Days In Male Mice Research Articles

Females display relatively preserved muscle quality compared with males during the onset and early stages of C26-induced cancer cachexia.

Cancer cachexia is clinically defined by involuntary weight loss >5% in <6 mo, primarily affecting skeletal muscle. Here, we aimed to identify sex differences in the onset of colorectal cancer cachexia with specific consideration to skeletal muscle contractile and metabolic functions. Eight-weeks old BALB/c mice (69 males, 59 females) received subcutaneous C26 allografts or PBS vehicle. Tumors were developed for 10-, 15-, 20-, or 25 days. Muscles and organs were collected, in vivo muscle contractility, protein synthesis rate, mitochondrial function, and protein turnover markers were assessed. One-way ANOVA within sex and trend analysis between sexes were performed, P < 0.05. Gastrocnemius and tibialis anterior (TA) muscles became atrophic in male mice at 25 days, whereas female mice exhibited no significant differences in muscle weights at endpoints despite presenting hallmarks of cancer cachexia (fat loss, hepatosplenomegaly). We observed lowered muscle contractility and protein synthesis concomitantly to muscle mass decay in males, with higher proteolytic markers in muscles of both sexes. mRNA of Opa1 was lower in TA, whereas Bnip3 was higher in gastrocnemius after 25 days in male mice, with no significant effect in female mice. Our data suggest relative protections to skeletal muscle in females compared with males despite other canonical signs of cancer cachexia and increased protein degradation markers; suggesting we should place onus upon nonmuscle tissues during early stages of cancer cachexia in females. We noted potential protective mechanisms relating to skeletal muscle contractile and mitochondrial functions. Our findings underline possible heterogeneity in onset of cancer cachexia between biological sexes, suggesting the need for sex-specific approaches to treat cancer cachexia.NEW & NOTEWORTHY Our study demonstrates biological-sex differences in phenotypic characteristics of cancer cachexia between male and female mice, whereby females display many common characteristics of cachexia (gonadal fat loss and hepatosplenomegaly), protein synthesis markers alterations, and common catabolic markers in skeletal muscle despite relatively preserved muscle mass in early-stage cachexia compared with males. Mechanisms of cancer cachexia appear to differ between sexes. Data suggest need to place onus of early cancer cachexia detection and treatment on nonmuscle tissues in females.

Effectiveness of Averrhoa bilimbi leaf extract on spermatogenic cells of mice (Mus Musculus L.) hyperglycemia

The purpose of this study was to determine the effect of giving Averrhoa bilimbi leaf extract on sperm quality of diabetic mice. This research is a pure experimental (true experimental) with a post-test-only control group design approach. This research was conducted by giving Averrhoa bilimbi leaf extract as a treatment for 42 days in male mice. Sperm quality parameters observed included viability, abnormalities, motility in sperm. In all variables, the results of the data showing a normal distribution with a p-value &gt; 0.05 were then carried out with a parametric test using one-way ANOVA. Averrhoa bilimbi leaf extract can increase the number of spermatogenic cells in male mice with hyperglycemia.

Hypertrophic preconditioning cardioprotection after myocardial ischaemia/reperfusion injury involves ALDH2-dependent metabolism modulation

Brief episodes of ischaemia and reperfusion render the heart resistant to subsequent prolonged ischaemic insult, termed ischaemic preconditioning. Here, we hypothesized that transient non-ischaemic stress by hypertrophic stimulation would induce endogenous cardioprotective signalling and enhance cardiac resistance to subsequent ischaemic damage. Transient transverse aortic constriction (TAC) or Ang-Ⅱ treatment was performed for 3–7 days in male mice and then withdrawn for several days by either aortic debanding or discontinuing Ang-Ⅱ treatment, followed by subsequent exposure to regional myocardial ischaemia by in situ coronary artery ligation. Following ischaemia/reperfusion (I/R) injury, myocardial infarct size and apoptosis were markedly reduced and contractile function was significantly improved in the TAC preconditioning group compared with that in the control group. Similar results were observed in mice receiving Ang-Ⅱ infusion. Mechanistically, TAC preconditioning enhanced ALDH2 activity, promoted AMPK activation and improved mitochondrial energy metabolism by increasing myocardial OXPHOS complex expression, elevating the mitochondrial ATP content and improving viable myocardium glucose uptake. Moreover, TAC preconditioning significantly mitigated I/R-induced myocardial iNOS/gp91phox activation, inhibited endoplasmic reticulum stress and ameliorated mitochondrial impairment. Using a pharmacological approach to inhibit AMPK signalling in the presence or absence of preconditioning, we demonstrated AMPK-dependent protective mechanisms of TAC preconditioning against I/R injury. Furthermore, treatment with adenovirus-encoded ALDH2 partially emulated the actions of hypertrophic preconditioning, as evidenced by improved mitochondrial metabolism, inhibited oxidative stress-induced mitochondrial damage and attenuated cell death through an AMPK-dependent mechanism, whereas genetic ablation of ALDH2 abrogated the aforementioned actions of TAC preconditioning. The present study demonstrates that preconditioning with hypertrophic stress protects the heart from I/R injury via mechanisms that improve mitochondrial metabolism, reduce oxidative/nitrative stress and inhibit apoptosis. ALDH2 is obligatorily required for the development of cardiac hypertrophic preconditioning and acts as the mediator of this process.

English

Bowdichia virgilioides Kunth (Fabaceae) is a tree species used in folk medicine against inflammatory disorders, and it is still unclear whether consumption of the B. virgilioides stem bark extract has any effect on immune cells. Thus, this study aimed to evaluate the effects of B. virgilioides stem bark extract (AEBv) on the functions of the thymus and B lymphocytes. The AEBv was obtained by decoction and administrated orally once a day in male mice for 7 days. After treatment, thymi were obtained for morphology analysis, relative weight, cellularity and deposition of laminin. Thymocytes obtained from control and treated animals were submitted to adhesion and migration assays followed by immunophenotyping. Also, mesenteric lymph nodes were removed, and the lymphoid cells were obtained to measure B cell proliferation and cytokine production. Mice treated with AEBv and their controls were also immunized with sheep red blood cells (SRBC), and 8 days post immunization the sera were collected to perform antibody titration using hemagglutination assays. AEBv administration decreased thymus weight and cellularity and increased the laminin deposition without any significant effect on the expression of the laminin receptor VLA-6 in the thymus microenvironment. Additionally, thymocytes of AEBv-treated mice showed reduced adhesion and migration on laminin substrates, and the B lymphocytes showed reduced proliferation, enhanced IL-10 secretion, decreased TNF-&alpha; production, and attenuated anti-SRBC antibody production. These findings showed that AEBv intake might cause modulatory effects on the functions of thymocytes and B-lymphocytes, contributing to the treatment of immune diseases. Key words: Thymocytes, cell migration, cell adhesion, immunosuppression, B cell. &nbsp;

Long-term metabolic consequences of acute dioxin exposure differ between male and female mice

Epidemiological studies have consistently shown an association between exposure to environmental pollutants and diabetes risk in humans. We have previously shown that direct exposure of mouse and human islets (endocrine pancreas) to the highly persistent pollutant TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) causes reduced insulin secretion ex vivo. Furthermore, a single high-dose of TCDD (200 µg/kg) suppressed both fasting and glucose-induced plasma insulin levels and promoted beta-cell apoptosis after 7 days in male mice. The current study investigated the longer-term effects of a single high-dose TCDD injection (20 µg/kg) on glucose metabolism and beta cell function in male and female C57Bl/6 mice. TCDD-exposed males displayed modest fasting hypoglycemia for ~4 weeks post-injection, reduced fasting insulin levels for up to 6 weeks, increased insulin sensitivity, decreased beta cell area, and increased delta cell area. TCDD-exposed females also had long-term suppressed basal plasma insulin levels, and abnormal insulin secretion for up to 6 weeks. Unlike males, TCDD did not impact insulin sensitivity or islet composition in females, but did cause transient glucose intolerance 4 weeks post-exposure. Our results show that a single exposure to dioxin can suppress basal insulin levels long-term in both sexes, but effects on glucose homeostasis are sex-dependent.

Hypothalamic leptin sensitivity and health benefits of time-restricted feeding are dependent on the time of day in male mice.

Synchronization between biologic clocks and metabolism is crucial for most species. Here, we examined the ability of leptin, important in the control of energy metabolism, to induce leptin signaling at the molecular as well as the behavioral level throughout the 24-h day in mice fed either a control or a high-fat diet (HFD). Furthermore, we investigated the effects of time-restricted feeding (TRF; a limitation of HFD access to 6 h each day) on energy metabolism during different periods throughout the 24-h day. In control mice, molecular leptin sensitivity was highest at zeitgeber time (ZT)0 (lights on), declining during the light phase, and increasing during the dark phase. Surprisingly, leptin resistance in HFD-fed mice was only present from the middle of the dark to the middle of the light period. Specifically, when TRF occurred from ZT21 to ZT3 (when leptin resistance in HFD-fed mice was most profound), it resulted in a disruption of the daily rhythms of locomotor activity and energy expenditure and in increased plasma insulin levels compared with other TRF periods. These data provide evidence that leptin sensitivity is controlled by the circadian rhythm and that TRF periods may be most efficient when aligned with the leptin-sensitive period.-Boucsein, A., Rizwan, M. Z., Tups, A. Hypothalamic leptin sensitivity and health benefits of time-restricted feeding are dependent on the time of day in male mice.

Abstract 2821: Impact of SCID mouse gender on tumorigenicity, xenograft growth and drug-responses in patient derived orthotopic xenograft models (PDOX) of malignant brain tumors

Abstract Brain tumor is the leading cause of cancer related death in children. To develop new therapies and to understand tumor biology, many efforts are made to develop patient derived orthotopic xenograft mouse models (PDOX). There are, however, some concerns about the potential impact of animal genders on tumor take rate, xenograft growth and in vivo drug responses. To systematically analyze such impact, we retrospectively examined &amp;gt;1,000 mice of 59 PDOX models, including glioblastoma (GBM), medulloblastoma (MB), primitive neuroectodermal tumor (PENT) and ependymoma (EPN) and diffuse intrinsic pontine glioma (DIPG), in which there were age-matched (within 2 wks differences) with relatively even numbers of male and female animals. All the animals were injected with identical number of tumor cells (1x10^5) in the mouse brains matching that in human patients. Animals were monitored daily and euthanized when they were moribund. Differences between male and female mice were analyzed by t-test. In 25 GBM models (15 adult and 10 pediatric) involving 419 mice, there was an average of number of mice was 17.8±9.5 mice per model (male: female = 9.88±5.8:7.93±3.6) (P&amp;gt;0.05). In 26 MB models with total 488 mice, the average animal number was 17.24±8.8 mice per model (male:female= 9.54±5.34 :7.71±3.45) (P&amp;gt;0.05). The tumor take rate was near &amp;gt;95% in both male and female mice (P&amp;gt;0.05). In GBM models, the survival times was 94.1±21.7 days in male mice and 92.3±20.8 days in female (P&amp;gt;0.05); wherease in MB models, they were 134.95±39.86 day in male and 121.08±31.68 days in female mice (P&amp;gt;0.05). The impact of animal gender on drug responses in vivo is essential to the preclinical drug testing. A total of 468 mice from 11 GBM, 5 MB, 3 EPN, 3 DIPG and 2 PNET were treated a series of standard and investigational drugs/compounds, including radiation, oncolytic virus (SVV-001), ABT888, BMI1-inhibitor, MLN8237, TMZ, Flavopiridol, SYC-435, SYC-719, SYC-836, Olig 2 inhibitor, VPA, SAHA, Echinomycin, MCB613, and PARPi. The overall survival times were 104.55±23.63 day in male mice, and 98.64±25.66 in female mice (P&amp;gt;0.05). The only model that exhibited the increased responses in male mice was found in IC-2305GBM treated with TMZ and PTC-596 (P&amp;lt;0.05). In conclusion, our data demonstrated that the gender of SCID mice does not have major impact on animal development nor in drug responses, and SCID mice of both genders are appropriate for brain tumor PDOX model development. Citation Format: Lin Qi, Mari Kogiso, Yuchen Du, Huiyuan Zhang, Frank Braun, Holly Lindsay, Sibo Zhao, Sarah Injac, Patricia Baxter, Zhigang Liu, Yujing Zhang, Jack Su, Adekunle Adesina, Andrew Walter, Jeffery Murray, Javad Nazarian, Will Parsons, Murali Chintagumpala, Xiao-nan Li. Impact of SCID mouse gender on tumorigenicity, xenograft growth and drug-responses in patient derived orthotopic xenograft models (PDOX) of malignant brain tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2821. doi:10.1158/1538-7445.AM2017-2821

Epididymal CYP2E1 plays a critical role in acrylamide-induced DNA damage in spermatozoa and paternally mediated embryonic resorptions†.

Acrylamide is a ubiquitous toxicant in human lives, due to its formation in many food products. Acrylamide induces dominant lethal mutations with administration of 25 mg/kg bw/day for 5 days in male mice. Cytochrome P450, family 2, subfamily E, polypeptide 1 (CYP2E1) is responsible for this dominant lethality. CYP2E1 is the only enzyme responsible for the conversion of acrylamide to the highly reactive metabolite glycidamide, which forms adducts with DNA. CYP2E1 is present predominantly in the liver, as well as the brain, kidney, intestines, and spleen. Within the male mouse reproductive tract, CYP2E1 localizes to spermatocytes. However, embryo resorptions have been demonstrated to occur only with exposure of the late stages of spermatogenesis and spermatozoa. It was determined that CYP2E1 is additionally expressed within the mouse epididymal epithelium, and this localization is responsible for acrylamide-induced dominant lethality. Further, an equivalent profile of CYP2E1 expression was identified in the human reproductive tract. While spermatozoa of both species were also established to possess CYP2E1, this did not contribute to acrylamide-induced DNA damage. In vitro studies strengthened these findings further, revealing that acrylamide exposure only induces DNA damage in human and mouse spermatozoa following metabolism by the mouse epididymal epithelial cell line (mECap18) to glycidamide. These findings emphasize, for the first time, the vital role of the epididymis in the reproductive toxicity associated with acute acrylamide exposure.

Amelioratory effect of coenzyme Q10 on potential human carcinogen Microcystin-LR induced toxicity in mice

Microcystins are a group of cyclic heptapeptide toxins produced by cyanobacteria. More than 100 microcystin analogues have been detected, among which microcystin-LR is the most abundant and toxic variant. Present study was designed to reveal whether potential human carcinogen microcystin-LR could imbalance the glycolytic-oxidative-nitrosative status of heart, kidney and spleen of mice and also to explore the amelioratory effect of coenzyme Q10 on microcystin-LR induced toxicity. Microcystin-LR was administered at a dose of 10 μg/kg bw/day, ip for 14 days in male mice. In microcystin-LR treated mice as compared to control, significant increase in the level of lipid peroxidation, hydrogen peroxide, lactate dehydrogenase, nitric oxide with a concomitant decrease in the level of glutathione was observed, suggesting microcystin-LR induced toxicity via induction of oxidative-nitrosative-glycolytic pathway. Although several studies have evaluated numerous antioxidants but still there is no effective chemoprotectant against microcystin-LR induced toxicity. When microcystin-LR treated mice were co-administered coenzyme Q10 (10 mg/kg bw/day, im) for 14 days, it was observed that coenzyme Q10 ameliorates microcystin-LR induced toxicity via modulation of glycolytic-oxidative-nitrosative stress pathway. Thus, the results suggest that coenzyme Q10 has a potential to be developed as preventive agent against microcystin-LR induced toxicity.

Retrospective Study of Spontaneous Osteosarcomas in the Nonobese Diabetic Strain and Nonobese Diabetic–Derived Substrains of Mice

In this retrospective study, spontaneous osteosarcomas were found in 85 of 1,202 (7.1%) nonobese diabetic (NOD) and NOD-derived mice. Gross tumors were evident at an average age of 155.8 days in male mice and 151.4 days in female mice. Compared with male mice, female mice had a statistically insignificant higher incidence: 56 cases (8.3% of 672) versus 28 cases (6.1% of 458). NOD/ShiLtJ mice had the highest incidence, with 39 cases among all the strains and substrains represented (3.2% of 1,202 necropsies), whereas NOD.SCID substrains had the highest incidence, with 16 cases among the various NOD-derived substrains (1.3% of 1,202 necropsies). There was a statistically significant difference in tumor incidence between NOD/ShiLtJ and NOD.SCID mice. Tumors were more frequent in the appendicular skeleton (55.7%) than in the axial skeleton (44.3%) and most often arose from the femurs. Histologically, osteoblastic osteosarcoma was the most common tumor type, with 79 cases (94%), followed by mixed osteosarcoma, with 5 cases (6%). Metastases were rare, with only 2 cases (2.3%).

Twenty‐Four‐Hour Variation of L‐Arginine/Nitric Oxide/Cyclic Guanosine Monophosphate Pathway Demonstrated by the Mouse Visceral Pain Model

The L‐arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway is known to be involved in central and peripheral nociceptive processes. This study evaluated the rhythmic pattern of the L‐arginine/NO/cGMP pathway using the mouse visceral pain model. Experiments were performed at six different times (1, 5, 9, 13, 17, and 21 h after light on) per day in male mice synchronized to a 12 h:12 h light‐dark cycle. Animals were injected s.c. with saline, 2 mg/kg L‐arginine (a NO precursor), 75 mg/kg L‐NG‐nitroarginine methyl ester (L‐NAME, a NOS inhibitor), 40 mg/kg methylene blue (a soluble guanylyl cyclase and/or NOS inhibitor), or 0.1 mg/kg sodium nitroprusside (a nonenzymatic NO donor) 15 min before counting 2.5 mg/kg (i.p.) p‐benzoquinone (PBQ)‐induced abdominal constrictions for 15 min. Blood samples were collected after the test, and the nitrite concentration was determined in serum samples. L‐arginine or L‐NAME caused both antinociception and nociception, depending on the circadian time of their injection. The analgesic effect of methylene blue or sodium nitroprusside exhibited significant biological time‐dependent differences in PBQ‐induced abdominal constrictions. Serum nitrite levels also displayed a significant 24 h variation in mice injected with PBQ, L‐NAME, methylene blue, or sodium nitroprusside, but not saline or L‐arginine. These results suggest that components of L‐arginine/NO/cGMP pathway exhibit biological time‐dependent effects on visceral nociceptive process.

AM1241, a cannabinoid CB 2 receptor selective compound, delays disease progression in a mouse model of amyotrophic lateral sclerosis

Effective treatment for amyotrophic lateral sclerosis (ALS) remains elusive. Motor neuron degeneration is the primary pathology in ALS; however non-neuronal cells contribute to the disease process. In particular, inflammatory processes have been shown to play an important role. AM1241 is a cannabinoid CB 2 receptor selective agonist that has been shown to be effective in models of inflammation and hyperalgesia. Here we report that treatment with AM1241 was effective at slowing signs of disease progression when administered after onset of signs in an ALS mouse model ( hSOD1 G93A transgenic mice). Administration at the onset of tremors delayed motor impairment in treated mice when compared to vehicle controls. Three conditions of ALS, the loss of motor function, paralysis scoring and weight loss, were analyzed using a mathematical model. Loss of motor function (as assessed by performance on a rotarod) was delayed by 12.5 days in male mice by AM1241. In female mice, AM1241 extended rotarod performance by 3 days, although this was not statistically significant. In male mice, AM1241 also extended by 5 days the time to reach the 50% point on a visually-assessed performance scale. AM1241 did not affect weight loss or survival (129.8 ± 1.7 days, vehicle; 129.1 ± 7.0 days, AM1241, n = 16). As AM1241 was well tolerated by the animals, cannabinoid CB 2 receptor-selective compounds may be the basis for developing new drugs for the treatment of ALS and other chronic neurodegenerative diseases.

Effects of Repeated Caffeine Administration on Voluntary Wheel Running in Mice

0144 Caffeine administration can have ergogenic effects on endurance exercise performance during a single bout of maximal forced exercise in both humans and animals. However, little is known about the effects of specific dosing regimens of caffeine on 24-hour voluntary physical activity patterns over multiple days. Purpose: To examine voluntary running patterns over 3 days in male mice in response to 3 consecutive doses of caffeine given at hourly intervals at the beginning of their inactive/sleep period (lights on). Methods: 7–9 week old male CD1 mice (n = 16) took part in 3 treatment conditions involving consumption of highly palatable food pellets (0.5g) containing no caffeine (CON), low dose caffeine (LO; 6mg/kg), or high dose caffeine (HI; 12mg/kg) at 7, 8, and 9 am for 3 days in a repeated measures design. Daily voluntary running patterns were recorded by computer with Vital View data collection/analysis software (Mini-Miter). Results: Repeated measures ANOVA showed no main effect of treatment (caffeine at two doses) on overall 24 hr wheel running (distance, time on wheel, and peak speed). However, there was a significant treatment × time interaction that showed differences between treatments at the beginning of the light period (when caffeine was given) and the subsequent dark period (when mice are most active). Between 7–11am when caffeine was administered, both HI and LO caffeine increased distance, time and peak speed above that of CON. There were no significant differences between HI and LO during this period. Conversely, during the subsequent 4-hr period at the beginning of the dark period there was a greater rebound decrease in distance, time, peak speed for HI compared to LO and CON. Conclusion: Both high and low doses of caffeine showed similar increases in voluntary running behavior when given at hourly intervals during the 1st 3 hours of the inactive/sleep period in mice. However, the lower dose of st caffeine would be recommended because it was without the possible disadvantage of reducing activity during the subsequent peak activity period. Information such as this could be useful when designing nutritional strategies to maximize worker performance in military or industrial settings when frequent changes in work schedule is necessary.

Rates of urine excretion by house mouse (Mus domesticus): Differences by age, sex, social status, and reproductive condition

A series of five experiments was used to test hypotheses about factors affecting excreted urine output per day in male and female house mice (Mus domesticus). Urine was collected in metabolism cages over a 24-hr period. Male house mice excrete urine at a rate 1.5-2.0 times that of females. Daily average urine output increases with age for both sexes and for mice of the same age; urine output per day is correlated with body mass. Females in estrus produce more urine than females in diestrus. Urine output per day increases during the latter two thirds of pregnancy and remains high throughout lactation. Density does not influence urine output per day for either sex over the range of densities tested. Castration reduces urine output per day in male mice, but ovariectomy in females does not alter rates of urine production. Dominant males produce more urine than subordinate males, but there are no similar effects for female mice. The findings have potential implications for our understanding of the functional and ecological uses of urine by house mice.

Quantitative Determinations of Critical Mortality Periods in Untreated and Treated Infections with the B Strain of Trypanosoma cruzi in Mice

The mortality patterns of untreated experimental infections with the B strain of T. cruzi in female, Manor-Swiss mice are described, following intraperitoneal or subcutaneous inoculation with varying numbers of parasites. A positive correlation occurred between initial parasite densities and degrees of virulence following both routes of inoculation, but greater relative degrees of virulence and more uniform mortality patterns resulted when the subcutaneous route was employed. Examples are given of the use of quantitative survival-time data for measuring chemotherapeutic activity in highly standardized experimental infections with the B strain of T. cruzi in mice. The B strain of Trypanosoma cruzi, originally isolated from a Brazilian patient in 1942, has been used extensively in this country for experimental studies on Chagas' disease (von Brand et al., 1949; Hauschka, 1949; Hauschka et al., 1950; Goble, 1951, 1952; Norman et al., 1959). Published reports on its degree of virulence in laboratory animals are divergent, although the techniques of inoculation and the number and frequency of in vivo or in vitro passages have varied considerably. Von Brand et al. (1949) stated that this strain killed all rats during the 3rd week of infection. Survival times in male mice inoculated subcutaneously with 500,000 parasites from mouse donors ranged from 11 to 32 days (in later experiments, 9 to 28 days) according to Hauschka (1949). Median survival times of 17 days in male mice and 21 days in female mice were observed by Goble (1951) after intraperitoneal inoculation with 30 million cultural forms. In a later paper Goble (1952) stated that the median survival time in Webster female mice inoculated with 30 million cultural forms was about 28 days, with 85% mortality within 60 days. Norman et al. (1959) obtained only transient infections in mice inoculated with cultural forms, stating that virulence was lost during a 5-year period in the laboratory. Since March 1948, the B strain of T. cruzi, obtained through the courtesy of the National Institutes of Health, has been used in our laboratories for studies on experimental chemotherapy. More than 500 consecutive animal passages have been carried out to date, usually in mice but occasionally in weanling rats, and little relative change in virulence has been noted during this period. The course of infections has been altered markedly in different experiments by changes in the size and route of the inocula, but predictable mortality patterns occurred in different groups of mice when comparable experimental conditions were maintained. This paper presents representative data on the characteristics of mouse infections with the B strain of T. cruzi under the experimental conditions employed in our laboratories during the past 2 years, with particular reference to (1) changes in degree of virulence following different initial parasite densities or different routes of inoculation and (2) responses to chemotherapy. Emphasis has been placed on the use of quantitative survival-time data, rather than measurements of peripheral parasitemia, for determinations of degree of virulence and alterations in the course of infections following the introduction of chemotherapeutic agents. MATERIALS AND METHODS Female white mice, Manor-Swiss strain, weighing from 18 to 22 g, were used as experimental hosts throughout the studies described herein. Received for publication 7 July 1962. * From the Experimental Therapeutics Research Section, Lederle Laboratories Division, American Cyanamid Company, Pearl River, New York.