student of Eastern Mediterranean University, Famagusta, TRNC.
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.
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 1,2.
Fig 1: Life cycle of Rattus norvegicus
Retrieved from (https://www.google.com.tr/search?q=life+cycle+of+rattus+norvegicus&source=lnms&tbm=isch&sa=X&ved=0ahUKEwil0emFzubXAhVKy6QKHQ4cCbkQ_AUICigB&biw=1366&bih=637#imgrc=mJPLniCyG_ppRM:)
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 4.
and Limitations as a model organism
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
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 4.
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.
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.
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). In the image below is the result of the experiment on
how the calculation was done to answer the question
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 5.
Model Approach Using Rattus Norvegicus
The image above
was an experiment that was done using Wistar rats. The 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 (Table 1).
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 shown below in Fig 1 (image).
Figure 1 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 (Fig 2).
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 6.
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.
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2675817/ Accessed 30 Nov. 2017.
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http://file:///C:/Users/HP/Downloads/80-80-1-PB.pdf Accessed 2 Dec. 2017.
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