ANIMALS USED IN PHARMACOLOGICAL EXPERIMENTS

Selection of an animal model is one of the most important steps in any of the experimental pharmacological study.
Animal model preferred for the study must be producing similar disease profile as in the human.

1. ANIMALS USED IN PHARMACOLOGICAL EXPERIMENTS SIVASWAROOP YARASI

2. INTRODUCTION : Selection of an animal model is one of the most important steps in any of the experimental pharmacological study. Animal model preferred for the study must be producing similar disease profile as in the human. Suitable animal model should be selected which follows three main objectives: Use of an animal phylogenetically closer to man or Use of an animal in which the process under investigation is as close as possible to that in man, The Anatomy, Physiology and Biochemistry are considered to be similar

3. Broadly experimental animals are divided into   three categories.

4. Rodents

5. Mouse as an Experimental Animal ( Mus musculus ) In the biomedical research mice are preferred which are the smallest rodents used in the laboratory. Several advantages over other species like They are easy to keep, handle and require small place for housing. Invaluable resource in identifying the several alleles which further develop in the research of mutagenesis. Only known species in which it is possible to grow totipotent embryonic stem (ES) cells in vitro, and can form germ line once re-injected into a developing embryo.

6. DBA/2 is the most ancient of all in bred strains, established by CC Little in 1909 whereas C57BL/6 was established by Miss Lathrop after 10 years. Other commonly used strains C3H, CBA and A were discovered by LC Strong. Common diseases: salmonellosis like mouse typhoid, small pox, streptobacillus infection.

7. Due to the large similarity in mice and human genome (>99% conserved), it provides good model for research not only on mammalian biology but also on a wide variety of human diseases like, cancer, diabetes, ageing, atherosclerosis, immunological disease, autoimmune disorders, neurological dysfunction and other endocrine diseases and several other diseases. Knockout and Knock in mice have been developed for the selective assessment at the genetic level. Knockout mice: Selective gene is taken out The first knockout mouse was created by Mario R Capecchi, Martin Evans and Oliver Smithies in 1989, for which they were awarded the Nobel Prize for Medicine in 2007. Knock in mice: gene of interest is introduced. In 2006, Nobel Prize was awarded to Andrew Z. Fire and Craig C Mello for RNA interference, in which genes are silenced or “ knocked down ” by short pieces of double stranded RNA.

8. Uses: acute toxicity studies, insulin assay, chemotherapeutic and teratogenic agents screening, cancer and genetic research, isolated preparations- vas deference, ileum

9. Rat as an Experimental Animal ( Rattus norvegicus ) Most commonly used animal in the biomedical research. “Wistar rat” and the “Sprague Dawley rat” are preferred because of easy handling, sensitivity and low cost. The Albino rat (officially known as the Pink-Eyed White or PEW) is most likely the very first mutation to be discovered and purposely bred. Nude rats resemble nude mice in their lack of a normal thymus and functionally mature T cells and are phenotypically hairless with possible fine-sparse hair growth and most preferred model in immunological research. Rat is preferably used in the research of behavior, pharmacology, physiology, neurosciences, immunogenetics, transplantation, cancer risk assessment, cardiovascular diseases and aging.

10. Uses: evaluation of psychopharmacological agents, toxicity studies, antiulcer agents, liver physiological studies, antihypertensive effects, hormones assay.

11. Some important phenotypic differences between baby rats and mice Baby rat has blunt and broad large head relative to body whereas mice have triangular, small head relative to body Baby rat has small ears relative to the head whereas mice have large ears In the baby rat hind paw and body ratio is larger as compared to mice Tail is thick and shorter than body length in baby rat while mice have thin and larger or same length tail as compared to body.

12. Guinea Pig (GP) as an Experimental Animal ( Cavia porcellus ) 3 major varieties of strains used in the experimental studies and is the member of rodents suborder “ Hystricomorpha . herbivorous and eats green foods, seeds and roots it is essential to add vitamin C (Ascorbic acid) in the chow, while it is important to note that GP are not able to synthesize the required vitamin C daily. It is recommended that when GP is provided with the greens, then ascorbic acid should be given at the rate of 1gm/L of drinking water on a weekly basis. If they are completely dependent on the chow then vitamin C should be added in the dose of 200 mg/L daily. Duration of estrous cycle will range from 13-20 days an average of 16 days can be further divided into the other stages such as proestrus, estrus, metestrus and diestrus .

13. It is sensitive to various diseases and infections which makes it suitable for the diagnostic tests. It is an ideal model for the enteric amoebiasis and widely used in the hypersensitivity, immune response, anaphylactic shock, encephalomyelitis, tuberculosis and ascorbic acid metabolism. Also widely used in the screening of local anesthetics and is good proposed model for bronchial asthma, COPD.

14. Used in evaluation of bronchodilators, sensitization studies like egg albumin and horse serum, study of local anaesthetics, bioassay of digitalis, histamine and acetylcholine, screening of spasmodic and antispasmodic compounds.

15. Gerbil as an Experimental Animal ( Meriones unguiculatus ) Mongolian gerbil is a small laboratory rodent, having length in between rat and mice. phenotypically related to rat and also known as “jirds” or “sand rat” preferred in the laboratory because of ease in handling, mild and quiet nature Housing space of the gerbils are different at different places such as in USA, it is said that 5 gallons (1 gallon = 231 cubic inches) are required for each gerbil, i.e. 10 gallons for 2 gerbil, 15 gallons for 3 gerbil, 20 gallons for 4 gerbil, etc. UK, for a pair of Gerbils a 15” by 12” by 10” tank is ideal.

16. Gerbil is widely used as a research animal in the field of stroke, epilepsy, auditory studies (hearing curve similar to man), parasite and bacterial infections and lipid metabolism and heart disease studies (high serum cholesterol levels). This animal is one of the few species which were originally developed in Japan as laboratory animals.

17. Hamster as an Experimental Animal ( Mesocricetus auratus) Hamster a brown to gold color animal has become the third most commonly used laboratory experimental animal in the biomedical research. They have different strainsnamely Syrian hamsters (Golden), Chinese hamster (striped back), European hamster and Armenian hamster (gray). Syrian hamster is the most commonly used in biomedical research because of availability and ease of reproduction. Relatively free from spontaneous disease and susceptible to many introduced pathogenic agents. European hamster (quite larger than other hamster species (300-400 gm)) is a more suitable model for highly concentrated and prolonged smoke inhalation studies than the Syrian hamster.

18. Armenian hamster is more specific for the research to mutagenic and carcinogenic agents and for studying meiosis due to its susceptibility. Chinese hamster has the lowest number of chromosomes compared to any other placental nurtured laboratory animal and it is useful for cytogenesis research. Spontaneous model of human diseases such as diabetes mellitus , Syrian hamster dystrophy (Autosomal recessive skeletal muscle degeneration, cardiomyopathy, cardiohypertrophy , and congestive heart failure), cholesterol cholelithiasis or gall stones, polycystic diseases, dental caries. Good model for physiology and pathogenesis of Duchenne’s dystrophy, variable sizes of muscle, fiber, centrally located nuclei, with fatty infiltration and fibrous connective tissue replacement.

19. Hamsters are used extensively in slow virus (Scrapie, chronic measles, etc.) type C, Onco virus, influenza virus, respiratory syncytial virus (RSV) studies and vaccine production (Foot and Mouth). Due to anatomical advantages, cheek pouches do not have intact lymphatic drainage and hence, they are an ideal site for tissue transplants, such as, tumors and grafts. Hamsters are used for in vivo and in vitro diagnostic techniques for numerous infectious agents (i.e. Clostridium spp ; Leptospirosis spp )

20. Non-rodents (Mammalians)

21. Rabbit as an Experimental Animal ( Oryctolagus cuniculus ) Most common strain in use is New Zealand white rabbit, followed by the Dutch, the Flemish Giant and other minor strains of the domestic rabbit. New Zealand white rabbits have been used in the screening of different drugs for diseases like diabetes, diphtheria, tuberculosis, cancer, and heart diseases. Biomedical research studies in which rabbits is commonly used are genetics, nutrition, toxicology, physiology, immunology and reproduction. Classically the rabbit has been utilized in human medicine to determine pregnancy in women by injecting the serum from the patient into the rabbit and thereby inducing ovulation in the doe. Apart from the drugs, effects of skin creams, cosmetics, special diets, and food additives have also been tested on New Zealand white rabbits.

22. Other important uses are: Standard animal for pyrogen testing of all solutions for human medical use To test toxic effects of cosmetics and pharmaceutical s Good model for the production of antibodies and antiserums. New Zealand white rabbits have a genetic deviation called albinism . Albinism is caused by ack of melanin. Rabbit is homologous to humans to react similarly to diseases and medications. A female rabbit (doe) is fertile all year long. The gestation period is around 28 to 31 days. Cervical dislocation which is generally preferred for the rodent is not suitable for the rabbit because of short neck. It has very simple cardiac conductive tissue, free of connective tissue and is an animal of choice for many cardiac studies

23. Monkey as an Experimental Animal ( Macaca mulatta ) Monkeys are one of the most commonly used mammalian in the experimental studies others in the cue are dogs and cats. The rhesus monkey ( Macaca mulatta ) commonly used is found in the South Asia. Adult body weight is 10-12 kg for male and 8-10 Kg for female. During the study animal can be maintained in the gang cages (single or in group). The determined dimension for the cage is 24" × 30" × 30" for single or individual housing and 28 ft × 10 ft for group housing (8 female and 1 male may be maintained). Diet consists of 18-20% protein, 65-70% carbohydrates, 4-6% fat, 4-5% crude fiber, minerals, and vitamins. Monkeys are widely used as primate model to study drug metabolism because they generally show a metabolic pattern similar to humans.

24. Dog as an Experimental Animal ( Canis familiaris ) After monkey, dog is the most preferred advantages being small alimentary tract and easily get trained Mongrel and Beagles are the most preferred for the experiment purpose due to manageable size, moderate length of hair coat, docile nature and ease to handle. Cardiovascular research is preferred in the dogs. Drugs acting on blood pressure and vascular system are preferably screened. It is also a good model for diabetes mellitus and reproduction. The dog is frequently used as a model for many human conditions in areas such as cardiovascular research, diabetes mellitus, ulcerative colitis, open heart surgery, organ transplantation, central nervous system (CNS), safety pharmacology and toxicology.

25. Cat as an Experimental Animal ( Felis catus ) Cat has similar physiological features which are common with humans than the laboratory rabbit or rodent, hence they have been extensively used in cardiovascular ( CVS), behavioral and biomedical research. Useful models in studying the transmission of vitamins and minerals to the fetus and new born. Cat has distinct nictiating membrane hence commonly used in the screening of ganglion blocking drugs. It is not a good model in the experiment on the loss of righting reflex, because it regain its righting reflex even fall from a high altitude.

26. Cats are mainly used in the field of behavioral studies, cardiovascular studies, nerve impulse transmission. e.g. reflexes of the respiratory system and spinal system, reflexes associated with nociception, light perception, sound perception and body reaction to exposure to chemical stimuli. Additionally, it is also used in the neuropharmacology (particularly the testing of psychotropic drugs), toxicology, oncology and chromosomal abnormalities studies.

27. Pig as an Experimental Animal ( Sus scrofa domestica ) Several isolated organ models, investigation of skin permeation and for digestive systems, etc. are few important areas of research with pig as an experimental animal. Preferred because, it is small sized at maturation as well as is selecting for less hair contains on the body In Europe, pigs are used in pharmaceutical R & D studies in place of dogs and primates. Dissimilarities are in the vascularization (rich in man, poor in the pig) and in the sebaceous glands. Humans have mostly eccrine sweat glands over the body surface, whereas the pig has only apocrine glands. Similar findings also observed while studying the skin of other domestic mammals. Pig has small lungs in relation to body size and is susceptible to bronchitis and pneumonia.

28. Zebra Fish ( Danio rerio ) Dr George Streisinger at the University of Oregon observed that the Zebra fish is a suitable model for studying vertebrate development and genetics in early 1970’s and is considered to be the “Father of Zebra fish research” Zebra fish are vertebrates and have a backbone like humans which gives more close relation to humans than commonly used invertebrate models, such as insects and worms Studying embryo development and genetics make it useful animal model as in genetic techniques. Due to its clear eggs which can be developed outside the mother’s body and allow watching a zebra fish egg grow into a newly formed fish under a microscope in 2-4 days it got popular. The lifespan of zebra fish is considered to be approximately 5 years and length of the adult fish is about 6 cm.

29. Character Mice Rat Guinea pig Hamster Gerbil Rabbit Scientific name Mus musculus Rattus norvegius Cavia porcellus Mesocricetus auratus Meriones unguiculatus Oryctolagus cuniculus Weight (g) 18-40 150-400 600-1200 85-150 55-100 1000-3500 Life span (year) 1.5-3 2-4 4-8 1-3 2-4 6-12 Gestation period (days) 19-21 21-23 59-72 15.6-16 24-26 31-32 Estrous cycle (days) 4-5 4-5 15-19 4 4-6 5-14 Rectal temperature ( °C ) 38-39 36-40 37.2-40 37-38 37-38.5 38.5-40 Heart rate (min) 310-840 250-450 230-380 250-500 360 130-325 Blood pressure 133-160/90-110 84-134/60 80-94/55-58 150/100 90-130/60-90 Blood volume (ml/kg) 58.5 54-70 69-75 78 66-78 57-65 Respiratory rate 60-220 70-115 42-104 35-135 90 30-60 Mating age 6-8 weeks 10-12 weeks 12-20 weeks 6-8 weeks 3 months 6-9 months

30. References: Medhi B, Prakash A. Practical manual of experimental and clinical pharmacology. Jaypee Brothers Medical Publishers; 2010.