Environmental variables differentially affect ethanol-stimulated activity in selectively bred mouse lines

Insensitivity of the analysis of variance to heredity-environment interaction

It makes sense to attribute a definite percentage of variation in some measure of behavior to variation in heredity only if the effects of heredity and environment are truly additive. Additivity is often tested by examining the interaction effect in a two-way analysis of variance (ANOVA) or its equivalent multiple regression model. If this effect is not statistically significant at the α = 0.05 level, it is common practice in certain fields (e.g., human behavior genetics) to conclude that the two factors really are additive and then to use linear models, which assume additivity. Comparing several simple models of nonadditive, interactive relationships between heredity and environment, however, reveals that ANOVA often fails to detect nonadditivity because it has much less power in tests of interaction than in tests of main effects. Likewise, the sample sizes needed to detect real interactions are substantially greater than those needed to detect main effects. Data transformations that reduce interaction effects also change drastically the properties ofthe causal model and may conceal theoretically interesting and practically useful relationships. If the goal ofpartitioning variance among mutually exclusive causes and calculating “heritability” coefficients is abandoned, interactive relationships can be examined more seriously and can enhance our understanding of the ways living things develop.

Chronic ethanol disrupts circadian photic entrainment and daily locomotor activity in the mouse

BACKGROUND

Chronic ethanol abuse is associated with disrupted circadian rhythms and sleep. Ethanol administration impairs circadian clock phase-resetting, suggesting a mode for the disruptive effect of alcohol abuse on circadian timing. Here, we extend previous studies to explore the effects of chronic forced ethanol on photic phase-resetting, photic entrainment, and daily locomotor activity patterns in C57BL/6J mice.

METHODS

First, microdialysis was used to characterize the circadian patterns of ethanol uptake in the suprachiasmatic (SCN) circadian clock and correlate this with systemic ethanol levels and episodic drinking of 10 or 15% ethanol. Second, the effects of chronic forced ethanol drinking and withdrawal on photic phase-delays of the circadian activity rhythm were assessed. Third, the effects of chronic ethanol drinking on entrainment to a weak photic zeitgeber (1 minute of 25 lux intensity light per day) were assessed. This method was used to minimize any masking actions of light that could mask ethanol effects on clock entrainment.

RESULTS

Peak ethanol levels in the SCN and periphery occurred during the dark phase and coincided with the time when light normally induces phase-delays in mice. These delays were dose-dependently inhibited by chronic ethanol and its withdrawal. Chronic ethanol did not impede re-entrainment to a shifted light cycle but affected entrainment under the weak photic zeitgeber and disrupted the daily pattern of locomotor activity.

CONCLUSIONS

These results confirm that chronic ethanol consumption and withdrawal markedly impair circadian clock photic phase-resetting. Ethanol also disturbs the temporal structure of nighttime locomotor activity and photic entrainment. Collectively, these results suggest a direct action of ethanol on the SCN clock.

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.

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.

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.

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.

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.

Differential activating effects of ethanol in C57BL/6Abg and DBA/2Abg mice

A characteristic pattern of ETOH-induced locomotor activation in the DBA/2Abg strain (D2), small activation or sedation in the C57BL/6Abg (B6), and an intermediate position of the F1s was found using a between-group design and 1.5 g/kg ETOH. This pattern was consistent for a variety of behavioral indices not previously examined, including distance, rest time, movement speed and length, as well as the traditional horizontal counts. Using a within-subject, multiple day, repeated-testing procedure, the same three genotypes were also assessed after manipulating drug administration order, where ETOH exposure (1.5 g/kg) was on either the first or second test day. Another experiment examined the effect of lighting level on the response to 1.5 g/kg ETOH using a within-subjects approach. Neither the testing order nor lighting condition had any major influence on the magnitude of activation as measured by locomotor activity (distance). The pattern of additive genotypic influences exhibited by the B6, D2, and F1 mice is remarkably resistant to these contextual and procedural manipulations.

Response to selection for ethanol-induced locomotor activation: genetic analyses and selection response characterization

Selectively bred FAST mice are highly susceptible, while SLOW mice are less susceptible, to the locomotor stimulant effects of ethanol. Heritability estimates indicate that approximately 15% of the variance in the FAST lines is of additive genetic origin, while low susceptibility is ostensibly nonheritable. Inbreeding has increased at the rate of 2% per generation, but fertility has been unaffected. Measurement reliability for sensitivity to this ethanol effect was high when measured in both circular (r = 0.6) and square (r = 0.7) open-fields. In addition, our results indicate that we have selected for differences in sensitivity to ethanol rather than for differences in habituation to the test environment. The difference in response to ethanol between FAST and SLOW mice extended to tests varying in duration, and to a range of ethanol doses. We conclude that the divergence between FAST and SLOW mice generalizes to related test parameters, and speculate that the genetic architecture underlying the locomotor stimulant response may be simpler than previously proposed.

Further characterization of LSxSS recombinant inbred strains of mice: activating and hypothermic effects of ethanol

Lines of mice selected for differential initial sensitivity to the anesthetic effects of ethanol also differ in their locomotor responses to lower doses of ethanol. Sixteen recombinant inbred strains of mice derived from long-sleep (LS) and short-sleep (SS) selected lines as well as inbred LS and SS mice were used in a genetic correlational study to investigate possible associations between high-dose and low-dose indices of initial sensitivity to ethanol. Measurements of high-dose (4.1 g/kg) effects of ethanol were hypothermia, sleep time, and blood ethanol content at regaining of righting response, and the index of low-dose (1.875 g/kg) sensitivity was distance traveled during a 5-min period immediately following intraperitoneal injection with ethanol. The results indicated wide genetic variation in hypothermia and ethanol-induced locomotor activation in a manner consistent with polygenic influence. Furthermore, correlations between low-dose locomotor activity and hypnotic dose effects tended to be low and nonsignificant, indicating independence of inherited mechanisms underlying high- and low-dose ethanol sensitivity.

Selected mouse lines, alcohol and behavior

The technique of selective breeding has been employed to develop a number of mouse lines differing in genetic sensitivity to specific effects of ethanol. Genetic animal models for sensitivity to the hypnotic, thermoregulatory, excitatory, and dependence-producing effects of alcohol have been developed. These genetic animal models have been utilized in numerous studies to assess the bases for those genetic differences, and to determine the specific neurochemical and neurophysiological bases for ethanol's actions. Work with these lines has challenged some long-held beliefs about ethanol's mechanisms of action. For example, lines genetically sensitive to one effect of ethanol are not necessarily sensitive to others, which demonstrates that no single set of genes modulates all ethanol effects. LS mice, selected for sensitivity to ethanol anesthesia, are not similarly sensitive to all anesthetic drugs, which demonstrates that all such drugs cannot have a common mechanism of action. On the other hand, WSP mice, genetically susceptible to the development of severe ethanol withdrawal, show a similar predisposition to diazepam and phenobarbital withdrawal, which suggests that there may be a common set of genes underlying drug dependencies. Studies with these models have also revealed important new directions for future mechanism-oriented research. Several studies implicate brain gamma-aminobutyric acid and dopamine systems as potentially important mediators of susceptibility to alcohol intoxication. The stability of the genetic animal models across laboratories and generations will continue to increase their power as analytic tools.