Amplification of evoked potentials recorded from mouse hippocampal slices by very low repetition rate pulsed magnetic fields

A little goes a long way: Neurobiological effects of low intensity rTMS and implications for mechanisms of rTMS

Repetitive transcranial magnetic stimulation (rTMS) is a widespread technique in neuroscience and medicine, however its mechanisms are not well known. In this review, we consider intensity as a key therapeutic parameter of rTMS, and review the studies that have examined the biological effects of rTMS using magnetic fields that are orders of magnitude lower that those currently used in the clinic. We discuss how extensive characterisation of "low intensity" rTMS has set the stage for translation of new rTMS parameters from a mechanistic evidence base, with potential for innovative and effective therapeutic applications. Low-intensity rTMS demonstrates neurobiological effects across healthy and disease models, which include depression, injury and regeneration, abnormal circuit organisation, tinnitus etc. Various short and long-term changes to metabolism, neurotransmitter release, functional connectivity, genetic changes, cell survival and behaviour have been investigated and we summarise these key changes and the possible mechanisms behind them. Mechanisms at genetic, molecular, cellular and system levels have been identified with evidence that low-intensity rTMS and potentially rTMS in general acts through several key pathways to induce changes in the brain with modulation of internal calcium signalling identified as a major mechanism. We discuss the role that preclinical models can play to inform current clinical research as well as uncover new pathways for investigation.

Planar coil-based contact-mode magnetic stimulation: synaptic responses in hippocampal slices and thermal considerations

Implantable magnetic stimulation is an emerging type of neuromodulation using coils that are small enough to be implanted in the brain. A major advantage of this method is that stimulation performance could be sustained even though the coil is encapsulated by gliosis due to foreign body reactions. Magnetic fields can induce indirect electric fields and currents in neurons. Compared to transcranial magnetic stimulation, the coil size used in implantable magnetic stimulation can be greatly reduced. However, the size reduction is accompanied by an increase in coil resistance. Hence, the coil could potentially damage neurons from the excess heat generated. Therefore, it is necessary to study the stimulation performance and possible thermal damage by implantable magnetic stimulation. Here, we devised contact-mode magnetic stimulation (CMS), wherein magnetic stimulation was applied to hippocampal slices through a customized planar-type coil underneath the slice in the contact mode. With acute hippocampal slices, we investigated the synaptic responses to examine the field excitatory postsynaptic responses of CMS and the temperature rise during CMS. A long-lasting synaptic depression was exhibited in the CA1 stratum radiatum after CMS, while the temperature remained in a safe range so as not to seriously affect the neural responses.

A consensus panel review of central nervous system effects of the exposure to low-intensity extremely low-frequency magnetic fields

BACKGROUND

A large number of studies explored the biological effects of extremely low-frequency (0-300 Hz) magnetic fields (ELF-MFs) on nervous system both at cellular and at system level in the intact human brain reporting several functional changes. However, the results of different studies are quite variable and the mechanisms of action of ELF-MFs are still poorly defined. The aim of this paper is to provide a comprehensive review of the effects of ELF-MFs on nervous system.

METHODS

We convened a workgroup of researchers in the field to review and discuss the available data about the nervous system effects produced by the exposure to ELF-MFs.

MAIN FINDINGS/DISCUSSION

We reviewed several methodological, experimental and clinical studies and discussed the findings in five sections. The first section analyses the devices used for ELF-MF exposure. The second section reviews the contribution of the computational methods and models for investigating the interaction between ELF-MFs and neuronal systems. The third section analyses the experimental data at cellular and tissue level showing the effects on cell membrane receptors and intracellular signaling and their correlation with neural stem cell proliferation and differentiation. The fourth section reviews the studies performed in the intact human brain evaluating the changes produced by ELF-MFs using neurophysiological and neuropsychological methods. The last section shows the limits and shortcomings of the available data, evidences the key challenges in the field and tracks directions for future research.

Clozapine functions through the prefrontal cortex serotonin 1A receptor to heighten neuronal activity via calmodulin kinase II-NMDA receptor interactions

Aberrant dopamine release in the prefrontal cortex (PFC) is believed to underlie schizophrenia, but the mechanistic pathway through which a widely used antipsychotic, clozapine (Clz), evokes neurotransmitter-releasing electrical stimulation is unclear. We analyzed Clz-evoked regulation of neuronal activity in the PFC by stimulating axons in layers IV and V and recording the electrical effect in the post-synaptic pyramidal cells of layers II and III. We observed a Clz-evoked increase in population spike (PS), which was mediated by serotonin 1A receptor (5-HT(1A)-R), phospholipase Cβ, and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Immunoblotting demonstrated that the Clz-activation of CaMKII was 5-HT(1A)-R-mediated. Intriguingly, the NMDA receptor (NMDA-R) antagonist (±)2-amino-5-phosphonovaleric acid (APV) eliminated the Clz-mediated increase in PS, suggesting that the 5-HT(1A)-R, NMDA-R and CaMKII form a synergistic triad, which boosts excitatory post-synaptic potential (EPSP), thereby enhancing PS. In corroboration, Clz as well as NMDA augmented field EPSP (fEPSP), and WAY100635 (a 5-HT(1A)-R antagonist), APV, and a CaMKII inhibitor eliminated this increase. As previously shown, CaMKII binds to the NMDA-R 2B (NR2B) subunit to become constitutively active, thereby inducing α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor recruitment to the post-synaptic membrane and an increase in fEPSP. Co-immunoprecipitation demonstrated that Clz potentiates interactions among CaMKII, NR2B, and 5-HT(1A)-R, possibly in the membrane rafts of the post-synaptic density (PSD), because pretreatment with methyl-β-cyclodextrin (MCD), an agent that disrupts rafts, inhibited both co-immunoprecipitation as well as fEPSP. In summary, Clz functions in the PFC by orchestrating a synergism among 5-HT(1A)-R, CaMKII, and NMDA-R, which augments excitability in the PFC neurons of layers II/III.

The influence of pulsed magnetic fields (PMFs) on nonsynaptic potentials recorded from the central and peripheral nervous systems in vitro

The influence of pulsed magnetic fields (PMFs) on nonsynaptic potentials recorded from the central and peripheral nervous system in vitro has been investigated. The population spikes (PSs) recorded from hippocampal slices during antidromic stimulation and compound action potentials (CAPs) recorded from the segments of the sciatic nerve were used as indicators of neuronal activity. The potentials recorded from both preparations were significantly and permanently enhanced following PMF (0.16 Hz, 15 mT) exposure. The increase in the antidromic PS occurred even in the presence of potassium channel blocker tetraethylammonium (TEA) and was accompanied by multiple spiking. Among all frequencies of PMF tested (0.5, 0.16, 0.07, 0.03, 0.0 Hz), the frequency of 0.5 Hz was the most effective in enhancement of potential amplitude. The influence of PMF on the amplitude of two CAPs evoked by the pair of electrical stimuli applied in rapid succession has also been evaluated. In control conditions the potential triggered by the second stimuli was slightly smaller expressing the phenomenon of short-term depression (STD). Although PMF exposure amplified the amplitude of both potentials, the increase in the size of the first potential was significantly greater increasing further the magnitude of STD. The blocking of potassium channels reversed STD into facilitation. One of the possible mechanisms involved in PMF action could be the modification of the axonal threshold, which was significantly reduced following exposure to PMF.

Does exposure to extremely low frequency magnetic fields produce functional changes in human brain?

Behavioral and neurophysiological changes have been reported after exposure to extremely low frequency magnetic fields (ELF-MF) both in animals and in humans. The physiological bases of these effects are still poorly understood. In vitro studies analyzed the effect of ELF-MF applied in pulsed mode (PEMFs) on neuronal cultures showing an increase in excitatory neurotransmission. Using transcranial brain stimulation, we studied noninvasively the effect of PEMFs on several measures of cortical excitability in 22 healthy volunteers, in 14 of the subjects we also evaluated the effects of sham field exposure. After 45 min of PEMF exposure, intracortical facilitation produced by paired pulse brain stimulation was significantly enhanced with an increase of about 20%, while other parameters of cortical excitability remained unchanged. Sham field exposure produced no effects. The increase in paired-pulse facilitation, a physiological parameter related to cortical glutamatergic activity, suggests that PEMFs exposure may produce an enhancement in cortical excitatory neurotransmission. This study suggests that PEMFs may produce functional changes in human brain.

Synaptosome behaviour is unaffected by weak pulsed electromagnetic fields

The present study examined the effect on rat cortical synaptosomes of a 2 h exposure to 50-Hz electromagnetic fields (EMFs) with a peak magnetic field of 2 mT. We measured modifications of synaptosomal mitochondrial respiration rate, ATP production, membrane potential, intrasynaptosomal Ca(2+) concentration and free iron release. The O(2) consumption remained unvaried in exposed synaptosomes at about 2 nM O(2)/min/mg proteins; ATP production was also unchanged. The intrasynaptosomal Ca(2+) concentration decreased slowly and there was a slight, but non-significant, depolarisation of the synaptosomal membrane. Finally, the free iron release by synaptosomal suspensions, a useful predictor of neuro-developmental outcome, remained unchanged after EMF exposure. On the whole, our results indicate that the physiological behaviour of cortical synaptosomes is not affected by weak pulsed EMFs.

Plasticity of 5‐HT1Areceptor‐mediated signaling during early postnatal brain development

The presence of serotonin 1A receptor (5-HT(1A)-R) in the hippocampus, amygdala, and most regions of the frontal cortex is essential between postnatal day-5-21 (P5-21) for the expression of normal anxiety levels in adult mice. Thus, the 5-HT(1A)-R plays a crucial role in this time window of brain development. We show that the 5-HT(1A)-R-mediated stimulation of extracellular signal-regulated kinases 1 and 2 (Erk1/2) in the hippocampus undergoes a transition between P6 and P15. At P6, a protein kinase C (PKC) isozyme is required for the 5-HT(1A)-R -->Erk1/2 cascade, which causes increased cell division in the dentate gyrus. By contrast, at P15, PKC alpha participates downstream of Erk1/2 to augment synaptic transmission through the Schaffer Collateral pathway but does not cause increased cell division. Our data demonstrate that the 5-HT(1A)-R -->Erk1/2 cascade uses PKC isozymes differentially, first boosting the cell division to form new hippocampal neurons at P6 and then undergoing a plastic change in mechanism to strengthen synaptic connections in the hippocampus at P15.

Modulation of learning and hippocampal, neuronal plasticity by repetitive transcranial magnetic stimulation (rTMS)

The influence of high-frequency repetitive transcranial magnetic stimulation (rTMS) on learning process in mice and on neuronal excitability of the hippocampal tissue obtained from stimulated animals were investigated. While the stimulation with rTMS at higher frequency (15 Hz) improved animals' performance in novel object recognition test (NOR), lower frequency (1 and 8 Hz) impaired the memory. The effect was observed when evaluated immediately after rTMS exposure and declined with time. In parallel to the results of behavioral test, there was a significant enhancement of the synaptic efficiency expressed as of the long-term potentiation (LTP) recorded from hippocampal slices prepared from the animals exposed to 15 Hz rTMS. The stimulation with 1 and 8 Hz had no influence on the magnitude of LTP. Our results demonstrate that rTMS modifies mechanisms involved in memory formation. The effects of rTMS in vivo are preserved and expressed in the hippocampus tested in vitro.

Modification of the synaptic glutamate turnover in the hippocampal tissue exposed to low-frequency, pulsed magnetic fields

The influence of pulsed magnetic fields (PMF) on the release and uptake of glutamate was investigated. While the release was examined using hippocampal slices, synaptosomes were chosen to characterize the uptake process. (3)H-D-aspartate was used as a marker of glutamergic transmission. The pulsed magnetic fields (9-15 mT) applied according to the pattern which induced epileptic discharges in hippocampus amplified and attenuated the release and uptake of glutamate, respectively. However, the magnetic fields which induced an increase in neuronal excitability without concomitant seizures amplified both processes. These results confirm our previous reports and indicate that the glutamergic synapses are the target of magnetic fields action.

Neurobehavioural effects of electromagnetic fields

Very few laboratory studies in children have explored the effects of exposure to low level electromagnetic fields (EMFs) on neurobehavioural function. Studies investigating effect on neurotransmitters, cognitive function and brain activity in adults and animals indicate that acute exposure to EMFs does not appear to engender any consistent physiological or behavioural impairment although a few subtle effects may occur. This suggests that exposure of children to low level EMFs may not cause significant detrimental effects on brain function. However the available evidence is not sufficient to draw any definite conclusions, and further laboratory studies are required. In particular, experiments investigating the effects of radiofrequency (RF) fields on the performance of well-characterised cognitive and behavioural tasks by immature and developing animals are recommended, if studies with children cannot be performed for ethical and practical reasons.