Introduction

Neuropathy refers to damage to nerve fibres, such as axons and dendrites. If this neuropathy exists in the peripheral nervous system, i.e., outside the brain and spinal cord, then it is called peripheral neuropathy. This can be mononeuropathy, polyneuropathy, or multifocal damage in peripheral nerves on the basis of the number of damaged peripheral nerves.¹ However, peripheral neuropathy affects 2.4% of the population and is expected to burgeon to 8% in aged people. Clinical symptoms develop on the basis of the type of affected peripheral nerves, like heart rate and sweating in the case of autonomic nerves, muscle incoordination in motor nerves, and pain or numbness in sensory nerves. Multiple research studies have been carried out in order to understand the pathogenesis of this disease and its treatment strategies at different dose patterns using rodent models of rats and mice.¹

Classification of peripheral neuropathy models

Peripheral neuropathy is studied through animal models: 

• In vivo studies
• In vitro studies

In vivo animal models 

In vivo means within the living organisms (animals). In these types of models, disease is induced in a healthy animal, and drugs are administered afterwards. The basic aim of such in vivo studies is to explore the mechanisms of diseases and investigate the therapeutic intervention in the whole body of animals. The following are the varieties of animal models that involve the induction of peripheral neurological diseases in rats or mice through different chemicals, surgery, and infections.^2,3,4

Diabetic peripheral neuropathy models

Diabetic peripheral neuropathy (DPN) models induce type I and type II diabetes in rats/mice.^2,3,4

Examples of DPN models

• Streptozotocin (STZ)-injected rats
• Non-obese diabetic (NOD) and Akita mice 
• Db/db mice, ob/ob mice, and Zucker diabetic fatty rats (ZDF)
• High-fat diet-induced diabetes

How do these DPN models cause neuropathy?

STZ is transported into pancreatic beta (𝝱) cells through the GLUT2 transporter, where it causes DNA alkylation and apoptosis (cell death) of pancreatic cells, that is, the loss of the insulin-producing capacity of 𝝱 (beta) cells, and hyperglycemia occurs. Hyperglycemia is high blood glucose levels, and it upregulates the polyol pathway, which leads to more production of sorbitol. 

This sorbitol, being poorly diffusable, resides in the Schwann cells and neurons, which ultimately pulls water into the intracellular compartment of nerve cells and glial cells. This osmotic stress damages the cell membrane through swelling. Moreover, STZ not only damages the pancreatic cells but also enters into every type of cell that has GLUT2, like hepatocytes and kidney cells. Metabolic impairments like advanced glycation end products (AGEPs), protein kinase C downstreaming signals, and mitochondrial dysfunction lead to the unleashing of reactive oxygen species. These cellular processes trigger inflammation and cause injury to microvasculature:^2,3,4

Examples of damage to peripheral nerves in DPN models

• Thickening and degeneration of neurons
• Atrophy and degeneration of axons
• Demyelination of nerves
• Thrombosis in blood vessels which supply nerves

Such morphological changes in nerves shift towards functional loss, like low conduction velocity of sensory fibres and motor nerves in the peripheral nervous system.^2,3,4

Mutations in the leptin gene of ZDF rats, ob/ob, and db/db mice; the Ins2 gene in Akita mice; and the cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) gene in NOD mice trigger autoimmune destruction of the beta cells in the pancreas and cause hypoinsulinemia (when insulin is lower than it’s normal level in the body), which leads to biochemically mediated membranal damage in peripheral nerves.^2,3,4

Chemotherapy-induced peripheral neuropathy (CIPN)

Drugs such as paclitaxel and vincristine disturb the function of microtubules, which in turn impairs the axonal transportation and causes an accumulation of organelles and proteins in the axons. This leads to the swelling of axons in the peripheral nervous system. Other inducers like cisplatin and oxaliplatin trigger oxidative stress and inflammatory processes through DNA damage within the dorsal root ganglion of sensory neurons, which exacerbates axonal degeneration in peripheral nerves and causes peripheral neuropathy.⁵

Surgical pain models

Similarly, a peripheral nerve is traumatised in three different ways, which are as follows:⁶

• Tightly ligating ½ of the sciatic nerve (partial sciatic nerve ligation) or loosely ligating the whole nerve in chronic constriction injury (CCI)
• Transecting a few branches of the sciatic nerve in spared nerve injury (SNI)

Infection-induced neuropathy

Infections caused by human immunodeficiency virus (HIV) and Mycobacterium leprae also behave like inducers of peripheral neuropathy.^7,8

How does HIV cause neuropathy?

HIV envelope glycoprotein (gp120) binds to C-X-C chemokine receptor type 4 (CXCR4) on sensory neurons and Schwann cells of the peripheral nervous system to release calcium ions, which in turn triggers apoptosis and causes the loss of synaptic transmission. Moreover, HIV stimulates microglia to release pro-inflammatory tumour necrosis factors such as TNF-α and damages nerves.⁷

Leprosy model in peripheral neuropathy

The bacteria that causes leprosy is Mycobacterium leprae. It enters Schwann cells and activates innate immune responses as well as adaptive immune cells like CD4+ Th1 cells, thereby releasing inflammatory mediators that cause the loss of the myelin sheath around them and result in nerve dysfunction.⁸

In vitro models of peripheral neuropathy 

Peripheral nerves are damaged by adding neuropathy inducers like chemotherapeutics, hydrogen peroxide (H₂O₂), and tumour necrosis factor alpha (TNF-α) into isolated cells present in glass settings. These isolated cells, derived from the normal dorsal root ganglion of sensory neurons in sciatic nerves or Schwann cells, are cultured in a culture medium like Dulbecco’s Modified Eagle Medium (DMEM) for the induction of peripheral neuropathy. Sometimes, isolated cells are tumour-derived and are called immortalised neuronal cell lines, like PC12 cells taken from rat phaeochromocytoma.⁹

Behavioural analysis in neuropathic rodents 

Behavioural tests are performed to evaluate neuronal deficits after the induction of peripheral neuropathic pain in rats/mice:¹⁰

Von Frey test

This test is used to evaluate nociception in animals where different plastic filaments are applied to the soles of rats or mice, and each von Frey filament exerts a different weight in grams. This test is used to determine the paw withdrawal threshold of rodents mechanically.

Thermal allodynia

Animals are placed on heated plates having a temperature of 50-55°C. This hot plate test is used to assess the time it takes for rats/mice to respond, such as paw licking and jumping, and to investigate the analgesic effects of drugs.

Cold plate test

In this test, metal plates are cooled down to 0-4°C to induce cold allodynia, and nocifensive actions like paw withdrawal effects and paw licking are recorded.

Motor coordination tests

To evaluate the motor coordination and balance of rats/mice, the rotarod test is used, in which animals are placed on a rod rotating at a constant speed or accelerating very fast. In it, parameters such as the duration of the staying period of animals on a rotating rod and latency fall are measured in animals with neurodegenerative diseases.¹¹

Electrophysiological activity

This test helps determine nerve conduction velocity (NCV) in neuropathic rats or mice. NCV is the speed of action potential propagation in nerves, and this speed will highlight nerve damage in animals.¹²

FAQs

What are the five stages of peripheral neuropathy?

Based on the severity of damage to the peripheral nerves, there are five stages of peripheral neuropathy, starting from stage 1, inconsistent pain; stage 2, constant pain, numbness, and tingling; stage 3, intense pain and persistent numbness; stage 4, severe pain and numbness; and ending at stage 5, complete loss of sensation.

What are the risk factors of peripheral nerve neuropathy?

People with alcohol misuse disorder, diabetic patients, and vitamin B12-deficient individuals, for example, are more prone to peripheral neuropathy.

How do you differentiate neuropathic pain from peripheral neuropathy?

Any damage to the nerves in the peripheral nervous system is called peripheral neuropathy, and pain that results from such damage is neuropathic pain.

Summary

Neuropathy in the peripheral nervous system is induced in animals through experimentation that mimics the human nervous system. Such in vivo models investigate the causes of neuropathy, pain intensity, and treatment approaches in peripheral neuropathies. In these models, inducers of neuropathy may include chemicals, trauma and ligation of nerves. Afterwards, different drugs are used to determine the efficacy and potency required to counteract this disease induction. Furthermore, behavioural and histological examination is conducted for the surveillance of disease and pharmacological treatment.