Neuronal signaling is the fundamental process by which the central nervous system senses, transmits, integrates, and adaptively regulates information. Through synaptic release, receptor activation, ion channel opening, second messenger generation, and gene expression regulation, neurons convert external stimuli and intercellular information into changes in membrane potential, synaptic plasticity, neural development, and behavioral output.

Keywords: neuronal signaling; GPCR; Notch signaling pathway; ion channel; synaptic plasticity; glial cells; neural stem cells; central nervous system disease

1 Basic Logic of Neuronal Signal Transduction

1.1 Levels of Neuronal Signaling

Neuronal signal transduction is not a single pathway, but a network composed of membrane excitability, synaptic transmission, receptor signaling, intracellular cascades, and transcriptional regulation.

(1) Membrane potential level

Neurons generate resting membrane potentials and action potentials through transmembrane movement of ions such as sodium, potassium, calcium, and chloride. Voltage-gated sodium channels mediate the rapid rising phase of action potentials, voltage-gated potassium channels participate in repolarization and firing frequency regulation, and voltage-gated calcium channels connect electrical activity with neurotransmitter release.

(2) Synaptic transmission level

Presynaptic neurons release signaling molecules such as glutamate, GABA, dopamine, 5-HT, acetylcholine, histamine, and neuropeptides. After receptors on the postsynaptic membrane recognize these neurotransmitters, excitatory or inhibitory responses are generated. Synaptic signals can rapidly alter membrane potential and can also produce long-term changes in synaptic strength through second messengers.

(3) Intracellular signaling level

After GPCRs, receptor tyrosine kinases, ion channels, Notch, and other receptors are activated, they can further connect to pathways such as cAMP/PKA, PLC/Ca²⁺/PKC, MAPK/ERK, PI3K/AKT, CaMK, CREB, and NF-κB, affecting neuronal excitability, survival, differentiation, and plasticity.

(4) Transcriptional and fate level

Long-term neural activity can alter the expression of CREB, NFAT, MEF2, Notch effector molecules, and neurotrophic factor-related genes, thereby affecting neuronal maturation, axonal and dendritic growth, synaptic remodeling, neural stem cell differentiation, and adaptive or injury responses under disease conditions.

Table 1 Major Levels of Neuronal Signal Transduction

Figure 1 Schematic diagram of neuronal synaptic receptors and intracellular signaling transduction

1.2 Coordination Between Neurons and Glial Cells

Information processing in the central nervous system is not completed by neurons alone. Astrocytes, microglia, oligodendrocytes, and neural stem cells all participate in signal regulation.

(1) Astrocytes

Astrocytes regulate glutamate clearance, potassium buffering, energy metabolism, and blood-brain barrier function. Their Ca²⁺ signaling can influence neurotransmitter homeostasis and synaptic activity.

(2) Microglia

Microglia are immune surveillance cells in the central nervous system. They can affect synaptic pruning, neuroinflammation, and neurodegenerative lesions through inflammatory factors, complement, and phagocytosis.

(3) Oligodendrocytes

Oligodendrocytes form myelin sheaths and improve neural conduction efficiency. Myelin injury alters the conduction velocity of nerve impulses and participates in multiple sclerosis, white matter injury, and neurodegenerative diseases.

(4) Neural stem cells

The proliferation, self-renewal, and differentiation of neural stem cells are regulated by signaling pathways such as Notch, Wnt, BMP, FGF, Shh, and neurotrophic factors. Notch signaling is especially closely associated with maintaining stem cells in an undifferentiated state and regulating neurogenesis.

2 GPCR-Mediated Neuronal Signaling

2.1 Localization of GPCRs in the Nervous System

G protein-coupled receptors (GPCRs) are one of the most important receptor families in the nervous system. 5-HT receptors, histamine receptors, opioid receptors, dopamine receptors, muscarinic acetylcholine receptors, adenosine receptors, and many neuropeptide receptors all belong to the GPCR family.

(1) Ligand recognition

GPCRs can recognize neurotransmitters, peptide ligands, lipid mediators, light signals, taste molecules, and metabolites. After activation, receptors undergo conformational changes and further regulate heterotrimeric G proteins.

(2) G protein branches

Gαs usually promotes cAMP generation, Gαi/o usually inhibits cAMP and regulates ion channels through Gβγ, Gαq/11 activates PLCβ and triggers Ca²⁺ and PKC signaling, and Gα12/13 is associated with RhoA and cytoskeletal regulation.

(3) Regulation of neuronal excitability

GPCRs can indirectly regulate voltage-gated calcium channels, potassium channels, TRP channels, and synaptic release mechanisms, thereby altering neuronal firing frequency, neurotransmitter release probability, and synaptic strength.

2.2 5-HT Receptor Signaling

(1) Mechanistic core

5-HT receptors participate in the regulation of mood, sleep, pain, appetite, gastrointestinal motility, and cognition. Most 5-HT receptors are GPCRs, and different subtypes can connect to Gαs, Gαi/o, or Gαq/11 pathways.

(2) Experimental indicators

Commonly detected indicators include cAMP, Ca²⁺, ERK phosphorylation, synaptic currents, neuronal firing frequency, receptor expression, and behavioral changes.

(3) Result interpretation

Although they are all 5-HT receptors, different subtypes may produce completely different effects. The receptor subtype should be clearly defined during research, and results should not be summarized simply as “enhanced or weakened 5-HT signaling.”

2.3 Opioid Receptor Signaling

(1) Mechanistic core

Opioid receptors mainly include μ, δ, and κ receptors. Most of them reduce cAMP levels through Gαi/o and regulate calcium channels and potassium channels through Gβγ. This signaling reduces neurotransmitter release and neuronal excitability and is an important mechanism in analgesia and addiction research.

(2) Experimental indicators

Commonly detected indicators include decreased cAMP, calcium channel inhibition, potassium channel opening, β-arrestin recruitment, receptor internalization, pain-related behavior, and reward-related behavior.

(3) Result interpretation

Opioid receptor research should not evaluate only analgesic effects; tolerance, dependence, respiratory depression, and β-arrestin-biased signaling should also be assessed. The bias of different ligands toward G protein pathways or β-arrestin pathways may determine differences in efficacy and adverse reactions.

2.4 Histamine Receptor Signaling

(1) Mechanistic core

Histamine receptors participate in wakefulness, allergic inflammation, gastric acid secretion, and neuroimmune regulation. H1 receptors commonly connect to Gαq/11-Ca²⁺ signaling, H2 receptors mostly connect to Gαs-cAMP signaling, and H3 receptors often act as presynaptic autoreceptors that regulate neurotransmitter release.

(2) Experimental indicators

Ca²⁺ transients, cAMP levels, neurotransmitter release, neuronal excitability, and inflammatory factor changes can be detected.

(3) Result interpretation

Histamine signaling shows marked cell type differences. Activation of presynaptic H3 receptors may reduce neurotransmitter release, whereas activation of postsynaptic receptors may alter neuronal excitability. Interpretation should therefore depend on receptor localization and subtype.

Table 2 Common GPCRs in the Nervous System and Their Signaling Characteristics

3 Ion Channels and Neuronal Excitability

3.1 Voltage-Gated Sodium Channels

(1) Mechanistic core

Voltage-gated sodium channels are responsible for rapid depolarization during action potentials. After channel opening, Na⁺ rapidly flows inward, causing the membrane potential to rise quickly and initiating and propagating nerve impulses.

(2) Experimental indicators

Patch-clamp recording is commonly used to detect sodium currents, current density, activation/inactivation kinetics, and action potential threshold. Nav channel expression and mutations can also be detected.

(3) Result interpretation

Enhanced sodium channel function usually increases neuronal excitability and may be associated with epilepsy, pain, and some neurodevelopmental disorders. Reduced function may lead to conduction defects or insufficient neural function.

3.2 Voltage-Gated Calcium Channels

(1) Mechanistic core

Voltage-gated calcium channels connect membrane electrical activity with neurotransmitter release. When an action potential reaches the presynaptic terminal, Ca²⁺ influx triggers synaptic vesicle fusion and neurotransmitter release.

(2) Experimental indicators

Calcium currents, intracellular Ca²⁺ transients, synaptic vesicle release, synaptic currents, and calcium channel subtype expression are commonly detected.

(3) Result interpretation

Enhanced calcium channel function can increase neurotransmitter release and neuronal excitability, whereas reduced function may decrease synaptic transmission. GPCR-mediated inhibition of voltage-gated calcium channels through Gβγ is one of the important mechanisms of neuromodulation.

3.3 TRP Channels

(1) Mechanistic core

Transient receptor potential (TRP) channels can sense temperature, mechanical stimuli, chemical stimuli, osmotic pressure, and inflammatory mediators, and are especially important in sensory neurons. TRPV, TRPA, TRPM, and other subtypes participate in pain, inflammation, temperature sensation, and neural injury responses.

(2) Experimental indicators

Ca²⁺ influx, membrane currents, pain behavior, inflammatory factors, channel expression, and drug sensitivity are commonly detected.

(3) Result interpretation

TRP channel activation usually indicates enhanced sensory neuron excitability, but interpretation should take inflammatory status and cell type into account. If TRP channel expression increases together with reduced pain threshold, its participation in pain sensitization is supported.

3.4 Chloride Homeostasis and Inhibitory Signaling

(1) Mechanistic core

GABA_A receptors and glycine receptors mediate Cl⁻ channel opening. In mature neurons, KCC2 maintains a low intracellular Cl⁻ concentration, making GABA signaling mainly inhibitory. If Cl⁻ homeostasis is disrupted, GABA signaling may shift from inhibitory to depolarizing.

(2) Experimental indicators

GABA-evoked currents, KCC2, NKCC1, intracellular Cl⁻ levels, and inhibitory synaptic currents are commonly detected.

(3) Result interpretation

GABA receptor expression does not necessarily mean that inhibitory function is normal. If KCC2 decreases or NKCC1 increases, GABA signaling may weaken or even shift toward depolarization. This is important in epilepsy, neurodevelopmental abnormalities, and post-injury changes in excitability.

4 Notch Signaling and Neural Development

4.1 Basic Process of Notch Signaling

(1) Mechanistic core

Notch signaling is a development-regulating pathway dependent on cell-cell contact. After a ligand binds to the Notch receptor, the receptor undergoes proteolytic cleavage and releases the Notch intracellular domain (NICD), which then enters the nucleus and regulates target gene expression.

(2) Key molecules

Notch receptors, Delta/Jagged ligands, ADAM proteases, γ-secretase, RBPJ, and Hes/Hey transcription factors are important components of this pathway. γ-Secretase participates in both Notch receptor cleavage and APP processing, whereas β-secretase is mainly involved in APP processing and Aβ generation; therefore, neurodegenerative disease research should distinguish Notch signaling from APP/Aβ-related secretase processes.

(3) Functional significance

Notch signaling can regulate neural stem cell proliferation, survival, self-renewal, and differentiation, maintain the undifferentiated cell pool, and affect lineage selection among neurons, astrocytes, and oligodendrocytes.

4.2 Notch and Neural Stem Cells

(1) Mechanistic core

When Notch activity is high, neural stem cells are more likely to maintain an undifferentiated state. When Notch signaling decreases, cells are more likely to enter the neuronal differentiation program. This process is important for embryonic development, adult neurogenesis, and post-injury repair.

(2) Experimental indicators

NICD, Hes1, Hes5, Nestin, Sox2, DCX, NeuN, GFAP, Olig2, and cell proliferation indicators are commonly detected.

(3) Result interpretation

Enhanced Notch signaling does not necessarily indicate enhanced neural repair. Long-term excessive Notch activation may maintain stem cells in an undifferentiated state and inhibit neurogenesis. Excessive Notch reduction may lead to depletion of the stem cell pool.

4.3 Notch and Nervous System Diseases

(1) Stroke

After ischemia, Notch signaling can affect neural stem cell activation, vascular responses, inflammation, and glial cell responses. Notch may have different roles at different stages, so the acute phase and repair phase should be analyzed separately.

(2) Alzheimer’s disease

γ-Secretase participates in both Notch receptor cleavage and amyloid precursor protein processing. Therefore, targeting secretases in neurodegenerative diseases requires simultaneous consideration of Aβ production and Notch-related side effects.

(3) Central nervous system tumors

Notch signaling can maintain stem cell-like characteristics in some tumors and promote proliferation, drug resistance, and invasion. Research should combine cell proliferation, stemness markers, differentiation status, and tumor-forming capacity for interpretation.

Table 3 Research Focuses of Notch Signaling in the Nervous System

5 Synaptic Plasticity and Intracellular Cascades

5.1 Ca²⁺/CaMK/CREB Pathway

(1) Mechanistic core

Ca²⁺ is the core second messenger that converts neuronal activity into intracellular responses. NMDA receptors, voltage-gated calcium channels, and some TRP channels can all cause Ca²⁺ elevation, further activating CaMK and CREB and regulating synaptic plasticity and gene expression.

(2) Experimental indicators

Intracellular Ca²⁺ transients, CaMKII phosphorylation, CREB phosphorylation, BDNF expression, dendritic spine density, and long-term potentiation (LTP) are commonly detected.

(3) Result interpretation

The significance of Ca²⁺ elevation depends on amplitude, duration, and spatial location. Transient local Ca²⁺ signals can promote synaptic plasticity, whereas sustained excessive Ca²⁺ may induce excitotoxicity and cell death.

5.2 MAPK/ERK Pathway

(1) Mechanistic core

The MAPK/ERK pathway can be activated by GPCRs, receptor tyrosine kinases, neurotrophic factors, and synaptic activity. It participates in neuronal differentiation, synaptic plasticity, memory formation, and cell survival.

(2) Experimental indicators

p-ERK, p-MEK, CREB, immediate early genes, neurite length, and synapse-related proteins are commonly detected.

(3) Result interpretation

Increased ERK phosphorylation should be interpreted according to the time point. Short-term activation may participate in signal transduction, whereas sustained abnormal activation may be associated with pathological excitation, inflammation, or tumor-like proliferation.

5.3 PI3K/AKT Pathway

(1) Mechanistic core

The PI3K/AKT pathway is related to neuronal survival, metabolic regulation, axonal growth, and anti-apoptotic responses. Neurotrophic factors, some GPCRs, and receptor tyrosine kinases can all activate this pathway.

(2) Experimental indicators

p-AKT, p-GSK3β, mTOR signaling, cell survival, neurite growth, and synaptic protein expression are commonly detected.

(3) Result interpretation

AKT activation is usually associated with enhanced survival signaling, but in tumor or abnormal proliferation contexts it may also promote pathological cell proliferation. In nervous system research, neuroprotection and abnormal proliferation should be distinguished according to cell type.

5.4 BDNF/TrkB Signaling

(1) Mechanistic core

After BDNF binds TrkB, it can activate the MAPK/ERK, PI3K/AKT, and PLCγ pathways, promoting neuronal survival, synaptic maturation, dendritic spine formation, and plasticity regulation.

(2) Experimental indicators

BDNF, TrkB phosphorylation, p-ERK, p-AKT, p-CREB, synaptic proteins, dendritic spine density, and behavioral learning and memory indicators are commonly detected.

(3) Result interpretation

Increased BDNF does not necessarily represent a purely protective effect. Depending on brain region, disease stage, and receptor status, BDNF may participate in compensatory repair or pathological excitation and epileptic sensitization.

6 Abnormal Neuronal Signaling and Disease

6.1 Epilepsy

(1) Mechanistic characteristics

Epilepsy is associated with excitation/inhibition imbalance, enhanced glutamatergic excitation, weakened GABAergic inhibition, ion channel abnormalities, Ca²⁺ overload, and neuroinflammation.

(2) Key indicators

NMDA/AMPA receptors, GABA_A receptors, KCC2, Nav channels, Ca²⁺ signaling, GFAP, Iba1, IL-1β, and TNF-α are commonly detected.

(3) Interpretation focus

Enhanced neuronal excitability should be judged together with electrophysiology, neurotransmitter receptors, ion channels, and behavioral seizures. Elevation of a single inflammatory factor cannot fully explain epileptogenesis.

6.2 Alzheimer’s Disease

(1) Mechanistic characteristics

Alzheimer’s disease involves Aβ, Tau pathology, synaptic injury, neuroinflammation, mitochondrial dysfunction, and abnormal neurotrophic signaling. γ-Secretase-related processes connect both Aβ production and Notch signaling, resulting in mechanistic complexity.

(2) Key indicators

Aβ, p-Tau, synaptic proteins, BDNF/TrkB, γ-secretase-related molecules, Notch/NICD, GFAP, Iba1, and cognitive behavior are commonly detected.

(3) Interpretation focus

Reducing Aβ is not necessarily equivalent to improving cognition. Synaptic function, neuroinflammation, neuronal survival, and behavioral results should be evaluated together.

6.3 Stroke and Ischemic Injury

(1) Mechanistic characteristics

After cerebral ischemia, insufficient energy supply leads to ion pump dysfunction, increased glutamate release, Ca²⁺ overload, oxidative stress, and inflammatory responses. Notch, PI3K/AKT, MAPK, and inflammatory pathways all participate in injury and repair processes.

(2) Key indicators

Infarct volume, neurological function score, Ca²⁺ signaling, ROS, cell death, inflammatory factors, Notch/NICD, neurogenesis, and angiogenesis indicators are commonly detected.

(3) Interpretation focus

Stroke research should distinguish the acute injury phase from the repair phase. In the acute phase, inhibiting excessive excitation and inflammation may be protective; in the recovery phase, neurogenesis, glial responses, and vascular remodeling should be emphasized.

6.4 Central Nervous System Tumors

(1) Mechanistic characteristics

Central nervous system tumors such as gliomas can use Notch, GPCR, PI3K/AKT, MAPK, and calcium signaling to maintain proliferation, invasion, stemness, and therapeutic tolerance.

(2) Key indicators

Notch/NICD, Hes1, GPCR expression, p-AKT, p-ERK, migration and invasion, tumor sphere formation, stemness markers, and therapeutic sensitivity are commonly detected.

(3) Interpretation focus

If only decreased proliferation is observed after targeting Notch or GPCRs, stemness, invasion, differentiation status, and in vivo tumor-forming capacity should be further confirmed to avoid misinterpreting general cytotoxicity as a specific pathway effect.

Table 4 Representative Products Related to Neuronal Signal Transduction

Table 5 Product Lines Related to Neural Signaling Pathways and Application Directions

The core of neuronal signal transduction is to correlate receptor recognition, ion channel activity, second messenger changes, intracellular cascades, and cell type-specific outcomes. GPCR and Notch signaling represent important directions in neuromodulation and developmental fate regulation, respectively. Together with ion channels, synaptic plasticity, glial responses, and neural stem cell states, they determine central nervous system development, homeostasis, and disease progression.

References

[1] Palczewski K, et al. Annu Rev Neurosci. 2013 Jul 8;36:139-164.

[2] Geppetti P, et al. Neuron. 2015 Nov 18;88(4):635-649.

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