4.9.1 Central Nervous System and Behavioural Disease Models
Central nervous system disease models are amongst the most extensively used in pharmacological research because behavioural and electrophysiological...
Central nervous system disease models are amongst the most extensively used in pharmacological research because behavioural and electrophysiological endpoints, unlike many peripheral disease markers, can often be measured non-invasively and repeatedly in the same animal, improving statistical power while reducing overall animal usage.
Epilepsy — Maximal Electroshock (MES) and Pentylenetetrazole (PTZ) Models
Epilepsy is a chronic neurological disorder characterised by recurrent, unprovoked seizures arising from abnormal, synchronous neuronal discharge. Two complementary acute seizure models are used to screen candidate anticonvulsants and probe distinct seizure mechanisms. In the Maximal Electroshock (MES) model, typically performed in Wistar rats or Swiss albino mice, a supramaximal electrical stimulus (approximately 50 mA for 0.2 seconds, delivered via corneal or ear electrodes) induces tonic hindlimb extension analogous to human generalised tonic-clonic seizures; abolition of the tonic extensor phase indicates anticonvulsant activity, and the model is considered predictive of drugs effective against generalised seizures acting via sodium-channel blockade or glutamatergic inhibition. In the pentylenetetrazole (PTZ) model, a chemoconvulsant dose of 80–90 mg/kg administered intraperitoneally antagonises GABA-A receptor-mediated inhibition, producing myoclonic jerks and clonic seizures; protection in this model is considered predictive of drugs enhancing GABAergic transmission, such as benzodiazepines and valproate. Efficacy in both models is quantified as the percentage of animals protected from the defined seizure endpoint, and standard reference drugs include phenytoin (MES-predictive) and diazepam or sodium valproate (PTZ-predictive). Limitations include the acute, single-seizure nature of both models, which does not reproduce the epileptogenesis process underlying chronic human epilepsy; genetic and kindling models (for example, the pilocarpine or kainate status-epilepticus models) are increasingly used as complementary chronic alternatives in advanced research.
Depression — Forced Swim Test (FST) and Tail Suspension Test (TST)
Depression is modelled behaviourally using tests of 'behavioural despair', in which rodents subjected to an inescapable stressor progressively adopt an immobile posture interpreted as analogous to the psychomotor retardation and hopelessness of human depression. In the Forced Swim Test, rats or mice are placed in a cylinder of water from which escape is impossible, and immobility time is recorded over a 5–6 minute observation period following an initial habituation swim; increased immobility indicates a depressive-like phenotype, while reduced immobility following drug treatment indicates antidepressant-like activity. The Tail Suspension Test applies an analogous logic to mice suspended by the tail for approximately 6 minutes, avoiding the confound of differential swimming ability across genotypes or treatments. Imipramine and fluoxetine are standard reference antidepressants in both assays. Both tests are valued for their speed, low cost, and sensitivity to clinically effective antidepressant drug classes, but are criticised for reflecting acute pharmacological reactivity rather than a genuine model of the chronic, multifactorial aetiology of clinical depression; chronic mild stress and chronic social defeat stress paradigms are increasingly used as more construct-valid alternatives in contemporary neuropsychopharmacology research.
Anxiety — Elevated Plus Maze (EPM) Model
Anxiety disorders are modelled by exploiting rodents' natural conflict between the drive to explore a novel environment and their innate aversion to open, elevated, and brightly lit spaces. The Elevated Plus Maze apparatus consists of two open arms and two enclosed arms arranged in a plus shape and elevated approximately 50 cm above the floor; an anxious animal spends proportionally less time in, and makes fewer entries into, the open arms. Anxiolytic drug activity is indicated by an increase in the percentage of time spent in, and entries into, the open arms, without a confounding non-specific increase in total locomotor activity (assessed by total arm entries). Diazepam is the standard positive control. The Open Field Test provides a complementary measure, in which reduced central-zone exploration and increased thigmotaxis (wall-hugging behaviour) indicate an anxious phenotype. Both tests are rapid, inexpensive, and require no chemical or surgical induction, but are sensitive to procedural variables such as handling stress, testing-room lighting, and prior test experience, all of which must be rigorously standardised across a study.
Alzheimer's Disease — Scopolamine-Induced Amnesia Model
Alzheimer's disease is a progressive neurodegenerative disorder characterised by cholinergic neuronal loss, amyloid-beta plaque deposition, and progressive memory impairment. The scopolamine-induced amnesia model exploits the central role of cholinergic transmission in memory consolidation: administration of the muscarinic antagonist scopolamine (typically 1 mg/kg intraperitoneally) in Wistar rats or Swiss mice produces reversible, pharmacologically induced amnesia that mimics the cholinergic deficit of Alzheimer's disease without requiring genetically modified animals. Cognitive impairment is quantified using the Morris Water Maze (escape latency to a submerged platform, and time spent in the target quadrant during a subsequent probe trial), the passive avoidance task (latency to re-enter a chamber previously associated with a mild foot shock), and biochemical acetylcholinesterase (AChE) activity assay in brain homogenate. Piracetam and donepezil, both clinically used cognitive enhancers, serve as standard reference drugs. The principal advantage of this model is its speed and low cost relative to genetically modified amyloid- or tau-transgenic mouse lines, which better recapitulate the underlying neuropathology but require considerably longer study durations and specialised breeding colonies; contemporary Alzheimer's research increasingly combines the scopolamine screening model for rapid throughput with transgenic models for definitive mechanistic and disease-modifying studies.
Parkinson's Disease — 6-OHDA and MPTP Models
Parkinson's disease results from progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta, producing bradykinesia, rigidity, and resting tremor. Two chemical lesioning approaches are standard. Stereotaxic unilateral intracerebral injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra or medial forebrain bundle of rats produces a selective, largely unilateral dopaminergic lesion; the resulting motor asymmetry is quantified using apomorphine- or amphetamine-induced rotational behaviour (rotations away from or towards the lesioned side, respectively), striatal dopamine content assay, and tyrosine hydroxylase (TH) immunohistochemistry as a marker of surviving dopaminergic neurons. Systemic administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), typically in mice, is metabolised to the active toxin MPP+ by glial monoamine oxidase-B and selectively taken up by dopaminergic neurons via the dopamine transporter, producing bilateral nigrostriatal degeneration that more closely parallels the bilateral presentation of human Parkinson's disease. Levodopa and dopamine agonists such as bromocriptine serve as reference standards for symptomatic reversal. Both models successfully reproduce dopaminergic degeneration but do not reproduce the Lewy body alpha-synuclein pathology characteristic of the human disease, motivating the increasing use of alpha-synuclein overexpression and pre-formed fibril seeding models in translational Parkinson's research.
Schizophrenia — Amphetamine- and Ketamine-Induced Models
Schizophrenia is modelled pharmacologically by exploiting the two principal neurochemical hypotheses of the disorder. The amphetamine-induced hyperlocomotion model, based on the dopamine hypothesis, uses systemic amphetamine administration to produce excessive dopaminergic signalling, resulting in stereotyped behaviour and hyperlocomotion in rodents that models the positive symptoms of schizophrenia; attenuation of this hyperlocomotion by a test compound is interpreted as antipsychotic-like activity, with haloperidol or clozapine as reference standards. The ketamine-induced model, based on the glutamate (NMDA receptor hypofunction) hypothesis, uses sub-anaesthetic doses of the NMDA receptor antagonist ketamine to produce both positive-symptom-like hyperlocomotion and negative/cognitive-symptom-like deficits in social interaction and working memory, offering somewhat broader construct validity than the amphetamine model alone. Prepulse inhibition of the acoustic startle reflex, a sensorimotor gating measure disrupted in both models and in human schizophrenia patients, provides a further translationally relevant endpoint. Both models are pharmacologically induced and acute, and therefore do not capture the neurodevelopmental origin of schizophrenia; neurodevelopmental models such as prenatal maternal immune activation are used in more specialised research settings.
Neuropathic Pain — Chronic Constriction Injury (CCI) Model
Neuropathic pain arises from injury or dysfunction of the somatosensory nervous system itself, rather than from ongoing tissue damage, and is characterised clinically by allodynia (pain from normally non-painful stimuli) and hyperalgesia (exaggerated pain from normally painful stimuli). The Chronic Constriction Injury model, performed in rats, involves surgically placing four loose ligatures around the common sciatic nerve, producing a controlled, partial nerve injury that induces robust and reproducible mechanical and thermal hypersensitivity in the ipsilateral hind paw within one week of surgery. Mechanical allodynia is quantified using von Frey filaments (the force required to elicit paw withdrawal), and thermal hyperalgesia is quantified using the Hargreaves radiant heat test. Gabapentin, pregabalin, and amitriptyline serve as standard reference analgesics, reflecting the distinct pharmacology of neuropathic (as opposed to nociceptive or inflammatory) pain. The CCI model is valued for closely reproducing the time course and quality of human neuropathic pain following nerve injury, though the surgical skill required and the inherent variability in ligature tightness between operators necessitate careful surgeon training and, ideally, blinded outcome assessment.
Cognitive Function — Morris Water Maze (MWM) Model
Beyond its application in Alzheimer's disease research described above, the Morris Water Maze is used more broadly as a general assay of spatial learning and memory, applicable to cognitive-enhancer screening, neurotoxicology, and ageing research. The apparatus consists of a circular pool filled with opacified water, containing a submerged, invisible escape platform whose location the animal must learn using extra-maze spatial cues over repeated training trials; the primary endpoints are escape latency (time to locate the platform) during training and time spent in the target quadrant during a probe trial, reflecting the strength of the spatial memory formed. Because the task depends on hippocampal function, the Morris Water Maze is particularly sensitive to compounds or lesions affecting hippocampal long-term potentiation. Advantages include its extensive historical validation and sensitivity; limitations include the physical stress of forced swimming, which can itself confound results in animals with impaired stress-coping ability, and the requirement for intact vision and swimming ability, which restricts its use in certain genetically or surgically modified animal lines.
General Behavioural Assessment — Open Field Test
The Open Field Test is among the most widely used general-purpose behavioural assays in pharmacology, serving simultaneously as a locomotor-activity assay, an anxiety-related measure, and a screen for sedative or stimulant drug effects. An animal is placed in a large, open, brightly lit arena, and its behaviour is recorded (manually or via automated video-tracking software such as ANY-maze or EthoVision) over a fixed observation period, typically five to ten minutes. Key parameters include total distance travelled and rearing frequency (locomotor activity), time spent in the central versus peripheral zone (anxiety-related thigmotaxis), and grooming frequency and duration. Because many CNS-active drugs alter locomotor activity, the Open Field Test is frequently run as a companion assay alongside more specific behavioural tests (such as the elevated plus maze or forced swim test) to rule out the possibility that an apparent anxiolytic or antidepressant effect is in fact a non-specific sedative or stimulant artefact. Its principal advantages are speed, minimal equipment requirement, and applicability across essentially any rodent strain or genotype.