Rattus Norvegicus and Its Usage as a Model Organism

Introduction

Rattus norvegicus is one of the commonly known muroids, this rat is also called brown rat, Norwegian rat etc. The mid-1500s was when it reached Europe and around 1750 it reached North American. This rodent coat color is brown or grey with its body size 25cm long equal to its tail length. Females weigh 250g and the males weigh 250g. This rodent originally originated in Asia which is now known as Northern China. Norwegian rat is now found all continents except for the Antarctica region. This rodent cause damage everywhere they are found by feeding foodstuff because they are forager that depend on a large amount of food to live.

Reproduction and Development

Norwegian rat practice polygynandrous i.e. males and females within a group mate with multiple partners. Males reach maturity around 13 weeks and the females are around ~19 weeks. The rate at which they breed in a warmer month is much higher compare to other months even though breeding is throughout the year. Eighteen hours after birth the female goes into estrus making the capable to have 7 litters yearly. During 6 hours period estrus which the females experience, they mate with up to 500X multiple competing males. Their gestation period lasts between 22-24 and litters average of 8 pups. It takes 2weeks for the offspring to open its eyes and nurtured for 3-4 before they are allowed to leave the nest. Litters might end up sharing the same nest with multiple females in which these females care for this litters of another female. They live up to 4 years in human care but in wild is ~2 years.

History as a Model Organism

The mouse has been used as a model organism for the past thirty years now because of available technologies that are used in studying mouse but the availability of the technology for rat will help scientist to pick which organism is the most appropriate model for basic biology, either rat or mice, or both of them. Although there are similarities between these species, an evolutionary separation between the species happened millions of years ago which cause a vital difference between the species.

Rat offers high advantage over mice when it comes to the model of human disease among other organisms. The fact about brown rat is that they were once the most broadly used species for medical research activities and successful generation of ES (embryonic stem) cells will increase their importance as model organism worldwide. The rat is a perfect model organism for studying cardiovascular disease, hypertension and stroke in particular which there are available strains for such research category. Their physiology is a bit similar to that of the humans and beside it is easy to monitor their physiology over time. Also, in case studies like cognition and memory, the rat is mostly used in such studies because their physiological system of learning and memory have been studied. Rats are more intelligent than mice which makes them capable of learning different tasks that are significant in cognition research.

Size proportionality is not the only thing considered in this organism basically because of surgical purposes but also significant organs substructures that affect how many organs were involved in experimental lesion and effect of drug administration to certain anatomical region specific to a central nervous system

Strengths and Limitations as a Model Organism

Strengths

• Rats are more intelligent than the mouse, so they are mostly used in cognitive research that has to do with learning and memory.
• Proportional size in rats makes them an excellent choice for not just only surgical purpose but on the effect on how many organs were involved in experimental lesion and effect of drug administration.
• Since rats have behavioral similarities to that of the humans, using the rat to study diabetes will help scientist to explain the mechanisms involved because the organism will display the phenotype similar to human disease.
• A serial blood draw is possible due to its size.
• Minimal lifespan is also an important point to be considered in studying human diseases because of wear and tear that the body undergoes over time permit susceptibility to chronic disease such as cardiovascular disease.

Limitations

In cardiovascular disease, the muscle tissue of the heart and blood vessel of the heart are affected which cause damage to this organ. Since the mammalian cardiomyocytes cannot regenerate, damage to the heart cause scarring and fibrosis and the only solution to this huge problem is heart transplant in which donors for this particle organ is very limited.

Cancer Study

Short lifespan in rodents wouldn't provide scientist of the long-term safety of the chemical compound because cancer prevention in humans require a long-term intake of chemical agent. Although the use of various chemical agents have played an excellent role preventing cancer in rats.

Developmental Questions Answered with Rattus Norvegicus

The above review paper compared rat age with that of the human's age due to basic questions that arise on 'How is a newborn rat good model for a new human offspring?' and 'How old is the rat with the human year?' by doing an experiment. A different technique was used in this review paper to answer the questions and it revealed that rat-human age correlates differently evidently at different life stages & development. They metaphorically describe the correlation using age period difference from weaning (42.4 rat days equal a year in humans) to an aged period (17.1 rat days equal a year in human).

Early period life comparison tends to differ severely from late middle age. The conclusion the review article was even though rat is an outstanding model organism in biomedical research, taking them as miniature isn't appropriate because the 3years a rat lives can't be directly said as miniature life is equivalent to the life humans live. So, an experiment that age is used as a deceive factor, physiological and developmental differences must be considered while correlating to human. Researchers need to understand that age-related differences depend mainly on the stage of life an organism is, therefore, factors to be analyzed need to be considered.

Disease Model Approach Using Rattus Norvegicus

The Wistar rats were intraperitoneally injected with toxic Leptospira interrogans serovar Copenhageni strain Fiocruz L1-130. A culture based immunofluorescence technique, silver staining was used to detect the presence of leptospires in the tissue sample. Fifty percent of the rats were inoculated with 10,000 of leptospires induced colonization in which the colonization continued for four months during the period of study. To investigate the formation and determine the carrier state, groups of rat were sacrificed on weekly bases and infected with 108 leptospires for up to a month and then monthly up to four months. According to the results derived from the renal tissue culture, immunofluorescence and silver staining renal colonization was formed as fast as one week post-infection and continued for almost four months.

Scanning electron microscopy (SEM) of kidney segments collected 4 weeks after infection confirmed the high density of leptospires in tabular lumens of chronically infected rats. It showed negative for the presence of leptospire in organs like the spleen, heart, lungs, and liver using the silver staining. Dissemination kinetic of leptospires studied in 2 experiments with three female rats per group that was infected with leptospires at 1, 3, 5, 7 & 9 days post-infection by peritoneal injection. The dissemination kinetic showed that the renal got colonized 7-9 days after being infected without any masked histopathology. The highest leptospires happened on the 5th day of post-infection, rapid removal followed in all tissue except for the kidneys where they saw dense leptospires aggregates that continued in the kidney tubules. The result of the experiment confirmed Rattus norvegicus actually showed resistance to this severe leptospirosis.

Conclusion

Rattus norvegicus is the most common model organism used in scientific research like psychological, medical and other biological research. Their main advantage is when they are in captivity, they can grow quite faster to sexual maturity, they are easy to breed & keep. Their disadvantages are strains that are specifically bred for certain research categories like cognitive diseases, neurological diseases, cardiovascular disease and so on.

References

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