Exploring Cortical Plasticity and Oscillatory Brain Dynamics via Transcranial Magnetic Stimulation and Resting-State Electroencephalogram

Transcranial magnetic stimulation (TMS) is a non-invasive, non-pharmacological technique that is able to modulate cortical activity beyond the stimulation period. The residual aftereffects are akin to the plasticity mechanism of the brain and suggest the potential use of TMS for therapy. For years, TMS has been shown to transiently improve symptoms of neuropsychiatric disorders, but the underlying neural correlates remain elusive. Recently, there is evidence that altered connectivity of brain network dynamics is the mechanism underlying symptoms of various neuropsychiatric illnesses. By combining TMS and electroencephalography (EEG), the functional connectivity patterns among brain regions, and the causal link between function or behaviour and a specific brain region can be determined. Nonetheless, the brain network connectivity are highly complex and involve the dynamics interplay among multitude of brain regions. In this review article, we present previous TMS-EEG co-registration studies, which explore the functional connectivity patterns of human cerebral cortex. We argue the possibilities of neural correlates of long-term potentiation/ depression (LTP-/LTD)-like mechanisms of synaptic plasticity that drive the TMS aftereffects as shown by the dissociation between EEG and motor evoked potentials (MEP) cortical output. Here, we also explore alternative explanations that drive the EEG oscillatory modulations post TMS. The precise knowledge of the neurophysiological mechanisms underlying TMS will help characterise disturbances in oscillatory patterns, and the altered functional connectivity in neuropsychiatric illnesses.

Long-term Treatment with Oriental Medicinal Herb Artemisia princeps Alters Neuroplasticity in a Rat Model of Ovarian Hormone Deficiency

Artemisia princeps (AP) is a flowering perennial used as a traditional medicine and dietary supplement across East Asia. No study has yet assessed its effects on synaptic plasticity in hippocampus and much less in a model of ovarian hormone deficiency. We examined the influence of chronic oral AP ethanol extract treatment in ovariectomized rats on the induction of long-term depression in a representative synapse (CA3-CA1) of the hippocampus. Ovariectomized rats demonstrated lower trabecular mean bone mineral densities than sham, validating the establishment of pathology. Against this background of pathology, AP-treated ovariectomized rats exhibited attenuated long-term depression (LTD) in CA1 relative to water-treated controls as measured by increased field excitatory post-synaptic potentials (fEPSP) activation averages over the post-stimulation period. While pathological significance of long-term depression (LTD) in ovariectomized rats is conflicting, that AP treatment significantly affected its induction offers justification for further study of its influences on plasticity and its related disorders.

Theta-burst Transcranial Magnetic Stimulation Alters the Functional Topography of the Cortical Motor Network

Background: Transcranial magnetic stimulation (TMS) is a non-invasive tool that is able to modulate the electrical activity of the brain depending upon its protocol of stimulation. Theta burst stimulation (TBS) is a high-frequency TMS protocol that is able to induce prolonged plasticity changes in the brain. The induction of plasticity-like effects by TBS is useful in both experimental and therapeutic settings; however, the underlying neural mechanisms of this modulation remain unclear. The aim of this study was to investigate the effects of continuous TBS (cTBS) on the intrahemispheric and interhemispheric functional connectivity of the resting and active brain. Methods: A total of 26 healthy humans were randomly divided into two groups that received either real cTBS or sham (control) over the left primary motor cortex. Surface electroencephalogram (EEG) was used to quantify the changes of neural oscillations after cTBS at rest and after a choice reaction time test. The cTBS-induced EEG oscillations were computed using spectral analysis of event-related coherence (ERCoh) of theta (4–7.5 Hz), low alpha (8–9.5 Hz), high alpha (10–12.5 Hz), low beta (13–19.5 Hz), and high beta (20–30 Hz) brain rhythms. Results: We observed a global decrease in functional connectivity of the brain in the cTBS group when compared to sham in the low beta brain rhythm at rest and high beta brain rhythm during the active state. In particular, EEG spectral analysis revealed that high-frequency beta, a cortically generated brain rhythm, was the most sensitive band that was modulated by cTBS. Conclusion: Overall, our findings suggest that cTBS, a TMS protocol that mimics the mechanism of long-term depression of synaptic plasticity, modulates motor network oscillations primarily at the cortical level and might interfere with cortical information coding.

Long-term Synaptic Plasticity: Circuit Perturbation and Stabilization

At central synapses, activity-dependent synaptic plasticity has a crucial role in information processing, storage, learning, and memory under both physiological and pathological conditions. One widely accepted model of learning mechanism and information processing in the brain is Hebbian Plasticity: long-term potentiation (LTP) and long-term depression (LTD). LTP and LTD are respectively activity-dependent enhancement and reduction in the efficacy of the synapses, which are rapid and synapse-specific processes. A number of recent studies have a strong focal point on the critical importance of another distinct form of synaptic plasticity, non-Hebbian plasticity. Non-Hebbian plasticity dynamically adjusts synaptic strength to maintain stability. This process may be very slow and occur cell-widely. By putting them all together, this mini review defines an important conceptual difference between Hebbian and non-Hebbian plasticity.

Cortical Depression and Potentiation: Basic Mechanisms for Phantom Pain

People experience the feeling of the missing body part long after it has been removed after amputation are known as phantom limb sensations. These sensations can be painful, sometimes becoming chronic and lasting for several years (or called phantom pain). Medical treatment for these individuals is limited. Recent neurobiological investigations of brain plasticity after amputation have revealed new insights into the changes in the brain that may cause phantom limb sensations and phantom pain. In this article, I review recent progresses of the cortical plasticity in the anterior cingulate cortex (ACC), a critical cortical area for pain sensation, and explore how they are related to abnormal sensory sensations such as phantom pain. An understanding of these alterations may guide future research into medical treatment for these disorders.

The enhancement effects of amyloid β protein on in vivo hippocampal long term depression in rats / 中华行为医学与脑科学杂志

Objective To investigate the exact protocol eliciting the hippocampal CA1 long-term depression (LTD) of rats in vivo and the effect of amyloid β-protein (Aβ) on the LTD.Method By applying test stimulation to Schaffer collateral in hippocampal CA1 region in rats,recorded the in vivo field excitatory postsynaptic potentials (fEPSPs) ;further,observed the induction of LTD with different low frequency stimulation (LFS) and investigated the effect of Aβ25-35 on the LTD.Results Prolonged LFS (1,5 and 10 Hz) but not paired-pulse stimulus (PPS) effectively elicited the LTD in the hippocampal CA1 region,with significantly decreased amplitude of fEPSPs after LFS ; 1 Hz 900 pulses group induced a stronger LTD,being (63.7 ± 3.8) ％ at 120 min post-LFS,lower (P ＜ 0.05) than (75.1 ± 3.2) ％ in 600 pulses group ; different frequencies (1,5 and 10 Hz) of LFS with same pulses induced similar degree of LTD,the amplitude of fEPSPs were (63.7 ± 3.8) ％,(61.2 ± 3.6) ％ and (59.8 ± 3.9) ％ respectively,without significant differences between any two groups (P ＞ 0.05) ; after applying 12.5 nmol and 25 nmol Aβ25-35,the amplitude of fEPSPs decreased to (63.2 ± 3.8) ％ and (46.8 ± 3.9) ％,respectively,and lower and than that in control ((73.9 ± 3.0) ％,P ＜ 0.05).Conclusion Prolonged LFS effectively induced in vivo hippocampal LTD of rats,which provides an important electrophysiological protocol for the study of synaptic plasticity; Aβ25-35 injection dont affect the baseline synaptic transmission,but dose-dependently enhance the in vivo hippocampal LTD of rats,indicating that Aβ-induced LTD facilitation may be involved the early impairment of learning and memory in Alzheimer's disease.

COX-2 modulates Δ~9-THC-induced inhibition of LTD in CA1 area / 西安交通大学学报(医学版)

Objective To explore the role of cyclooxgense-2 (COX-2) in tetrahydrocannabinol (THC)-induced inhibition of long-term depression (LTD) in CA1 area in vitro. Methods The hippocampal slices were prepared at 24 h after intraperitoneal injection of Δ~9-THC (10 mg/kg) or COX-2 inhibitor NS398 (10 mg/kg). Field excitatory post-synaptic potentials (fEPSP) were recorded in the stratum radiatum of the CA1 area in vitro to observe the effect of NS398, a selective inhibitor of COX-2, on the THC-induced inhibition of LTD and THC's effect on membrane excitability in pyramidal neurons by using the whole-cell patch clamp technique. Results ① Low-frequency stimulation (1 Hz, 15 min) induced-LTD in CA1 area was significantly attenuated by Δ9-THC. ② Δ~9-THC did not affect the basal synaptic transmission and membrane excitability (including membrane resting potentials, input resistance and firing property). ③ THC-induced inhibition of LTD was reversed by NS398. ④ THC-induced inhibition of LTD was robustly impaired in COX-2 knockout mice. Conclusion THC-induced inhibition of LTD in CA1 area was mediated via COX-2.

Transducción de la señal, pivote de la integración neurobiológica de la memoria: Una propuesta diferente a la tradicional hipótesis del sistema colinérgico / Signal transduction, pillar of the neurobiological integration of memory: An alternative view to the cholinergic hypothesis

Los procesos neurofisiológicos, bioquímicos y moleculares descritos en la integración de la memoria, más que estar relacionados con la actividad colinérgica involucran fundamentalmente a neurotransmisores como la serotonina y el glutamato, así como a diversos canales iónicos como los del calcio y los del potasio. De hecho, los receptores de estos neurotransmisores están ligados directamente con la activación de la potenciación a largo plazo (LTP), mecanismo que contribuye a la preservación de la memoria. De esta forma que la activación del receptor 5HT desencadena una señal de transducción que al influenciar bioquímicamente al núcleo produce diversos cambios presinápticos con los que se expulsa al magnesio del área postsináptica, despolarizando a la neurona y activando simultáneamente a los receptores N metilD Aspartato dependientes (NMDAR), contribuyendo en esta forma a perpetuar el mecanismo de LTP en sus distintas fases: LTP1 que depende de la activación de proteincinasas; LTP2 ligada con la traslación genética; y LTP3 relacionada con la transcripción. A este poderoso mecanismo de activación neuronal, se contrapone el fenómeno de depresión a largo plazo (LTD), que se inicia cuando la neurona pre sináptica activa al inhibidor 1 en el momento en que detecta una reducción en el influjo de calcio, promoviendo en esta forma la defosforilación de una proteincinasa tipo II calcio calmodulin dependiente, lo que detiene el desarrollo del proceso de autofosforilación y con ello, el mecanismo de LTP. No obstante lo difundido de la hipótesis colinérgica en la enfermedad de Alzheimer, la integración de la memoria depende fundamentalmente de la intervención de otros sistemas de neurotransmisión como lo son el serotonérgico y el glutamatérgico, los que no han sido debidamente considerados en el tratamiento de esta enfermedad; sin embargo más allá de estos sistemas, se encuentran los mecanismos de autofosforilación de distintas proteincinasas cuyo control, además de repercutir sobre la expresión genética, podría restituir algunos de los trastornos que afectan la función cognoscitiva.

Neurophysiological, biochemical and molecular processes described in the integration of memory are closely related with neurotransmitters such as glutamate and serotonin (5HT) and with the function of calcium and potassium ion channels more than with cholinergic activity. In fact, glutamate and 5 HT receptors are closely related with Long-Term Potentiation (LTP) processes, the mechanism by which memory is preserved throughout time. That is, the activation of the 5 HT4 receptor triggers a transduction signal that after influencing nuclear cell activity, provokes several presynaptic changes, which leads to the displacement of magnesium from the postsynaptic area depolarizing the neuron and leading to the activation of N methyl -D-aspartate receptors (NMDA). As a whole, this process contributes to the support and perpetuation of LTP, which consists of the following processes: LTP1 that depends on protein kinase activity; LTP2 linked to translation of genes; and LTP3 closely related to genes transcription. On the opposite side but in perfect balance, we find the mechanism of Long Term depression (LTD), which is triggered instead when the Ca++ flow decreases in the presynaptic neuron activating the inhibitor 1 enzyme that promotes the dephosphorylation of a calmodulin dependent protein kinase II and as a result, the inhibition of autophosphorylation and consequently of LTP too. Despite the widespread dissemination of the cholinergic hypothesis in Alzheimer's disease, memory build up rather than involving acetylcholine essentially depends on the participation of other neurotransmitters such as 5 HT and glutamate, which have not been adequately considered in the treatment of this disease. However, beyond neurotransmission, it is the cellular mechanism of autophosphorylation of several protein kinases, the process susceptible of being activated or controlled by the action of distinct substances. In such a case, it would be possible to exert some influence on gene expression improving perhaps, some of the physiopathological deficits that characterize memory disruption.

COX-2 modulates ?~9-THC-induced inhibition of LTD in CA1 area / 西安交通大学学报(医学版)

Objective To explore the role of cyclooxgense-2(COX-2) in tetrahydrocannabinol(THC)-induced inhibition of long-term depression(LTD) in CA1 area in vitro.Methods The hippocampal slices were prepared at 24 h after intraperitoneal injection of ?9-THC(10 mg/kg) or COX-2 inhibitor NS398(10 mg/kg).Field excitatory post-synaptic potentials(fEPSP) were recorded in the stratum radiatum of the CA1 area in vitro to observe the effect of NS398,a selective inhibitor of COX-2,on the THC-induced inhibition of LTD and THC's effect on membrane excitability in pyramidal neurons by using the whole-cell patch clamp technique. Results ① Low-frequency stimulation(1 Hz,15 min) induced-LTD in CA1 area was significantly attenuated by ?9-THC.② ?9-THC did not affect the basal synaptic transmission and membrane excitability(including membrane resting potentials,input resistance and firing property).③ THC-induced inhibition of LTD was reversed by NS398.④ THC-induced inhibition of LTD was robustly impaired in COX-2 knockout mice.Conclusion THC-induced inhibition of LTD in CA1 area was mediated via COX-2.