A comparison of 129S2/SvHsd and C57BL/6JOlaHsd mice on a test battery assessing sensorimotor, affective and cognitive behaviours: implications for the study of genetically modified mice

GluR-A-dependent synaptic plasticity is required for the temporal encoding of nonspatial information

Four related experiments studied operant performance of mice on differential reinforcement of low rates of responding (DRL) paradigms. Experiment 1 showed that excitotoxic hippocampal lesions impaired performance of a 10-s DRL schedule (DRL-10). Experiments 2 and 3 showed that GluR-A AMPA receptor subunit knockout mice, which are deficient in CA3-CA1 long-term potentiation (LTP), were markedly impaired at 15 s (DRL-15), but less impaired at DRL-10. Experiment 4 compared DRL-15 performance in mice from the 2 strains from which the GluR-A colony was derived and showed that they did not differ. The results show that GluR-A-containing AMPA receptors are required for normal performance on hippocampus-dependent, nonspatial working memory tasks, consistent with a role for GluR-A in the temporal encoding (what happened when) of nonspatial information.

Hippocampal lesions, species-typical behaviours and anxiety in mice

The hippocampus is believed to play an important role in spatial cognition and anxiety. Much of the supporting evidence is derived from rat studies. Recent reports on hippocampal lesioned mice also showed impairments in spatial function, but anxiety was not uniformly diminished. There were, however, striking impairments in several "species typical" behaviours; lesioned mice made poorer nests, and hoarded and burrowed less. In the present experiments, mice with excitotoxic hippocampal lesions were tested in a well-established anxiety paradigm, the light-dark box. As in previous anxiety tests, the results were mixed; some measures (reduced dark time) suggested lesioned mice were less anxious; others (fewer light-dark transits) suggested greater anxiety. However, lesioned mice only made fewer transits when the door was small. This suggested that the tendency to enter small holes, so characteristic of small rodents, was reduced; subsequent tests showed lesioned mice preferred to explore in an alley rather than enter its attached tunnels. Further tests of "species typical" behaviours revealed that lesioned mice spent less time digging and climbing, and made less use of cardboard shelters in their cages. This was not due to inactivity; lesions did not reduce grooming or locomotion. Finally, tests of hyponeophagia showed hippocampal lesions reduced this measure of anxiety, so long as the control baseline was sufficiently high. Overall, the results suggest that the hippocampus is important in many species-typical behaviours, potentially influencing performance in a range of behavioural tests. However, species-typical behaviours offer easy and economical ways to test for hippocampal dysfunction, for example, in genetically modified mice.

Home cage activity and activity-based measures of anxiety in 129P3/J, 129X1/SvJ and C57BL/6J mice

We investigated the home cage activity and emotional behavior in mouse strains used as background for many studies of altered genes [C57BL/6J (B6, n=20), 129X1/SvJ (X1, n=20) and 129P3/J (P3, n=19)]. In their home cages, X1 and P3 mice exhibited less locomotion than did B6 mice, and the X1 mice showed significantly greater rearing than B6 and P3 mice did. A battery of three tests conducted in an open field (open field, emergence and novel object) revealed strain rankings of B6>X1>P3 or B6>X1=P3 in most activity variables. Significant correlations were found between home cage activity and activity in each of three tests, but not in all observation periods. Strain rankings on the elevated zero maze test were B6=X1>P3 in the number of stretched-attend body postures (SAPS) during the initial 6-min exposure for naive mice. Naive and nonnaive mice showed significantly different behaviors on the elevated zero maze. The results suggest that rankings on anxiety are P3>X1>B6 and that B6 mice have greater exploration in a novel environment compared with X1 and P3 mice. However, anxiety-like behaviors differed among strains in open-field-based tests and in the zero maze, and testing experience impacted performance on the zero maze. The findings illustrate that test variations and experience can influence performance and suggest the need for the consideration of how these factors interact with background strains in assessing gene-altered mice.

Long-term individual housing in C57BL/6J and DBA/2 mice: assessment of behavioral consequences

The aim of the present study was to investigate the effects of individual housing on mouse behavior. The male mice of the C57BL/6J and DBA/2 strains were separated at the age of 4 weeks and kept in individual housing for 7 weeks until behavioral testing began. Their behavior was compared to the group-housed mice in a battery of tests during the following 7 weeks. The single-housed mice were hyperactive and displayed reduced habituation in the tests assessing activity and exploration. Reduced anxiety was established in the elevated plus-maze, but an opposite effect was observed in the dark-light (DL) and hyponeophagia tests. Immobility in the forced swimming test was reduced by social isolation. The DBA mice displayed higher anxiety-like behavior than the B6 mice in the plus-maze and DL exploration test, but hyponeophagia was reduced in the DBA mice. Moreover, all effects of individual housing on the exploratory and emotional behavior were more evident in the DBA than in the B6 mice. Novel object recognition and fear conditioning (FC) were significantly impaired in the single-housed mice, whereas water-maze (WM) learning was not affected. Marked strain differences were established in all three learning tests. The B6 mice performed better in the object recognition and FC tasks. Initial spatial learning in the WM was faster and memory retention slightly enhanced in the B6 mice. The DBA mice displayed lower preference to the new and enhanced preference to the old platform location than the B6 mice after reversal learning in the WM. We conclude that individual housing has strong strain- and test-specific effects on emotional behavior and impairs memory in certain tasks.

Genetic dissection of a behavioral quantitative trait locus shows that Rgs2 modulates anxiety in mice

Here we present a strategy to determine the genetic basis of variance in complex phenotypes that arise from natural, as opposed to induced, genetic variation in mice. We show that a commercially available strain of outbred mice, MF1, can be treated as an ultrafine mosaic of standard inbred strains and accordingly used to dissect a known quantitative trait locus influencing anxiety. We also show that this locus can be subdivided into three regions, one of which contains Rgs2, which encodes a regulator of G protein signaling. We then use quantitative complementation to show that Rgs2 is a quantitative trait gene. This combined genetic and functional approach should be applicable to the analysis of any quantitative trait.

A mouse model of AChR deficiency syndrome with a phenotype reflecting the human condition

The two subtypes of mammalian muscle nicotinic acetylcholine receptors (AChR) are generated by the substitution of the epsilon (adult) subunit for the gamma (fetal) subunit within the AChR pentamer. Null mutations of the adult AChR epsilon-subunit gene are the most common cause of the AChR deficiency syndrome. This is a disorder of neuromuscular transmission characterized by non-progressive fatigable muscle weakness present throughout life. In contrast with the human disorder, mice with AChR epsilon-subunit null mutations die between 10 and 14 weeks of age. We generated transgenic mice that constitutively express the human AChR gamma-subunit in an AChR epsilon-subunit 'knock-out' background. These mice, in which neuromuscular transmission is mediated by fetal AChR, live well into adult life but show striking similarities to human AChR deficiency syndrome. They display fatigable muscle weakness, reduced miniature endplate potentials and endplate potentials, reduced motor endplate AChR number and altered endplate morphology. Our results illustrate how species differences in the control of ion-channel gene expression may affect disease phenotype, demonstrate that expression of adult AChR subtype is not essential for long-term survival, and suggest that in patients with AChR deficiency syndrome, up-regulation of the gamma-subunit could be a beneficial therapeutic strategy.

Cognition and immunity; antibody impairs memory

Patients with lupus (SLE) experience progressive cognitive loss without evidence of CNS vascular disease or inflammation. SLE patients produce anti-DNA antibodies that crossreact with NMDA receptors and are capable of mediating excitotoxic death. We now show that mice induced by antigen to express these antibodies have no neuronal damage until breakdown of the blood-brain barrier occurs. Following administration of lipopolysaccharide (LPS) to immunized mice, antibodies gain access to the brain. They bind preferentially to hippocampal neurons and cause neuronal death with resulting cognitive dysfunction and altered hippocampal metabolism on magnetic resonance spectroscopy. Memantine, an NMDA receptor antagonist, given prior to LPS administration, prevents neuronal damage. Thus, systemic immune responses can cause cognitive impairment in the absence of an inflammatory cascade, implicating the immune system in yet another arena of human pathobiology. Furthermore, NMDA receptor antagonists prevent antibody-mediated damage and may constitute a new approach to therapy in SLE.

Impaired motor coordination on static rods in BSE-infected mice

Scrapie and bovine spongiform encephalopathy (BSE) are both progressive neurodegenerative diseases that are transmissible to mice. The onset of clinical symptoms is more subtle and variable in murine BSE than in murine scrapie. Assessment of behavioural changes that occur throughout disease would aid early diagnosis of disease so that more consistent end points could be made and potential therapies could be investigated. C57BL/6J mice inoculated via the intraperitoneal route with 301C BSE or control inoculum were monitored on a fortnightly basis. The end point was when a mouse showed clinical signs as opposed to behavioural signs of BSE for two consecutive observations. Significant loss of motor function, as assessed by mice balancing on a static rod, was observed consistently from approximately 40 days prior to death. No significant differences in home cage activity (locomotion, rearing) or cognitive function (T-maze alternation) were observed. However, there was an increase in digging by BSE-infected mice from an early stage. This data will aid the standardisation of behavioural tests to characterise and assess the onset of BSE.

Behavioural profiles of inbred mouse strains used as transgenic backgrounds. I: motor tests

One of the characteristic manifestations in several neurodegenarative diseases is the loss of voluntary motor control and the development of involuntary movements. In order to determine the suitability of six mouse strains as transgenic background strains we investigated performance on a variety of tasks designed to identify subtle changes in motor control. On both the accelerating and the staggered speed rotarod all six mouse strains performed well. However, latency to fall from the rod was sensitive to both rotarod speed and repeated exposure to the apparatus. Performance of the DBA/2 mouse strain was highly variable across the time points used. On the acoustic startle test CBA mice showed the greatest degree of reactivity to the acoustic startle stimuli with both the C57 and DBA showing the least. Complex strain differences were also identified on measures of habituation to the startle stimuli and variations in the prepulse noise level, and prepulse/startle delay. Gait analysis using the footprint test did not reveal strain differences on measures of base width, overlap or stride length but the 129S2/Sv strain took significantly longer to traverse the runway than the other mouse strains. Finally, the swim tank test detected complex strain differences in swim speed, and the number of fore- and hindpaw paddles required to swim the length of the tank. These data taken together suggest that choice of background strain is a crucial consideration for the repeated behavioural assessment of motor deficits in transgenic mouse models of disease.

Mapping of a quantitative trait locus for morphine withdrawal severity

Chronic morphine exposure results in physical dependence, manifested by physical symptoms during naloxone-precipitated withdrawal. Jumping frequency is widely considered the most sensitive and reliable index of withdrawal intensity in mice. Inbred mouse strains surveyed for naloxone-precipitated withdrawal display large and significant strain differences in jumping frequency, including an approximately tenfold difference between C57BL/6 and 129P3 mice. In the present study, (B6 x 129)F2 hybrid mice were given daily morphine injections for four days using an escalating dosing schedule, and naloxone-precipitated withdrawal on day 5 was measured. A full-genome scan for linkage to phenotypic data was performed using polymorphic microsatellite markers. Significant linkage was observed between withdrawal jumping frequencies and a 28 cM-wide region of Chromosome 1 (32-60 cM; peak at 51 cM), accounting for 20% of the overall phenotypic variance. Two other suggestive QTLs were found, on Chromosomes 5 and 10, and an additive model fitting all three loci accounted for 43% of the total variance. F2 mice were also assessed for changes in morphine analgesic potency using the tail-withdrawal test in dose-response studies on days 1 and 4. No linkage was observed between Chromosomes 1, 5, and 10 and morphine analgesic tolerance, suggestive of genetic dissociation of naloxone-precipitated withdrawal from morphine and chronic morphine intake per se. The significant quantitative trait locus for naloxone-precipitated withdrawal severity in morphine-dependent mice, which we name Depmq1, may prove to be of considerable heuristic value once the underlying gene or genes are identified.

Behavioral and physiological mouse models for anxiety: effects of flesinoxan in 129S6/SvEvTac and C57BL/6J mice

Serotonin(1A) (5-HT(1A)) receptors are involved in anxiety. This study focuses on the role of genetic factors on the anxiety-related effects of 5-HT(1A) receptor stimulation using both a within subject design. The effects of 5-HT(1A) receptor activation were studied in high- and low-anxiety mice (129S6/SvEvTac (S6) and C57BL/6J (B6), respectively) in behavioral and physiological anxiety-related assays. These two strains were also selected because they are frequently used in gene-targeting studies. Mice were treated with the selective 5-HT(1A) receptor agonist flesinoxan (0-0.3-1.0-3.0 mg/kg s.c.) and tested in either the open-field activity test, the light-dark exploration test, or the stress-induced hyperthermia paradigm. Flesinoxan unexpectedly increased anxiety, but also decreased activity on several behavioral measures in B6 mice. Flesinoxan produced only minimal effects in the behavioral tests in the high-anxiety S6 strain. In contrast, the physiological hyperthermia response showed anxiolytic-like effects of flesinoxan in both strains. Our data indicate that the role of 5-HT(1A) receptor activation on anxiety-related responses is dependent on genetic background and selected paradigm used to assess anxiety. These findings indicate that it is critical to use a multi-level approach to develop mouse models for human diseases. In addition, the implication of such findings for studies on genetically modified mice is discussed.

Spatial reference memory in GluR-A-deficient mice using a novel hippocampal-dependent paddling pool escape task

Genetically modified mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit, GluR-A (GluR1), and deficient in hippocampal CA3-CA1 long-term potentiation (LTP), were assessed on a novel, hippocampal-dependent spatial reference memory, paddling pool escape task. The mice were required to use the extramaze cues around the laboratory to find a hidden escape tube that was in a constant location at one of 12 possible positions around the perimeter of the paddling pool, in order to escape from shallow water. The knockout mice performed well on this task. They displayed a small initial impairment (in terms of both escape latencies and choice errors), but they were soon as efficient as the wild-type mice in escaping from the water. This was further demonstrated by performance during a 20-s probe trial in which the exit tube was blocked. Both groups of mice spent most of the time searching in the quadrant of the pool in which the exit tube had previously been located. In a subsequent experiment, entirely normal spatial acquisition was observed in the knockout mice when the paddling pool was moved to a novel spatial environment. The GluR-A -/- mice were also unimpaired in a further reversal phase in which the correct exit location was moved by 180 degrees around the perimeter wall. These results are consistent with previous watermaze studies, providing further demonstration of intact hippocampus-dependent spatial reference memory in GluR-A knockout mice. They contrast strikingly with the profound deficits in hippocampus-dependent, short-term, flexible spatial working memory observed in these knockout mice. This study also demonstrates a novel behavioral task for assessing spatial memory in genetically modified mice. This task shares the behavioral profile of the well-established watermaze paradigm, but may have advantages for the study of genetically modified mice.

Performance of different mouse strains in an object recognition task

In the present study, we tested the memory performance of different mouse strains (129/Sv, BALB/c, C57BL and Swiss) in an object recognition task. In this one-trial learning task, mice showed a good object memory performance when a 1-h delay was interposed between the first and second trial. However, when a 24-h delay was used, the mice did not discriminate between the novel and the familiar object in the second trial, indicating that the mice did not remember the object, which was presented in the first trial. Using a 4-h delay, the discrimination performance was at an intermediate level, suggesting a delay-dependent forgetting in this task. Although strain differences were found in the absolute level of exploration activity, no strain differences were found on the relative discrimination index (d2). The present data show that object memory can be assessed in mice and, in contrast to other memory tasks, appears to be less strain-dependent. The reliability of the discrimination measures is discussed.

Behavioral alterations induced by repeated testing in C57BL/6J and 129S2/Sv mice: implications for phenotyping screens

The C57BL/6JOlaHsd and 129S2/SvHsd mice were tested in a battery designed for behavioral phenotyping of genetically modified mice. The study was performed in order to reveal the effect of training history on the behavior by comparison with the experimentally naive mice in the same tests. Significant strain differences were obtained in all experiments. Previous handling and testing reduced exploratory activity and emotionality significantly in the mice. The coordination ability was better and nociceptive sensitivity was increased in the trained mice. The contextual fear was reduced whereas the cued fear was enhanced in the experienced mice. The training history did not alter initial learning in the water maze. However, after reversal learning the naive mice displayed significant preference for both old and new platform locations, whereas the battery animals did not exhibit preference to the old location. The experienced mice appeared to be less active in the forced swimming test and exhibited decreased conditioned taste aversion. The influence of test history was strain-dependent in certain cases. Therefore, the experience has substantial consequences on the behavior, mainly by reducing exploratory activity, and the previous experience of the animals has always to be considered in the analysis of genetically modified mice.

Abnormal anxiety-related behavior in serotonin transporter null mutant mice: the influence of genetic background

Serotonin transporter (5-HTT) null mutant mice provide a model system to study the role genetic variation in the 5-HTT plays in the regulation of emotion. Anxiety-like behaviors were assessed in 5-HTT null mutants with the mutation placed on either a B6 congenic or a 129S6 congenic background. Replicating previous findings, B6 congenic 5-HTT null mutants exhibited increased anxiety-like behavior and reduced exploratory locomotion on the light <--> dark exploration and elevated plus-maze tests. In contrast, 129S6 congenic 5-HTT null mutant mice showed no phenotypic abnormalities on either test. 5-HTT null mutants on the 129S6 background showed reduced 5-HT(1A) receptor binding (as measured by quantitative autoradiography) and reduced 5-HT(1A) receptor function (as measured by 8-OH-DPAT-induced hypothermia). These data confirm that the 5-HTT null mutation produced alterations in brain 5-HT function in mice on the 129S6 background, thereby discounting the possibility that the absence of an abnormal anxiety-like phenotype in these mice was due to a suppression of the mutation by 129 modifier genes. Anxiety-like behaviors in the light <--> dark exploration and elevated plus-maze tests were significantly higher in 129S6 congenic +/+ mice as compared to B6 congenic +/+ mice. This suggests that high baseline anxiety-like behavior in the 129S6 strain might have precluded detection of the anxiety-like effects of the 5-HTT null mutation on this background. Present findings provide further evidence linking genetic variation in the 5-HTT to abnormalities in mood and anxiety. Furthermore, these data highlight the utility of conducting behavioral phenotyping of mutant mice on multiple genetic backgrounds.

A Comparison of GluR-A-Deficient and Wild-Type Mice on a Test Battery Assessing Sensorimotor, Affective, and Cognitive Behaviors

Previous studies have demonstrated a spatial working memory deficit in glutamate receptor (GluR)-A (GluR1) AMPA receptor subunit knockout mice. The present study evaluated male and female wild-type and GluR-A-/- mice on a test battery that assessed sensorimotor, affective, and cognitive behaviors. Results revealed a behavioral phenotype more extensive than previously described. GluR-A-/- mice were hyperactive, displayed a subtle lack of motor coordination, and were generally more anxious than wild-type controls. In addition, they showed a deficit in spontaneous alternation, consistent with previous reports of a role for GluR-A-dependent plasticity in hippocampus-dependent, spatial working memory. Although changes in motor coordination or anxiety cannot explain the dissociations already reported within the spatial memory domain, it is clear that they could significantly affect interpretation of results obtained in other kinds of behavioral tasks.

Genotypic differences in ethanol sensitivity in two tests of motor incoordination

Motor incoordination is frequently used as a behavioral index of intoxication by drugs that depress the central nervous system. Two tasks that have been used to assay incoordination in mice, the balance beam and the grid test, were evaluated to optimize aspects of apparatus and testing procedures for studying genetic differences. Mice of eight inbred strains were given one of several doses of ethanol or saline and tested for intoxication. Strains differed in sensitivity to ethanol in both tests, indicating a significant influence of genotype on ethanol sensitivity. For the balance beam, the width of the beam affected the strain sensitivity pattern, and only the widest beam worked well at all doses. For the grid test, both ethanol dose and the time after drug injection affected strains differentially. Although the behavioral sign of intoxication recorded for both tests was a foot-slip error, the correlations of strain means for ethanol sensitivity across the two tasks were generally not significant. This suggests that the genes influencing ethanol sensitivity in the two tasks are mostly different. These results make clear that a single set of task parameters is insufficient to characterize genetic influences on behavior. Several other issues affect the interpretation of data using these tests.

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.

Effects of medial prefrontal cortex cytotoxic lesions in mice

Mice (C57BL/6J strain, females) with cytotoxic lesions of the medial wall of the prefrontal cortex were given a battery of tests to assess emotional, species-typical, cognitive, motor and other behaviours. Lesioned mice showed a profile of reduced anxiety, both on a plus-maze, and a similar, novel test, the successive alleys. There was no evidence, however, for attenuation of anxiety in tests of hyponeophagia, and lesioned mice, like controls, preferred the black to the white area of an enclosed alley. Their locomotor activity tended to be higher than that of the controls, particularly when the test surroundings were novel or relatively so. Species-typical behaviours were similar to those of control mice, except lesioned mice displaced ('burrowed') less food pellets from a tube in their home cage. They were not impaired at learning a spatial Y-maze reference memory task, which is profoundly affected by cytotoxic hippocampal lesions in the same strain, or at learning a multi-trial passive avoidance test. Their strength and co-ordination in motor performance tests was also normal. The results show that cytotoxic medial prefrontal cortex lesions in mice produce a clear but restricted anxiolytic action. The marked reduction in burrowing, in the absence of any detectable impairment of motor ability, demonstrates the sensitivity of this behavioural index.

Differential learning abilities of 129T2/Sv and C57BL/6J mice as assessed in three water maze protocols

Knockout mice are generated by using ES cells from 129 mouse strains and are frequently backcrossed with other strains, like C57BL/6. It is important to characterise the physiological and, in particular, the behavioural profile of each strain in order to correctly analyse the functional contribution of a single gene mutation on the 'cognitive' phenotype. The present study compared 129T2/Sv (129) and C57BL/6J (C57) mice in three different spatial learning protocols in the water maze, using a hidden platform. In the 'standard' reference memory protocol, 129 and C57 attained an equivalent level of performance as assessed by accuracy in reaching the platform (path length), despite a faster swim speed exhibited by C57 mice. In a stepwise learning task, C57 mice showed poorer performances over all stages of learning. However they performed better than 129 in a massed learning protocol which taxes short-term memory, and in which they exhibited lower levels of perseveration. The results emphasize the importance of using various tasks differing in cognitive demand, but using the same experimental environment and motivation, in order to 1) evaluate strain- or mutation-dependent learning abilities, and 2) dissociate the roles played by cognitive and non-cognitive factors in the behavioural requirements of the tasks.

Absence of anxiolytic response to chlordiazepoxide in two common background strains exposed to the elevated plus-maze: importance and implications of behavioural baseline

Although genetic background is acknowledged as a potentially important determinant of mutant phenotypes, publications on genetically modified mice far outnumber those on progenitor strains. We have recently reported major differences in basal anxiety levels (elevated plus-maze & light/dark exploration) among three strains (C57BL/6JOlaHsd, 129/SvEv and 129S2/SvHsd) employed as progenitor stock in European laboratories (Rodgers et al. in press). Furthermore, the phenotypes of these inbred strains differed significantly from that of an outbred strain (Swiss-Webster) commonly used in behavioural pharmacology. In view of these findings, the present study assessed possible differences in the anxiolytic efficacy of chlordiazepoxide (0, 7.5 & 15.0 mg/kg, IP) in three of these strains (Swiss-Webster (SW), C57BL/6JOIaHsd (C57) & 129S2/SvHsd (129)). Experimentally naive mice were exposed to the elevated plus-maze, sessions were videotaped and behaviour analysed using ethological software. The performance of control subjects confirmed significant strain differences in basal levels of activity (SW > C57 > 129) and anxiety-related behaviours (129 = SW > C57), with hypolocomotion dominating the 129 profile. SW mice displayed an anxioselective response to both doses of chlordiazepoxide (CDP), with significant reductions in open arm avoidance and risk assessment observed in the absence of any change in general activity. In direct contrast, the lower dose of CDP (7.5 mg/kg) was without effect in either inbred strain, whereas treatment with 15.0 mg/kg induced a profile indicative of muscle relaxation/mild sedation in C57 mice and virtually abolished all behavioural activity in 129 mice. Although the absence of an anxiolytic response to CDP in C57 mice may be attributed to their low basal anxiety levels, the profile of 129 mice strongly suggests an abnormality in benzodiazepine/GABAA receptor function. The implications of these findings for research on mutant mice are discussed.

Contrasting phenotypes of C57BL/6JOlaHsd, 129S2/SvHsd and 129/SvEv mice in two exploration-based tests of anxiety-related behaviour

Knockout mice are typically generated on a mixed genetic background and, as such, detailed behavioural characterisation of these background strains is essential to the valid interpretation of mutant phenotypes. In this context, recent research has revealed significant differences in anxiety-like behaviour among the most commonly used background strains (C57BL/6J and various 129 substrains), leading to the possibility that at least certain mutant phenotypes may not after all be due to the targeted mutation. However, these findings derive largely from behavioural test batteries in which there may well be an experiential confound, while the widely reported hypolocomotor profile of most 129 substrains may compromise the principal indices of anxiety-like behaviour. In the present study, we have compared the behavioural profiles of three commonly used background strains (C57BL/6JOlaHsd, 129/SvEv and 129S2/SvHsd) in two of the most popular animal models of anxiety-the elevated plus-maze (EPM) and light/dark exploration (LDE) tests. Naive animals were used for each procedure, ethological scoring methods were employed throughout, and the inbred phenotypes were also compared with that of an outbred strain (Swiss-Webster) widely employed in test validation and behavioural pharmacology. Our results show that, despite their hypolocomotor profile, both 129 substrains display higher levels of anxiety-like behaviour (conventional and/or ethological measures) relative to the C57BL/6JOlaHsd strain. Furthermore, all three inbred strains were less active in both tests when compared with the outbred Swiss-Webster strain. However, whereas C57BL/6JOlaHsd mice displayed lower levels of anxiety-like behaviour than their Swiss-Webster counterparts (both tests), 129S2/SvHsd (but not 129/SvEv) mice exhibited evidence of higher anxiety, particularly in the LDE test. The implications of these findings are discussed in relation to both the behavioural and pharmacological phenotyping of mutant mice.

Effects of cytotoxic hippocampal lesions in mice on a cognitive test battery

Mice received cytotoxic lesions which selectively removed all of the hippocampus and dentate gyrus except the most ventral portions. They were impaired on both spontaneous and rewarded discrete-trial alternation in T-mazes. Acquisition of reference memory for the location of a hidden platform in the Morris water maze was impaired in lesioned mice. On an elevated Y-maze reference memory task, in which only one arm was rewarded, lesioned mice showed no evidence of learning. In a Lashley III maze task, however, where maze rotation demonstrated that control performance was independent of distal spatial cues, acquisition in the lesioned mice was unimpaired. Control levels of continuous spontaneous alternation in a Y-maze were too low to reveal a hippocampal deficit. A small impairment in acquisition of a multiple-trial passive avoidance task was seen in lesioned mice, despite a small but significant increase in reactivity to the footshock. These results are largely consistent with findings in hippocampal lesioned rats on the same or similar tasks, and reflect a major impairment of spatial cognition, with relative sparing of non-spatial task performance.

Peptide nucleic acids targeted to the mouse proNPFF(A) reveal an endogenous opioid tonus

Pharmacological studies have implicated the anti-opioid neuropeptide FF (NPFF) in the modulation of pain transmission. Since its physiological role has not yet been fully elucidated, the present study examined whether antisense peptide nucleic acid (PNA) complementary to the NPFF precursor (proNPFF(A)) modified pain sensitivity. Mice received three intraperitoneal (i.p.) injections (10mg/kg) of antisense PNA (As-proNPFF(A)) over a period of 24h. As-proNPFF(A) treatment significantly increased the basal tail withdrawal latency in the tail-flick test. This analgesia persisted during 2 days and was completely reversed by naloxone. Thus, antisense PNAs, by decreasing anti-opioid effects, revealed a basal endogenous opioid activity. Our results evidence a physiological interplay between NPFF and opioid systems and further support the use of PNA as effective antisense agents, for studying gene function in vivo.

Hippocampal cytotoxic lesion effects on species-typical behaviours in mice

The behavioural effects of hippocampal lesions have been extensively documented in rats. However, paradigms developed for rats cannot be assumed to transfer straightforwardly to mice; the behaviour of the two species differs in many respects. Mice are currently the species of choice for targeted genetic manipulations. A number of these programs aim to modulate hippocampal function. The present studies were therefore designed to provide a behavioural profile of selective, cytotoxic hippocampal lesions in tasks appropriate for mice. The lesions abolished food hoarding from a source outside the home base, and reduced the tendency to displace food pellets from a tube inside the home cage (burrowing). Lesioned mice showed reductions of directed exploration (rearing and head dipping), but not locomotor activity, in a holeboard and open field, and explored the edges of their home cages less when the lids were removed. Nest construction was also impaired. These effects were not due to gross motor impairments, as formal tests revealed no deficiencies in co-ordination or strength. There were suggestions of changes in emotionality, although a more consistent finding was that lesioned mice were often slower to initiate behaviour in novel surroundings, which may be congruent with the other deficits we observed. These results may aid interpretation of the many genetic manipulations that target the hippocampus, and of neurodegenerative conditions that induce hippocampal pathology.

Strain differences in activity and emotionality do not account for differences in learning and memory performance between C57BL/6 and DBA/2 mice

This study examined emotionality, activity, learning and memory, as well as the influence of emotionality and activity on learning and memory performance in C57BL/6 and DBA/2 mice using a mouse-test battery. DBA/2 mice performed more poorly than C57BL/6 mice in complex learning tasks such as the water maze and object recognition tasks. In contrast, C57BL/ 6 mice showed attenuated habituation tonovelty in the open field apparatus and poorer performance in the step-down passive avoidance task. The C57BL/6 mice were less exploratory and more anxious than the DBA/ 2 mice. The anxiety score (open arm entries in the elevated plus maze) was significantly correlated with all measures of learning and memory in the object recognition task, and some measures in the passive avoidance and water maze tasks. Analysis of covariance (with open arm entries as a covariate) revealed that some measures on trial 1 of the object recognition task, but not the memory scores on trial 2,were confounded by anxiety. No confounding factors of anxiety were found in the water maze or passive avoidance tasks. Similar results were obtained with the activity scores (line crossing and rearing in the open field). In conclusion, strain differences in activity and anxiety did not account for strain differences in learning and memory performance of C57BL/6 and DBA/2 mice. Nonetheless, the importance of using complete behavioural test batteries should be stressed to ensure that strain differences in learning and memory tasks are not confounded by non-cognitive factors.

Faster is not surer--a comparison of C57BL/6J and 129S2/Sv mouse strains in the watermaze

In recent years the use of genetic manipulations to investigate the molecular mechanisms underlying learning and memory has become a common approach. In a great many cases, the spatial learning ability of mutant mice has been assessed using the Morris watermaze task. The performance of these mice may, however, be strongly influenced by their genetic background and, therefore, the interpretation of their phenotype requires a preliminary characterization of the parental strains. The present study compared 129S2/Sv and C57/BL/6J inbred mouse strains, which have been widely used in deriving lines of genetically modified mice, on the hidden platform version of the watermaze task. During acquisition, the C57 mice displayed shorter escape latencies to find the platform than the 129S2s. Further analysis revealed, however, that the C57 mice also swam faster than the 129S2s. The analysis of path lengths was thus a more reliable measure of spatial learning, and revealed an equal level of performance in the two strains. This conclusion was confirmed during the two probe trials with both strains showing a similar spatial preference for the training site. These results suggest that the 129S2 substrain is no less proficient than the C57 substrain in terms of spatial learning in the watermaze, and also demonstrates the importance of not relying solely on escape latency as a measure of watermaze performance.