Endogenous targeted, frequency-specific terahertz photon neuromodulation

The storage and retrieval of information encoded in the nervous system are often accompanied by morphological changes in the participating neurons. During brain development, neurite elongation and branching establish a connective network that interlinks different components of the nervous system. Neuronal outgrowth and synaptogenesis are extremely important in the hippocampus, a brain structure deeply involved in diverse cognitive functions. Thus, modulating these processes of hippocampal neurons holds significant promise for the restoration and improvement of cognitive functions, including learning and memory.

THz photons have emerged as a promising tool, garnering significant attention in biosensing and neural research. We proposed a non-drug and non-thermal THz modulation approach to enhance neuronal growth and synaptogenesis. Our research revealed the frequency-dependent effects of THz photon modulation on neurons and explored the mechanism of its long-term effects.

Neuronal growth and synapse formation induced by THz photons

Frequency screening experiments showed that 34.5 THz photon stimulation specifically and effectively promoted neurite elongation and branching, and synapse formation in primary cultured rat hippocampal neurons. Immunofluorescence analysis revealed a 17.0% increase in average branch length following exposure to 34.5 THz photons. WB frequency screening experiments showed that 34.5 THz photon stimulation increased postsynaptic density protein 95 (PSD95) expression by 26.0% compared to the control group, while THz irradiation at other frequencies did not significantly affect PSD95 expression. Taken together, these findings suggest a frequency-dependent effect of THz photons in promoting neuronal growth and synapse formation.

Modulation of cAMP signaling pathway

Exposure to 34.5 THz photons increased cAMP levels by 45.9% in primary neuron cultures, indicating that THz stimulation modulated neuronal growth through the cAMP pathway. The  results suggested the existence of a direct upstream modulation of cAMP by THz photons rather than a purely slow feedback process completely resulting from new gene expression. As the direct upstream molecule of cAMP, adenylyl cyclase (AC) catalyzes the conversion of ATP to cAMP, thereby initiating the cAMP signaling pathways. Among its family, AC1 is predominantly expressed in the brain, particularly in regions such as the hippocampus, neocortex, and cerebellar cortex. Overexpression of AC1 in HEK 293T cells resulted in a significant 25.2% elevation in cAMP levels following THz stimulation, suggesting that AC1 is the possible direct target of THz modulation.

We also performed additional experiments to explore whether THz photons can act upstream of AC1, such as the NMDA receptor or calcium ion level. The resluts indicated that THz photons do not directly target the NMDA receptor or alter calcium ion levels in neurons. Based on our experimental and simulation findings, we tentatively propose that 34.5 THz radiation increases cAMP levels by targeting AC1, thereby activating downstream signaling pathways. Although the enhancement of AC1-ATP binding by THz photons tends to be an instant and reversible effect, it may prolong through cascade amplification of cAMP signaling pathways, potentially regulating gene expression, and modulating the cytoskeleton.

Molecular dynamics simulations of AC1-ATP binding

Molecular dynamics simulations revealed that 34.5 THz photons resonate with the guanidinium groups of arginine residues in the AC1 protein’s binding pocket. This resonance drove ATP to bind more tightly towards the side containing divalent metal ions and reduced the overall binding free energy (ΔG = -243 kJ/mol). The MD simulations indicate that THz photons could promote the binding of AC1 and ATP by resonating with the binding pocket of AC1, thereby accelerating cAMP production.

Synaptogenesis and cognitive improvement

Increased dendritic spine density and cAMP levels illustrated that 34.5 THz photon stimulation could upregulate cAMP signaling and promote synaptogenesis in vivo. These molecular changes were accompanied by significant improvements in learning and memory. In all behavioral tests, mice in the sham group exhibited behaviors similar to those in the control group, while mice in the THz stimulation group demonstrated enhanced learning and memory abilities. These findings indicate that 34.5 THz photons can promote synaptogenesis and enhance cognitive performance.

In summary, we provided a non-drug and non-thermal THz photon modulation approach that can effectively promote the outgrowth and synaptogenesis of hippocampal neurons. Our study elucidates the underlying mechanisms of THz photon modulation, revealing its interaction with AC1 and subsequent activation of the cAMP signaling pathway. Importantly, in vivo experiments demonstrated the translational potential of THz photon stimulation to improve cognition, as evidenced by improved learning and memory abilities in animals. Accordingly, THz photon modulation may serve as a potential physical treatment method, and these findings provide a theoretical foundation for future clinical applications of this technology.