Human cortical excitability can be modified by repetitive transcranial magnetic stimulation (rTMS), but the cellular mechanisms are largely unknown. Here, we show that the pattern of delivery of theta-burst stimulation (TBS) (continuous versus intermittent) differently modifies electric activity and protein expression in the rat neocortex. Intermittent TBS (iTBS), but not continuous TBS (cTBS), enhanced spontaneous neuronal firing and EEG gamma band power. Sensory evoked cortical inhibition increased only after iTBS, although both TBS protocols increased the first sensory response arising from the resting cortical state. Changes in the cortical expression of the calcium-binding proteins parvalbumin (PV) and calbindin D-28k (CB) indicate that changes in spontaneous and evoked cortical activity following rTMS are in part related to altered activity of inhibitory systems. By reducing PV expression in the fast-spiking interneurons, iTBS primarily affected the inhibitory control of pyramidal cell output activity, while cTBS, by reducing CB expression, more likely affected the dendritic integration of synaptic inputs controlled by other classes of inhibitory interneurons. Calretinin, the third major calcium-binding protein expressed by another class of interneurons was not affected at all. We conclude that different patterns of TBS modulate the activity of inhibitory cell classes differently, probably depending on the synaptic connectivity and the preferred discharge pattern of these inhibitory neurons.

Modified cortical excitability following repetitive transcranial magnetic stimulation (rTMS) may be related to short- or long-term synaptic plasticity of neuronal excitation but could also affect cortical inhibition. Therefore, in the rat we tested how three different rTMS protocols, intermittent and continuous theta-burst (iTBS, cTBS), and low-frequency 1 Hz stimulation, change the expression of GAD65, GAD67 and GAT-1 which are expressed in cortical inhibitory interneurons in an activity-dependent manner. Acutely (2 h), all protocols reduced the expression of GAD67 in frontal, motor, somatosensory and visual cortex but increased that of GAD65 and GAT-1 to different degree, with iTBS having the strongest acute effect. The initial decrease in GAD67 reversed after 1 day, leading to a strong increase in GAD67 expression for up to 7 days primarily in the frontal cortex in case of iTBS, cTBS and in all studied areas following 1 Hz rTMS. While also GAD65 and GAT-1 expression reversed after 1 day in case of iTBS and cTBS, 1 Hz rTMS induced a steady increase in GAD65 and GAT-1 expression during the 7 days investigated. Our data demonstrate that rTMS affects the expression of activity-dependent proteins of the cortical inhibitory interneurons. Besides common effects of low- (1 Hz) and high-frequency (TBS) stimulation on protein expression, differences in quantity and time course of changes point to differences in the contribution of possible neuronal subsystems. Further studies are needed to distinguish cell-type specific effects.

Transcranial magnetic stimulation (TMS) has become a popular method to non-invasively stimulate the human brain. The opportunity to modify cortical excitability with repetitive stimulation (rTMS) has especially gained interest for its therapeutic potential. However, details of the cellular mechanisms of the effects of rTMS are scarce. Currently favoured are long-term changes in the efficiency of excitatory synaptic transmission, with low-frequency rTMS depressing it, but high-frequency rTMS augmenting. Only recently has modulation of cortical inhibition been considered as an alternative way to explain lasting changes in cortical excitability induced by rTMS. Adequate animal models help to highlight stimulation-induced changes in cellular processes which are not assessable in human rTMS studies. In this review article, we summarize findings obtained with our rat models which indicate that distinct inhibitory cell classes, like the fast-spiking cells characterized by parvalbumin expression, are most sensitive to certain stimulation protocols, e.g. intermittent theta burst stimulation. We discuss how our findings can support the recently suggested models of gating and homeostatic plasticity as possible mechanisms of rTMS-induced changes in cortical excitability. Abbreviations CaBP, calcium-binding protein; CB, calbindin; CR, calretinin; CSP, cortical silent period; c-Fos, cellular DNA-binding proteins encoded by the c-fos genes; FS, fast-spiking cell; GAD65/GAD67, 65 kD and 67 kD isoforms of the glutamic acid decarboxylase; GAT-1, GABA transporter 1; LICI, long-interval cortical inhibition; LTD, long-term depression; LTP, long-term potentiation; MEP, motor evoked potential; MUA, multi-unit activity; PPAI, paired-pulse afferent inhibition; PV, parvalbumin; SEP, somatosensory evoked potential; SICI, short-interval cortical inhibition; rTMS, repetitive transcranial magnetic stimulation; TBS, theta-burst stimulation; cTBS, continuous TBS; iTBS, intermittent TBS; zif268, zinc finger transcription factor.Klaus Funke finished studying biology at the Ruhr-University in Bochum (RUB) with a diploma (1983) and doctoral thesis work (1988) on the central somatosensory system of pigeons. As a postdoc he joined the group of Ulf Eysel (Dept Neurophysiology, Medical School, RUB) studying the state-dependent modulation of retinogeniculate signal transmission (habilitation in physiology 1996). Since then he has built up a research team working on visual cortex plasticity and, currently, on the cellular mechanisms of transcranial magnetic stimulation. Alia Benali studied biology at the RUB. Her diploma (1996) and doctoral thesis (2001) were focused on somatosensory cortex plasticity in rats, while her postdoc studies (Ulf Eysel, Dept Neurophysiology) were devoted to lesion-induced visual cortex plasticity and the cellular effects of transcranial magnetic stimulation. In 2010 she joined the groups of Cornelius Schwarz (Werner Reichardt Centre for Integrative Neuroscience, University of Tübingen) and Jason Kerr (MPI for Bio...

Repetitive transcranial magnetic stimulation (rTMS) has been shown to alter cortical excitability depending on the stimulus-frequency used, with high frequency (5 Hz and higher) increasing it but low frequency (usually 1 Hz or lower) reducing it. To determine the efficiency of different rTMS protocols in inducing cortical network activity, we tested the acute effect of one low-frequency rTMS protocol (1 Hz) and two different high-frequency protocols (10 Hz and intermittent theta-burst stimulation, iTBS) on the expression of the two immediate early gene (IEG) proteins c-Fos and zif268 in the rat brain. The cortical expression of both IEGs was specifically changed in an rTMS-dependent manner. One and 10 Hz rTMS enhanced c-Fos protein expression in all cortical areas tested, while iTBS was effective only in limbic cortices. Zif268 expression was increased in almost all cortical areas after iTBS, while 10 Hz rTMS was effective only in the primary motor and sensory cortices. One Hertz rTMS had no effect on cortical zif268 expression. Furthermore, sham-rTMS had no effect on zif268 expression but increased c-Fos in limbic cortices. This is the first study demonstrating that cortical zif268 and c-Fos expression can be specifically modulated by acute rTMS depending on the pattern of stimulation applied.

No abstract

Repetitive transcranial magnetic stimulation (rTMS) can modulate cortical excitability in a stimulus-frequency-dependent manner. Two kinds of theta burst stimulation (TBS) [intermittent TBS (iTBS) and continuous TBS (cTBS)] modulate human cortical excitability differently, with iTBS increasing it and cTBS decreasing it. In rats, we recently showed that this is accompanied by changes in the cortical expression of proteins related to the activity of inhibitory neurons. Expression levels of the calcium-binding protein parvalbumin (PV) and of the 67-kDa isoform of glutamic acid decarboxylase (GAD67) were strongly reduced following iTBS, but not cTBS, whereas both increased expression of the 65-kDa isoform of glutamic acid decarboxylase. In the present study, to investigate possible functional consequences, we applied iTBS and cTBS to rats learning a tactile discrimination task. Conscious rats received either verum or sham rTMS prior to the task. Finally, to investigate how rTMS and learning effects interact, protein expression was determined for cortical areas directly involved in the task and for those either not, or indirectly, involved. We found that iTBS, but not cTBS, improved learning and strongly reduced cortical PV and GAD67 expression. However, the combination of learning and iTBS prevented this effect in those cortical areas involved in the task, but not in unrelated areas. We conclude that the improved learning found following iTBS is a result of the interaction of two effects, possibly in a homeostatic manner: a general weakening of inhibition mediated by the fast-spiking interneurons, and re-established activity in those neurons specifically involved in the learning task, leading to enhanced contrast between learning-induced and background activity.

The anterior cingulate cortex (ACC) represents a phylogenetically ancient region of the mammalian brain that has undergone recent adaptive changes in humans. It contains a large spindle-shaped cell type, referred to as von Economo neuron (VEN) that has been shown to be involved in the pathophysiology of various neuropsychiatric disorders. Schizophrenia is a group of disorders that is, in part, characterised by a disruption of neuronal migration in early ontogeny and presumably secondary degeneration after the first psychotic episode in some patients. Accordingly, we tested the hypothesis that the density of VENs is reduced in a neurodevelopmental subtype of schizophrenia, which we defined by an early onset of the disorder. The density of VENs was estimated in layer Vb of Brodmann's area 24 in 20 subjects diagnosed with schizophrenia. The results were compared with 19 specimens from patients with bipolar disorder as a clinical control and 22 non-psychiatric samples. The density of VENs did not differ between the three groups. However, the VEN density in the right ACC correlated with the age at onset, and inversely with the duration of the illness in schizophrenia, but not in bipolar disorder. Thus, patients with early onset schizophrenia (and longer duration of illness) had a reduced VEN density. Age, sex, postmortem interval, brain weight, and cortical thickness had no significant impact on the results. These findings suggest that VENs in the ACC are involved in neurodevelopmental and perhaps neurodegenerative processes specific to schizophrenia.

Transcranial magnetic stimulation (TMS) is a widely used non-invasive tool to study and modulate human brain functions. However, TMS-evoked activity of individual neurons has remained largely inaccessible due to the large TMS-induced electromagnetic fields. Here, we present a general method providing direct in vivo electrophysiological access to TMS-evoked neuronal activity 0.8–1 ms after TMS onset. We translated human single-pulse TMS to rodents and unveiled time-grained evoked activities of motor cortex layer V neurons that show high-frequency spiking within the first 6 ms depending on TMS-induced current orientation and a multiphasic spike-rhythm alternating between excitation and inhibition in the 6–300 ms epoch, all of which can be linked to various human TMS responses recorded at the level of spinal cord and muscles. The advance here facilitates a new level of insight into the TMS-brain interaction that is vital for developing this non-invasive tool to purposefully explore and effectively treat the human brain.