects discourage use, and in nonhumans, greater genetic risk for drug intake is predicted by reduced sensitivity to drug aversive effects; but whether these initial responses affect escalation of drug use, and the development of dependence is currently unknown. Although early use of a drug is a necessary step in the progression to abuse and dependence, other variables may be of greater importance in the transition from use to abuse. Alternatively, the same variables that predict initial acute drug effects and early use may significantly contribute to continued use, escalation and dependence. Here we review the existing evidence for relations between initial direct drug effects, early use, and continued use. Ultimately, these relations can only be determined from systematic longitudinal studies with comprehensive assessments from early drug responses to progression of problem drug use. In parallel, additional investigation of initial responses in animal models as predictors of drug use will shed light on the underlying mechanisms.

Intracranial self-stimulation in FAST and SLOW mice: effects of alcohol and cocaine

RATIONALE

Sensitivity to the stimulant and rewarding effects of alcohol may be genetically correlated traits that predispose individuals to develop an alcohol use disorder.

OBJECTIVE

This study aimed to examine the effects of alcohol and cocaine on intracranial self-stimulation (ICSS) in FAST and SLOW mice, which were selectively bred for extremes in alcohol stimulation.

METHODS

Male FAST and SLOW mice were conditioned to respond for reinforcement by direct electrical stimulation of the medial forebrain bundle (i.e., brain stimulation reward). ICSS responses were determined immediately before and after oral gavage with water or alcohol (0.3-2.4 g/kg) or intraperitoneal injection with saline or cocaine (1.0-30.0 mg/kg). In separate FAST and SLOW mice, the locomotor effects of these treatments were measured in activity chambers.

RESULTS

Alcohol dose-dependently lowered the threshold for self-stimulation (θ (0)) and the frequency that maintained 50% of maximal responding (EF50) in FAST mice but did not significantly affect these parameters in SLOW mice. The largest effects of alcohol were after the 1.7- and 2.4-g/kg doses and were about 40% compared to water injection. Alcohol did not affect MAX response rates, but dose-dependently stimulated locomotor activity in FAST mice. Cocaine lowered thresholds equally in FAST and SLOW mice, although cocaine-stimulated locomotor activity was higher in the FAST than in the SLOW mice.

CONCLUSIONS

Selective breeding for alcohol locomotor stimulation also renders the mice more sensitive to the effects of alcohol, but not cocaine, on ICSS.

Ethanol- and cocaine-induced locomotion are genetically related to increases in accumbal dopamine

Neuroanatomical research suggests that interactions between dopamine and glutamate within the mesolimbic dopamine system are involved in both drug-induced locomotor stimulation and addiction. Therefore, genetically determined differences in the locomotor responses to ethanol and cocaine may be related to differences in the effects of these drugs on this system. To test this, we measured drug-induced changes in dopamine and glutamate within the nucleus accumbens (NAcc), a major target of mesolimbic dopamine neurons, using in vivo microdialysis in selectively bred FAST and SLOW mouse lines, which were bred for extreme sensitivity (FAST) and insensitivity (SLOW) to the locomotor stimulant effects of ethanol. These mice also show a genetically correlated difference in stimulant response to cocaine (FAST > SLOW). Single injections of ethanol (2 g/kg) or cocaine (40 mg/kg) resulted in larger increases in dopamine within the NAcc in FAST compared with SLOW mice. There was no effect of either drug on NAcc glutamate levels. These experiments indicate that response of the mesolimbic dopamine system is genetically correlated with sensitivity to ethanol- and cocaine-induced locomotion. Because increased sensitivity to the stimulating effects of ethanol appears to be associated with greater risk for alcohol abuse, genetically determined differences in the mesolimbic dopamine response to ethanol may represent a critical underlying mechanism for increased genetic risk for alcoholism.

Combined scopolamine and ethanol treatment results in a locomotor stimulant response suggestive of synergism that is not blocked by dopamine receptor antagonists

BACKGROUND

Muscarinic acetylcholine receptors (mAChRs) are well positioned to mediate ethanol's stimulant effects. To investigate this possibility, we examined the effects of scopolamine, a receptor subtype nonselective mAChR antagonist, on ethanol-induced stimulation in genotypes highly sensitive to this effect of ethanol. We also investigated whether the dopamine D1-like receptor antagonist, SCH-23390 or the dopamine D2-like receptor antagonist, haloperidol, could block the extreme stimulant response found following co-administration of scopolamine and ethanol.

METHODS

Scopolamine (0, 0.0625, 0.125, 0.25, or 0.5 mg/kg) was given 10 minutes prior to saline or ethanol (0.75 to 2 g/kg) to female FAST (Experiment I) or DBA/2J (Experiment II) mice that were then tested for locomotion for 30 minutes. In Experiments III and IV, respectively, SCH-23390 (0, 0.015, or 0.03 mg/kg) was given 10 minutes prior, and haloperidol (0, 0.08, or 0.16 mg/kg) was given 2 minutes prior, to scopolamine (0 or 0.5 mg/kg), followed 10 minutes later by saline or ethanol (1.5 g/kg) and female DBA/2J mice were tested for locomotion for 30 minutes.

RESULTS

FAST and DBA/2J mice displayed a robust enhancement of the locomotor effects of ethanol following pretreatment with scopolamine that was suggestive of synergism. SCH-23390 had no effect on the response to the scopolamine + ethanol drug combination, nor did it attenuate ethanol- or scopolamine-induced locomotor activity. Haloperidol, while attenuating the effects of ethanol, was not able to block the effects of scopolamine or the robust response to the scopolamine-ethanol drug combination.

CONCLUSIONS

These results suggest that while muscarinic receptor antagonism robustly enhances acute locomotor stimulation to ethanol, dopamine receptors are not involved in the super-additive interaction of scopolamine and ethanol treatment. They also suggest that in addition to cautions regarding the use of alcohol when scopolamine is clinically prescribed due to enhanced sedative effects, enhanced stimulation may also be a concern.

Attenuation of the stimulant response to ethanol is associated with enhanced ataxia for a GABA, but not a GABA, receptor agonist

BACKGROUND

The gamma-aminobutyric acid (GABA) system is implicated in the neurobiological actions of ethanol, and pharmacological agents that increase the activity of this system have been proposed as potential treatments for alcohol use disorders. As ethanol has its own GABA mimetic properties, it is critical to determine the mechanism by which GABAergic drugs may reduce the response to ethanol (i.e., via an inhibition or an accentuation of the neurobiological effects of ethanol).

METHODS

In this study, we examined the ability of 3 different types of GABAergic compounds, the GABA reuptake inhibitor NO-711, the GABA(A) receptor agonist muscimol, and the GABA(B) receptor agonist baclofen, to attenuate the locomotor stimulant response to ethanol in FAST mice, which were selectively bred for extreme sensitivity to ethanol-induced locomotor stimulation. To determine whether these compounds produced a specific reduction in stimulation, their effects on ethanol-induced motor incoordination were also examined.

RESULTS

NO-711, muscimol, and baclofen were all found to potently attenuate the locomotor stimulant response to ethanol in FAST mice. However, both NO-711 and muscimol markedly increased ethanol-induced ataxia, whereas baclofen did not accentuate this response.

CONCLUSIONS

These results suggest that pharmacological agents that increase extracellular concentrations of GABA and GABA(A) receptor activity may attenuate the stimulant effects of ethanol by accentuating its intoxicating and sedative properties. However, selective activation of the GABA(B) receptor appears to produce a specific attenuation of ethanol-induced stimulation, suggesting that GABA(B) receptor agonists may hold greater promise as potential pharmacotherapies for alcohol use disorders.

Relationship between ethanol's acute locomotor effects and ethanol self-administration in male Long-Evans rats

BACKGROUND

Human studies have suggested an important relationship between ethanol sensitivity and risk of alcoholism. These studies have led some to hypothesize that a low initial sensitivity to ethanol's depressant effects and/or an elevated response to ethanol's stimulant effects may represent important risk factors associated with the development of abusive drinking behavior. Unfortunately, elucidating neurobiologic mechanisms that may underlie these relationships between ethanol sensitivity and ethanol drinking have been hampered by difficulties in modeling some of these interactions in animals. In this study, we re-examined some of these relationships in an outbred strain of rats using continuous access two-bottle choice drinking and a limited-access operant procedure that engenders pharmacologically relevant levels of ethanol intake and permits the discrete assessment of appetitive and consummatory measures of ethanol drinking behavior.

METHODS

Twenty-three male Long-Evans rats were habituated to a locomotor activity box and then tested for their response to a stimulant (0.5 g/kg) and depressant (1.5 g/kg) ethanol dose. Rats were then trained to complete a lever pressing requirement to gain access to 10% ethanol for 20-minute sessions conducted 5 d/wk for 5 weeks. Appetitive behavior was assessed after 2.5 and 4.5 weeks using 20-minute extinction trials in which ethanol was not presented and lever responses were recorded. Home-cage ethanol preference was also assessed prior to and immediately following the 5-week self-administration regimen using a continuous access, two-bottle choice procedure.

RESULTS

A significant increase in home-cage ethanol preference was observed following the self-administration procedure, however, neither measure of ethanol preference correlated with average daily ethanol intake during the operant self-administration sessions or with initial sensitivity to ethanol's stimulant or depressant effects. Notably, a significant negative correlation was observed between sensitivity to ethanol's locomotor depressant effect and daily intake during the operant self-administration sessions. No significant relationships were noted between sensitivity to ethanol's locomotor effects and extinction responding.

CONCLUSIONS

The results of these studies suggest that the well-established relationship between a low level of response to ethanol and increased ethanol consumption reported in human studies can be observed in an outbred rodent strain using a limited-access operant self-administration procedure, but not with home-cage ethanol drinking.

A role for neuronal nicotinic acetylcholine receptors in ethanol-induced stimulation, but not cocaine- or methamphetamine-induced stimulation

RATIONALE

Cocaine (COC), ethanol (EtOH), and methamphetamine (MA) are widely abused substances and share the ability to induce behavioral stimulation in mice and humans. Understanding the biological basis of behavioral stimulation to COC, EtOH, and MA may provide a greater understanding of drug and alcohol abuse.

OBJECTIVES

In these studies we set out to determine if neuronal nicotinic acetylcholine receptors were involved in the acute locomotor responses to these drugs, our measure of behavioral stimulation.

METHODS

A panel of acetylcholine receptor antagonists was used to determine if nicotinic receptors were involved in EtOH- and psychostimulant-induced stimulation. We tested the effect of these drugs in genotypes of mice (FAST and DBA/2J) that are extremely sensitive to this drug effect. To determine which acetylcholine receptor subunits may be involved in this response, relative expression of the alpha3, alpha6, beta2, and beta4 subunit genes was examined in mice selectively bred for high and low response to EtOH.

RESULTS

Mecamylamine, but not hexamethonium, attenuated the acute locomotor response to EtOH. The acetylcholine receptor antagonist dihydro-beta-erythroidine and methyllycaconitine had no effect on this response. The alpha6 and beta4, but not alpha3 or beta2, subunits of the acetylcholine receptor were differentially expressed between mice bred for extreme differences in EtOH stimulation. Mecamylamine had no effect on psychostimulant-induced locomotor activity.

CONCLUSIONS

Neuronal nicotinic receptors are involved in EtOH, but not psychostimulant, stimulation. These studies suggest a lack of involvement of some nicotinic receptor subtypes, but more work is needed to determine the specific receptor subtypes involved in this behavior.

Behavioral genetic contributions to the study of addiction-related amphetamine effects

Amphetamines, including methamphetamine, pose a significant cost to society due to significant numbers of amphetamine-abusing individuals who suffer major health-related consequences. In addition, methamphetamine use is associated with heightened rates of violent and property-related crimes. The current paper reviews the existing literature addressing genetic differences in mice that impact behavioral responses thought to be relevant to the abuse of amphetamine and amphetamine-like drugs. Summarized are studies that used inbred strains, selected lines, single-gene knockouts and transgenics, and quantitative trait locus (QTL) mapping populations. Acute sensitivity, neuroadaptive responses, rewarding and conditioned effects are among those reviewed. Some gene mapping work has been accomplished, and although no amphetamine-related complex trait genes have been definitively identified, translational work leading from results in the mouse to studies performed in humans is beginning to emerge. The majority of genetic investigations have utilized single-gene knockout mice and have concentrated on dopamine- and glutamate-related genes. Genes that code for cell support and signaling molecules are also well-represented. There is a large behavioral genetic literature on responsiveness to amphetamines, but a considerably smaller literature focused on genes that influence the development and acceleration of amphetamine use, withdrawal, relapse, and behavioral toxicity. Also missing are genetic investigations into the effects of amphetamines on social behaviors. This information might help to identify at-risk individuals and in the future to develop treatments that take advantage of individualized genetic information.

Selection for contextual fear conditioning affects anxiety-like behaviors and gene expression

Conditioned fear and anxiety-like behaviors have many similarities at the neuroanatomical and pharmacological levels, but their genetic relationship is less well defined. We used short-term selection for contextual fear conditioning (FC) to produce outbred mouse lines with robust genetic differences in FC. The high and low selected lines showed differences in fear learning that were stable across various training parameters and were not secondary to differences in sensitivity to the unconditioned stimulus (foot shock). They also showed a divergence in fear potentiated startle, indicating that differences induced by selection generalized to another measure of fear learning. However, there were no differences in performance in a Pavlovian approach conditioning task or the Morris water maze, indicating no change in general learning ability. The high fear learning line showed greater anxiety-like behavior in the open field and zero maze, confirming a genetic relationship between FC and anxiety-like behavior. Gene expression analysis of the amygdala and hippocampus identified genes that were differentially expressed between the two lines. Quantitative trait locus (QTL) analysis identified several chromosomal regions that may underlie the behavioral response to selection; cis-acting expression QTL were identified in some of these regions, possibly identifying genes that underlie these behavioral QTL. These studies support the validity of a broad genetic construct that includes both learned fear and anxiety and provides a basis for further studies aimed at gene identification.

GABAB receptor stimulation accentuates the locomotor effects of morphine in mice bred for extreme sensitivity to the stimulant effects of ethanol

Mice selectively bred for divergent sensitivity to the locomotor stimulant effects of ethanol (FAST and SLOW) also differ in their locomotor response to morphine. The GABA(B) receptor has been implicated in the mediation of locomotor stimulation to both ethanol and morphine, and a reduction in ethanol-induced stimulation has been found with the GABA(B) receptor agonist baclofen in FAST mice. We hypothesized that GABA(B) receptor activation would also attenuate the locomotor stimulant responses to morphine in these mice. In order to test this hypothesis, baclofen was administered to FAST-1 and FAST-2 mice 15 min prior to morphine, and activity was recorded for 30 min. Baclofen attenuated stimulation to 32 mg/kg morphine in FAST-1 mice, but only at a dose that also reduced saline activity. There was no stimulant response to 32 mg/kg morphine in FAST-2 mice, or to 16 mg/kg or 48 mg/kg morphine in FAST-1 mice, but the combination of baclofen with these morphine doses accentuated locomotor activity. Therefore, it appears that GABA(B) receptor activation is not a common mechanism for the locomotor stimulant responses to ethanol and morphine in FAST mice; however, these data suggest that GABA(B) receptor activation may instead enhance some of the behavioral effects of morphine.

Naloxone does not attenuate the locomotor effects of ethanol in FAST, SLOW, or two heterogeneous stocks of mice

RATIONALE

Previous studies suggest that some behavioral effects of ethanol and morphine are genetically correlated. For example, mice bred for sensitivity (FAST) or insensitivity (SLOW) to the locomotor stimulant effects of ethanol differ in their locomotor response to morphine.

OBJECTIVE

To evaluate a possible common mechanism for these traits, we examined the effect of naloxone, an opioid receptor antagonist, on ethanol- and morphine-induced locomotion in FAST and SLOW mice, as well as on ethanol-induced locomotion in two heterogeneous stocks of mice.

METHOD

In experiments 1 and 2, naloxone was given to FAST and SLOW mice 30 min prior to 2 g/kg ethanol or 32 mg/kg morphine, and locomotor activity was measured for 15 min (ethanol) or 30 min (morphine). In experiments 3 and 4, naloxone was administered 30 min prior to 1.25 g/kg ethanol, and locomotor activity was assessed in FAST mice and in a heterogeneous line of mice [Withdrawal Seizure Control (WSC)]. Experiment 5 assessed the effect of naloxone on ethanol-induced stimulation in outbred National Institutes of Health (NIH) Swiss mice.

RESULTS

There was no effect of naloxone on the locomotor response to ethanol in FAST, SLOW, WSC, or NIH Swiss mice. However, naloxone did significantly attenuate the locomotor effects of morphine in FAST and SLOW mice.

CONCLUSIONS

These results suggest that a common opioidergic mechanism is not responsible for the correlated locomotor responses to ethanol and morphine in FAST and SLOW mice, and that activation of the endogenous opioid system is not critical for the induction of ethanol-induced alterations in activity.

Sensitivity to ketamine, alone or in combination with ethanol, is altered in mice selectively bred for sensitivity to ethanol's locomotor effects

BACKGROUND

Sensitivity to erthanol's locomotor activating and reinforcing effects may be influenced by some common neural mechanisms. Mice selectively bred in replicate for increased (FAST-1 and FAST-2) and decreased (SLOW-1 and SLOW-2) sensitivity to ethanol's locomotor stimulant effects are useful for investigating the neural substrates of ethanol's effects. Previous studies have suggested that differences in N-methyl-d-aspartate (NMDA) receptors may underlie differences in ethanol-induced locomotion in these mice. This study examined the responses of FAST and SLOW mice to ketamine, a fast-acting NMDA antagonist. In addition, reverse-selected lines (r-FAST-1, r-FAST-2, r-SLOW-1, and r-SLOW-2) were tested as a means of verifying correlations detected in the forward-selected lines. Two initial studies characterized ketamine-induced locomotion in DBA/2J (D2) mice, an inbred strain chosen for its high sensitivity to ethanol-induced locomotion.

METHODS

After a 2- to 3-day period of habituation to test procedures, mice were given intraperitoneal injections of ketamine alone (0, 5, 10, 20, 30, and 60 mg/kg) or in combination with 1 or 2 g/kg ethanol. Locomotor activity was measured for 20 to 30 min in automated activity monitors.

RESULTS

When administered alone, ketamine dose-dependently stimulated the locomotor activity of D2 mice and also reduced the amount of ethanol-induced stimulation. Ketamine stimulated locomotion more in FAST mice than in SLOW mice. Reverse selection abolished these differences, because r-FAST and r-SLOW mice did not differ in their responses to ketamine. Ketamine potentiated ethanol's locomotor effects within FAST mice and potentiated ethanol's locomotor depressant effect within one replicate of SLOW mice.

CONCLUSIONS

We propose that sensitivities to ethanol- and ketamine-induced locomotion are genetically correlated and that the combined effects of ethanol and ketamine in FAST mice reflect a leftward shift in ethanol's biphasic dose-response curve.

Open field activity and EtOH activation of gamma-PKC null mutants

Null mutants of the neural-specific gamma-isotype of protein kinase C (gamma-PKC) have demonstrated differential responses to acute administration of ethanol in comparison with wild-type animals. Previous studies have shown that the mutants are less sensitive to ethanol-induced loss of righting response. Null mutants also consume more ethanol and exhibit less behavioral inhibition. In order to determine if these sensitivity differences extend to ethanol activation of locomotor activity in an open-field arena, baseline activity and the effect of two low doses of ethanol were assessed in gamma-PKC null mutants and wild-type littermates. Null mutants demonstrated higher levels of baseline activity than did their wild-type counterparts. Further analysis revealed that a 1.0 g/kg dose of ethanol increased locomotor activity in males and females of both genotypes, whereas only null mutant males were activated by a 1.25 g/kg ethanol dose. The current study demonstrates that male gamma-PKC null mutants exhibit increased sensitivity to activating doses of ethanol in contrast to previous findings of decreased sensitivity to higher, depressive doses. This reflects the pleiotropic effects of the gamma-PKC null mutation on the behavioral effects of ethanol.

Reverse Selection for Differential Response to the Locomotor Stimulant Effects of Ethanol Provides Evidence for Pleiotropic Genetic Influence on Locomotor Response to Other Drugs of Abuse

BACKGROUND

Addictive drugs share the ability to induce euphoria, which may be associated with their potential for abuse. Replicate mouse lines with high (FAST-1, FAST-2) and low (SLOW-1, SLOW-2) sensitivity to ethanol-induced psychomotor stimulation (a possible animal model for the euphoria experienced by humans) have provided evidence for common genetic influences (pleiotropy) on sensitivity to the effects of ethanol and of GABA-A receptor acting compounds on locomotor activity. Differences between FAST and SLOW mice in locomotor response to certain other drugs were found later in selection. Reverse selection produced lines (r-FAST-1, r-FAST-2, r-SLOW-1, r-SLOW-2) with similar locomotor responses to ethanol. These lines are well suited for asking whether the same alleles that influence sensitivity to ethanol are also responsible for these later arising differences in drug sensitivity.

METHODS

Two replicate sets of forward- and reverse-selected FAST and SLOW lines were tested for the effects of multiple doses of morphine, cocaine, methamphetamine, nicotine, and scopolamine on their locomotor behavior. We predicted that differences in drug sensitivity between the FAST and SLOW lines would be reduced or eliminated in the reverse-selected lines.

RESULTS

Differences in sensitivity to morphine, cocaine, methamphetamine, and nicotine that arose in earlier generations of the FAST-1 and SLOW-1 lines ultimately also appeared in the FAST-2 and SLOW-2 lines. However, some differences between the FAST-2 and SLOW-2 lines (those in response to cocaine and methamphetamine) were not seen until several generations after selection had been relaxed. In lines reverse-selected for sensitivity to ethanol, differences in sensitivity to the other drugs were decreased, eliminated, or even reversed. No differences in scopolamine response were found in the replicate 1 forward- or reverse-selected lines. However, a small difference in scopolamine response in the replicate 2 lines was reversed.

CONCLUSIONS

Genes that influence the locomotor response to ethanol also influence locomotor response to other drugs with stimulant effects in the FAST and SLOW mice. The current data most strongly support this conclusion for sensitivity to morphine and nicotine.

Genetic analysis of the psychomotor stimulant effect of ethanol

Genetic influences on the psychomotor stimulant effect of ethanol may be a key feature of abuse liability. While earlier work has shown the activational effects of ethanol to be under the influence of a relatively uncomplicated additive genetic system, preliminary data from our laboratory suggested the possibility of nonadditive genetic variance. In the present study, a full Mendelian cross was conducted to further characterize gene action and search for quantitative trait loci (QTL) influencing the psychomotor stimulant properties of ethanol. We tested 3062 mice of the six Mendelian cross genotypes (P1, P2, F1, F2, BC1 and BC2) derived from a cross between the C57BL/6J (B6) and C3H/HeJ (C3H) inbred strains of mice. On day 1, mice were injected with saline, put in a holding cage for 5 min, then placed in an activity monitor for 5 min. On day 2, mice were injected with 1.5 g/kg ethanol, and activity again monitored for 5 min. Analysis showed the expected activation in the C3H strain and little activation in the B6 strain, with no effect of sex. Biometrical genetic analysis showed a best-fit model that included the mean (m), additive effect (a), and an epistatic parameter (i = homozygote by homozygote interaction). Analysis showed good evidence for QTL on chromosomes 1 (logarithm of odds (LOD) 3.4-7.5, 88-100 cM), 6 (LOD 9.1-10.4, 46-50 cM) and 15 (LOD 7.3-8.8, 28-32 cM). While the regions on chromosomes 1 and 6 have previously been implicated in several different ethanol-related phenotypes, this is the first report of a QTL influencing the psychomotor stimulant properties of ethanol on chromosome 15. Other studies have identified QTL in this region of chromosome 15 mediating locomotor activation caused by other psychostimulants, including cocaine, amphetamine and phencyclidine.

Ventral tegmental area region governs GABA(B) receptor modulation of ethanol-stimulated activity in mice

Locomotor stimulation in response to ethanol in mice may model human ethanol-induced euphoria. The associated neural substrates, possibly relevant to alcoholism, have not been fully elucidated. Systemic injection of baclofen, a GABA(B) receptor agonist, attenuates ethanol's stimulant effects. GABA(B) receptors on dopamine cell bodies in the ventral tegmental area (VTA) may modulate ethanol-induced dopamine release, a postulated mechanism for ethanol's stimulant effects. However, baclofen's attenuating effects could be associated with peripheral receptor actions. Baclofen was injected i.c.v. or into the VTA of FAST mice, bred for extreme sensitivity to ethanol-induced locomotor stimulation, to test the hypotheses that (1) central GABA(B) receptors influence baclofen's effects on ethanol-stimulated activity, and (2) VTA GABA(B) receptors specifically modulate ethanol's stimulant effects. I.c.v. baclofen dose-dependently attenuated ethanol stimulation, supporting a central locus for baclofen's effects. Anterior VTA baclofen also attenuated ethanol stimulation. However, more posterior VTA infusions unexpectedly potentiated ethanol stimulation. In SLOW mice, bred for resistance to ethanol stimulation, posterior intra-VTA baclofen did not alter EtOH response. However, anterior VTA baclofen alone produced a locomotor depressant effect in SLOW mice, not seen in FAST mice. GABA(B) receptor autoradiography using [(3)H]CGP 54626, a potent GABA(B) receptor antagonist, did not reveal line differences in binding density in the VTA, or in the substantia nigra pars compacta, a nearby brain structure associated with motor control. These results suggest that anterior VTA GABA(B) receptors play a role in baclofen's attenuation of ethanol's stimulant effects, and that posterior VTA GABA(B) receptors serve an opposite role that is normally masked. Selection for differential ethanol stimulant sensitivity has altered VTA GABA(B) systems that influence locomotor behavior. However, differences in GABA(B) receptor densities in the VTA or substantia nigra pars compacta cannot explain the selected line difference.

Locomotor activity responses to ethanol, other alcohols, and GABA-A acting compounds in forward- and reverse-selected FAST and SLOW mouse lines

Mice selectively bred for high (FAST) or low (SLOW) locomotor stimulant response to ethanol have been found to differ in response to drugs with gamma-aminobutyric acid (GABA)-ergic actions. Reverse selection produced lines that are similar in sensitivity to ethanol stimulation (r-FAST and r-SLOW) and provided a unique model for testing hypotheses about shared genetic influence on sensitivity to ethanol and GABAergic drugs. FAST mice were more stimulated than SLOW mice by all drugs tested: ethanol, methanol, n-propanol, t-butanol, pentobarbital, diazepam, and allopregnanolone. In contrast, r-FAST and r-SLOW mice differed in sensitivity to only a few isolated drug doses. Locomotor responses of each reverse-selected line were significantly different from the responses of their respective forward-selected line for all drugs. Results support an effect of selection for ethanol sensitivity on allosteric modulation of the GABA-A receptor.

Strain and sex differences in repeated ethanol treatment-induced motor activity in quasi-congenic mice

The B6.C quasi-congenic Recombinant QTL Introgression (RQI) strains of the b4i5 series have similar genetic background, but differ in about 5% of their genome from the C57BL/6ByJ (B6) background strain because they carry short chromosome segments introgressed from the BALB/cJ (C) donor strain. These RQI strains were derived from mouse lines selectively bred for high activity of mesencephalic tyrosine hydroxylase (TH/MES), therefore genetic variation in dopamine system-related behaviours, such as ethanol-induced motor activity, can be expected. Males and females of 17 RQI and two progenitor strains were tested for initial motor activity for 15 min after a habituating injection of saline, which was followed by an i.p. injection of saline or ethanol (2 g/kg) and an additional test of motor activity for 30 min. This procedure was repeated during 4 subsequent days. In all strains, the first-day ethanol treatment showed an inhibitory effect. With repetition of the treatment the inhibitory effect decreased, and a stimulatory effect could be observed with significant strain- and sex-dependent variation. Females exhibited higher activity in the saline group than males, and reached an equilibrium of inhibition and stimulation sooner than males with repetition of the ethanol treatment. The highest (> 25-fold) difference in activity after repeated ethanol treatment was detected between females of the two strains B6.Cb4i5-Alpha4/Vad and B6.Cb4i5-Beta13/Vad. These results firstly suggest that females are more sensitive to repeated ethanol exposure than males, secondly they support the observations that ethanol has both inhibitory and stimulatory effects on motor activity, which are affected by sex, genotype, and repetition of treatment, and thirdly offer new quasi-congenic animal models with highly different responses to ethanol allowing one to more quickly move to gene detection.

Ethanol-induced activation and rapid development of tolerance may have some underlying genes in common

Ethanol exerts biphasic effects on behavior, stimulant at low doses and depressant at higher doses. In the present study we used two mouse genetic models to investigate the relationships among activating and depressant responses to alcohol. The first model was a panel of nine isogenic genotypes. FAST and SLOW mice, selectively bred for high and low ethanol-induced motor activation, respectively, were used as a second model. We used loss of righting reflex to assess initial sensitivity and acute functional tolerance to a hypnotic dose of ethanol (3 g/kg, 20% v/v). Blood ethanol concentration at the onset of loss of righting reflex was used as an estimate of initial sensitivity, while the difference between concentration values at the recovery and loss of righting represented an acute functional tolerance score. Mean initial sensitivity and acute functional tolerance values of the nine strains were correlated with a previously obtained measure of ethanol-induced locomotor activation. Activation correlated significantly with both initial sensitivity (rg = 0.80; P < 0.05) and acute functional tolerance (rg = 0.77; P < 0.05). Thus, inbred genotypes that were activated more by a low dose of ethanol were also more sensitive to and developed more acute tolerance to a high dose. FAST mice had initial sensitivity values similar to those of SLOW mice, but developed more pronounced tolerance, indicating that ethanol-induced activation and acute functional tolerance may be regulated by some common genetic mechanisms. In summary, these results supported a genetic association between ethanol-induced activation and rapid development of tolerance.

Sensitivity to the locomotor stimulant effects of ethanol and allopregnanolone is influenced by common genes

Allopregnanolone is a neuroactive steroid that, like ethanol (EtOH), has stimulant, anxiolytic, ataxic, and depressant effects. Two experiments tested the hypothesis that sensitivity to the locomotor stimulant effects of these drugs is influenced by a common set of genes. Sensitivity to the locomotor stimulant effects of allopregnanolone was determined in 24 BXD recombinant inbred (RI) strains. Strain means were positively correlated with extant means for EtOH stimulation in 20 of the same strains. Quantitative trait locus (QTL) analysis provisionally identified many loci, including several known to influence sensitivity to EtOH. Sensitivity to allopregnanolone was also measured in FAST and SLOW mice, which were selectively bred for differential locomotor response to EtOH, to determine whether selection has also altered allopregnanolone sensitivity. FAST mice were more sensitive to the stimulant effects of allopregnanolone compared with SLOW mice. These data suggest that sensitivity to the locomotor stimulant effects of these drugs is influenced by common genes.

Sensitivity to ethanol-induced motor incoordination in FAST and SLOW selectively bred mice

Earlier studies using the grid test have indicated a negative genetic correlation between sensitivity to ethanol-induced locomotor stimulation and ethanol-induced motor incoordination in FAST and SLOW mice, lines selectively bred for differential sensitivity to ethanol's stimulant effects. Because different tests of motor coordination may not measure the same behavioral competencies or physiological substrates, the present experiments tested adult ethanol- or saline-exposed FAST and SLOW mice of two replicates (FAST-1, FAST-2, SLOW-1, and SLOW-2) using three additional tests of coordination: a stationary dowel, fixed-speed rotarod, and accelerating rotarod. After ethanol treatment, FAST-1 mice fell from the stationary dowel at shorter latencies than SLOW-1 mice, suggesting that they had relatively greater sensitivity to ethanol. However, brain ethanol concentrations (BrECs) were similar at time of fall, and no differences were found between replicate-2 lines. SLOW-1 mice fell from the fixed-speed rotarod at lower BrECs than FAST-1 mice, suggesting possibly greater sensitivity of the SLOW-1 line. Again, no replicate-2 line differences were found. No significant differences were detected for the accelerating rotarod. These results provide little support for a negative genetic relationship between sensitivity to the stimulant and ataxic effects of ethanol using these measures of motor coordination.

Identification of an acute ethanol response quantitative trait locus on mouse chromosome 2

A two-stage strategy was used to identify and confirm quantitative trait loci (QTLs) associated with the changes in locomotor activity induced by a 1.5 gm/kg ethanol challenge. For stage 1, putative QTLs were identified by analysis of the strain means for 25 strains of the BXD recombinant inbred (RI) series (males only). QTLs were identified on chromosomes 1, 2, 4, and 6. The activity response to chlordiazepoxide generated similar QTLs on chromosomes 2 and 6. None of the QTLs were similar to those generated from analysis of the saline response data. For stage 2, 900 male C57BL/6J (B6) x DBA/2J (D2) F2 intercross animals were phenotyped for ethanol response, and the phenotypic extremes (those animals > and <1 SD from the mean) were identified. These extremes differed by >10,000 cm/15 min in their response to ethanol. The extreme progeny were used for a genome-wide scan both to confirm the putative RI-generated QTLs and to detect new QTLs. The F2 analysis generated no new QTLs with logarithm of the likelihood for linkage (LOD) scores >3. For RI-generated QTLs, only the QTL on chromosome 2 was confirmed (LOD = 5.3). The position of the peak LOD was estimated to be 47 cM with a 20 cM 1 LOD support interval; this QTL accounted for 6% of the phenotypic variance. The 1 LOD support interval overlaps with QTLs previously identified for alcohol preference and acute ethanol withdrawal (;; ).

Ontogeny of ethanol-induced locomotor activity and hypothermia differences in selectively bred FAST and SLOW mice

The replicate lines of selectively bred FAST and SLOW mice differ in locomotor response to 2 g/kg ethanol (EtOH). FAST mice show enhanced locomotion; SLOW mice exhibit no change or locomotor depression. Little is known about the responses of FAST and SLOW mice to EtOH during development. We assessed the locomotor responses of FAST and SLOW mice at postnatal days (P) 10, 15, 30, and 60. A genetically correlated response, EtOH-induced hypothermia, was also investigated. Although all animals demonstrated their respective selection phenotypes in adulthood, developing FAST mice exhibited ethanol stimulation by P15 (replicate 1) or P30 (replicate 2). At these ages, responses of FAST mice differed from those of SLOW. The stimulant response in FAST mice was adult-like at P30. EtOH-induced hypothermia was seen in SLOW mice by P15. These data suggest that sensitivity to the locomotor stimulant effects of EtOH changes during postnatal development, and may mirror developmental profiles for certain neurotransmitter systems.

Behavioral sensitization to ethanol: genetics and the effects of stress

Some aspects of drug abuse syndromes may be influenced by sensitization to some drug effects. This enhancement of drug effect has been associated with prior drug exposure and with exposure to stressful stimuli. It has been postulated that sensitization to psychomotor stimulant drug effects influences sensitivity to drug reward. The drugs of abuse best characterized for sensitization phenomena include cocaine, amphetamine, and morphine. In general, ethanol's molecular mechanisms of action have been difficult to define relative to drugs with known receptor or transporter binding sites and, likewise, ethanol sensitization has been less thoroughly examined. Evidence supporting the existence of behavioral sensitization to ethanol, for genetic differences in the occurrence of ethanol sensitization, and for the influence of corticosterone on the development of ethanol sensitization is reviewed herein. There appear to be different genetic determinants of acute drug sensitivity and sensitization. Cross-sensitization between stress and ethanol suggest a potential role for hypothalamic-pituitary-adrenal (HPA) axis associated changes in ethanol sensitization, consistent with mechanisms likely contributing to sensitization to other abused drugs. Furthermore, glucocorticoid receptors appear to mediate both ethanol- and stress-induced sensitization to ethanol. A biological link between drug reward and drug sensitization involving HPA axis hormones may exist and, thus, study of the sensitization process may elucidate mechanisms relevant to drug abuse.

Dose- and conditioning trial-dependent ethanol-induced conditioned place preference in Swiss-Webster mice

The motivational effects of ethanol were examined in Swiss-Webster mice using an unbiased place conditioning, design. Adult male Swiss-Webster mice received six 5-min pairings of a tactile stimulus with different doses of ethanol (1, 2, 3, or 4 g/kg. IP). A different tactile stimulus was paired with saline injections. A 60-min preference test was given after the first four conditioning trials and an additional 30-min preference test after the sixth conditioning trial. During conditioning, ethanol initially produced locomotor stimulation at the 2 g/kg dose and locomotor depression at the 4 g/kg dose. However, after repeated ethanol exposure, all doses produced overall increases in activity relative to saline, suggesting sensitization to ethanol's stimulant effect. After four conditioning trials ethanol-induced conditioned place preference was noted in mice receiving 3 and 4 g/kg ethanol. After two additional conditioning trials all ethanol doses produced conditioned place preference. These results indicate that ethanol has dose-dependent rewarding effects measured in an unbiased place-conditioning paradigm using a standard outbred mouse strain. Further, additional place-conditioning trials enhance the development of preference at lower (1 or 2 g/kg) ethanol doses.

Correlated responses to selection in FAST and SLOW mice: effects of ethanol on ataxia, temperature, sedation, and withdrawal

A replicated bidirectional selective breeding program has produced lines of mice that differ in locomotor response to ethanol (EtOH). FAST mice were bred for high locomotor activation, whereas SLOW mice were bred for low or depressed locomotor activity in response to 2.0 g/kg of EtOH. We tested FAST and SLOW mice for differences in sensitivity to the incoordinating (1.5 to 2.5 g/kg), hypothermic (3.0 g/kg), and sedative (4.0 g/kg) effects of EtOH, and for differences in sensitivity to withdrawal after acute and chronic EtOH exposure. SLOW mice were more ataxic in a grid test and developed greater tolerance than FAST mice at 2.0 g/kg of EtOH, were more hypothermic than FAST mice, and were more sensitive to the sedative effects of EtOH than FAST mice, as measured by latency to and duration of loss of righting reflex, and by blood ethanol concentrations at regain of the righting reflex. FAST mice had more severe withdrawal seizures after chronic exposure, but did not differ from SLOW mice in withdrawal severity after an acute injection of EtOH. These data suggest that FAST mice are generally more sensitive to central nervous system excitation, and SLOW mice are generally more sensitive to central nervous system sedation by EtOH, and further suggest genetic overlap with respect to genes that mediate locomotor responses to EtOH and genes determining sensitivity to EtOH-induced ataxia, hypothermia, sedation, and withdrawal severity after chronic exposure. Our current observations are in contrast to observations made earlier in selection, in which few line differences in sensitivity to EtOH effects other than locomotor activity were found. Thus, it seems that continued selection for differences in locomotor response to EtOH has produced genetically correlated differences in other EtOH responses.

Ethanol self-administration is genetically independent of locomotor stimulation in fast and slow mice

One substance abuse hypothesis proposes that rewarding effects of drugs are causally related to their psychostimulant effects. We examined this hypothesis by comparing operant self-administration of ethanol in mice selectively bred for either high (Fast) or low (Slow) locomotor stimulation response to ethanol. Mice were trained to lever press for ethanol using postprandial induction and were then tested over a range of conditions to determine the degree of self-administration. There were no significant differences between Fast and Slow mice in either the amount of work produced to obtain ethanol or the amount of ethanol consumed. In general, none of the groups of mice showed robust ethanol-reinforced behavior. This is in contrast with C57BL/6J mice tested concurrently, which showed substantial ethanol-reinforced behavior. Further analysis revealed individual differences in responding within each of the selected lines. However, there was no systematic pattern within or between groups for these individual differences, suggesting that the genes mediating ethanol-reinforced behavior are segregating in a manner independent from genes mediating the locomotor stimulant response to ethanol, and thus, the mechanistic processes mediating reinforcement from ethanol are distinct from those that influence the psychomotor stimulant response to this drug.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Bidirectional selective breeding for ethanol effects on locomotor activity: characterization of FAST and SLOW mice through selection generation 35

Increased recognition of the advantages of genetic animal models has led to heightened interest in their use and development. A replicated bidirectional selective breeding project has produced lines of mice that differ in their locomotor responses to 2.0 g/kg ethanol. FAST-1 and FAST-2 mice are highly stimulated by ethanol (EtOH), whereas SLOW-1 and SLOW-2 mice are either not affected or respond with locomotor depression. Current heritability estimates indicate that approximately 6-8% of the response variance in the FAST lines and 2-10% of the response variance in the SLOW lines is of additive genetic origin. Little systematic response to selection has occurred in recent generations, which implies that the limits of selection have been reached. Analysis of saline activity over 35 generations of selection indicates that baseline activities have not changed during the course of selection in three of the lines, whereas baseline activity of FAST-1 mice has increased slightly. In EtOH dose-response studies (0.5-3.0 g/kg), FAST mice had biphasic dose-response curves, whereas the locomotor activity of SLOW mice was either unaffected or depressed by all doses of EtOH. In addition, FAST mice spent more time in motion, traveled farther per movement, traversed greater distances in the center of the test chamber, and ambulated more quickly than SLOW mice when given EtOH. FAST and SLOW mice differed in EtOH clearance rates; however, the differences were slight relative to the large difference in locomotor response. We encourage the use of FAST and SLOW mice to investigate neurophysiological factors underlying sensitivity to the behavioral effects of EtOH, with a view to further testing of the postulated homology between locomotor stimulant effects and addiction potential of drugs of abuse.

Strategies for mapping and identifying quantitative trait loci specifying behavioral responses to alcohol

Most responses to alcohol in both humans and animals are heritable, and this genetic sensitivity to ethanol is determined by multiple genes. However, the number of genes, their identities, and just how they determine susceptibility to the actions of alcohol are unknown. Herein, we describe a multistage strategy for mapping quantitative trait loci (QTLs) using recombinant inbred strains and F2 mice. Precise mapping of the chromosome positions of these QTLs should increase our understanding of the genetic causes for individual differences in behavioral sensitivity to alcohol by (1) identifying genomic markers associated with sensitivity to alcohol, (2) allowing the genes specifying behavior to be cloned by position, and (3) elucidating "candidate" genes demonstrating linkage to markers associated with behavioral responses to alcohol. Syntenic conservation between the mouse and human genomes should facilitate the eventual mapping and cloning of human homologs of these QTLs. Ultimately, cloning of these genes may allow the development of gene therapies or other therapeutic interventions for management or prevention of alcoholism and alcohol abuse.

Localization of genes influencing ethanol-induced conditioned place preference and locomotor activity in BXD recombinant inbred mice

Genetic differences in ethanol's ability to induce conditioned place preference were studied in 20 BXD Recombinant Inbred (RI) mouse strains and in the C57BL/6J and DBA/2J progenitor strains. Male mice from each strain were exposed to a Pavlovian conditioning procedure in which a distinctive floor stimulus (CS+) was paired four times with ethanol (2 g/kg). A different floor stimulus (CS-) was paired with saline. Control mice were injected only with saline. Floor preference testing without ethanol revealed significant genetic differences in conditioned place preference, with some strains spending nearly 80% time on the ethanol-paired floor while others spent only 50% (i.e., no preference). Control mice showed genetic differences in unconditioned preference for the floor cues, but unconditioned preference was not genetically correlated with conditioned preference. There were also substantial genetic differences in ethanol-stimulated activity, but contrary to psychomotor stimulant theory, ethanol-induced activity on conditioning trials was not positively correlated with strength of conditioned place preference. However, there was a significant negative genetic correlation (r = -0.42) between test session activity and preference. Quantitative trait loci (QTL) analyses showed strong associations (P < 0.01) between conditioned place preference and marker loci on chromosomes 4, 8, 9, 18 and 19. Weaker associations (0.01 < P < 0.05) were identified on several other chromosomes. Analysis also yielded several significant QTL for unconditioned preference, ethanol-stimulated activity, and sensitization. Overall, these data support the conclusion that genotype influences ethanol-induced conditioned place preference, presumably via genetic differences in sensitivity to ethanol's rewarding effects. Moreover, several chromosomal regions containing candidate genes of potential relevance to ethanol-induced conditioned place preference have been identified.

Effects of acute and repeated ethanol exposures on the locomotor activity of BXD recombinant inbred mice

Investigations of ethanol's (EtOH's) complex response profile, including locomotor and other effects, are likely to lead to a more in-depth understanding of the constituents of alcohol addiction. Locomotor activity responses to acute and repeated EtOH (2 g/kg, ip) exposures were measured in BXD recombinant inbred (RI) mice and their C57BL/6J (B6) and DBA/2J (D2) progenitors. Both the acute response and the change in initial EtOH response with repeated treatments were strain-dependent. The coefficient of genetic determination was 0.38-0.49 for initial locomotor response to EtOH, and 0.29 for change in response. Changes in response were largely attributable to sensitization of locomotor stimulation. Quantitative trait loci (QTL) analyses identified significant marker associations with basal activity, acute locomotor response, and change in response. Markers were for QTL on several chromosomes, and there was only one case of overlap in marker associations among phenotypes. Acute locomotor response and locomotor sensitization were negatively correlated with 3% EtOH preference drinking data collected in BXD RI strains. Overall, these results demonstrate locomotor sensitization induced by EtOH, suggest independence of genetic determination of locomotor responses to acute and repeated EtOH exposure, and partially support a relationship between reduced sensitivity to the locomotor stimulant/sensitizing effects of EtOH and EtOH consumption.