Strain differences in three measures of ethanol intoxication in mice: the screen,dowel and grip strength tests

Mice from 8 to 21 inbred strains were tested for sensitivity to ethanol intoxication using a range of doses and three different measures: the screen test, the dowel test and a test of grip strength. Strains differed under nearly all conditions. For the dowel test, two dowel widths were employed, and mice were tested immediately or 30 min after ethanol. For the dowel and screen tests, low doses failed to affect some strains, and the highest doses failed to discriminate among mice, maximally affecting nearly all. For grip strength, a single ethanol dose was used, and mice of all strains were affected. Pharmacokinetic differences among strains were significant, but these could not account for strain differences in intoxication. For doses and test conditions in the middle range, there were only modest correlations among strain means within a test. In addition, genotypic correlations across tests were modest to quite low. These results suggest that different specific versions of a test reflect the influence of different genes, and that genetic influences on different tests were also distinct.     

Characterization of the parallel rod floor apparatus to test motor incoordination in mice

Impairment of motor coordination, or ataxia, is a prominent effect of alcohol ingestion in humans. To date, many models have been created to examine this phenomenon in animals. Evidence suggests that the tasks thought to measure this behavior in mice actually measure different components of this complex trait. We have characterized the parallel rod floor apparatus to quantify ethanol-induced motor incoordination. Using genetically heterogeneous mice, we evaluated the influence of rod diameter and inter-rod distance on dose-related ethanol-induced motor incoordination to select parameters that optimized testing procedures. We then used the DBA/2J and C57BL/6J inbred strains of mice to examine the effect of 2 g/kg of ethanol, by serially testing mice on two floor types, separated by 1 week. Finally, we tested eight inbred strains of mice on four floor types to examine patterns of strain sensitivity to 2 g/kg of intraperitoneal ethanol and determined the test parameters that maximized strain effect size. Motor incoordination varied depending on the floor type and strain. When data from strain 129S1/SvlmJ were removed from the analyses because of their extreme behavior, the greatest strain effect size was observed on one floor type during the first 10 min of testing after 2 g/kg of intraperitoneal ethanol. These findings suggest that the parallel rod floor apparatus provides a useful means for examining ethanol-induced motor incoordination in mice but that specific testing procedures are important for optimizing detection of motor incoordination and genetic influences.     

Comparison between multiple behavioral effects of peripheral ethanol administration in rats: sedation, ataxia, and bradykinesia

Although low doses of systemic ethanol stimulate locomotion in mice, in rats the typical response to peripheral ethanol administration is a dose-dependent suppression of motor activity. In the present study, male rats received acute doses of ethanol IP (0.0, 0.25, 0.5, 1.0 or 2.0 g/kg) and were tested on several behavioral tasks related to the motor suppressive or sedative effects of the drug. This research design allowed for comparisons between the effects of ethanol on different behavioral tasks in order to determine which tasks were most sensitive to the drug (i.e., which tasks would yield deficits that appear at lower doses). In the first two experiments, rats were evaluated on a sedation rating scale, and ataxia/motor incoordination was assessed using the rotarod apparatus. Administration of 2.0 g/kg ethanol produced sedation as measured by the sedation scale, and also impaired performance on the rotarod. In a third experiment, ethanol reduced locomotion in the stabilimeter at several doses and times after IP injection, with 0.25 g/kg being the lowest dose that produced a significant decrease in locomotion. Finally, experiment four studied the effects of ethanol on operant lever pressing reinforced on a fixed ratio 5 (FR5) schedule for food reinforcement. Data showed suppressive effects on lever pressing at doses of 1.0, and 2.0 g/kg ethanol. Analysis of the interresponse time distribution showed that ethanol produced a modest slowing of operant responding, as well as fragmentation of the temporal pattern of responding and increases in pausing. Taken together, these results indicate that rats can demonstrate reduced locomotion and slowing of operant responding at doses lower than those that result in sedation or ataxia as measured by the rotarod. The detection of subtle changes in different motor test across a broad range of ethanol doses is important for understanding ethanol effects in other cognitive, motivational or sensory processes.     

Genetically determined differences in ethanol sensitivity influenced by body temperature during intoxication

The present study investigated the importance of body temperature during intoxication in mediating differences between five inbred strains of mice (C57BL/6J; BALB/cJ; DBA/2J; A/HeJ; 129/J) in their acute sensitivity to the hypnotic effects of ethanol. Mice exposed to 22 degrees C after ethanol injection became hypothermic and exhibited statistically significant differences between strains in rectal temperatures at the return of the righting reflex (RORR), duration of loss of the righting reflex (LORR), and blood and brain ethanol concentrations at RORR. Exposure to 34 degrees C after injection offset ethanol-hypothermia and markedly reduced strain-related differences in rectal temperatures and blood and brain ethanol concentrations at RORR. Brain ethanol concentrations at RORR were significantly lower in C57, BALB, DBA and A/He mice exposed to 34 degrees C compared to mice exposed to 22 degrees C during intoxication suggesting that offsetting hypothermia increased ethanol sensitivity in these strains. Taken with previous in vitro studies, these results suggest that genetically determined differences in acute sensitivity to the behavioral effects of ethanol reflect differences in body temperature during intoxication as well as differences in sensitivity to the initial actions of ethanol at the cellular level.     

Neuroprotective effect of small heat shock protein,Hsp27, after acute and chronic alcohol administration

Alcohol induces degeneration of neurons and inhibits neurogenesis in the brain. Small heat shock proteins are able to protect neurons in cerebral ischemia and oxidative stress. In this study, we investigated the neuroprotective effect of small heat shock protein, Hsp27, after acute and chronic ethanol administrations using transgenic mice overexpressing the human Hsp27 protein. Transgenic mice and wild-type littermates were injected with 2 g/kg ethanol intraperitoneally, and then motor coordination and muscle strength were analyzed using different behavioral tests, such as footprint analysis, balance beam, and inverted screen tests. Ethanol-injected transgenic mice showed similar footprints to control saline-injected mice, did not fall of the beam, and were able to climb to the top of the inverted screen, while wild-type mice showed ataxia and incoordination after ethanol injection. The effect of Hsp27 on chronic ethanol consumption was also investigated. Drinking water of mice was replaced by a 20% ethanol solution for 5 weeks, and then brain sections were stained with Fluoro Jade C staining. We found significantly lesser amount of degenerating neurons in the brain of ethanol-drinking transgenic mice compared to wild-type mice. We conclude that Hsp27 can protect neurons against the acute and chronic toxic effects of ethanol.     

Effects of ethanol on locomotor and anxiety-like behaviors and the acquisition of ethanol intake in Lewis and spontaneously hypertensive rats

The purpose of the present study was to investigate whether Lewis (LEW) and spontaneously hypertensive rats (SHR), characterized in numerous behavioral tests as strains with high-anxiety and low-anxiety, respectively, could differ in their sensitivity to the effects of ethanol in the elevated plus maze (EPM) and the open field (OF), two classical models of anxiety/emotionality, as well as in the acquisition of ethanol drinking behavior. It was also of interest to examine the relationship between sweet and bitter fluids preference and ethanol intake. SHR and LEW rats were given saline or ethanol injections (0.6 or 1.2 g/kg, ip.) and tested in the EPM and OF. Subsequently the same animals were given continuous free choice between water and ethanol solution (2-8%). Additional groups of animals were exposed to a free-choice regimen between saccharin (0.002-0.09%) or quinine (0.0001-0.0015%) and water. The low dose of ethanol (0.6 g/kg) induced anxiolytic-like effects and intensive locomotor activation mainly in SHR rats tested in the OF arena. Overall, LEW counterparts were unaffected in OF test. In oral self-administration paradigm, SHR rats consumed significantly more ethanol than LEW rats. Concerning other solutions, SHR rats consumed large amounts of saccharin compared with LEW rats. These data indicate that the SHR preference for ethanol intake may be positively related to their differential sensitivity to the anxiolytic/stimulant effects of ethanol and to the sensitivity of this strain for saccharin reinforcement. In addition, these findings provide evidence that the SHR strain may represent a useful genetic and pharmacological tool to investigate ethanol drinking traits.     

Alcohol intoxication: ion channels and genetics

Acute in vitro exposure to ethanol and other intoxicant-anesthetics activates gamma-aminobutyric acid (GABA)-stimulated chloride channels and inhibits voltage-dependent calcium and sodium channels of isolated brain membranes. The question of whether these neurochemical actions are responsible for intoxication in vivo has been addressed using animal populations displaying genetic differences in sensitivity to alcohol and benzodiazepine intoxication. These genetic approaches include inbred strains, selected lines, recombinant inbred strains, and heterogeneous stocks. Genetic differences in ion channel function provide strong evidence for a role of the GABA-stimulated chloride channel in ethanol and benzodiazepine intoxication; the role of calcium and sodium channels is less clear.     

Mouse inbred strain differences in ethanol drinking to intoxication

Recently, we described a simple procedure, Drinking in the Dark (DID), in which C57BL/6J mice self-administer ethanol to a blood ethanol concentration (BEC) above 1 mg/ml. The test consists of replacing the water with 20% ethanol in the home cage for 4 h early during the dark phase of the light/dark cycle. Three experiments were conducted to explore this high ethanol drinking model further. In experiment 1, a microanalysis of C57BL/6J behavior showed that the pattern of ethanol drinking was different from routine water intake. In experiment 2, drinking impaired performance of C57BL/6J on the accelerating rotarod and balance beam. In experiment 3, 12 inbred strains were screened to estimate genetic influences on DID and correlations with other traits. Large, reliable differences in intake and BEC were detected among the strains, with C57BL/6J showing the highest values. Strain means were positively correlated with intake and BEC in the standard (24 h) and a limited (4 h) two-bottle ethanol vs. water test, but BECs reached higher levels for DID. Strain mean correlations with other traits in the Mouse Phenome Project database supported previously reported genetic relationships of high ethanol drinking with low chronic ethanol withdrawal severity and low ethanol-conditioned taste aversion. We extend these findings by showing that the correlation estimates remain relatively unchanged even after correcting for phylogenetic relatedness among the strains, thus relaxing the assumption that the strain means are statistically independent. We discuss applications of the model for finding genes that predispose pharmacologically significant drinking in mice.     

Repair of brain damage size and recovery of neurological dysfunction after ischemic stroke are different between strains in mice: evaluation using a novel ischemic stroke model

In the current study, we established a novel murine ischemic brain damage model using a photochemical reaction to evaluate the recovery of neurological dysfunction and brain repair reactions. In this model, reproducible damage was induced in the frontal lobe of the cortex, which was accompanied by neurological dysfunction. Sequential changes in damage size, microglial accumulation, astrocyte activation, and neurological dysfunction were studied in C57BL/6J and BALB/c mouse strains. Although the initial size of damage was comparable in both strains, the extent of damage was later reduced to a greater extent in C57BL/6J mice than that in BALB/c mice. In addition, C57BL/6J mice showed later edema clearance until day 7, less microglial accumulation, and relatively more astrocyte activation on day 7. Neurologic dysfunction was evaluated by three behavioral tests: the von Frey test, the balance beam test, and the tail suspension test. The behavioral abnormalities evaluated by these tests were remarkable following the induction of damage and recovered by day 21 in both strains. However, the abnormalities were more prominent and the recovery was later in C57BL/6J mice. These findings demonstrate that our novel ischemic stroke model is useful for evaluating brain repair reactions and the recovery of neurological dysfunction in mice with different genetic backgrounds. In addition, we found that both the brain repair reactions and the recovery of neurological dysfunction after comparable ischemic brain damage varied between strains; in that, they both occurred later in C57BL/6J mice.     

Individual differences in sensitivity and tolerance to alcohol

Abstract

Human responses to alcohol—especially sensitivity and acute behavioral tolerance—are being studied within a behavioral genetic design involving comparisons of scores from monozygotic twins, dizygotic twins, nontwin siblings, and unrelated (adoptee) pairs reared in the same family from infancy. The planned genetic analyses must await completion of data collection. The present sample is, however, adequate for analyses of means and for some analyses of individual differences in responses. For most of the measures being used, we find the usual mean decrement in performance after alcohol dosing to 0.100 BAC, but individual differences in response to alcohol are large, and a few individuals actually improve in performance after dosing. Also, on two tests, Cancellation and Block Rotations, there is a significant mean improvement in performance immediately after dosing. As yet we do not have a satisfactory explanation for this phenomenon. On most but not all of the tests, performance improves after the initial decrement during a 3‐hour period in which blood alcohol levels are maintained by additional hourly doses. This improvement may be due in part to practice effects, as well as to the development of acute behavioral tolerance to ethanol (ABTE). We are still exploring ways in which these effects may be disentangled, using data from concurrent placebo control subjects and from pre‐dosing test sessions. It is already apparent, nevertheless, that most of the variability in sensitivity and ABTE is related to pre‐existing individual variability rather than to gender, age, height, weight, or drinking history. By mailing annual questionnaires to all participants, we hope to be able to test the hypothesis that those who were relatively insensitive to ethanol or who showed a relatively large amount of ABTE during the test sessions may be at increased risk for heavy alcohol consumption.     

Observer-rated ataxia: rating scales for assessment of genetic differences in ethanol-induced intoxication in mice.

Identification of genetic and physiological mechanisms underlying a drug's or mutation's effects on motor performance could be aided by the existence of a simple observation-based rating scale of ataxia for mice. Rating scales were developed to assess ataxia after ethanol (2.75, 3.0, and 3.25 g/kg) in nine inbred mouse strains. Each scale independently rates a single behavior. Raters, blinded to dose, scored four behaviors (splay of hind legs, wobbling, nose down, and belly drag) at each of four time points after injection. The severities of hind leg splaying and wobbling were quantifiable, whereas nose down and belly dragging were expressed in all-or-none fashion. Interrater reliabilities were substantial (0.75 0 at some time), but all doses were equally effective. Incidence of nose down and belly dragging behaviors increased strain dependently after ethanol, but strains did not differentially respond to dose. Ethanol-induced splaying was modestly, and negatively, genetically correlated with wobbling. Nose down and belly dragging tended to be associated with splaying and wobbling at later times. Four distinct ataxia-related behaviors were sensitive to ethanol. Strains differed in ethanol sensitivity for all measures. Modest strain mean correlations among behaviors indicate that these behaviors are probably under control of largely different genes and that ataxia rating scales should rate separate behaviors on discrete scales.     

Different data from different labs: lessons from studies of gene-environment interaction

It is sometimes supposed that standardizing tests of mouse behavior will ensure similar results in different laboratories. We evaluated this supposition by conducting behavioral tests with identical apparatus and test protocols in independent laboratories. Eight genetic groups of mice, including equal numbers of males and females, were either bred locally or shipped from the supplier and then tested on six behaviors simultaneously in three laboratories (Albany, NY; Edmonton, AB; Portland, OR). The behaviors included locomotor activity in a small box, the elevated plus maze, accelerating rotarod, visible platform water escape, cocaine activation of locomotor activity, and ethanol preference in a two-bottle test. A preliminary report of this study presented a conventional analysis of conventional measures that revealed strong effects of both genotype and laboratory as well as noteworthy interactions between genotype and laboratory. We now report a more detailed analysis of additional measures and view the data for each test in different ways. Whether mice were shipped from a supplier or bred locally had negligible effects for almost every measure in the six tests, and sex differences were also absent or very small for most behaviors, whereas genetic effects were almost always large. For locomotor activity, cocaine activation, and elevated plus maze, the analysis demonstrated the strong dependence of genetic differences in behavior on the laboratory giving the tests. For ethanol preference and water escape learning, on the other hand, the three labs obtained essentially the same results for key indicators of behavior. Thus, it is clear that the strong dependence of results on the specific laboratory is itself dependent on the task in question. Our results suggest that there may be advantages of test standardization, but laboratory environments probably can never be made sufficiently similar to guarantee identical results on a wide range of tests in a wide range of labs. Interpretations of our results by colleagues in neuroscience as well as the mass media are reviewed. Pessimistic views, prevalent in the media but relatively uncommon among neuroscientists, of mouse behavioral tests as being highly unreliable are contradicted by our data. Despite the presence of noteworthy interactions between genotype and lab environment, most of the larger differences between inbred strains were replicated across the three labs. Strain differences of moderate effects size, on the other hand, often differed markedly among labs, especially those involving three 129-derived strains. Implications for behavioral screening of targeted and induced mutations in mice are discussed.     

Genotype modulates age‐related alterations in sensitivity to the aversive effects of ethanol: an eight inbred strain analysis of conditioned taste aversion

Adolescent individuals display altered behavioral sensitivity to ethanol, which may contribute to the increased ethanol consumption seen in this age‐group. However, genetics also exert considerable influence on both ethanol intake and sensitivity. Currently there is little research assessing the combined influence of developmental and genetic alcohol sensitivities. Sensitivity to the aversive effects of ethanol using a conditioned taste aversion (CTA) procedure was measured during both adolescence (P30) and adulthood (P75) in eight inbred mouse strains (C57BL/6J, DBA/2J, 129S1/SvImJ, A/J, BALB/cByJ, BTBR T⁺tf/J, C3H/HeJ and FVB/NJ). Adolescent and adult mice were water deprived, and subsequently provided with access to 0.9% (v/v) NaCl solution for 1 h. Immediately following access mice were administered ethanol (0, 1.5, 2.25 and 3 g/kg, ip). This procedure was repeated in 72 h intervals for a total of five CTA trials. Sensitivity to the aversive effects of ethanol was highly dependent upon both strain and age. Within an inbred strain, adolescent animals were consistently less sensitive to the aversive effects of ethanol than their adult counterparts. However, the dose of ethanol required to produce an aversion response differed as a function of both age and strain.     

Strain-specific alteration of zebrafish feeding behavior in response to aversive stimuli

Behavioral management of risk, in which organisms must balance the requirements of obtaining food resources with the risk of predation, has been of considerable interest to ethologists for many years. Although numerous experiments have shown that animals alter their foraging behavior depending on the levels of perceived risk and demand for nutrients, few have considered the role of genetic variation in the trade-off between these variables. We performed a study of four zebrafish (Danio rerio (Hamilton, 1822)) strains to test for genetic variation in foraging behavior and whether this variation affected their response to both aversive stimuli and nutrient restriction. Zebrafish strains differed significantly in their latency to begin foraging from the surface of the water under standard laboratory conditions. Fish fed sooner when nutrients were restricted, although this was only significant in the absence of aversive stimuli. Aversive stimuli caused fish to delay feeding in a strain-specific manner. Strains varied in food intake and specific growth rate, and feeding latency was significantly correlated with food intake. Our results indicate significant genetic variation in foraging behavior and the perception of risk in zebrafish, with a pattern of strain variation consistent with behavioral adaptation to captivity.     

Influence of test duration on the sensitivity of the two-bottle choice test

The long-term two-bottle choice test is commonly used as a simple screen to examine the acceptance of taste solutions by rodents. As part of an investigation of factors influencing the sensitivity of the two-bottle choice test, we determined the extent to which test duration influenced test sensitivity. C57BL6/J and 129X1/SvJ mice received four series of eight two-bottle tests, with each test lasting 1, 2, 4 or 6 days. Each series involved sequential tests with water, 2 mM saccharin, 5 and 50 mM citric acid, 30 and 300 micro M quinine hydrochloride, 75 mM NaCl and 10% ethanol. There were significant differences between the strains in intake of saccharin, 5 and 50 mM citric acid, NaCl and ethanol in 4 and 6 day tests, but only saccharin and ethanol in 2 day tests, and 5 mM citric acid and ethanol in 1 day tests. To compare the sensitivity of the tests, we developed an analytical approach based on the comparison of deviations of individual 129X1/SvJ mice from the C57BL6/J strain mean. Our results suggest that to discriminate between strains or treatments when using 'standard' laboratory conditions and methods, 1 day tests are generally inadequate and 2 day tests are useful only if large effects are anticipated. Tests lasting 4 or 6 days are more sensitive, but conducting 6 day tests provides little additional benefit and sometimes is detrimental relative to conducting 4 day tests.     

Housing mice in the individually ventilated or open cages—Does it matter for behavioral phenotype?

Individually ventilated caging (IVC) systems for rodents are increasingly common in laboratory animal facilities. However, the impact of such substantial change in housing conditions on animal physiology and behavior is still debated. Most importantly, there arise the questions regarding reproducibility and comparison of previous or new phenotypes between the IVC and open cages. The present study was set up for detailed and systematic comparison of behavioral phenotypes in male and female mice of three widely used inbred strains (C57BL/6JRccHsd, DBA/2JRccHsd, 129S2/SvHSd) after being kept in two housing environments (IVC and open cages) for 6 weeks (since 4 weeks of age) before behavioral testing. The tests addressed exploratory, anxiety‐like and stress‐related behavior (light‐dark box, open field, forced swim test, stress‐induced hyperthermia), social approach and species‐specific behavior (nest building, marble burying). In all tests, large and expected strain differences were found. Somewhat surprisingly, the most striking effect of environment was found for basal body temperature and weight loss after one night of single housing in respective cages. In addition, the performance in light‐dark box and open field was affected by environment. Several parameters in different tests showed significant interaction between housing and genetic background. In summary, the IVC housing did not invalidate the well‐known differences between the mouse strains which have been established by previous studies. However, within the strains the results can be influenced by sex and housing system depending on the behavioral tasks applied. The bottom‐line is that the environmental conditions should be described explicitly in all publications.     

Molecular Genetic Analysis of Ethanol Intoxication in Drosophila melanogaster

Recently, the fruit fly Drosophila melanogaster has been introducedas a model system to study the molecular bases of a varietyof ethanol-induced behaviors. It became immediately apparentthat the behavioral changes elicited by acute ethanol exposureare remarkably similar in flies and mammals. Flies show signsof acute intoxication, which range from locomotor stimulationat low doses to complete sedation at higher doses and they developtolerance upon intermittent ethanol exposure. Genetic screensfor mutants with altered responsiveness to ethanol have beencarried out and a few of the disrupted genes have been identified.This analysis, while still in its early stages, has alreadyrevealed some surprising molecular parallels with mammals. Theavailability of powerful tools for genetic manipulation in Drosophila,together with the high degree of conservation at the genomiclevel, make Drosophila a promising model organism to study themechanism by which ethanol regulates behavior and the mechanismsunderlying the organism's adaptation to long-term ethanol exposure.     

Behavioral profiles of inbred strains on novel olfactory,spatial and emotional tests for reference memory in mice

Studying the behavior of genetic background strains provides important information for the design and interpretation of cognitive phenotypes in mutant mice. Our experiments examined the performance of three commonly used strains (C57BL/6J, 129S6, DBA/2J) on three behavioral tests for learning and memory that measure very different forms of memory, and for which there is a lack of data on strain differences. In the social transmission of food preference test (STFP) all three strains demonstrated intact memory for an odor-cued food that had been sampled on the breath of a cagemate 24 hours previously. While C57BL/6J and 129S6 mice showed good trace fear conditioning, DBA/2J mice showed a profound deficit on trace fear conditioning. In the Barnes maze test for spatial memory, the 129S6 strain showed poor probe trial performance, relative to C57BL/6J mice. Comparison of strains for open field exploratory activity and anxiety-like behavior suggests that poor Barnes maze performance reflects low exploratory behavior, rather than a true spatial memory deficit, in 129S6 mice. This interpretation is supported by good Morris water maze performance in 129S6 mice. These data support the use of a C57BL/6J background for studying memory deficits in mutant mice using any of these tasks, and the use of a 129S6 background in all but the Barnes maze. A DBA/2J background may be particularly useful for investigating the genetic basis of emotional memory using fear conditioning.     

Behavioral Assessment of the Aging Mouse Vestibular System

Age related decline in balance performance is associated with deteriorating muscle strength, motor coordination and vestibular function. While a number of studies show changes in balance phenotype with age in rodents, very few isolate the vestibular contribution to balance under either normal conditions or during senescence. We use two standard behavioral tests to characterize the balance performance of mice at defined age points over the lifespan: the rotarod test and the inclined balance beam test. Importantly though, a custom built rotator is also used to stimulate the vestibular system of mice (without inducing overt signs of motion sickness). These two tests have been used to show that changes in vestibular mediated-balance performance are present over the murine lifespan. Preliminary results show that both the rotarod test and the modified balance beam test can be used to identify changes in balance performance during aging as an alternative to more difficult and invasive techniques such as vestibulo-ocular (VOR) measurements.     

Statistical Parameters in Behavioral Tasks and Implications for Sample Size of C57BL/6J:129S6/SvEvTac Mixed Strain Mice

Most mixed strain progeny from gene-knockout experiments typically originate from C57BL/6J and one of the 129 substrains, frequently 129S6/SvEvTac. The results of this behavioral survey suggest that C57BL/6J:129S6/SvEvTac mixed strain mice are amenable to behavioral testing. The variability in behavioral tasks for subjects arising from this mixed strain genetic background does not preclude screening with a battery of behavioral tests. With clues provided by a screen of mixed strain subjects, follow-up analyses with isogenic, congenic, or F1 hybrid animals may be targeted to specific behavioral themes.