In Alzheimer’s disease models, cholinergic detection should not focus only on measuring acetylcholine or acetylcholinesterase separately. The key is to assess the relationship among neurotransmitter levels, enzyme activity changes, cholinergic neuronal injury, and inflammatory responses. When decreased acetylcholine, abnormal AChE activity, reduced ChAT expression, and elevated inflammatory factors occur together, they provide stronger support for cholinergic dysfunction and neuroinflammation involvement in the model phenotype.

Keywords: Alzheimer’s disease model; acetylcholine detection; acetylcholinesterase; AChE activity; choline acetyltransferase; ChAT; neuroinflammation; IL-1β; TNF-α; IL-6; microglia; hippocampus; cortex

1 Detection Logic of Cholinergic Indicators in Alzheimer’s Disease Models

1.1 Core readouts of cholinergic function

(1) Acetylcholine content

Acetylcholine is a direct neurotransmitter readout of cholinergic neurotransmission and is commonly used to evaluate cholinergic functional status in cognition-related brain regions such as the hippocampus and cortex. A decrease in ACh usually suggests insufficient cholinergic transmitter supply, but it should be interpreted together with AChE, ChAT, and behavioral results to avoid mistaking sample degradation or sampling differences for model effects.

(2) Acetylcholinesterase activity

AChE hydrolyzes acetylcholine and is an important enzyme responsible for transmitter clearance in the cholinergic synaptic cleft. Increased AChE activity may accelerate ACh degradation. Decreased AChE activity may also reflect cholinergic neuronal loss, reduced enzyme expression, or residual drug inhibition. Therefore, “enhanced enzyme function” and “reduced cholinergic neurons” should be distinguished.

(3) Choline acetyltransferase expression

ChAT is responsible for acetylcholine synthesis and is an important marker of cholinergic neuronal function and cholinergic synthetic capacity. If ACh decreases together with ChAT reduction, impaired cholinergic synthesis is more strongly supported. If ACh decreases but ChAT does not change significantly, AChE activity, choline uptake, sample handling, and methodological differences should be examined.

(4) Changes in inflammatory factors

IL-1β, TNF-α, IL-6, IL-10, NLRP3, Caspase-1, GFAP, and Iba1 can be used to evaluate neuroinflammation and glial activation. If increased inflammatory factors are accompanied by decreased ACh, reduced ChAT, and cognitive behavioral abnormalities, this may suggest that the inflammatory environment participates in cholinergic functional impairment.

1.2 Model type affects indicator interpretation

(1) Transgenic models

APP/PS1, 5xFAD, 3xTg-AD, and other models are more suitable for observing cholinergic degenerative changes, Aβ deposition, Tau pathology, and glial activation during chronic pathological progression. In these models, ACh, AChE, ChAT, Iba1, and GFAP should be analyzed together with age, brain region, and pathological burden.

(2) Pharmacologically induced models

The scopolamine model is often used to establish learning and memory impairment related to cholinergic blockade and is suitable for evaluating cholinergic drugs, AChE inhibitors, or cognition-enhancing interventions. This model focuses on functional blockade and is not equivalent to complete Alzheimer’s disease pathology. Its results should not be directly interpreted as long-term neurodegenerative changes.

(3) Aβ- or inflammation-induced models

Aβ oligomers, Aβ injection, LPS, or STZ models are often used to observe inflammation, oxidative stress, synaptic damage, and cholinergic functional changes. In these models, inflammatory factors, oxidative stress indicators, and synaptic proteins should be detected together with cholinergic indicators.

Table 1 Detection Combinations of Cholinergic and Inflammatory Indicators in Alzheimer’s Disease Models

2 Acetylcholine Detection: Neurotransmitter Level and Sample Stability

2.1 Applicable scenarios

(1) Evaluation of cholinergic function in brain regions

The hippocampus and cortex are commonly used brain regions in Alzheimer’s disease models. Decreased ACh content is often associated with learning and memory impairment, weakened synaptic function, and insufficient cholinergic input. If basal forebrain cholinergic projection injury is studied, separate detection of the basal forebrain, hippocampus, and cortex can be added.

(2) Pharmacodynamic evaluation

After intervention with AChE inhibitors, cholinergic receptor modulators, anti-inflammatory drugs, or neuroprotective agents, ACh content can serve as a direct readout of cholinergic functional recovery. AChE activity should be detected simultaneously in pharmacodynamic analysis to avoid observing an increase in ACh without being able to determine its source.

(3) Cell model analysis

In neurons, neuron-like cells, or co-culture systems, ACh detection can be used to evaluate changes in cholinergic function. If the cells themselves have a weak cholinergic phenotype, ACh readings may be low, and ChAT, VAChT, or related receptor expression should be detected simultaneously.

2.2 Detection methods

(1) ELISA or enzymatic assay kits

These methods are suitable for rapid screening of multiple samples. They are relatively simple to operate and can be used for brain tissue homogenates, serum, cerebrospinal fluid, or cell supernatants. They are suitable for comparison among treatment groups, but are sensitive to sample matrix, extraction efficiency, and standard curve quality.

(2) HPLC or LC-MS/MS

These methods are suitable for higher-specificity ACh quantification and can also be combined with detection of choline, betaine, amino acids, or other neurotransmitters. If the research focuses on neurotransmitter metabolic networks or drug effects on choline metabolism, chromatographic or mass spectrometric methods have greater advantages.

(3) Supplementary tissue localization

ACh itself is difficult to localize in tissue and is usually not used as a routine immunolocalization target. If cholinergic neurons or cholinergic terminals need to be localized, ChAT, VAChT, AChE histochemistry, or related cholinergic receptors should be prioritized.

2.3 Control points

(1) Sampling speed

ACh is susceptible to enzymatic degradation and sample processing effects. After sampling, brain regions should be rapidly separated, processed at low temperature, and frozen promptly. Delayed sampling or repeated freeze-thaw cycles increase result variation.

(2) Normalization method

Brain tissue samples are commonly normalized by protein amount or tissue weight. If the model shows obvious brain atrophy, tissue edema, or cell death, tissue weight, protein concentration, and consistency of sampled brain regions should be recorded simultaneously.

(3) Drug residue

After AChE inhibitor intervention, increased ACh may result from reduced degradation. The interval between the last dose and sampling should be recorded, and AChE activity and ChAT expression should be combined for interpretation.

3 Acetylcholinesterase Detection: Enzyme Activity Changes and Cholinergic Pathway Status

3.1 Applicable scenarios

(1) Evaluation of cholinergic degradation capacity

AChE activity detection is used to determine changes in ACh hydrolysis capacity. If AChE activity increases and ACh decreases in the model group, enhanced ACh degradation may contribute to cholinergic dysfunction. If AChE activity decreases but ACh still decreases, ChAT expression, cholinergic neuronal number, and the degree of tissue injury should be examined.

(2) Pharmacodynamic validation of AChE inhibitors

After intervention with donepezil, galantamine, huperzine A, or candidate drugs, AChE activity is a key pharmacodynamic indicator. If AChE activity decreases, ACh increases, and behavior improves, the cholinergic pathway may participate in the drug effect. If AChE is inhibited but behavior does not improve, inflammatory, synaptic, and neuronal injury indicators should be further analyzed.

(3) Auxiliary analysis of peripheral samples

Serum or plasma AChE activity can serve as a peripheral cholinesterase-related readout, but it cannot be directly equated with brain cholinergic function. In Alzheimer’s disease model studies, brain-region detection usually has higher priority than peripheral samples.

3.2 Detection methods

(1) Ellman method

The Ellman method uses acetylthiocholine as the substrate and detects thiocholine production through DTNB color development. It is a classical method for AChE activity detection. This method is suitable for brain tissue homogenates and cell lysates, and results are usually expressed as enzyme activity per unit protein.

(2) Micro method and colorimetric method

The micro method is suitable for limited samples from hippocampus or cortex subregions. The colorimetric method is suitable for conventional spectrophotometer or microplate platforms. Method selection should be based on sample amount, detection throughput, and instrument conditions.

(3) Histochemical method

AChE histochemistry can be used to observe the distribution of AChE activity in brain regions and is suitable for analyzing cholinergic fibers, neuronal regions, and their relationship with tissue structure. This method is more oriented toward localization and morphology and is not suitable as a substitute for enzyme activity quantification.

3.3 Control points

(1) Substrate specificity

Some detection systems may be affected by butyrylcholinesterase or other esterases. If BChE activity is high in the sample, appropriate substrates, inhibitors, or methods should be selected to distinguish AChE from BChE.

(2) Protein normalization

AChE activity should be normalized by protein amount, tissue weight, or cell number. When brain atrophy or cell death is obvious, calculation based only on tissue weight may lead to interpretive bias.

(3) Inhibitor interference

If residual AChE inhibitors are present in drug intervention groups, ex vivo detection values may be affected. The time interval after the last dose, tissue washing, homogenization conditions, and detection systems should be kept consistent.

Table 2 Comparison of Acetylcholine and AChE Detection Methods

4 Detection of Cholinergic Neurons and Receptor Pathways

4.1 ChAT detection

(1) Applicable scenarios

ChAT is the key enzyme at the acetylcholine synthesis side and is suitable for evaluating cholinergic neuronal function and transmitter synthesis capacity. Decreased ChAT in the basal forebrain, hippocampus, and cortex often suggests impairment of the cholinergic system.

(2) Detection methods

Western blot is suitable for quantitative comparison of protein expression. Immunohistochemistry and immunofluorescence are suitable for locating cholinergic neurons or projection regions. qPCR can detect Chat mRNA changes. ELISA can be used to detect ChAT levels in tissue homogenates.

(3) Interpretation points

When ChAT decreases together with ACh, impaired synthesis is more strongly supported. If ChAT remains unchanged while ACh decreases, AChE activity, choline uptake, and sample stability should be analyzed carefully.

4.2 VAChT detection

(1) Applicable scenarios

VAChT loads acetylcholine into synaptic vesicles and is an indicator related to cholinergic release capacity. If synaptic release, cholinergic terminal function, or drug effects on cholinergic transmission are studied, VAChT provides supplementary value.

(2) Detection methods

Immunofluorescence is suitable for observing cholinergic terminal distribution. Western blot is suitable for overall expression comparison. qPCR can be used for transcriptional analysis. Combined detection with synaptic proteins such as synaptophysin and PSD95 can evaluate synaptic structural changes.

(3) Interpretation points

A decrease in VAChT suggests possible impairment of vesicular loading or cholinergic terminal function, but it cannot alone represent actual ACh release. If release function needs to be analyzed, microdialysis, cellular release assays, or electrophysiological indicators can be combined.

4.3 Cholinergic receptor detection

(1) Applicable scenarios

M1 mAChR, M2 mAChR, α7 nAChR, and other receptors participate in cognition, synaptic plasticity, inflammation regulation, and neuroprotection. In Alzheimer’s disease models, receptor expression changes may reflect cholinergic signal sensitivity and downstream regulatory capacity.

(2) Detection methods

Western blot, qPCR, immunofluorescence, and immunohistochemistry can all be used for receptor expression analysis. If functional regulation is the focus, downstream signals such as ERK, CREB, NF-κB, NLRP3, or synaptic proteins should be combined.

(3) Interpretation points

Increased receptor expression may be a compensatory response, or it may reflect inflammation or changes in cell-type proportions. Interpretation should be combined with neuronal markers, glial markers, and brain-region localization.

Table 3 Extended Indicators of the Cholinergic Pathway and Their Application Positioning

5 Inflammatory Factor Detection: Interaction between Neuroinflammation and Cholinergic Injury

5.1 Detection of pro-inflammatory factors

(1) IL-1β

IL-1β is often associated with microglial activation, the NLRP3 inflammasome, and synaptic injury. In Alzheimer’s disease models, increased IL-1β accompanied by decreased ACh, reduced ChAT, and cognitive impairment can support inflammatory involvement in cholinergic functional damage.

(2) TNF-α

TNF-α participates in neuroinflammatory amplification, synaptic functional regulation, and cell death-related processes. Increased TNF-α should be analyzed together with Iba1, CD68, NF-κB, and neuronal injury indicators, rather than being interpreted alone as a measure of model severity.

(3) IL-6

IL-6 can reflect inflammatory responses and may also participate in repair and glial reactions. When IL-6 increases in the model group, its pro-inflammatory or compensatory nature should be judged together with time points and other inflammatory factors.

5.2 Detection of anti-inflammatory and regulatory factors

(1) IL-10

IL-10 can serve as an anti-inflammatory regulatory indicator. If IL-10 increases after intervention while pro-inflammatory factors decrease and ACh or ChAT improves, anti-inflammatory regulation may participate in cholinergic functional recovery.

(2) TGF-β

TGF-β is associated with immune regulation, glial responses, and tissue repair. Its changes should be analyzed together with GFAP, Iba1, and extracellular matrix-related indicators to avoid simply classifying it as protective or damaging.

(3) Cholinergic anti-inflammatory pathway

α7 nAChR is closely related to the cholinergic anti-inflammatory pathway. If ACh level, α7 nAChR expression, and inflammatory factors improve simultaneously, this can support cholinergic regulation of inflammatory responses.

5.3 Glial cells and inflammatory pathways

(1) Microglial activation

Iba1, CD68, TMEM119, and morphological changes can be used to analyze microglial activation. Increased Iba1-positive area does not necessarily indicate enhanced phagocytosis or pro-inflammatory activation and should be interpreted together with CD68, inflammatory factors, and morphological features.

(2) Astrocytic response

Increased GFAP reflects reactive changes in astrocytes. If GFAP increases together with inflammatory factors and decreased synaptic proteins, neuroinflammation and glial responses may participate in cognition-related pathology.

(3) Inflammatory signaling pathways

NF-κB, NLRP3, ASC, Caspase-1, iNOS, and COX-2 can be used to analyze inflammatory mechanisms. If drug intervention improves cholinergic indicators while suppressing these pathways, it supports a cholinergic-inflammatory interaction mechanism.

Table 4 Integrated Interpretation of Inflammatory Factors and Cholinergic Indicators

6 Sample Design and Experimental Quality Control

6.1 Brain region selection

(1) Hippocampus

The hippocampus is closely related to learning and memory and is a core brain region for detecting ACh, AChE, ChAT, inflammatory factors, and synaptic proteins. Behavioral results from the Morris water maze, Y-maze, novel object recognition, and similar tests are usually more suitable for interpretation together with hippocampal indicators.

(2) Cortex

The cortex can reflect changes in cognitive processing, Aβ burden, and inflammatory responses. If the model shows obvious cortical pathology, cortical ACh, AChE, ChAT, and inflammatory factors should also be detected.

(3) Basal forebrain

The basal forebrain is an important source of cholinergic neurons. If degeneration of cholinergic neurons is studied, ChAT immunostaining and cell counting in the basal forebrain have high value.

6.2 Time-point selection

(1) Acute pharmacological models

Acute models such as scopolamine should focus on ACh, AChE, and behavioral changes within a short period after administration. Inflammatory factors may not be the earliest changing indicators, so time points should be set according to the model mechanism.

(2) Chronic transgenic models

Transgenic models should be designed by age, such as early pathology, the cognitive decline phase, and the obvious pathological phase. Aβ, Tau, inflammation, and cholinergic indicators may not change synchronously and should be analyzed by stage.

(3) Intervention experiments

Drug intervention should include a model group, positive drug group, low-, medium-, and high-dose groups, and a normal control group. Cholinergic indicators should be sampled at the same or adjacent time points as behavioral, pathological, and inflammatory indicators.

6.3 Sample handling

(1) Low-temperature and rapid operation

ACh and enzyme activity detection should be performed under low-temperature and rapid conditions as much as possible. After brain-region separation, brain tissues should be frozen immediately or processed for extraction to avoid prolonged exposure at room temperature.

(2) Avoid repeated freeze-thaw cycles

Repeated freeze-thaw cycles affect ACh, enzyme activity, and protein stability. Samples should be aliquoted according to detection purpose. ACh, AChE, Western blot, and ELISA samples can be stored separately.

(3) Unified normalization

ACh, AChE, and inflammatory factors should be clearly normalized by protein amount, tissue weight, or volume. When different indicators use different normalization methods, the interpretation logic should remain consistent.

Table 5 Common Problems and Optimization Directions

7 Product Selection for Cholinergic and Neuroinflammatory Detection in Alzheimer’s Disease Models

Table 6 Product Selection for Cholinergic Indicators and Neuroinflammation Detection in Alzheimer’s Disease Models

8 Common Questions

8.1 Is acetylcholine detection always necessary in Alzheimer’s disease models?

Not necessarily. If the research focus is cholinergic function, cognitive impairment, or the pharmacodynamic effect of AChE inhibitors, ACh detection is important. If the focus is Aβ, Tau, or inflammatory mechanisms, ACh can serve as a supplementary indicator and should be interpreted together with ChAT, AChE, and behavioral results.

8.2 Does increased AChE activity necessarily indicate cholinergic injury?

Not necessarily. Increased AChE may lead to enhanced ACh degradation, but decreased AChE may also occur when cholinergic neurons are lost or enzyme expression is reduced. ACh, ChAT, VAChT, and brain-region histology should be combined for interpretation.

8.3 Should the micro method or colorimetric method be used for acetylcholine detection?

The micro method is suitable for experiments with limited sample amounts, such as hippocampal and cortical subregions. The colorimetric method is suitable for routine detection when sample volume is relatively sufficient and the number of groups is large. If higher specificity or combined detection of choline and other neurotransmitters is required, HPLC or LC-MS/MS can be used for further validation.

8.4 Should inflammatory factors be measured in brain tissue or serum?

For neuroinflammation studies, hippocampus, cortex, and other brain tissues should be prioritized. Serum inflammatory factors can serve as references for peripheral inflammatory status, but they cannot directly represent brain inflammation levels. Cerebrospinal fluid or subregion brain tissue detection can be added when conditions allow.

8.5 Which better represents cholinergic neuronal injury, ChAT or AChE?

ChAT is more related to ACh synthesis capacity and cholinergic neuronal function, whereas AChE is more related to ACh degradation capacity and cholinesterase activity. For judging cholinergic neuronal injury, ChAT localization or protein expression is usually more valuable. For judging drug effects on ACh degradation, AChE activity is more critical.

8.6 Can the scopolamine model represent cholinergic injury in Alzheimer’s disease?

The scopolamine model mainly simulates cognitive impairment caused by cholinergic receptor blockade and is suitable for cholinergic pharmacodynamic screening. However, it cannot fully represent Aβ deposition, Tau pathology, chronic inflammation, and neurodegenerative changes in Alzheimer’s disease. The model boundary should be clearly stated during interpretation.

8.7 Can elevated inflammatory factors prove cholinergic system damage?

Not by themselves. Elevated inflammatory factors indicate activation of inflammatory responses, but whether the cholinergic system is damaged requires joint support from ACh, AChE, ChAT, VAChT, receptor expression, and behavioral results.

Cholinergic detection in Alzheimer’s disease models should establish an indicator combination around “neurotransmitter content-degrading enzyme activity-synthetic enzyme expression-inflammatory response-behavioral phenotype.” ACh reflects neurotransmitter level, AChE evaluates degradation-side changes, ChAT and VAChT assess cholinergic neuronal function, and inflammatory factors and glial markers help explain the neuroinflammatory environment behind cholinergic injury.