Preclinical CNS Screening Models
This dense, exam-critical unit is the backbone of pharmacological screening. It begins with the essential practical fundamentals—dose calculation, drug preparation, and control group design—then systematically covers the complete battery of preclinical in-vivo models used to evaluate drugs acting on the Central Nervous System. From the Tail Flick Test for analgesics to the Forced Swim Test for antidepressants, each model provides a specific, measurable pharmacological endpoint.
Syllabus & Topics
- 1Preclinical Study Design Fundamentals: Dose Selection & Calculation: Based on LD50 values, therapeutic indices, and pilot study data. Inter-species Dose Conversion: Using FDA body surface area (BSA) conversion factors (e.g., Human dose × Km factor). Drug Solution Preparation: Dissolving in appropriate vehicles—aqueous (0.9% Normal Saline, Distilled Water), organic (DMSO, Tween 80 suspension), or CMC (0.5% Carboxymethyl Cellulose for suspensions). Grouping: Minimum groups: (1) Normal/Sham (no disease induction, no treatment), (2) Negative/Disease Control (disease induced + vehicle only), (3) Positive Control (disease induced + standard drug), (4) Test Groups (disease induced + new drug at different doses). Rationale for Animal/Sex Selection: Species chosen based on model relevance. Sex: Use both sexes unless hormonal interference (e.g., estrogen’s cardioprotective effect may mask results in female CVS models).
- 2Analgesic Screening Models: Central Analgesic Models (Opioid-type pain): Tail Flick Test: A radiant heat beam or hot water is applied to the mouse/rat tail. The time taken to ‘flick’ the tail away (reaction time/latency) is measured. An analgesic drug increases the latency. Hot Plate Test: The mouse is placed on a heated metal surface (55°C). The latency to lick paws or jump is measured. Analgesics increase this latency. Peripheral Analgesic Models (NSAID/non-opioid): Acetic Acid-Induced Writhing Test: Injecting dilute acetic acid (0.6%) IP into mice induces characteristic abdominal constrictions (‘writhes’). An analgesic reduces the number of writhes. Formalin Test: Formalin injection into the hind paw produces a biphasic pain response (early neurogenic phase + late inflammatory phase).
- 3Antipyretic & Anti-inflammatory Screening Models: Antipyretic Model: Brewer’s Yeast-Induced Pyrexia: SC injection of 20% Brewer’s yeast suspension into rats causes fever (pyrexia) within 18-24 hours by inducing prostaglandin E2 production. The rectal temperature is measured. An antipyretic drug (like Paracetamol) reduces the elevated temperature back toward normal. Anti-inflammatory Models: Acute Inflammation: Carrageenan-Induced Paw Edema (Winter’s Method): Sub-plantar injection of 1% Carrageenan into the rat hind paw causes a measurable swelling (paw volume increase measured by Plethysmometer). An anti-inflammatory drug reduces paw volume. Chronic Inflammation: Cotton Pellet Granuloma Method: Sterile cotton pellets implanted subcutaneously in rats. After 7 days, the granuloma tissue weight is measured. Anti-inflammatory drugs reduce granuloma weight.
- 4Sedative-Hypnotic & Antipsychotic Screening Models: Sedative-Hypnotic: Actophotometer Test (Locomotor Activity): The animal is placed in a photocell-equipped chamber that records movements as counts. Sedatives dramatically reduce locomotor activity. Rota-Rod Test (Motor Coordination): The animal is placed on a rotating rod. Sedatives impair the ability to stay on, causing the animal to fall off sooner. Pentobarbitone-Induced Sleeping Time: A sub-hypnotic dose of Pentobarbitone is given IP. Sedatives potentiate its effect, prolonging the sleeping time. Antipsychotic: Haloperidol-Induced Catalepsy (Bar Test): The animal’s forepaws are placed on an elevated bar. The time it remains immobile (cataleptic) is measured. Antipsychotics induce catalepsy (extrapyramidal effect). Apomorphine/Amphetamine-Induced Stereotypy: Dopamine agonists cause repetitive behaviors (stereotypy). Antipsychotics (D2 blockers) inhibit this stereotypy.
- 5Antidepressant & Nootropic Screening Models: Antidepressant: Forced Swim Test (Porsolt’s Test): A mouse/rat is placed in an inescapable cylinder of water. After initial struggling, the animal becomes immobile (‘behavioral despair’). The total immobility time in a 5-6 minute test is measured. Antidepressants (like Imipramine, Fluoxetine) reduce immobility time. Tail Suspension Test: The mouse is suspended by its tail from a height. Immobility time is recorded. Antidepressants reduce immobility. Nootropic (Cognitive Enhancement): Elevated Plus Maze: Tests learning and memory (Transfer Latency). The time for the animal to move from an open arm to a closed arm is measured on Day 1 (acquisition) and Day 2 (retention). Nootropics reduce Day 2 latency. Morris Water Maze: A rat must find a hidden platform in a water pool using spatial cues. Nootropics improve escape latency and platform-zone time.
- 6Antiepileptic & Anti-Parkinsonian Screening Models: Antiepileptic: Maximal Electroshock Seizure (MES) Test: A brief electrical stimulus is delivered through ear-clip electrodes to induce tonic hind-limb extension (THLE) in mice. Antiepileptics (like Phenytoin) abolish THLE. Tests generalized tonic-clonic seizures. Pentylenetetrazol (PTZ)-Induced Seizure Test: PTZ (a GABA antagonist) is injected SC/IP to induce clonic seizures. Antiepileptics (like Diazepam, Valproate) increase the seizure threshold or abolish seizures. Tests absence and myoclonic seizures. Anti-Parkinsonian: Haloperidol-Induced Catalepsy: Haloperidol (D2 blocker) induces Parkinsonian-like catalepsy. Anti-Parkinsonian drugs (Levodopa, Trihexyphenidyl) reverse the catalepsy. Reserpine-Induced Akinesia: Reserpine depletes dopamine, causing akinesia (inability to move), tremor, and rigidity. Anti-Parkinsonian drugs reverse these symptoms. 6-OHDA Lesioned Rat Model: Chemical destruction of nigrostriatal dopamine neurons—a more sophisticated chronic Parkinson’s model.
Learning Objectives
Exam Prep Questions
Q1. Why does the “Forced Swim Test” work as an antidepressant screening model?
In the forced swim test, a rodent placed in an inescapable container of water initially attempts to escape but eventually becomes immobile, which is interpreted as “behavioral despair.” Antidepressant drugs such as SSRIs (e.g., fluoxetine) and TCAs (e.g., imipramine) reduce this immobility by increasing neurotransmitters like serotonin and norepinephrine, thereby restoring escape-oriented behavior. Although it does not replicate true human depression, it serves as a rapid and reliable behavioral model for screening antidepressant activity.
Q2. Why is the Carrageenan Paw Edema model the gold standard for anti-inflammatory screening?
The carrageenan-induced paw edema model produces a well-defined, biphasic inflammatory response. The early phase (0–1 hour) is mediated by histamine, serotonin, and bradykinin, while the late phase (1–6 hours) is primarily driven by prostaglandins. Since non-steroidal anti-inflammatory drugs (NSAIDs) specifically inhibit prostaglandin synthesis, they effectively reduce swelling in the second phase. The inflammation can be quantitatively measured using instruments like a plethysmometer, making this model highly reproducible and reliable.
Q3. What is the difference between the MES and PTZ seizure models?
The MES (Maximal Electroshock) model induces generalized tonic-clonic seizures using an electrical stimulus and is useful for identifying drugs that block voltage-gated sodium channels, such as phenytoin and carbamazepine. In contrast, the PTZ (Pentylenetetrazol) model induces seizures by inhibiting GABA-A receptors, leading to clonic or absence-like seizures. Drugs that enhance GABAergic activity, such as diazepam or valproic acid, are effective in this model. Using both models provides a broader evaluation of anticonvulsant drug activity across different seizure mechanisms.