Flavor preferences conditioned by intragastric glucose but not fructose or galactose in C57BL/6J mice

The present study determined if mice, like rats, differ in their flavor conditioning responses to intragastric (IG) infusions of three common monosaccharide sugars. In Experiment 1, C57BL/6J mice were trained to drink a flavored saccharin solution (the CS+) paired with intragastric (IG) self-infusions of 16% glucose, fructose or galactose and a different flavored solution (the CS-) paired with IG water infusions during 22 h/day training sessions. The glucose infusions increased CS+ intakes during training and produced a strong CS+ preference (~87%) in two-bottle choice tests. In contrast, the fructose and galactose infusions reduced CS training intakes and did not condition a CS+ preference. Experiment 2 determined if reducing fructose and galactose concentration would enhance conditioning. However, IG infusions of 8% sugar also failed to condition CS+ preferences. The robust conditioning response to IG glucose confirms results obtained with rats, but the indifference of mice to IG fructose and galactose contrasts with preference and avoidance responses observed in rats. The effectiveness of glucose to condition preferences suggests an important role for glucose-specific sensors rather than gut "sweet" taste receptors in the postoral modulation of carbohydrate appetite.     

Flavor preferences conditioned by intragastric fat infusions in rats

The present series of experiments determined if rats would learn to prefer cue flavors that were paired with intragastric (IG) infusions of fat. Adult female rats were fitted with gastric catheters and were trained to drink a flavored (CS+) solution, e.g., cherry-water, associated with IG infusions of a corn oil emulsion. On other days, an alternately flavored (CS-) solution, e.g., grape-water, was paired with IG infusions of water. The preference between the CS solutions was then assessed with 2-bottle choice tests. In Experiment 1, food-restricted rats were given saccharin-sweetened CS solutions for 10 min/day followed by IG infusions of a 7.1% oil emulsion or water. In subsequent choice tests they displayed a small (60%) but significant preference for the CS+ solution. In Experiments 2-5, rats had access to the CS solutions for 20 or 23 hr/day; drinking the CS solutions automatically triggered IG infusions of oil or water (electronic esophagus preparation). Infusions of 7.1% oil failed to produce a significant preference for the CS+ solution in rats given ad lib access to chow, and offered unsweetened CS flavors. Increasing the oil concentration (14.5% and 29%) did not facilitate the formation of a preference, nor did infusing the rats with a preingested oil emulsion. However, rats restricted to 2 hr/day access to chow, and 20 hr/day access to the CS solutions acquired a significant (85%) preference for the CS+ flavor; subsequent training and testing with saccharin-sweetened CS solutions increased the preference for the CS+ flavor to 95%. Ad lib fed rats trained with saccharin-sweetened CS solutions also developed a reliable preference (76%) for the CS+ flavor paired with IG oil; this preference persisted during four extinction tests when both CS+ and CS- flavors were paired with IG water. These results demonstrate that, as with other macronutrients (carbohydrate, protein), the postingestive effects of fats can condition flavor preferences, and suggest that the postingestive actions of nutrients play an important role in food selection and reward.     

Amino acid and carbohydrate preferences in C57BL/6ByJ and 129P3/J mice

Compared with mice from the 129P3/J (129) inbred strain, mice from the C57BL/6ByJ (B6) inbred strain have higher consumption of several sweet-tasting amino acids and carbohydrates. To examine the relative contribution of taste and nutritive properties in these strain differences, we measured responses of B6 and 129 mice to eight sweet and non-sweet amino acids and carbohydrates in two-bottle preference tests with water. Mice from the two strains did not differ in consumption of non-sweet l-valine and l-histidine. Compared with 129 mice, B6 mice had higher consumption and lower preference thresholds for sweet amino acids l-glutamine, l-alanine and l-threonine, monosaccharides glucose and fructose, and maltooligosaccharide. These data suggest that differences in gustatory responsiveness are an important factor underlying higher consumption of some amino acids and carbohydrates by B6 mice compared with 129 mice. It is likely that in B6 mice, higher sweet taste responsiveness results in increased consumption of sweet-tasting amino acids and sugars, and higher taste responsiveness to complex carbohydrates results in increased consumption of maltooligosaccharide. However, postingestive processes also influence nutrient consumption and may be responsible for higher intake of carbohydrates compared with sweet-tasting amino acids. Results of this study set the stage for genetic analysis of differences between B6 and 129 mice in taste responsiveness and macronutrient consumption.     

Asbestos-induced autoimmunity in C57BL/6 mice

Environmental impacts on autoimmunity have significant public health implications. Epidemiological studies have shown associations between exposure to airborne silicates, such as crystalline silica or asbestos, and autoimmunity, but the etiology remains unclear. The purpose of this study was to test the hypothesis that asbestos could lead to a specific pattern of autoantibodies and pathology indicative of systemic autoimmune disease (SAID). Female C57Bl/6 mice were instilled intratracheally with 2 doses x 60 microg/mouse of amphibole asbestos (tremolite), wollastonite (a non-fibrogenic control fiber), or saline alone. Serum samples were collected and urine was checked for protein bi-weekly for 7 months. By 26 weeks, the asbestos-instilled animals had a significantly higher frequency of positive anti-nuclear antibody (ANA) tests compared to wollastonite and saline groups. The majority of positive ANAs showed homogeneous or combined homogeneous/speckled patterns, and tested positive for antibodies to dsDNA and SSA/Ro 52. Serum isotyping showed no significant changes in IgM, IgA, or IgG subclasses. However, there was an overall decrease in the mean IgG serum concentration in asbestos-instilled mice. IgG immune complex deposition was demonstrated in the kidneys of asbestos-instilled mice, with evidence of glomerular and tubule abnormalities suggestive of glomerulonephritis. Flow cytometry demonstrated moderate changes in the percentages of CD25+ T-suppressor cells and B1a B-cells in the superficial cervical lymph nodes of the asbestos-instilled mice. These data demonstrate that asbestos leads to immunologic changes consistent with the development of autoimmunity. This study provides a non-autoimmune prone murine model for use in future elucidation of mechanisms involved in asbestos-induced autoimmune disease.     

A/J mice are susceptible and C57BL/6 mice are resistant to Listeria monocytogenes infection by intragastric inoculation

Previous studies demonstrated that the innate resistance of mice to Listeria monocytogenes infection by intravenous or intraperitoneal inoculation is regulated principally by the Hc locus on mouse chromosome 2. The A/J and C57BL/6 mouse strains were identified as prototype L. monocytogenes-susceptible and -resistant strains, respectively. In the present study, we compared the relative susceptibilities of A/J and C57BL/6 mice to intragastric (i.g.) inoculation with L. monocytogenes. The results of our study indicate that A/J mice are significantly more susceptible than C57BL/6 mice to an i.g. challenge with L. monocytogenes. This was reflected in the estimated 50% lethal doses for the two strains (10(6) and 10(8) CFU for A/J and C57BL/6 mice, respectively) and a more rapid and severe dissemination of the infection to the spleen and liver in A/J mice than in C57BL/6 mice. Histopathological examination of tissues from the infected mice confirmed the greater severity of disease in A/J mice. Clearance of a primary infection enhanced the resistance of both A/J and C57BL/6 mice to reinfection with L. monocytogenes via the gastrointestinal tract. However, the relative difference in susceptibility between the two strains was evident even after immunization. The A/J mouse holds promise as a model for investigating the pathogenesis of gastrointestinal listeriosis because of its ability to develop systemic infection following challenge with numbers of organisms similar to those recovered from some L. monocytogenes-contaminated food products.     

Pregnancy reduces the genetic resistance of C57BL/6 mice to Listeria monocytogenes infection by intragastric inoculation

In this study, we compared genetically resistant C57BL/6 and susceptible A/J mice for their resistance to Listeria monocytogenes infection during pregnancy. Intragastric infection with modest numbers of bacterial cells (10⁵ CFU) caused reproducible fetal infection and abortion in both mouse strains. Bioluminescence imaging demonstrated dissemination of L. monocytogenes cells from maternal to fetal organs within 3 days of intragastric infection. Although non-pregnant C57BL/6 mice were significantly more resistant to infection than non-pregnant A/J mice, C57BL/6 and A/J mice had similar microbial loads (CFU) in maternal and fetal tissues during pregnancy. Inflammation and necrosis, however, were more severe in A/J mice as evaluated by semi-quantitative histopathology. Although the microbial load in fetal tissues was similar for all fetuses within a single uterus, inflammation and necrosis varied among individual fetuses and placentas. We also noted that the uterus is a target for L. monocytogenes infection in non-pregnant mice.     

Flavor preferences conditioned by intragastric fructose and glucose: differences in reinforcement potency

Many prior conditioning studies indicate that fructose, unlike glucose, has minimal postingestive reinforcing effects. Using a new training procedure, food-restricted rats were trained in alternate 20-h/day sessions with one flavored solution (CS+F) paired with intragastric (IG) infusions of 16% fructose and another flavor (CS-) paired with IG water. In subsequent two-bottle tests they showed a robust (85%) preference for the CS+F over the CS-. A third flavor (CS+G) was then paired with IG 16% glucose, and it was strongly preferred to the CS+F. When retrained 30 min/day with new flavors paired with IG fructose, glucose, or water the rats learned only a CS+G preference. When training was extended to 20 h/day, a CS+F preference developed. New rats trained 20 h/day with two-bottle access to CS+F and CS- paired with IG fructose and water failed to acquire a CS+F preference. Other rats rapidly developed a strong preference when trained with concurrent access to CS+G and CS- paired with IG glucose and water. These data indicate that both fructose and glucose generate postingestive reinforcing signals, but that the fructose signals are weaker and/or delayed relative to those produced by glucose.     

Flavor preferences conditioned by intragastric nutrient infusions in food restricted and free-feeding rats

The role of deprivation state in flavor preference conditioning by nutrients was investigated in rats fitted with intragastric (IG) catheters. In different experiments, food restricted (FR) and food ad libitum (AL) groups were trained to drink one flavored solution (CS+) paired with IG infusions of maltodextrin, corn oil, or casein and another flavored solution (CS-) paired with IG water infusions. Training intakes of the CS solutions were limited to equate the exposure of the FR and AL groups. The IG nutrient infusions conditioned flavor preferences in FR and AL groups which, in three of four experiments, were of similar magnitude. Food restriction did, however, increase the overall intake of the CS+ solutions during testing. Rats trained with one CS+ while food restricted and a second CS+ while food unrestricted showed similar preferences for the two CS+ flavors. Prefeeding AL rats to satiety with chow prior to daily training sessions did not prevent them from developing a preference for a CS+ paired with IG maltodextrin. These findings indicate that the postoral actions of nutrients are reinforcing in food sated as well as hungry rats.     

Behavioral changes in aging female C57BL/6 mice

Using a range of tests we have studied alterations in behavior with advancing age in female C57BL/6 (of Jackson origin), the golden standard on which most genetically engineered mice are back-crossed. In parallel, growth and survival data were collected. In a protected environment the 90% and 75% cohort survival age was 20 and 25 months, respectively, and the 50% cohort survival was 32 months. In mice, body weight increases continuously until 15-20 months of age, while in advanced age whole body weight drops. The body mass loss in senescence is associated with emergence of other aged phenotype features such as kyphosis, balding and loss of fur-color.Our behavioral data show that aging modulates certain aspects of basic behavior in a continuous manner, like explorative and locomotor activities. Advanced age associates with an acceleration of behavioral impairments evident in most of the tests used, including motor skill acquisition and memory consolidation. However, certain domains of mouse behavior were well preserved also in advanced age such as thermal noxious threshold and working memory as assessed by an object recognition task. The decreased drive to explore is suggested to be a key factor underlying many aspects of reduced performance including cognitive capacity during aging. Behavioral aging affects genetically closely related individuals housed under strictly standardized conditions differentially (Collier, T.J., Coleman, P.D., 1991. Divergence of biological and chronological aging: evidence from rodent studies. Neurobiol. Aging, 12, 685-693; Ingram, D.K., 1988. Motor performance variability during aging in rodents. Assessment of reliability and validity of individual differences. Ann. N.Y. Acad. Sci., 515, 70-96). Consistent with this a subpopulation of the 28-month-old mice showed an explorative activity similar to young-adult mice and a significantly stronger preference for a novel object than aged mice with a less explorative behavior. Thus, subtle environmental factors and epigenetic modifications may be important modulators of aging.     

The superior olivary complex in C57BL/6 mice

The cellular and cytoarchitectural features of the lateral superior olive, the medial superior olive, the superior paraolivary nucleus and the medial, lateral and ventral nuclei of the trapezoid body are described in C57BL/6 mice using Nissl, Bodian and Golgi techniques. Principal, spindle and marginal cells are present in a well-defined lateral superior olive. The dendrites of these cells run primarily within rostrocaudal sheets as in the cat. The principal cells of the medial nucleus of the trapezoid body are similar to the principal cells in the cat. Large multipolar cells characterize the lateral nucleus of the trapezoid body and bipolar cells with a medial-lateral orientation are found in the medial superior olive. The largest neurons are found in the superior paraolivary nucleus and the lateral superior olive, and the medial and ventral nuclei of the trapezoid body. While brain weight and neuronal packing density change with development, the characteristic location of cell groups and the shape and Nissl-staining pattern of neurons in the youngest brains examined were essentially unchanged in the adult mice, although dendritic maturation had occurred. The homologies of the C57BL/6 superior olivary complex nuclei with the same areas described in other mouse strains, rat and cat are discussed. This study expands our understanding of the organization of the superior olivary complex in an inbred strain of Mus musculus and relates it to other species. The data about changes occurring during postnatal maturation may aid in the interpretation of behavioral and physiological studies of neonatal plasticity of the auditory system.     

Age and brightness discrimination learning by C57BL/6J mice

Groups of 20–34 C577BL/6J mice, aged 75, 225, 375, and 525 days, learned a brightness discrimination for water. The error scores of the 75- and 525-day groups were almost identical, and both made significantly more errors than the 225-day-old mice. Both immature and older mice appear to be deficient in learning ability compared to young adults.     

Simvastatin enhances hippocampal long-term potentiation in C57BL/6 mice

Statins inhibit 3-hydroxy-3-methylglutaryl CoA reductase, the rate-limiting enzyme in the cholesterol biosynthetic pathway, and they are widely used to control plasma cholesterol levels and prevent cardiovascular disease. However, emerging evidence indicates that the beneficial effects of statins extend to the CNS. Statins have been shown to improve the outcome of stroke and traumatic brain injury, and statin use has been associated with a reduced prevalence of Alzheimer's disease (AD) and dementia. However, prospective studies with statins in AD have produced mixed results. Recently, we reported that simvastatin, a widely used statin in humans, enhances learning and memory in non-transgenic mice as well as in transgenic mice with AD-like pathology on a mixed genetic background. However, the cellular and molecular mechanisms underlying the beneficial effects of simvastatin on learning and memory remain elusive. The present study was undertaken to investigate the effect of acute simvastatin treatment on hippocampal long-term potentiation (LTP), a cellular model of learning and memory, in brain slices from C57BL/6 mice. Our results demonstrate that a prolonged in vitro simvastatin treatment for 2–4 h, but not a short-term 20-min exposure, significantly increases the magnitude of LTP at CA3–CA1 synapses without altering basal synaptic transmission or the paired-pulse facilitation ratio in hippocampal slices. Furthermore, we show that phosphorylation of Akt (protein kinase B) is increased significantly in the CA1 region following 2-hour treatment with simvastatin, and that inhibition of Akt phosphorylation suppresses the simvastatin-induced enhancement of LTP. These findings suggest activation of Akt as a molecular pathway for augmented hippocampal LTP by simvastatin treatment, and implicate enhancement of hippocampal LTP as a potential cellular mechanism underlying the beneficial effects of simvastatin on cognitive function.     

Heroin-induced locomotor activity and conditioned place preference in C57BL/6J and 129P3/J mice

Differences in the locomotor stimulating and rewarding properties of drugs of abuse have been described in several inbred strains of mice, and comparisons of inbred strains with differing responses to drugs of abuse may provide crucial insight into the question of individual vulnerability to the effects of drugs of abuse. The present study was designed to examine the rewarding and locomotor-stimulating effects of heroin in C57BL/6J and 129P3/J mice. Heroin produced a robust dose-dependent locomotor stimulation in both strains. Both strains also developed conditioned place preference to heroin, again in a dose-dependent manner. However C57BL/6J mice developed conditioned place preference to only the two lowest doses of heroin tested, while the 129P3/J counterparts showed conditioned place preference to only the three highest doses tested. These studies indicate that 129P3/J mice are less sensitive to the rewarding effects of heroin than are age-matched C57BL/6J mice.     

Dose-related induction of hepatic preneoplastic lesions by diethylnitrosamine in C57BL/6 mice

The C57BL/6 mouse strain (or derivation of this strain) is used as a background for many transgenic mouse models. This strain has a relatively low susceptibility to chemically induced hepatocarcinogenesis compared with other commonly used experimental mouse strains. In the present study, the authors treated C57BL/6 mice with 25, 50, and 75 mg/kg of diethylnitrosamine (DEN) for 4 or 8 weeks by intraperitoneal injection to investigate the dose-response pattern of preneoplastic and neoplastic lesion formation in the liver. DEN induced preneoplastic lesions and cytokeratin 8/18-positive foci in a dose-dependent manner. In the 75 mg/kg for 8 weeks treatment group, hepatocellular adenoma, cholangioma and hemangioma, and cytokeratin 19-positive foci were also induced, but a significant decrease in body weight was observed. The suitable DEN treatment range for this strain was concluded to be from 75 mg/kg for 4 weeks (total amount = 300 mg/kg) to 50 mg/kg for 8 weeks (total amount = 400 mg/kg). These results should prove useful for future studies investigating hepatocarcinogenesis in both the background C57BL/6 strain and other transgenic mouse models derived from it.     

Differences in resistance of C57BL/6 and C57BL/10 mice to infection by Mycobacterium avium are independent of gamma interferon

After infection with a low-virulence strain of Mycobacterium avium, C57BL/6 and C57BL/10 mice had clear differences in the control of the infection in their livers and spleens. This difference in susceptibility was not associated with differences in the H-2 complex. It was dependent on the activity of CD4(+) T cells but unrelated to the ability of these cells to secrete gamma interferon or to the development of delayed-type hypersensitivity responses at 3 weeks of infection. It was associated with lower total numbers of CD4(+) cells present in infected spleens and was related to an earlier induction of protective T cells, as measured by adoptive-transfer assays. These data further strengthen the notion of gamma-interferon-independent mechanisms of protection against mycobacteria.     

Enhancement of maze learning in old C57BL/6 mice by dietary lecithin

Maze learning was studied in young and old C57BL/6 mice. One group of old animals was fed lecithin (supplemented with vitamin E). A second group of old mice and the young mice received standard laboratory mouse chow. The mice were tested on a three-choice-point maze. Young mice learned the maze significantly more easily than either group of old mice. Learning in the lecithin-fed old mice was significantly better than in the control group on a standard diet.     

Cytogenetic studies of ethyl acrylate using C57BL/6 mice

The clastogenicity of ethyl acrylate (EA) was examined in vivo by injecting i.p. five male C57BL/6 mice per dose group with either 125, 250, 500 or 1000 mg/kg EA dissolved in saline. Controls received solvent only. Acrylamide (100 mg/kg), because of its similarity in structure and mode of action to EA, was used as a positive control. Twenty-four hours after injection, the animals were anesthetized and the spleens aseptically removed. Splenocytes were isolated on density gradients and cultured with concanavalin A to stimulate cell division. In half the cultures bromodeoxyuridine was added at 21 h for analysis of chromosome aberrations (CAs) in first division cells and sister chromatid exchange (SCE) in second division cells. In the remaining cultures cytochalasin B was added to produce binucleated cells for scoring of micronuclei (MN). There was no significant increase in SCE or CAs at any of the doses of EA examined. At the highest dose examined (1000 mg/kg), EA did cause a small but significant increase in binucleated cell MN. Acrylamide caused an increase in MN and SCEs in splenocytes. Because others have found EA to be clastogenic in vitro, isolated splenocytes were exposed to a wide range of concentrations of EA during the G0 stage of the cell cycle or 23 h after mitogen stimulation during the late G1 or early S phase of the cell cycle. Although EA was toxic for both exposure regimens, significant increases in chromatid-type aberrations were found only when the target cells were treated 23 h after mitogenic stimulation. No statistically significant increase in SCE frequency was found after either treatment regimen. These data suggest that EA is only clastogenic at near toxic concentrations during a specific stage of the cell cycle.     

Varenicline ameliorates nicotine withdrawal-induced learning deficits in C57BL/6 mice

Varenicline, a partial agonist for a4ss2 nicotinic acetylcholine receptors (nAChRs) and full agonist for a7 nAChRs, has been approved for the treatment of smoking cessation. Although recent clinical trials support the efficacy of varenicline for managing global nicotine withdrawal symptoms and for smoking cessation, its effects on animal models of specific withdrawal-associated behaviors have not been tested. The present study evaluated the effects of varenicline on contextual fear conditioning and its effects on nicotine (6.3 mg/kg/day) withdrawal-induced deficits in contextual fear conditioning. Varenicline (0.01, 0.1, 1.0 mg/kg) had no effect on contextual fear conditioning when administered alone, but (0.1 mg/kg) prevented nicotine withdrawal-associated deficits in contextual fear conditioning. These data demonstrate, for the first time, that varenicline reverses nicotine withdrawal-induced deficits in an animal model and suggest that varenicline may be effective at treating nicotine withdrawal-associated deficits in learning and memory.     

Development of high-grade lymphoma in Helicobacter pylori-infected C57BL/6 mice

Helicobacter pylori infection is associated with chronic gastritis, peptic ulcer disease, gastric adenocarcinoma and MALT lymphoma. Mice with H. pylori infection develop severe gastritis and atrophic changes in their stomachs after 6 months. We followed H. pylori-infected animals for 13 months to find out whether dysplasia, carcinoma or lymphoma developed. Six-week-old C57BL/6 mice were infected with the CagA-positive and VacA-positive H. pylori mouse-passaged strain 119/95, fed a low antioxidant diet, and kept in microisolated cages. Histopathological changes were examined after 13 months' infection. All H. pylori-inoculated mice (n = 5) developed a gastric squamous papilloma with nagging of the lamina muscularis after 13 months. Three out of five animals developed high-grade B-cell lymphoma derived from a MALT lymphoma at the squamous-corpus border with manifestations also in the liver, spleen and kidney. There was a suspicion of local gastric lymphoma in the two remaining mice but with no significant changes in the liver, spleen or kidney. The normal control mice showed no pathological changes in any of these organs. It is concluded that this mouse model with infection by the CagA-positive, vac-toxin-producing H. pylori strain 119/95 is suitable for use in the study of lymphoma development and also development of squamous cell papilloma with proliferative features.