Introduction 

The Metabolic Cage is used to collect and separate rodents’ urine and feces and allows measurement of their food and water intake for qualitative and quantitative studies. It has an open floor with a grid mesh, allowing the waste products to fall below the cage and into a funnel. Urine flows down the middle of the funnel into a collection tube outside the cage. The feces roll down the side of the funnel into a separate feces collection tube, which prevents it from mixing with the urinary waste. The metabolic cage is also equipped with a feeding chamber and a water bottle, allowing food and water intake to be easily measured by subtracting the weight of the feeding chamber and measuring the volume of the water after food and water intake. 

In standard animal housing cages, the bedding absorbs the urine, and the feces requires manual separation from it. This causes contamination between the two waste products, which reduces their accuracy in analyzing each sample. On the other hand, the metabolic cage effectively separates each sample, avoiding contamination. Urine and fecal samples are used to verify the health and physiological status of the animal. Metabolic cages can also be used for several other purposes, such as, it can be used in drug studies to measure the compounds excreted in the urine or feces of rodents or to determine the amount of ingested substances absorbed by examining what remains in the fecal pellets. It can also be used to study rodents’ feeding and exercise activities. 

Apparatus and Equipment 

The Metabolic cage measures 21 cm in length, 20.5 cm in width, and 32 mm in height. It is made of acrylic and comprises an Upper Chamber made of smooth, gnaw-proof materials. A Feeder Chamber and a water bottle are equipped on the outer portion of the apparatus. The feeder chamber contains a sliding drawer, which can be pulled from the outside for refilling the feeder chamber without disturbing the rodents. A collection funnel and separating cone are below the cage floor, connected to a urine and feces collection tube. The metabolic cage is fitted on a stainless steel table with a hole in the center, which precisely fits the cage.

Literature Review 

Investigation of the effect of prostatic inflammation on voiding behavior

Lee, Yang, and Bushman (2015) investigated the effect of prostatic inflammation on voiding behavior in adult C57BL/6J mice. The mice were used as an animal model of bacterial-induced, isolated prostatic inflammation. Transurethral inoculation of uropathogenic E. coli 1677 was used to induce prostatic inflammation. The metabolic cage was used to examine the voiding frequency and volume per void of the subjects. It was observed that the subjects displayed a significant increase in voiding frequency on three out of the four testing days. However, a decrease in voiding volume was observed during all time points compared with the control mice. Overall results indicated that urinary frequency and nocturia symptoms could cause prostatic inflammation. 

Investigation of the effect of a synthetic live bacterial therapeutic preventing phenylketonuria (PKU)

Isabella et al. (2018) engineered Escherichia coli Nissle to investigate its effects in preventing PKU, a genetic disease that prevents the metabolism of phenylalanine (Phe). The synthetic strain (SYNB1618) of bacteria was developed to express genes encoding Phe-metabolizing enzymes in response to anoxic conditions in the mammalian gut. The synthetic strain of bacteria was administered to the Pahenu2/enu2 PKU mouse model. The metabolic cage was used to house three mice per cage, which were on a Phe-deficient diet. The mice’s urine was collected every four hours and analyzed for hippuric acid (HA), which served as a biomarker for phenylalanine ammonia-lyase (PAL) in vivo. The results indicated that administering activated  SYNB1618 ameliorated an increase in the levels of serum Phe in the mice that were on a Phe-deficient diet following Phe injection and an increase in urinary HA. This suggested that, regardless of dietary protein intake, enterorecirculating Phe breakdown effectively lowers serum Phe levels.

Investigation of the effect of Fibroblast growth factor 21 (FGF21) on rodent fluid homeostasis 

Turner et al. (2018) investigated the effect of FGF21 on rodent water intake, urinary output, and blood pressure. The rats were housed in metabolic cages, which were used to measure their urine output and fluid consumption. The rats were administered a single intravenous injection of PF-05231023, a long-acting analog of FGF21, which was used to assess the subjects’ fluid homeostasis. The results indicated a 20% increase in cumulative water intake at 48 and 72 hours post-dosing compared to the control subjects. This increase in water intake was accompanied by an increase in urine output. However, the increased urine output preceded the increase in fluid intake. The experimental rats had 40% more urine output than the control subjects after 24 hours of treatment and thereafter. A telemeter was implanted in the rats to measure their heart rate and blood pressure. It was observed that FGF21 significantly and quickly raised heart rate and blood pressure before alterations in urine output and water consumption was observed.

Data Analysis 

The following parameters can be observed using the metabolic cage

• The volume of urinary output 
• The amount of fecal matter output
• The volume of water intake 
• The amount of food intake 

Strengths and Limitations 

The metabolic cage is widely used in pharmacodynamic and pharmacokinetic studies in biomedical research. It allows urine and feces to be completely separated from each other without contamination, which allows them to be accurately measured. Moreover, it can also measure food intake and water intake. The design of the food chamber helps minimize stress because of its drawer design that allows easy filling of the food from outside the cage. Moreover, the component below the cage floor can also be removed without disturbing the subjects, further minimizing stress. 

However, rodents might experience stress in the metabolic cage due to social isolation, the absence of substrate and enrichment, and smaller living space, which can affect their food and water intake, and urine and fecal output. 

Summary 

• The metabolic cage collects and measures rodents’ urine and fecal output and their food and water intake. 

• It consists of an apparatus equipped with a feeder chamber and water bottle. The cage floor is made of a mesh grid, allowing urine and fecal waste to fall below. The waste falls into a funnel which is connected to a collection tube. 

• The metabolic cage separates urine waste from fecal matter, preventing contamination between the two, which can be used to analyze both samples individually for various studies. 

• The metabolic cage can be used for pharmacodynamic and pharmacokinetic studies in biomedical research. 

References 

1. Lee, S., Yang, G., & Bushman, W. (2015). Prostatic inflammation induces urinary frequency in adult mice. PloS one, 10(2), e0116827. https://doi.org/10.1371/journal.pone.0116827
2. Isabella, V. M., Ha, B. N., Castillo, M. J., Lubkowicz, D. J., Rowe, S. E., Millet, Y. A., Anderson, C. L., Li, N., Fisher, A. B., West, K. A., Reeder, P. J., Momin, M. M., Bergeron, C. G., Guilmain, S. E., Miller, P. F., Kurtz, C. B., & Falb, D. (2018). Development of a synthetic live bacterial therapeutic for the human metabolic disease phenylketonuria. Nature biotechnology, 36(9), 857–864. https://doi.org/10.1038/nbt.4222
3. Turner, T., Chen, X., Zahner, M., Opsahl, A., DeMarco, G., Boucher, M., Goodwin, B., & Perreault, M. (2018). FGF21 increases water intake, urine output and blood pressure in rats. PloS one, 13(8), e0202182. https://doi.org/10.1371/journal.pone.0202182