Medications, as essential tools in healthcare, undergo laboratory evaluation before being applied in treatment. Since cellular and organ functions are largely consistent across animals and humans, many processes in animal cells mirror those in human cells. From a biological standpoint, humans are categorized as animals. In experimental pharmacology, animals are utilized to examine disease effects on living tissues, evaluate new treatments, including animal medications, and assist in the training of future health professionals and scientists. They have significantly contributed to research progress, enabling the discovery of various new drugs through animal experimentation. However, animal testing presents specific challenges, including issues related to animal procurement, the complexity of experimental procedures, strict regulatory guidelines, ethical concerns, and, frequently, the perspectives of students. Consequently, modern advancements, such as computer simulations, are increasingly being adopted as substitutes for animal use. This article highlights the role of animals in experimental pharmacology while also acknowledging proposed alternatives for pharmacology education.
Animal experimentation [1, 2] refers to the utilization of animals in scientific research, training, and academic instruction. The term vivisection [3, 4], commonly used by critics of animal testing, denotes the act of dissecting or performing surgery on a live animal. Reports indicate that animals are often subjected to distressing and unnecessary practices during educational and training exercises. Concerns have been raised about the methods of euthanasia used in these so-called “irrelevant tests,” particularly due to the heavy reliance on animals in toxicity testing and dermatological research. The CPCSEA works to guarantee that animals do not undergo avoidable pain or suffering before, during, or after experimentation. In India, animals are extensively used in a wide range of research activities [5]. They remain a fundamental component in drug testing, bioassay screening, and preclinical research, including comprehensive toxicity assessments [6–8]. This preclinical data on safety and efficacy must be provided to drug regulatory agencies before authorization for clinical trials in humans can be granted [9–11].
In the realm of pure research, both the diversity and number of animals used exceed those of applied research [12]. For instance, studies involving embryogenesis, developmental genetics, behavior, and breeding are frequently performed on organisms such as fruit flies, nematodes, mice, and rats. Applied research, typically conducted in pharmaceutical companies or academic institutions, is focused on addressing specific scientific questions. The University Grants Commission (UGC), CPCSEA, and Medical Council of India (MCI) advocate the principles of the three Rs [13] in animal experimentation—replacement, reduction, and refinement—with a fourth R, rehabilitation, being proposed to support animal care post-experimentation [14–18]. With evolving practices in science, the development of alternative methods has become essential. In numerous Indian medical colleges, the use of live animals in experiments [19] is steadily declining. These practices are being substituted with cost-effective and educationally effective alternatives [20, 21].
This article highlights the role of animals in experimental pharmacology while also acknowledging proposed alternatives for pharmacology education.
Albino rats are among the most widely used laboratory species due to their manageable size and heightened responsiveness to various drugs. A broad head, coarse fur, and elongated ears are characteristic of them. Their tail is usually shorter than their body length, and they possess a long, cylindrical torso with a slender tail and very short limbs.
The head features two slit-like nostrils and a pointed snout, a narrow mouth with a divided upper lip and a short lower jaw, as well as two small, prominent eyes positioned for both forward and side vision. Numerous long whiskers are present. The rat’s neck is short, and its trunk is marginally broader than its head. Notably, rats lack a vomiting center, which means they are unable to vomit, and they also do not possess a gall bladder.
Rats are frequently utilized in analgesic and anticonvulsant evaluations, hormone bioassays for agents like insulin, oxytocin, and vasopressin, chronic blood pressure monitoring, gastric acid secretion studies, and in acute and chronic toxicity assessments. They are particularly suitable for studying drug-induced teratogenicity and carcinogenicity.
Rat tissues are extensively employed to assess a range of drug actions.
Blood Collection: Small quantities of blood can be collected from the tail vein by trimming the tip of the tail. For larger volumes, blood is drawn from anesthetized rats via cardiac puncture or by accessing the orbital sinus.
For many years, the guinea pig has served as a standard laboratory animal in human experimental research due to its calm temperament. This species is a small, tailless rodent with short ears and a rectangular, blunt head when viewed from the side. It has a thick, short neck that merges into the trunk, and its limbs are of unequal length, with the forelegs shorter than the hind legs. The guinea pig exhibits several physiological differences from other commonly used lab rodents. Notably, it requires dietary Vitamin C and is highly prone to anaphylactic reactions and tuberculosis. It also displays high sensitivity to histamine. Symbolically, the guinea pig is often used to represent a subject of scientific experimentation. Although once commonly employed to distinguish bacterial strains, modern laboratories now favor mice and rats due to their faster reproduction rates.
Guinea pigs are primarily employed to test bronchodilator drugs in models of asthma induced by histamine or acetylcholine aerosols. They are also instrumental in immunology, particularly in exploring delayed hypersensitivity responses using antigens such as horse serum or egg albumin. These animals have been widely applied in research on local anesthetics and in the bioassay of digitalis. Due to their sensitive cochlea, guinea pigs are useful in auditory studies, as well as in oxygen consumption experiments. Their similarity to humans in requiring an external source of Vitamin C makes them valuable in studies on vitamin C metabolism. As natural hosts for Mycobacterium, they are highly suitable for tuberculosis research.
Isolated organ systems from guinea pigs—such as lung and intestinal tissues—have been widely used. These preparations played a significant role in the early development of medications for treating peptic ulcers and beta-blockers for hypertension. The terminal ileum is particularly responsive and is favored in the preliminary testing of spasmodic and antispasmodic agents, as well as in the detection and measurement of histamine and related substances.
Small volumes of blood (less than 0.25 ml) can be drawn through venesection of the marginal ear vein. Larger quantities may be obtained from anesthetized guinea pigs via cardiac puncture or by accessing the metatarsal veins. For repeated sampling of small amounts, the orbital sinus is commonly used.
Albino mice represent the smallest commonly used laboratory animals and can be bred in a standardized manner. They have a sleek coat, slender bodies, long pointed snouts, large, rounded ears, and long, flat front teeth. Mice are low-cost and easy to manage in laboratory settings.
Mice are extensively used in acute toxicity testing. They are also crucial in insulin and analgesic bioassays, and they serve as general models for evaluating chemotherapeutic agents. Specially bred strains of mice are essential for studying genetic disorders and cancer. Mice are the preferred species for teratogenicity testing. A specific strain known as nude mice, which lack a thymus gland, is crucial in studies of tissue immunity and transplantation biology.
In experimental setups, mouse tissues are used, although only limited organs—particularly the vas deferens and ileum—are practical due to their small and delicate structure.
Rabbits are gentle and easy to manage in laboratory settings. Their bodies are covered with fine, smooth hair except in certain regions such as the nose tip, parts of the scrotum, and the inguinal area. The rabbit has well-defined musculature and large, upright external ears (pinna). Its lower lip is undivided, while the upper lip is split, and both connect to a small external opening of the mouth. The nostrils are oval-shaped and join the cleft in the upper lip. Long, noticeable whiskers are found around the nose, above each eye, and on one or two spots on each cheek. Rabbits have large, laterally placed eyes, often pink in albino varieties. Their hind limbs are longer, stronger, and more muscular compared to the forelimbs.
Anatomically, rabbits possess a large caecum and an elongated appendix. A genetic link exists between fur color and the presence of atropinesterase. Because this enzyme is found in the rabbit’s liver and plasma, rabbits can tolerate relatively high doses of belladonna.
Rabbits are primarily used in pyrogen testing for intravenous solutions. Substances affecting capillary permeability are assessed via intracutaneous injection, followed by intravenous administration of dyes like Evans blue. Rabbits are also utilized to evaluate the effects of miotics, mydriatics, insulin, and other antidiabetic drugs, along with curare and sex hormones. They play an essential role in serological studies and in screening for embryo-toxic agents. Due to nonspontaneous ovulation and ease of semen collection, they are commonly used in reproductive research. Additionally, the marginal ear vein is ideal for injections and blood withdrawals, making rabbits suitable for bioavailability investigations.
Commonly used isolated tissues include the heart, jejunum, and ileum for evaluating drug effects.
Blood Collection: Blood samples are often obtained from the marginal ear vein. Other collection sites include the jugular vein and orbital sinus. For larger volumes (around 20 ml), cardiac puncture under anesthesia is the preferred method.
The species Mesocricetus auratus, known as the Syrian or Golden hamster, belongs to the family Cricetidae, subfamily Cricetinae, and the genus Mesocricetus. These hamsters are primarily nocturnal and are most active during the night, engaging in behaviors like digging, burrowing, chewing, and playing. Their natural sleep-wake cycles make them particularly suitable for circadian rhythm studies.
Syrian hamsters are highly territorial. When new hamsters are introduced into their space, aggressive behavior such as biting, fighting, and even killing may occur. This behavior generally begins to emerge between 8 and 10 weeks of age.
Hamsters are extensively employed in research fields such as oncology, immunology, and physiology. They are especially valuable in IVF studies. Their reproductive system is distinct, involving a regular 4-day estrous cycle typical of non-primate mammals, and they have a notably short gestation period of 16 days.
The domestic cat (Felis catus) is a member of the Felidae family and inhabits nearly all parts of the world except Australia and Antarctica. Typically reaching lengths of up to 28 inches (71 cm), cats are the smallest representatives of the feline family. As obligate carnivores, their primary diet consists of animal meat, and their digestive systems are well-suited to processing raw flesh. While cats often rely on human caretakers for food, they also exhibit hunting behaviors to supplement their diet. Although not as frequently used in scientific studies, cats are valuable research models because they develop many of the same diseases as humans, such as leukemia, Alzheimer’s disease, heart disease, infections, and immunodeficiency conditions. Their relatively long lifespan—up to 20 years—makes them particularly suitable for investigating age-related and progressive disorders.
Cats possess a well-developed nictitating membrane, which contracts in response to stimulation of the cervical sympathetic trunk and to drugs such as adrenaline and histamine. This contraction is measured in studies examining ganglionic blocking agents. Morphine has an excitatory effect on the central nervous system in cats.
Cats are used in acute pharmacological studies focused on drugs that influence blood pressure. Both anesthetized and spinal preparations are utilized, the latter especially useful for catecholamine assays. The nictitating membrane’s response is recorded to assess ganglionic blocking drug actions.
They are key models for exploring neural centers within the brain. Cats’ susceptibility to methemoglobinemia makes them appropriate for toxicity evaluations involving compounds like acetanilide. Furthermore, they are extensively used in neurological research areas, including balance, movement, auditory studies, motor neuron function, and investigations of spinal cord injury. Due to similarities in brain structure, cats have also been employed in brain mapping research. Additionally, they serve as effective models for studying viral disease syndromes.
Frogs belong to the class Amphibia and have been utilized in biological studies for over two centuries. These amphibians are safe and manageable for laboratory use, but cannot reproduce under laboratory conditions. In India, the largest species is Rana tigrina, which measures between 5 and 17 cm in length. They exhibit a variety of colors, including yellow, olive green, and grey, with dark, uneven markings. Frogs possess a pointed snout, muscular and elongated hind limbs, and a proportionately sized head. Their toes are nearly completely webbed, and their tympanum (eardrum) is prominent. The distance between the nose and the mouth exceeds that between the eye and the nose. The space between the eyes is smaller than the upper eyelid, and the tympanum is about two-thirds the diameter of the eye. Males are distinguishable by their darker coloring, the presence of breeding pads on their first finger, and two lateral vocal sacs that appear as folds of skin on either side of the throat. Females are generally larger in size.
Frogs are commonly used in the fields of physiology, pharmacology, and toxicology. One significant advantage of using frogs is that their isolated tissue preparations do not require a constant temperature of 37 ℃ and can be maintained at room temperature. In frogs, adrenaline serves as the neurotransmitter for the sympathetic nervous system.
Frogs are used to examine the effects of drugs on the central nervous system, study isolated tissues such as the rectus abdominis muscle, and prepare heart preparations. They are also employed in evaluating medicines that act on the CNS and the neuromuscular junction, as well as in testing drugs for retinal toxicity. The African species Xenopus laevis has long been used as a biological assay for human pregnancy detection.
This species is also widely used to model human diseases and for studies in vertebrate embryology, development, molecular biology, genomics, neurobiology, and toxicology. Due to the unique characteristics of Xenopus eggs and embryos, they are considered highly effective tools for biomedical research.
Dogs (Canis familiaris), members of the Canidae family, are recognized as among the earliest domesticated species. They have been part of scientific research for over a hundred years. While naturally carnivorous, dogs can thrive on a well-balanced, properly formulated omnivorous diet in domestic settings. As large laboratory animals, dogs are particularly valuable because they can be easily tamed and trained.
Their long-standing role in research is mainly due to their physiological similarities with humans. Dogs possess a comparable number of genes, and their entire genome has been sequenced, making them ideal subjects for genetic research. Furthermore, they are affected by several human-like conditions, including diabetes, epilepsy, autoimmune disorders, cancers, and various eye diseases, making them relevant animal models for studying such illnesses.
Dogs are involved in acute experiments related to drugs that impact blood pressure and intestinal motility. They are also used in studies of gastric acid secretion, pharmacokinetics, and investigations involving antidiabetic agents.
Surgically prepared chronic gastric fistulas and pouches are frequently used in dogs for examining gastric secretion processes.
Monkeys and apes are classified as primates—the most advanced order of mammals—which also includes humans. In research, macaques and marmosets are the most commonly used primate species. Structurally and functionally, monkeys closely resemble humans, making them highly valuable in medical studies. Although their use is limited in number, primates have contributed to significant scientific advancements, such as the development of the polio vaccine, neonatal life support technologies, and deep brain stimulation techniques for Parkinson’s disease.
Primates are applied in diverse areas such as virology, parasitology, immunology, nutrition, and reproductive studies. Ongoing primate research particularly targets infectious diseases, including the development of HIV/AIDS vaccines and therapeutic strategies. They are also utilized in testing the safety of new pharmaceutical drugs and vaccines. Table 1 shows the details of some common animals used in laboratories. An overview of commonly used laboratory species, including monkeys, hamsters, dogs, cats, and frogs, is presented in Table 2.
Table 1. Details of some common animals used in laboratories
| Mice | Rat | Guinea Pig | Rabbit |
Scientific name | Mus musculus | Rattus norvegius | Cavia porcellus | Oryctolagus cuniculus |
Order | Rodentia | Rodentia | Rodentia | Lagomorpha |
Body temperature | 37.4 ºC | 37.5-39 ºC | 37.6-38.9 ºC | 38.7-39.1 ºC |
Respiration rate | 90-230/minute | 70-180/minute | 40-110/minute | 38-55/minute |
Heart rate | 300-750/ minute | 260-500/minute | 240-400/minute | 135-300/minute |
Blood pressure | 120/75 | 130/90 | 75-52 | 130-90 |
Blood volume (ml/kg) | 7-9/55-80 | 6-7/ 64 (50-70) | 6-12/75 (67-92) | 4-8/56 (44-70) |
Food consumption | 15 gm/100 gm/day | 10 gm/100 gm/day | 6 gm/100 gm/day | 5 gm/100 gm/day |
Water consumption (body weight /day) | 15 ml/100 gm/day | 10-12ml /100 gm/day | 10ml/100gm/day | 5-10 ml/100 gm/day |
Life span | 1-3 years | 2-3.5 years | 4-5 years | 4-5 up to 15 years |
Prefer humidity | 60-70% | 44-60% | 45% | 40%-50% |
Room temperature | 20-27 ºC | 18.5-27 ºC | 18.5-27 ºC | 15.5-18.5 ºC |
Mating age | 6-8 weeks | 70-84 days | 12-20 weeks | 5-6 months |
Esterous cycle | 4-5 days | 4 to 5 days | 15-19 days | There is no regular estrous cycle. Receptivity periods last between 5 and 14 days. |
Gestation periods | 19-21 days | 21-23 days | 59-72 days | 31 days |
Body weight | 25-40 gm | 250-500 gm | 200-1000 gm | 2-6 kg |
Table 2. Details of some common animals used in laboratories
| Monkey | Hamster | Dog | Frog | Cat |
Scientific name | Macaca mullata | Mesocricetus auratus | Canis familiaris | Rana tigrana | Felis catus |
Order | primates | Rodentia | Carnivora | Anura | Carnivora |
Body temperature | 37-39 ºC | 36.2-37.5 ºC | 37.7 ºC | 26-8 °C | 38.06-39.17 °C |
Respiration rate | 76-90 | 74/minute | 14-28/minute | 66-104/minute | 24-42/minute |
Heart rate | Up to 150/minute | 280-412/ minute | 77-138/minute | 64 times per minute | 140-220/minute |
Blood pressure (mmHg) | 130/100 | 94 99/67 | 140/80 | 25-35 systolic to 18-28 diastolic | 140-90 |
Blood volume | 54 ml/kg | 78 ml/kg | 86 (79-90) ml/kg | 15,93,600 in females and 10,29,700 per mm3 in male | 60 ml/kg |
Food consumption | 1-2.4 kg/day | 12 gm/day | 3-4 meals/day | Five crickets per meal. (0.2 to 0.8 grams/cricket) | 40 gms /kg of body weight 10 or more meals/day |
Water consumption | 18.5 ml/kg/day | 20 ml/day | 20-70 ml/kg /day | 45 ml/kg/day | |
Life span | 30 years | 2-3 years | 10-13 years | 10-12 years | 2-16 years |
Room temperature | 37-40 ºC | 37 ºC | 24-27 ºC | 18-25 ºC | 21.111 °C |
Mating age | 4-5 years | 6-8 weeks | 90 days | 4 years | 6 months-female 8 months-male |
Esterous cycle | 26-28 days | 4 days | 180 days | 40 days | 21 days |
Gestation periods | 165 days | 15-18 days | 62 days | 33 days | 58-67 days |
Body weight | About 5000-6000 gm | 110-140 gm | 1.5-75 kg | 25-500 gms | 4.1-5.4 kg |
In the context of biomedical studies, alternatives to the use of live animals refer to any approach that either fully or partially eliminates the need for living animal subjects in experiments [22, 23]. Numerous substitute techniques have now gained acceptance globally as viable options for replacing animal testing.
One notable example is the Zebrafish model [24, 25], a modern vertebrate used effectively for toxicity screening. These fish are small, reproduce rapidly, and are easy to maintain in large populations. According to findings [26], 90% of the chemicals tested on zebrafish led to specific toxic effects on tissues, organs, or behavior. Chemical exposure in zebrafish can occur either through the aquatic environment or via microinjections. The presence of PS1 and PS2 orthologs in zebrafish has made them particularly beneficial in Alzheimer’s disease research [27]. It is increasingly anticipated that such complete organism models may serve as partial or full alternatives to traditional animal models in pharmacology training.
Several countries have adopted computer-based alternatives for experimental purposes. In India, two prominent software systems—Expharm and Xcology—are available in both free and paid versions [28, 29]. These programs have undergone extensive testing and have been widely implemented. Many countries utilize computer simulations to some extent, and evaluations have demonstrated that these tools are cost-effective and time-saving compared to live animal experiments. Furthermore, students reported that these digital methods enhanced their understanding of drug actions. Although improved versions of existing software are accessible, no major breakthrough program has yet emerged. So far, international regulatory organizations have validated and accepted nearly 50 alternative testing methods.
The Three Rs principle guides the ethical framework for humane use of animals in science. Researchers intending to use animals must first justify the necessity and outline how they plan to reduce animal usage and minimize any suffering involved:
· Replacement: This includes both absolute alternatives, like in silico computer modeling [30] and in vitro systems, and relative alternatives that avoid the use of ‘protected’ animals. Examples include immortalized animal cell lines, primary cells, tissues, and organs from already deceased animals, slaughterhouse byproducts, invertebrates such as Drosophila and nematode worms, amphibian and fish larvae, bacteria, fungi, and other microorganisms.
· Reduction: Involves minimizing the number of animals needed while still achieving valid statistical results. This can be achieved through refined experimental designs, effective statistical approaches, novel imaging technologies, avoiding repeated tests, and encouraging data and resource sharing.
· Refinement [31]: Aims to lessen the severity of procedures and improve overall animal welfare. This includes enhanced tools and practices, better pain control, and efforts to reduce or prevent discomfort and distress [32–34]. Non-invasive methods and the application of appropriate anesthetic and analgesic protocols are examples of this approach.
· Rehabilitation, often referred to as the fourth R, stresses the importance of ensuring animals receive appropriate care and recovery after their use in experiments.
Animals serve multiple purposes for humans, including their use in scientific research. Ensuring the well-being of animals involved in experiments is crucial. However, eliminating animals from laboratory studies would hinder our understanding of health and disease, as well as slow the development of vital and innovative treatments. Many countries have adopted computer-based alternatives to varying extents. Evaluations of these software systems have shown that such options are practical to implement and help reduce both the cost and time associated with animal experimentation. The three Rs provide a framework that researchers must adhere to to minimize the suffering of animals used in laboratories. The term “alternative” refers to any modification in animal testing that achieves one or more of the three Rs: replacing animals, reducing the number of animals used, and refining techniques to lessen pain or distress. It also highlights the importance of the fourth R, which is the rehabilitation of animals following their use.
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