Electromagnetic induction was described by Michael Faraday. This principle, formulated mathematically by James C. Maxwell, states that fluctuating magnetic fields can induce electric current in conductors placed nearby. Recently, this principle has been applied to induce electric current in the adult human brain, in a technique known as transcranial magnetic stimulation (TMS) [for review see refs 45]. A variant of TMS, in which the stimulus can be repeated for a few seconds at frequencies up to 50 Hz, is referred to as rTMS 456. rTMS is an experimental tool for stimulating neurons via brief magnetic pulses delivered by a coil placed on the scalp. rTMS can non-invasively interfere with neural functions related to a target cortical area with high temporal accuracy 456. The key features of the technique are that rTMS machine delivers a large current in a short period of time – the current in the rTMS coil then produces a magnetic field which, if changing rapidly enough, will induce an electric field sufficient to stimulate neurons or change the resting membrane potential in the underlying cortex 45678. In short, rTMS can be used to induce a transient interruption of normal brain activity in a relatively restricted area of the brain 45678. Because of these unique and powerful features, rTMS has been popular in various fields, including cognitive neuroscience and clinical application [for review see refs 4567]. However, despite its utility, the mechanisms of how rTMS stimulates neurons and interferes with neural functions are still unknown.

Animal studies have broadened our understanding of how rTMS affects brain functioning (Fig. 1). There is evidence that rTMS causes changes in neuronal circuits as reflected by behavioral changes 45678. These alterations include regional changes in the release of neurotransmitters, transsynaptic efficiency, signaling pathways and in gene transcription.

A possible mechanism by which rTMS exerts and effect is through gene induction 8910. Different studies support that rTMS modulates the expression of immediate early genes such as c-fos and c-jun 8910. These genes are activated transiently and rapidly in response to a wide variety of cellular stimuli. They represent a standing response mechanism that is activated at the transcription level in the first round of response to stimuli, before any new proteins are synthesized 8910. A single rTMS train increased c-fos mRNA in the paraventricular nucleus of the thalamus, and to a lesser extent in the frontal and cingulate cortices, but not in the parietal cortex 9. In contrast to these acute effects, 14 daily rTMS treatments increased c-fos mRNA in the parietal cortex 10. No change was seen in Brain-derived neurotrophic factor (BDNF) mRNA expression 10. Another study found that a longer treatment (5-day series separated by 2-day intervals for 11 weeks) significantly enhanced BDNF mRNA in the hippocampus and the parietal and piriform cortices 11. The BDNF is the most abundant and widely distributed neurotrophin in the CNS and plays regulatory roles in many neuronal functions including survival, neurogenesis, and synaptic plasticity. Effects on neurotrophic factors could possibly explain preliminary findings of neuroprotective and neuroplastic effects of rTMS, such as mossy fiber sprouting in the hippocampus following chronic rTMS 12. Possible effects of rTMS on neurotrophic factors might be relevant to new theories about the mechanisms of action of antidepressant medications 13.

Importantly, specific effects of rTMS on neurotransmitter system have been repeatedly reported. Acute treatment with rTMS in rodents modulates monoamine content and turnover 1415, but no effects on neurotransmitter levels or metabolites have been reported after chronic stimulation 16. Shortly after rTMS, dopamine is reported to be reduced in the frontal cortex and increased in the striatum 14 and the hippocampus 1415. Increased serotonin (5-HT) in the hippocampus was found in brain homogenates with HPLC 14, but not in an in vivo microdialysis study 15. Reductions in arginine vasopressin release and increases in taurine, aspartate, and serine were reported in the hypothalamic paraventricular nucleus with rTMS 15.

The dopaminergic system might therefore be one of the primary candidate neurotransmitter systems which are directly and selectively modulated by rTMS of frontal brain regions. It ventral tegmental area (VTA) and the substantia nigra, i.e. the regions of origin of the mesolimbic and mesostriatal dopaminergic pathways 17. These neuroanatomical connections may explain how stimulation of frontal brain regions enhances dopamine efflux in axon terminal areas originating from mesencephalic dopaminergic cell groups. Apart from the hippocampus, the ventral (i.e. nucleus accumbens) and dorsal striatum receive dense dopaminergic projections from the VTA and substantia nigra respectively, 1819 and therefore might be candidate regions for possible rTMS-induced changes in interneuronal communication. Consistent with the hypothesis that stimulation of frontal brain regions by rTMS may increase dopaminergic neurotransmission in areas other than the hippocampus, it has been reported that direct electrical stimulation of the prefrontal cortex enhances dopamine release in the dorsal striatum and nucleus accumbens 2021.

The neurochemical changes induced by the exposure to rTMS in vivo are preserved and expressed in the tissue in vitro. rTMS initiates the biosynthesis of new molecules which persist in the tissue beyond the period of stimulation 2223. The activation of a rat's brain with rTMS enhanced the magnitude of long-term potentiation (LTP) recorded in vitro 24 and made the hippocampal slices prepared from exposed brains much more resistant to ischemic damage 2526. This data suggest a protective effect of BDNF for synaptic transmission after transient forebrain ischemia.

Given the beneficial effects of rTMS in the treatment of affective disorders, the mesolimbic dopaminergic system is of particular interest as it comprises the major components of the neural circuitry of reward and incentive motivation 27. With respect to PD, activation of the mesostriatal dopaminergic pathway is likely to be a possible candidate mechanism behind the therapeutic effects of rTMS that have been reported 282930.

Several studies in humans have shown that acute and chronic rTMS treatments of the frontal cortex improve the symptoms of PD, which is thought to result from subcortical dopamine dysfunction 31. Interestingly, acute rTMS treatment of the prefrontal cortex (PFC) in humans was demonstrated by functional neuroimaging to induce a release of endogenous dopamine in the ipsilateral dorsal striatum 32. Animal studies have shown that descending pathways from the frontal cortex modulate dopamine release in subcortical areas, such as the striatum 33. Modulation of dopamine release may be relevant to the pathophysiology of PD 31.

The dogma that the adult mammalian brain is incapable of cellular self-repair has finally been overcome, when the generation of new neurons in the adult brain was described for the subventricular zone (SVZ) as well as for the dentate gyrus subgranular zone (SGZ) of the hippocampus 3343536 (Fig. 2A). The SVZ is an important germinal layer that forms during development adjacent to the telencephalic ventricular zone. This layer is most prominent in the lateral wall of the lateral ventricle facing the developing ganglionic eminences. The SVZ contains the largest pool of dividing neural progenitors/stem cells (NSCs) in the adult brain of all mammals, including humans 3343536. In rodents, the adult SVZ contains four cell types defined by their morphology, ultrastructure, molecular markers and electrophysiological properties: 1) migrating neuronal progenitor cells (type A cells), 2) SVZ protoplasmic astrocytes (type B cells), 3) transit amplifying progenitor cells (type C cells), and 4) ependymal cells (type E cells) that separated the SVZ from the ventricular cavity, whose function is to circulate the cerebrospinal fluid (Fig. 2B) 3343536. The organization of the adult human SVZ is significantly different compared to that of rodents (Fig. 2C). In adult rodents, SVZ astrocytes (Type B cells) are located next to the ependymal layer. In contrast, in the adult human brain, SVZ astrocytes are not found adjacent to the ependymal cells (Fig. 2D). Instead, the cell bodies of human SVZ astrocytes accumulate in a band or ribbon separated from the ependymal layer by a gap that is largely devoid of cells 37.

Different growth/trophic factor and cytokines have been identified in vivo as modulators of the numeric expansion and as fate determinants of NSCs, such as epidermal growth factor (EGF), transforming growth factor α (TGF-α), fibroblast growth factor (FGF-2), nerve growth factor (NGF), BDNF, glial cell line-derived neurotrophic factor (GDNF), basic fibroblast growth factor (bFGF), insulin-like growth factor-1 (IGF-1), erythropoietin (EPO) and others 3343536. These agents, however, appear to act solely as mitogens without changing the fate determination of the NSCs and their progeny.

Studies in various animal disease models have convincingly shown that the SVZ can respond to insults in the adult brain by producing new progenitor cells that can migrate to sites that have been affected by neurodegenerative pathology or brain injury 336. In response to neurodegeneration in Huntington's disease, epilepsy, multiple sclerosis and stroke, there is an upregulation of progenitor cell production, cytokine levels and migratory proteins in the SVZ, leading to an increase in the number of adult-born neurons. By contrast, in Alzheimer's disease and PD there are fewer proliferating cells in the SVZ 336.

With regard to the development of a cell replacement therapy in PD, it is intriguing to know that dopaminergic neurons are constitutively regenerated by adult neurogenesis in the olfactory bulb of the adult brain 334353637. These neurons are born in the SVZ and migrate via the rostral migratory stream to the olfactory bulb, where they integrate as interneurons in the glomerular layer 334353637. A detailed understanding of the molecular signals governing the proliferation, targeted migration and dopaminergic differentiation of these precursor cells may offer the exciting perspective that the endogenous NSCs in the SVZ may ultimately be instrumentalized for non-invasive replacement of the degenerating nigrostriatal system by autologous dopaminergic neurons 3.

Dopaminergic innervation and signalling in the SVZ profoundly increases the SVZ's proliferative capacity. A recent study used a 6-hydroxidopamine (6-OHDA) lesion model of PD, in which the medial forebrain bundle (also know as the nigrostriatal pathway) is destroyed by an excitotoxin, and measured proliferation in the SVZ 38 (Fig. 3A). The number of proliferating cells was found to have decreased by approximately 40%, as measured by bromodeoxyuridine (BrdU)-positive cells counts. Furthermore, this study demonstrated that the amount of proliferation in the SVZ after a 6-OHDA lesion was proportional to the amount or remaining dopaminergic innervation in the striatum 38. After the dopamine precursor L-DOPA was administered, normal proliferation levels were restored in the side ipsilateral to the lesion, however, on the contralateral side, L-DOPA had no effect 38. In a hypothesis-based attempt to identify such factors, intrastriatal infusion of TGFα in a rodent model of PD has been shown to induce massive proliferation and migration of cells from the SVZ toward the TGFα infusion site 39. This treatment has been reported to result in increased numbers of dopaminergic-like neurons in the striatum. In behavioral experiments, there was a significant reduction of apomorphine-induced rotations in animals receiving the TGFα. However, a more recent attempt to replicate this finding has reproduced the precursor cell immigration, but failed to show a neuronal phenotype of the newborn striatal cells 40. In this later study, intrastriatal TGFα infusion induced a significant increase in SVZ proliferation and substantial migratory waves of nestin-positive progenitor cells from the adult SVZ into the striatum of dopamine-depleted rats, but there was no dopaminergic differentiation of the newborn neurons 40. In a similar way, infusion of BDNF into the lateral ventricle of the adult rat has been reported to induce migration of SVZ-derived neuroblasts into the parenchyma of the striatum 41.

In sharp contrast, there appears to be a reduction in precursor cell proliferation in the SVZ of PD patients and experimental animals as a consequence of dopamine depletion 338. Recent studies have suggested that 60 daily rTMS treatments (60 Hz, 0.7 mT; Fig. 1) induce an in situ differentiation of SVZ-derived precursors in dopamine-producing neurons in rats with unilateral 6-OHDA lesions of the substantia nigra (Fig. 3) 4243. In behavioral experiments, there was a significant reduction of amphetamine-induced rotations in animals receiving the rTMS. The number of new dopaminergic cells was correlated with better locomotor activity (Fig. 3B). Patch-clamp studies suggested that a small percentage of SVZ-derived dopaminergic-like cells exhibited the electrophysiological properties of mature dopaminergic neurons and presented spontaneous postsynaptic potentials 43. Further work is required to identify the key molecular players regulating this phenomenon, in order to develop novel cell therapies for PD based on rTMS.

On the other hand, it has been demonstrated that cerebral ischemia increases neurogenesis both in the SGZ and in the SVZ of the adult brain 444546. This increase has been associated with the activation of the NMDA receptor 47. The neuronal precursors of the SVZ migrate to the ischemic zone of the adjacent striatum 4546 and through the rostral migratory stream (RMS) and the lateral cortical stream to the ischemic zone of the cerebral cortex where the damaged neurons are differentiated and replaced 444546. Clinical trials using low-frequency rTMS applied to the unaffected hemisphere demonstrated decreased interhemispheric inhibition of the affected hemisphere with the associated behavioral changes in stroke patients 4849. Despite there being clinical evidence suggesting that electrical cortical DC stimulation to the affected hemisphere1 and low frequency rTMS to the unaffected hemisphere can modulate the cortical excitability and produce measurable hand motor improvement in stroke patients, the efficacy of high-frequency rTMS on the corticomotor excitability and the acquisition of motor skills in chronic stroke patients is being explored 484950. Although the effect of rTMS on neurogenesis in post-stroke is not known, it is a study that should be explored.