Synaptic plasticity – the biological basis for lasting TMS effects
One of the defining characteristics of repetitive transcranial magnetic stimulation (rTMS) is that its effects can persist well beyond the stimulation period [1]. These sustained effects are thought to arise from synaptic plasticity, the brain’s ability to modify the strength and organization of neural connections in response to activity or external stimulation [2].
Synaptic plasticity occurs over different timescales. Short-term synaptic plasticity (STSP) supports rapid information processing and is primarily mediated by transient changes in neurotransmitter release [3, 4]. In contrast, long-term synaptic plasticity (LTSP) involves more persistent structural and molecular adaptations, including dendritic spine remodeling, and forms the biological basis for learning, memory and long-term behavioural adaptation [4].
Traditionally, LTSP has been described within the framework of long-term potentiation (LTP) and long-term depression (LTD), which represent increases and decreases in synaptic efficacy, respectively [5, 6]. However, synaptic plasticity extends beyond this simplified model. Hebbian plasticity describes the activity-dependent strengthening of synaptic connections between neurons that are repeatedly activated together, often summarized as “cells that fire together, wire together” [7]. To prevent excessive excitation or inhibition, homeostatic plasticity acts as a counterbalance by globally adjusting synaptic strength to maintain network stability [8]. In addition, meta plasticity regulates the threshold for future potentiation or depression, thereby influencing how neurons respond to subsequent stimulation [8, 9].
Although these concepts describe the functional outcome of plasticity, the underlying biological response involves multiple interconnected molecular signaling pathways that collectively regulate neuronal communication, structural remodeling and long-term adaptation.
Glutamatergic signaling and calcium influx – initiating synaptic plasticity
Synaptic plasticity results from the coordinated interaction of calcium (Ca²⁺) signaling, neurotransmitter release, gene expression, protein synthesis and neuron–glia interactions [2, 3, 11]. Among these processes, glutamatergic neurotransmission plays a central role.
Glutamate is the principal excitatory neurotransmitter in the central nervous system and exerts its effects primarily through NMDA and AMPA receptors. NMDA receptors (NMDARs) are particularly important because their activation allows calcium ions (Ca²⁺) to enter the neuron during membrane depolarization. Rather than simply altering neuronal excitability, calcium functions as a second messenger that initiates intracellular signaling cascades required for the induction of long-term plasticity [12, 13].
AMPA receptors (AMPARs) mediate fast excitatory synaptic transmission. Changes in the trafficking and clustering of AMPA receptors at the postsynaptic membrane determine how strongly neurons respond to glutamate and therefore contribute to the expression of synaptic strengthening or weakening [6].
Evidence from fundamental neuroscience supports the involvement of these pathways. Gandolfi et al. used electrical theta-burst stimulation in acute cerebellar slices and observed changes in CREB phosphorylation, Creb and c-Fos transcription, and c-FOS protein expression. These changes were not observed in the presence of an NMDA-receptor antagonist. This provides evidence for NMDA-dependent plasticity in an experimental slice model, rather than direct evidence of a TMS mechanism [14]. Separately, magnetic-stimulation experiments in rat models identified changes in gene expression across several pathways, including glutamatergic and GABAergic signaling, ion channels and mitochondrial function [15].
BDNF–TrkB signaling – a central regulator of neuroplasticity
Beyond glutamatergic signaling, one of the most extensively studied molecular pathways involved in TMS-induced plasticity is brain-derived neurotrophic factor (BDNF) signaling. BDNF belongs to a family of proteins known as neurotrophins, which support neuronal survival, differentiation, dendritic growth and synapse formation. Because BDNF is released in an activity-dependent manner, it is considered one of the key regulators of learning and long-term synaptic adaptation. Following release, BDNF binds to its high-affinity receptor, tropomyosin receptor kinase B (TrkB), initiating intracellular signaling pathways that regulate gene transcription, protein synthesis, neuronal survival and structural remodeling [16,17].
Multiple preclinical studies have demonstrated activation of the BDNF pathway following rTMS. High-frequency stimulation increased BDNF expression together with activation of ERK signaling, whereas lower-frequency stimulation produced more limited effects [18]. Similarly, chronic 15 Hz rTMS in a rat model of depression increased hippocampal BDNF protein levels and ERK1/2 phosphorylation, with effects still detectable two weeks after treatment ended [19].
Additional evidence has been reported in disease models. In experimental Alzheimer’s disease, prolonged 10 Hz stimulation increased BDNF expression together with enhanced cholinergic signaling [20]. In a model of ischemic stroke, repeated 10 Hz stimulation increased BDNF–TrkB signaling, activated downstream Akt signaling and increased the Bcl-2/Bax ratio, suggesting simultaneous activation of neuroplasticity and neuronal survival pathways [21].
Kim and colleagues showed that both 1 Hz and 10 Hz magnetic stimulation increased phosphorylation of several signaling proteins, including ERK, Akt, STAT proteins and CREB, in neuronal cell models. Responses were generally stronger and persisted longer after 10 Hz stimulation under the tested conditions [22]. Wang and colleagues reported enhanced BDNF–TrkB signaling and increased association between TrkB and NMDA receptors following repeated 5 Hz rTMS. In rats, these signaling changes occurred without increased BDNF or TrkB protein expression in the examined brain regions. In healthy human participants, signaling changes were measured in peripheral lymphocytes, rather than directly in the brain [23].
Intracellular signaling pathways – translating neuronal activity into long-term adaptation
Activation of NMDA receptors and BDNF–TrkB signaling initiates several intracellular signaling pathways that convert transient neuronal activity into long-lasting cellular adaptations.
One of the best-characterized pathways is ERK (extracellular signal-regulated kinase), which transmits signals from the cell membrane to the nucleus and regulates activity-dependent gene transcription. Another important pathway is PI3K/Akt, which promotes neuronal growth, metabolism and survival while suppressing apoptotic signaling. Downstream of Akt lies mTOR (mechanistic target of rapamycin), a master regulator of protein synthesis that enables the production of proteins required for synaptic remodeling. Additional signaling molecules, including STAT3 and STAT5, contribute to cellular differentiation and survival, while the transcription factor CREB regulates the expression of genes involved in long-term plasticity [18,19,22,24].
Preclinical magnetic-stimulation studies have reported changes in several of these pathways. Findings include increased BDNF levels and ERK activation in selected experimental models [18,19], and increased phosphorylation of ERK, Akt, STAT proteins and CREB in neuronal cells [22]. In a trimethyltin-induced model of hippocampal neurodegeneration, iTBS increased expression of the PI3K regulatory subunit p85, restored Akt and mTOR phosphorylation, and increased ERK1/2 phosphorylation. The authors proposed that BDNF–TrkB signaling might contribute to these effects, but did not directly establish that mechanism [24].
Collectively, these intracellular pathways provide the molecular link between neuronal activation during stimulation and the longer-lasting structural changes that characterize synaptic plasticity.
Gene expression and structural remodeling
Long-term consolidation of some forms of synaptic plasticity involves changes in gene expression and protein synthesis. However, early phases of plasticity can also involve modification and redistribution of existing proteins [3,5].
Magnetic-stimulation experiments have reported changes in plasticity-related genes and proteins in specific experimental models. Kim et al. measured changes in immediate early genes, including c-Fos and Arc, alongside phosphorylation of signaling proteins such as CREB [22]. Ismail et al. reported transient changes in several proteins, including BDNF and synapsin-1, after iTBS in a neuronal cell line [26]. Separately, Podda et al. demonstrated effects on Bdnf regulation and synaptic plasticity using anodal tDCS in mice. That study provides evidence about electrical stimulation, rather than TMS [25].
Transcriptomic studies further demonstrate that repeated magnetic stimulation alters genes involved in GABAergic, glutamatergic, cholinergic and calcium signaling, together with receptor subunits, ion channels and mitochondrial proteins [15]. In a Neuro-2a cell model, iTBS produced time-dependent changes in receptor-related and plasticity-associated proteins. BDNF and the phosphorylated-to-total TrkB ratio increased at 30 minutes, but these changes were not sustained at later assessment points [26].Additional studies have shown increased expression of glutamatergic receptor subunits, activity-dependent transcription factors and genes associated with long-term synaptic remodeling following iTBS [27].
Together, these findings suggest that magnetic stimulation induces widespread transcriptional changes that extend beyond individual signaling pathways and support long-term structural remodeling of neural circuits.
Glial cells, inflammation and neuronal survival
Neuroplasticity is increasingly recognized as a process involving not only neurons but also glial cells, including astrocytes and microglia. Astrocytes contribute to synaptogenesis, neurotransmitter homeostasis and metabolic support, whereas microglia participate in synaptic pruning and regulation of the neuroimmune environment [28–33].
In a rat model of cerebral ischemia/reperfusion injury, 28 days of iTBS reduced astrocyte activation and markers associated with pro-inflammatory M1 microglia, while increasing markers associated with M2 microglia. These changes occurred alongside reduced neuronal apoptosis and improved neurological outcomes. The findings suggest that modulation of the inflammatory environment may contribute to recovery in this experimental injury model [34].
Several rTMS studies similarly reported increased Bcl-2/Bax ratios, indicating a shift towards neuronal survival following stimulation [21,22,35]. While these findings primarily originate from models of stroke and neurodegenerative disease, they suggest that pathways regulating apoptosis and neuroprotection may overlap with those responsible for synaptic remodeling.
Summary
Current evidence suggests that TMS-induced neuroplasticity cannot be explained by a single biological mechanism. Instead, magnetic stimulation appears to engage a complex network of interconnected pathways involving glutamatergic neurotransmission, calcium signaling, BDNF–TrkB signaling, intracellular kinase cascades, gene transcription, protein synthesis, glial regulation and neuronal survival. These pathways converge to produce long-lasting changes in synaptic strength and neural network organization.
Importantly, much of the mechanistic evidence remains preclinical, and direct confirmation of these complete molecular cascades in the human brain is still limited. Nevertheless, the consistency of findings across cellular and animal studies provides a biologically plausible framework for understanding the sustained effects of TMS and guiding future translational research.
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