Diverse effects of stanozolol in C57BL/6J and A/J mouse strains

Although skeletal muscle is the principal target for androgenic anabolic steroids (AAS) other physiological and behavioral processes are also affected. Wide variations in response to AAS are known to exist in individuals but the genetic basis of this has hardly been explored. Female mice from the A/J and C57BL/6J strains were divided into four experimental groups: CTRL-Sham, housed in a regular mouse cage and subjected to a sham operation mimicking implantation of steroids; CTRL-AAS, mice similarly housed and implanted with a pellet containing stanozolol (release rate, 4.6 mg/kg/day); EX-Sham, sham operated mice housed in a cage with two towers which required mice to climb 1 m to obtain food or water; EX-AAS, mice similarly housed and implanted with a stanozolol pellet. The experimental treatment was initiated at 10 weeks of age and lasted for 7 weeks. Body weight was assessed periodically during the experiment (time effect), systolic blood pressure (BP) and heart rate (HR) were measured after 6 weeks of treatment, and weights of gastrocnemius (GAST), soleus, tibialis anterior (TA), extensor digitorum longus (EDL), quadriceps femoris (QF) and biceps brachii (BB) muscles, heart, liver, kidney and abdominal fat were measured after 7 weeks of treatment. AAS treatment significantly increased weight of GAST (P ≪ 0.001), TA (P < 0.01), EDL (P < 0.01) and QF (P ≪ 0.001) muscles in both of the strains. Several of the measured indices were differentially affected in the two strains by AAS (body weight, Time × Strain × AAS P < 0.02; BP and HR, Strain × AAS P < 0.03 and P < 0.01, respectively). These findings encourage the view that recombinant inbred strains and chromosome substitution strains derived from the A/J and C57BL/6J mice can be utilized to explore the genetic architecture of these interactions in order to elucidate the mechanism underlying both the positive and negative health-related effects of AAS.     

Age-dependent and tissue-specific structural changes in the C57BL/6J mouse genome

We tested the hypothesis that structural changes in the genome parallel age- and organ-specific phenotypes in conjunction with the differential transposition activities of retroelements. The genomes of the liver from C57BL/6J mice were larger than other organs, coinciding with an increase in genomic copies of certain retroelements. In addition, there were differential increments in the genome size of the liver with increasing age, which peaked at 5 weeks. The findings that the genome structure of an individual is variable depending on age and organ type in association with the transposition of retroelements may have broad implications in understanding biologic phenomena.     

Calpain/calpastatin activities and substrate depletion patterns during hindlimb unweighting and reweighting in skeletal muscle

Unloading of skeletal muscle by hindlimb unweighting (HU) is characterized by atrophy, protein loss, and an elevation in intracellular Ca²⁺ levels that may be sufficient to activate Ca²⁺-dependent proteases (calpains). In this study, we investigated the time course of calpain activation and the depletion pattern of a specific structural protein (desmin) with unloading and subsequent reweighting. Rats underwent 12 h, 24 h, 72 h or 9 days of HU, followed by reweighting for either 0, 12 or 24 h. Total calpain-like activity was elevated with HU in skeletal muscle (P < 0.05) and was further enhanced with reweighting (P < 0.05). The increases in calpain-like activity were associated with a proportional increase in activity of the particulate fraction (P < 0.05). Activity of the μ-calpain isoform was elevated with 12 and 24 h of HU (P < 0.05) and returned to control levels thereafter. With reweighting, activities of μ-calpain were elevated above control levels for all HU groups except 9 days (P < 0.05). In contrast, minimal changes in m-calpain and calpastatin activity were observed with HU and reweighting. Although desmin depletion levels did not reach statistical significance, a significant inverse relationship was found between the μ-calpain/calpastatin ratio and the amount of desmin in isolated myofibrils (R = −0.83, P < 0.001). The results suggest that calpain activation is an early event during unloading in skeletal muscle, and that the majority of the increase in calpain activity can be attributed to the μ-isoform.     

Spontaneous central apneas occur in the C57BL/6J mouse strain

Despite the clinical significance of central apneas in a wide range of disorders little is known about their pathogenesis. Research in this field has been hindered by the lack of appropriate animal models. Our goal was to determine whether the C57BL/6J mouse strain, which has an inherited predisposition for dysrhythmic breathing, exhibits spontaneous apneas. In vivo plethysmography of unanesthetized, unrestrained adult C57BL/6J mice revealed a regular occurrence of spontaneous apneas. In situ recordings from respiratory outputs (phrenic, vagal, hypoglossal nerves) in the working heart-brainstem preparation (WHBP) also showed spontaneous central apneas accompanied by laryngeal closure as indicated by tonic vagal postinspiratory activity and increase in subglottal pressure. The apneas were further characterized by a hypoglossal discharge with delayed onset compared to the tonic vagal postinspiratory activity. We conclude that spontaneous central apneas with active laryngeal closure occur in C57BL/6J mice. This mouse strain is a useful animal model to study neuronal mechanisms that underlie the generation of spontaneous central apneas.     

A multimodal, multidimensional atlas of the C57BL/6J mouse brain

Strains of mice, through breeding or the disruption of normal genetic pathways, are widely used to model human diseases. Atlases are an invaluable aid in understanding the impact of such manipulations by providing a standard for comparison. We have developed a digital atlas of the adult C57BL/6J mouse brain as a comprehensive framework for storing and accessing the myriad types of information about the mouse brain. Our implementation was constructed using several different imaging techniques: magnetic resonance microscopy, blockface imaging, classical histology and immunohistochemistry. Along with raw and annotated images, it contains database management systems and a set of tools for comparing information from different techniques. The framework allows facile correlation of results from different animals, investigators or laboratories by establishing a canonical representation of the mouse brain and providing the tools for the insertion of independent data into the same space as the atlas. This tool will aid in managing the increasingly complex and voluminous amounts of information about the mammalian brain. It provides a framework that encompasses genetic information in the context of anatomical imaging and holds tremendous promise for producing new insights into the relationship between genotype and phenotype. We describe a suite of tools that enables the independent entry of other types of data, facile retrieval of information and straightforward display of images. Thus, the atlas becomes a framework for managing complex genetic and epigenetic information about the mouse brain. The atlas and associated tools may be accessed at http://www.loni.ucla.edu/MAP.     

Age-correlated changes in dopaminergic nigrostriatal perikarya of the C57BL/6NNia mouse

Alterations in neurotransmitter systems of the basal ganglia have been postulated to contribute to the disruption of motor function and balance associated with aging. This study examined nigrostriatal (A9) and mesolimbic (A10) dopamine neurons for qualitative age-correlated changes using fluorescence histochemistry for catecholamines and immunocytochemical techniques for the catecholamine-synthesizing enzyme, tyrosine hydroxylase. Results from this study suggest that age-correlated morphological changes in A9 but not all A10 neurons in the midbrain are present in mature adult (10-month) C57BL/6NNia mice and show a progressive increase in severity until at least 30 months of age. These changes are characterized by a progressive accumulation of lipofuscin in dopamine-containing perikarya, a markedly reduced dopamine content per cell as determined visually by histofluorescence, and an increase in the number of large, fluorescent axonal dilations in dopamine-containing fibers of the mesolimbic and nigrostriatal systems. These data suggest that heterogeneous morphological aging patterns exist within dopamine-containing neurons of the midbrain and that based upon their terminal projection sites, various regions of the striatum and cortex may be differentially affected in the aged brain. In addition, these findings support the belief that age-related changes in neural structure are not generalized to an entire brain nucleus or cell type but are selective for individual cells within an affected area.     

Abnormal distribution of fiber types in the slow-twitch soleus muscle of the C57BL/6J DY2J/DY2J dystrophic mouse during postnatal development

The postnatal development of extrafusal fibers in the slow-twitch soleus muscle of genetically dystrophic C57BL/6J dy^2J/dy^2J mice and their normal age-matched controls was investigated by histochemical and quantitative methods at selected ages of 4, 8, 12, and 32 weeks. The majority of fibers in the soleus consisted of two kinds, fast-twitch oxidative-glycolytic (FOG) and slow-twitch oxidative (SO), according to reactions for alkaline-stable and acid-stable myosin ATPase and the oxidative enzyme, NADH-tetrazolium reductase. A minor population of fibers, stable for both alkaline- and acid-preincubated ATPase, but variable in staining intensity for NADH-TR, were designated “atypical” fibers. With age, the normal soleus exhibited a gradual increase in the number and proportion of SO fibers and a reciprocal, steady decline in the percentage of FOG fibers. Atypical fibers were numerous at 4 weeks, but were substantially diminished at later ages. Since total extrafusal fiber number remained relatively constant between the periods examined, this change in relative proportions reflects an adaptive transformation of fiber types characteristic of normal postnatal growth. A striking alteration in the number and distribution of fiber types was associated with the dystrophic soleus. At 4 weeks an 18% reduction in total fiber number was already noted. Subsequently, by 32 weeks a further 22% diminution in overall fiber number had occurred. With age, the absolute number and proportion of dystrophic SO fibers were drastically reduced. In contrast, the percentage of dystrophic FOG fibers increased significantly while their absolute numbers between 4 and 32 weeks remained relatively constant. Atypical fibers in the dystrophic solei were found in elevated numbers at all age groups, particularly at 12 weeks. They may, in part, represent attempts at regeneration or an intermediate stage in fiber-type transformation. Microscopically, both of the major fiber types appeared affected, albeit differently, by the dystrophic process. We suggest that a failure or retardation in the normal postnatal conversion of fiber types within the soleus muscle occurs in this murine model for muscular dystrophy.     

Development of inferior colliculus response properties in C57BL/6J mouse pups

Summary Response properties of inferior colliculus (IC) neurons were studied in tranquilized C57BL/6J mice during a period of rapid auditory system development between 12 and 17 days of age. In IC units of the youngest mice, spontaneous activity was absent, a disproportionate number of onset responses was observed, and many units were not securely driven by sound. Frequency response ranges were restricted to relatively low frequencies, sharpness of tuning was poor, and thresholds at best frequencies (BFs) were quite high. Dynamic intensity ranges were restricted, but nonmonotonic functions were observed. By 15–17 days of age, spontaneous activity was appreciable, incidences of response patterns were near adult proportions, and most units in the ventrolateral nucleus were securely driven by tones. Response ranges had expanded markedly to include high frequencies, sharpness of tuning increased, and thresholds had decreased. Dynamic intensity ranges and intensity functions were similar to those observed in adult mice.     

8-OH-DPAT suppresses spontaneous central apneas in the C57BL/6J mouse strain

Apneas are common and prognostically relevant disorders of the central control of breathing, but pharmacological interventions are dissatisfying. The respiratory phenotype of C57BL/6J mice is characterized by the occurrence of spontaneous central apneas with laryngeal closure. In the present study we investigated the impact of the 5-HT(1A) receptor agonist 8-OH-DPAT on apneas in C57BL/6J mice, because of the important role of serotonin in the regulation of breathing and previous reports showing that serotonergic drugs can affect central apneas. Whole-body plethysmography in awake, unrestrained mice revealed that intraperitoneal application of 8-OH-DPAT (10microgkg(-1)) decreased the occurrence of spontaneous apneas from 1.91+/-0.25 to 1.05+/-0.05 apneas min(-1). The efficacy of 5-HT(1A) receptor activation was further verified in the in situ working heart-brainstem preparation. Here the apneas occurred at a frequency of 1.33+/-0.19min(-1). Intra-arterial perfusion with 1-2microM 8-OH-DPAT completely abolished spontaneous apneas. These results suggest that 5-HT(1A) receptor activation may be a potential treatment option for central apneas.     

Adenosine receptor antagonists inhibit the development of morphine sensitization in the C57BL/6 mouse.

We examined the effects of adenosine antagonists on the development of morphine sensitization in C57BL/6 mice. Adenosine antagonists or vehicle were repeatedly co-administered intraperitoneally with morphine (10 mg/kg, s.c.) to mice once every other day for 9 days. Two days later, a 10 mg/kg morphine-only challenge was administered to each group. Consistent with sensitization, mice receiving morphine alone developed enhanced ambulatory activity responses to subsequent morphine administrations and, upon morphine-only challenge, had a significantly greater response to morphine than vehicle pretreated animals. The nonselective adenosine antagonist, caffeine, at 10 and 20 mg/kg but not at 5 mg/kg, attenuated the development of sensitization during co-administration with morphine and also following morphine-only challenge. The adenosine A1 selective antagonist 1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine (PACPX), at 0.001 and 0.002 mg/kg but not at 0.2 mg/kg, similarly attenuated the development of morphine sensitization. We propose a mechanism which involves an adenosine receptor role in the mesolimbic dopamine system.     

Post-sigh breathing behavior and spontaneous pauses in the C57BL/6J (B6) mouse

The purpose was to examine sighs and spontaneous pauses in regard to the stability of resting breathing in the B6 strain, compared to the A/J strain. A 5-HT_1A receptor agonist (buspirone) and a chromosomal substitution strain (B6a1) were used to further alter breathing patterning. Ten-minute recordings of room air breathing were collected from unanaesthetized B6, A/J, and B6a1 mice. Despite no differences between strains in the magnitude and incidence of sighs, post-sigh apneas, the variation for duration of expiration (T_e) after sighs, and the number of spontaneous pauses were greater in the B6, while Shannon Entropy (nonlinear metrics) for T_e after sighs was lower in B6, compared to the other strains. Buspirone and chromosomal substitution eliminated post-sigh apneas and decreased spontaneous pauses. A greater irregularity and the lower complexity of post-sigh breathing in B6 are reversed by elements on A/J chromosome 1 and by increased 5-HT_1A serotonergic tone.     

Histamine H2 receptors mediate morphine-induced locomotor hyperactivity of the C57BL/6J mouse

Morphine-induced locomotor hyperactivity of the C57BL/6J mouse was challenged with intracranial injections of antihistamines or the opiate antagonist naloxone. When cimetidine (H2 receptor blocker) was injected into the nucleus accumbens/stria terminalis, it significantly reduced opiate-stimulated locomotion. However, ventricular injections of cimetidine did not significantly alter either morphine or amphetamine hyperactivity, nor did cimetidine depress spontaneous locomotion. Although naloxone eliminated morphine-induced locomotion when injected into either the nucleus accumbens or the ventricles, chlorpheniramine (H1 receptor blocker) failed to reduce this behavior. These data suggest that opiate-stimulated locomotion of the C57BL/6J mouse may be partially mediated by histamine H2 receptors of the nucleus accumbens or closely adjacent structures.     

Maternal entrainment in the circadian activity rhythm of laboratory mouse (C57BL/6J)

Maternal entrainment of circadian activity rhythm was studied in the laboratory mouse (C57BL/6J). Pregnant mice gave birth in constant dim light and the mother raised the pups until Postnatal Day 18 (weaning). The wheel running activity of these pups was individually monitored from Day 18. It was found that the phases of pups' activity on the day of weaning were similar to the phases of their mother's rhythm (p < 0.001), indicating that maternal entrainment occurs in the C57BL/6J pups. When the mother mouse was cyclically presented to the pups for 12 h of a day, thereby creating presence-absence (PA) cycles of 12:12 h, it was found that the pups' activity rhythm entrained to the imposed cycles. The onset of activity of the pups coincided with the beginning of the mother's 12-h absence period. It is proposed that the social contact between the mother and the pups is taken by the pups as subjective day (rest time) and absence of the mother as subjective night (activity time). This maternal (nonphotic) entrainment, however, does not continue beyond Postnatal Days 23-26, despite ongoing PA cycles. These results indicate that the PA cycles of the mother are a transient zeitgeber, and are effective in entraining the rhythm of pups only for about 4 weeks after birth.     

Aging changes in the β-endorphin neuronal system in the preoptic area of the C57BL/6J mouse: Ultrastructural analysis

In hypothalami of aging rodents, β-endorphin (β-EP) neuron number and content are reduced. The objectives of this study were: first, to analyze ultrastructurally the population of neuronal elements in a selected region of the preoptic area (POA) in young and old mice: second, to study the β-EP neuronal system in the same region to determine whether or or not this population remains stable with age. Vibratome sections from the most caudal POA through the diagonal band of Broca were examined by light microscopy and immunochemistry in mature, cycling (5–6 months old) and old, acyclic, disease-free (24–26 months old) mice. A subset of β-EP-like perikarya and associated structures was observed in the periventricular POA. When this subregion was examined at the ultrastructural level, there was a significant decrease in the number of recipient dendrites [3.78 ± 0.04 SEM/μm² youngs vs. 0.82 ± 0.03/gmm ² old; p < 0.007, analysis of variance (ANOVA)], but a significant in the number of nonmyelinated axons 20.0 ± 2.6/gmm² youngs vs. 26.8 ± 0/07/μm² old; p <0.05). Immunolabeled that contained a synapse comprised 2.56 ± 0.08% of all terminal with synapses in young mice but not only 0.34 ± 0.04% in old ones when corrected for surface area examined (ifp < 0.03). A significant age-related loss was also observed in the nonmyelinated β-EP-labeled axons population (1.50 ± 0.10% young vs. 0.40 ± 0.01% old; p < 0.009, ANOVA). We conclude that there are critical changes in the microenvironment of the POA in old, noncycling female mice that are likely to affect neuron function.     

The distribution of intracellular calcium concentration in isolated single fibres of mouse skeletal muscle during fatiguing stimulation

Fluorescence microscopy has been used to examine the distribution of intracellular calcium concentration ([Ca²⁺]_i) in isolated single fibres from the mouse flexor brevis muscle during fatiguing stimulation. Under control conditions there was a virtually uniform distribution of [Ca²⁺]_i in fura-2 loaded fibres either at rest or during short (0.35 s, 100 Hz) tetani. Fatigue produced by repeated short tetani was accompanied by an early rise, followed by a marked fall, in tetanic [Ca²⁺]_i. Throughout the period of fatiguing stimulation the distribution of [Ca²⁺]_i remained uniform with no detectable gradients observed. In contrast, when fatigue was produced by continuous 100 Hz stimulation, a small gradient of [Ca²⁺]_i developed across the fibre with the [Ca²⁺]_i in the centre of the fibre lower than that at the edge of the fibre. This gradient was apparent after 1.7 s, persisted for at least 11 s and was superimposed on a rise followed by a fall in spatially averaged [Ca²⁺]_i. Reduction of the extracellular Na⁺ to 50% caused reduced force production and a reduced [Ca²⁺]_i in the centre of the fibre. To assess the contribution of reduced response of the myofibrillar proteins to [Ca²⁺]_i during continuous tetani, the relation between [Ca²⁺]_i and force throughout the long tetanus was compared with that obtained in short, unfatigued tetani. These results show that in long tetani, reduced tetanic [Ca²⁺]_i and reduced responsiveness of the myofibrillar proteins to [Ca²⁺]_i each make important contributions to the decline of force, whereas the gradients of [Ca²⁺]_i make only a small contribution.     

Exercise alters SIRT1, SIRT6, NAD and NAMPT levels in skeletal muscle of aged rats

Silent information regulators are potent NAD⁺-dependent protein deacetylases, which have been shown to regulate gene silencing, muscle differentiation and DNA damage repair. Here, changes in the level and activity of sirtuin 1 (SIRT1) in response to exercise in groups of young and old rats were studied. There was an age-related increase in SIRT1 level, while exercise training significantly increased the relative activity of SIRT1. A strong inverse correlation was found between the nuclear activity of SIRT1 and the level of acetylated proteins. Exercise training induced SIRT1 activity due to the positive effect of exercise on the activity of nicotinamide phosphoribosyltransferase (NAMPT) and thereby the production of sirtuin-fueling NAD⁺. Exercise training normalized the age-associated shift in redox balance, since exercised animals had significantly lower levels of carbonylated proteins, expression of hypoxia-inducible factor-1alpha and vascular endothelial growth factor. The age-associated increase in the level of SIRT6 was attenuated by exercise training. On the other hand, aging did not significantly increase the level of DNA damage, which was in line with the activity of 8-oxoguanine DNA glycosylase, while exercise training increased the level of this enzyme. Regular exercise decelerates the deleterious effects of the aging process via SIRT1-dependent pathways through the stimulation of NAD⁺ biosynthesis by NAMPT.     

Voltage-dependent K+ channels in the sarcolemma of mouse skeletal muscle

The voltage-dependent K⁺ channels of the mammalian sarcolemma were studied with the patch-clamp technique in intact, enzymatically dissociated fibres from the toe muscle of the mouse. With a physiological solution (containing 2.5 mM K⁺) in the pipette, depolarizing pulses imposed on a cell-attached membrane patch activated K⁺ channels with a conductance of about 17 pS. No channel activity was observed when the pipette solution contained 2mM tetraethylammonium (TEA), or 2 mM 4-aminopyridine (4-AP). Whole cell recordings from these very small muscle fibres showed the well-known delayed rectifier K⁺ outward current with a threshold of about -40mV. The whole-cell current was completely blocked by 2 mM TEA in the bath, suggesting that the TEA-sensitive channels in the patch were also delayed rectifier channels. The inactivation properties of the channels were studied in the cell-attached mode. Averaged single-channel traces showed at least two types of channels discernible by their inactivation time course at a test potential of 60 mV. The fast type inactivated with a time constant of about 150ms, the slow type with a time constant of about 400 ms. A little channel activity always remained during pulses lasting several minutes, indicating either the presence of a very slowly inactivating third type of K⁺ channel, or the tendency of the fast inactivating channels to re-open at constant voltage. No difference was seen in the single-channel amplitudes of the different types of K⁺ channels. The well characterized adenosine-5-triphosphate-(ATP)-sensitive and Ca²⁺-dependent K⁺ channels, although present, were not active under the conditions used. The results suggest that in mouse skeletal muscle the delayed rectifier channels to not only carry the outward current during excitation but are also responsible for the resting K⁺ conductance.     

Histological changes in the spleen and liver of C57BL/6 and A/J mice during Plasmodium chabaudi AS infection

The level of resistance to infection in inbred mice with the murine malaria species Plasmodium chabaudi AS is genetically determined. Resistant C57BL/6, which are able to eliminate the parasite by 4 weeks, develop marked splenomegaly and survive the infection. Susceptible A/J mice, which succumb to infection (mean survival time = 10 days), develop only minimal splenomegaly. In order to determine if gross differences in the organization, number, and type of spleen cells are related to the outcome of infection with P. chabaudi AS, the development of splenomegaly was examined by enzyme and immunohistochemical methods during the first week after infection. Cryostat sections of spleens removed from normal animals of both strains and at 4 and 7 days after intraperitoneal infection with 10(6) parasitized erythrocytes were stained for enzyme (acid phosphatase and nonspecific esterase) and immunohistochemistry with conventional monoclonal antibodies against T cells, B cells, and macrophages as well as with novel rat anti-mouse monoclonal antibodies which define discrete subpopulations of macrophages in the mouse spleen. The livers of normal and infected animals of each strain were also examined. The results of this study demonstrate (1) differences between normal, uninfected B6 and A/J mice in the organization and number of one subpopulation of macrophages in the spleen, the marginal metallophilic macrophages, and (2) marked histological changes in the spleen and liver during the course of infection in both resistant C57BL/6 and susceptible A/J mice. These changes include depletion of cells from the marginal zone of the spleen which, in the case of the marginal metallophilic macrophages, appears to be more severe in susceptible A/J mice.     

Cell proliferation in hippocampal formation during the development and aging of C57/BL6 mouse

目的:探讨C57/BL6小鼠海马结构发育及衰老过程中细胞增殖的变化规律.方法:采用免疫组织化学和免疫印迹结合体视学方法对不同发育阶段C57/BL6小鼠海马结构中的细胞增殖进行系统观察和定量分析.结果:胚胎12天(E12 d)海马结构原基中见大量双皮质素抗原(DCX)阳性细胞.E18 d阳性细胞弥漫分布.生后1～14天(P1 d～P14 d)阿蒙角(CA)锥体层外周阳性细胞排列成1条致密带,P1 d最宽,至P14 d消失.P1 d～P7 d阳性细胞弥散分布于齿状回(DG)各层,P14 d主要分布于颗粒层内1/2,P21 d～18个月主要分布于亚颗粒细胞层.CA锥体层体积P1 d～P3 d呈上升趋势,P7d降低.DG颗粒层体积P1 d～P14 d增大;P21 d～18个月呈下降趋势.CA各区锥体细胞层阳性细胞Vv值P1 d CA3区下降,CA1区上升,P7d各区均降低.E18 d～P3 d,DCX蛋白表达逐渐增多;P3 d～18个月呈下降趋势.结论:小鼠海马结构细胞增殖、分化、迁移高峰,CA区位于生前,DG区生后两周仍有一高峰.     

Abnormality in the cerebellar folial pattern of C57BL/6J mice

The characteristic folial pattern of the mouse cerebellum is formed during postnatal development. We observed this process in C57BL/6J (B6) mice in detail, and found an abnormal folial pattern in a specific region (lobules VIII and IX of the vermis) in a substantial number of B6 mice. The frequency of this abnormality increased during postnatal development and reached 55% in the adult. Thus, the present study showed an abnormality in the cerebellar folial pattern of B6 mice, a mouse widely used in knockout studies, and called for caution in the phenotypic analysis of knockout mice of the B6 genetic background.