Long-term depression and long-term potentiation in horizontal connections of the barrel cortex

Probing Our Built-in Calculator: A Systematic Narrative Review of Noninvasive Brain Stimulation Studies on Arithmetic Operation-Related Brain Areas

This systematic review presented a comprehensive survey of studies that applied transcranial magnetic stimulation and transcranial electrical stimulation to parietal and nonparietal areas to examine the neural basis of symbolic arithmetic processing. All findings were compiled with regard to the three assumptions of the triple-code model (TCM) of number processing. Thirty-seven eligible manuscripts were identified for review (33 with healthy participants and 4 with patients). Their results are broadly consistent with the first assumption of the TCM that intraparietal sulcus both hold a magnitude code and engage in operations requiring numerical manipulations such as subtraction. However, largely heterogeneous results conflicted with the second assumption of the TCM that the left angular gyrus subserves arithmetic fact retrieval, such as the retrieval of rote-learned multiplication results. Support is also limited for the third assumption of the TCM, namely, that the posterior superior parietal lobule engages in spatial operations on the mental number line. Furthermore, results from the stimulation of brain areas outside of those postulated by the TCM show that the bilateral supramarginal gyrus is involved in online calculation and retrieval, the left temporal cortex in retrieval, and the bilateral dorsolateral prefrontal cortex and cerebellum in online calculation of cognitively demanding arithmetic problems. The overall results indicate that multiple cortical areas subserve arithmetic skills.

Cortical direct current stimulation improves signal transmission between the motor cortices of rats

Transcranial direct current (DC) stimulation is a noninvasive brain stimulation technique that is now widely used to improve motor and cognitive function. The neuromodulatory effects of DC is considered to extend to nearby as well as remote brain areas from the site of stimulation because of current flowing into the brain and/or signal transmission in neuronal networks. However, the effects of DC on cortico-cortical neuronal transmission are not well known. In the present study, we focused on signal transmission from the primary (M1) to secondary (M2) motor cortex of rats. Intra-cortical microstimulation (ICMS) was applied to the M1 under DC conditions, and changes in synaptic activity in the M2 were examined using current-source density analyses. The synaptic input to the M2 superficial layers was enhanced during DC stimulation, while the synaptic input to the M2 deeper layers was increased after DC stimulation. These results suggest that DC stimulation improves cortico-cortical neuronal transmission from M1 to M2, and that the effectiveness of DC may be different among different projection neuron types in the M1.

Personalized Transcranial Magnetic Stimulation in Psychiatry

Transcranial magnetic stimulation (TMS) is a noninvasive brain stimulation technique that allows for modulating the activity of local neural populations and related neural networks. TMS is touted as a viable intervention to normalize brain activity and alleviate some psychiatric symptoms. However, TMS interventions are known to be only moderately reliable, and the efficacy of such therapies remains to be proven for psychiatric disorders other than depression. We review new opportunities to personalize TMS interventions using neuroimaging and computational modeling, aiming to optimize treatment to suit particular individuals and clinical subgroups. Specifically, we consider the prospect of improving the efficacy of existing TMS interventions by parsing broad diagnostic categories into biologically and clinically homogeneous biotypes. Biotypes can provide distinct treatment targets for optimized TMS interventions. We further discuss the utility of computational models in refining TMS personalization and efficiently establishing optimal cortical targets for distinct biotypes. Personalizing cortical stimulation targets, treatment frequencies, and intensities can improve the therapeutic efficacy of TMS and potentially establish noninvasive brain stimulation as a viable treatment for psychiatric symptoms.

Efficacy of theta burst stimulation (TBS) for major depression: An exploratory meta-analysis of randomized and sham-controlled trials

Theta burst stimulation (TBS) has been proposed as a novel treatment for major depression (MD). However, randomized and sham-controlled trials (RCTs) published to date have yielded heterogeneous clinical results and we have thus carried out the present systematic review and exploratory meta-analysis of RCTs to evaluate this issue. We searched the literature for RCTs on TBS for MD from January 2001 through September 2016 using MEDLINE, EMBASE, PsycINFO, and CENTRAL. We then performed a random-effects meta-analysis with the main outcome measures including pre-post score changes in the Hamilton Depression Rating Scale (HAM-D) as well as rates of response, remission and dropout. Data were obtained from 5 RCTs, totalling 221 subjects with MD. The pooled Hedges' g for pre-post change in HAM-D scores was 1.0 (p = 0.003), indicating a significant and large-sized difference in outcome favouring active TBS. Furthermore, active TBS was associated with significantly higher response rates when compared to sham TBS (35.6% vs. 17.5%, respectively; p = 0.005), although the groups did not differ in terms of rates of remission (18.6% vs. 10.7%, respectively; p = 0.1) and dropout (4.2% vs. 7.8%, respectively; p = 0.5). Finally, subgroup analyses indicated that bilateral TBS and unilateral intermittent TBS seem to be the most promising protocols. In conclusion, although TBS is a promising novel therapeutic intervention for MD, future studies should identify more clinically-relevant stimulation parameters as well as neurobiological predictors of treatment outcome, and include larger sample sizes, active comparators and longer follow-up periods.

Theta-burst stimulation: a new form of TMS treatment for depression?

Major depressive disorder (MDD) is a common debilitating condition where only one third of patients achieve remission after the first antidepressant treatment. Inadequate efficacy and adverse effects of current treatment strategies call for more effective and tolerable treatment options. Transcranial magnetic stimulation (TMS) is a noninvasive approach to manipulate brain activity and alter cortical excitability. There has been more than 15 years of research on the use of repetitive form of TMS (rTMS) for the treatment of patients with depression, which has shown it to be an effective antidepressant treatment. Even though rTMS treatment has shown efficacy in treating depression, there is a high degree of interindividual variability in response. A newer form of rTMS protocol, known as theta-burst stimulation (TBS), has been shown to produce similar if not greater effects on brain activity than standard rTMS. TBS protocols have a major advantage over standard rTMS approaches in their reduced administration duration. Conventional rTMS procedures last between 20 and 45 min, as compared to TBS paradigms that require 1 to 3 min of stimulation. Recently, a small number of studies have suggested that TBS has similar or better efficacy in treating depression compared to rTMS. Optimization, identification of response predictors, and clarification of neurobiological mechanisms of TBS is required if it is to be further developed as a less time intensive, safe, and effective treatment for MDD.

Frequency-specific response facilitation of supra and infragranular barrel cortical neurons depends on NMDA receptor activation in rats

Sensory experience has a profound effect on neocortical neurons. Passive stimulation of whiskers or sensory deprivation from whiskers can induce long-lasting changes in neuronal responses or modify the receptive field in adult animals. We recorded barrel cortical neurons in urethane-anesthetized rats in layers 2/3 or 5/6 to determine if repetitive stimulation would induce long-lasting response facilitation. Air-puff stimulation (20-ms duration, 40 pulses at 0.5-8Hz) was applied to a single whisker. This repetitive stimulation increased tactile responses in layers 2/3 and 5/6 for 60min. Moreover, the functional coupling (coherence) between the sensory stimulus and the neural response also increased after the repetitive stimulation in neurons showing response facilitation. The long-lasting response facilitation was due to activation of N-methyl-d-aspartate (NMDA) receptors because it was reduced by APV ((2R)-amino-5-phosphonovaleric acid, (2R)-amino-5-phosphonopentanoate) and MK801 application. Inactivation of layer 2/3 also blocked response facilitation in layer 5/6, suggesting that layer 2/3 may be fundamental in this synaptic plasticity processes. Moreover, i.p. injection of eserine augmented the number of layer 2/3 neurons expressing long-lasting response facilitation; this effect was blocked by atropine, suggesting that muscarinic receptor activation favors the induction of the response facilitation. Our data indicate that physiologically repetitive stimulation of a single whisker at the frequency at which rats move their whiskers during exploration of the environment induces long-lasting response facilitation improving sensory processing.

Imaging the spatio-temporal dynamics of supragranular activity in the rat somatosensory cortex in response to stimulation of the paws

We employed voltage-sensitive dye (VSD) imaging to investigate the spatio-temporal dynamics of the responses of the supragranular somatosensory cortex to stimulation of the four paws in urethane-anesthetized rats. We obtained the following main results. (1) Stimulation of the contralateral forepaw evoked VSD responses with greater amplitude and smaller latency than stimulation of the contralateral hindpaw, and ipsilateral VSD responses had a lower amplitude and greater latency than contralateral responses. (2) While the contralateral stimulation initially activated only one focus, the ipsilateral stimulation initially activated two foci: one focus was typically medial to the focus activated by contralateral stimulation and was stereotaxically localized in the motor cortex; the other focus was typically posterior to the focus activated by contralateral stimulation and was stereotaxically localized in the somatosensory cortex. (3) Forepaw and hindpaw somatosensory stimuli activated large areas of the sensorimotor cortex, well beyond the forepaw and hindpaw somatosensory areas of classical somatotopic maps, and forepaw stimuli activated larger cortical areas with greater activation velocity than hindpaw stimuli. (4) Stimulation of the forepaw and hindpaw evoked different cortical activation dynamics: forepaw responses displayed a clear medial directionality, whereas hindpaw responses were much more uniform in all directions. In conclusion, this work offers a complete spatio-temporal map of the supragranular VSD cortical activation in response to stimulation of the paws, showing important somatotopic differences between contralateral and ipsilateral maps as well as differences in the spatio-temporal activation dynamics in response to forepaw and hindpaw stimuli.

Dynamic, experience-dependent modulation of synaptic zinc within the excitatory synapses of the mouse barrel cortex

Increasing evidence suggests that synaptic zinc, found within the axon terminals of a subset of glutamatergic neurons in the cerebral cortex, is intricately involved in cortical plasticity. Using the vibrissae/barrel cortex model of cortical plasticity, we have previously shown manipulations of sensory input leads to rapid changes in synaptic zinc levels within the corresponding regions of the somatotopic map in the cortex. Here, using electron microscopy, we show how some of these changes are mediated at the synaptic level. We found that the density of zincergic synapses increased significantly in layers II/III, IV, and V. In layers IV and V, this change occurred in the absence of a significant increase in excitatory synapse density, which seems to indicate that excitatory synapses, which previously did not contain synaptic zinc, begin to newly house zinc within its synaptic vesicles. Our results show that excitatory neurons can dynamically change the phenotype of the vesicular content of their synapses in response to changes in sensory input. Given the range of modulatory effects zinc can have on neurotransmission, such a change in the complement of vesicular contents presumably allow these neurons to utilize synaptic zinc to facilitate plasticity. Thus, our results further support the role of zinc as an active participant in the processes contributing to experience-dependent cortical plasticity.

Sensory deprivation unmasks a PKA-dependent synaptic plasticity mechanism that operates in parallel with CaMKII

Calcium/calmodulin kinase II (CaMKII) is required for LTP and experience-dependent potentiation in the barrel cortex. Here, we find that whisker deprivation increases LTP in the layer IV to II/III pathway and that PKA antagonists block the additional LTP. No LTP was seen in undeprived CaMKII-T286A mice, but whisker deprivation again unmasked PKA-sensitive LTP. Infusion of a PKA agonist potentiated EPSPs in deprived wild-types and deprived CaMKII-T286A point mutants but not in undeprived animals of either genotype. The PKA-dependent potentiation mechanism was not present in GluR1 knockouts. Infusion of a PKA antagonist caused depression of EPSPs in undeprived but not deprived cortex. LTD was occluded by whisker deprivation and blocked by PKA manipulation, but not blocked by cannabinoid antagonists. NMDA receptor currents were unaffected by sensory deprivation. These results suggest that sensory deprivation causes synaptic depression by reversing a PKA-dependent process that may act via GluR1.

Cholinergic modulation of sensory interference in rat primary somatosensory cortical neurons

Sensory interaction was studied using extracellular recordings from 275 neurons in the primary somatosensory (SI) cortex of pentobarbital-anesthetized rats. Tactile stimulation was applied to the receptive field using a 1 mm diameter probe that indented the skin for 20 ms, at 0.5 Hz, (test stimulus). Tactile test responses of SI neurons decreased during simultaneous application of a gentle tickling (distracter stimuli) continuously for 60 s on a separate receptive field located in the same or the contralateral hindlimb (ipsi- or contralateral distraction). This decrease in neural response produced by distracter stimuli was interpreted as "sensory interference". Sensory interference was observed in 66% and 61% of recorded SI neurons when ipsi- or contralateral distracters were applied, respectively and was blocked by a novel stimulus obtained by increasing the stimulation frequency of the test tactile stimuli from 0.5 to 2 Hz. The number of neurons showing sensory interference in response to a contralateral distracter was not modified after corpus callosum transection, suggesting that interhemispheric connections are not crucial for sensory interference. In contrast, the number of neurons showing sensory interference decreased in animals with 192 IgG-saporin basal forebrain lesions that decreased the number of cortical cholinergic fibers. This finding indicates that cholinergic afferents from the basal forebrain are fundamental to sensory interference and suggests that the associative cortices - basal forebrain - sensory cortices network may be implicated in sensory interference.

A homeostatic rule for inhibitory synapses promotes temporal sharpening and cortical reorganization

Experience with transient stimuli leads to stronger neural responses that also rise and fall more sharply in time. This sharpening enhances the processing of transients and may be especially relevant for speech perception. We consider a learning rule for inhibitory connections that promotes this sharpening effect by adjusting these connections to maintain a target homeostatic level of activity in excitatory neurons. We analyze this rule in a recurrent network model of excitatory and inhibitory units. Strengthening inhibitory-->excitatory connections along with excitatory-->excitatory connections is required to obtain a sharpening effect. Using the homeostatic rule, we show that repeated presentations of a transient signal will "teach" the network to respond to the signal with both higher amplitude and shorter duration. The model also captures reorganization of receptive fields in the sensory hand area after amputation or peripheral nerve resection.

Deficits in experience-dependent cortical plasticity and sensory-discrimination learning in presymptomatic Huntington's disease mice

Huntington's disease (HD) is one of a group of neurodegenerative diseases caused by an expanded trinucleotide (CAG) repeat coding for an extended polyglutamine tract. The disease is inherited in an autosomal dominant manner, with onset of motor, cognitive, and psychiatric symptoms typically occurring in midlife, followed by unremitting progression and eventual death. We report here that motor presymptomatic R6/1 HD mice show a severe impairment of somatosensory-discrimination learning ability in a behavioral task that depends heavily on the barrel cortex. In parallel, there are deficits in barrel-cortex plasticity after a somatosensory whisker-deprivation paradigm. The present study demonstrates deficits in neocortical plasticity correlated with a specific learning impairment involving the same neocortical area, a finding that provides new insight into the cellular basis of early cognitive deficits in HD.

Timing-dependent plasticity in human primary somatosensory cortex

Animal experiments suggest that cortical sensory representations may be remodelled as a consequence of changing synaptic efficacy by timing-dependent associative neuronal activity. Here we describe a timing-based associative form of plasticity in human somatosensory cortex. Paired associative stimulation (PAS) was performed by combining repetitive median nerve stimulation with transcranial magnetic stimulation (TMS) over the contralateral postcentral region. PAS increased exclusively the amplitude of the P25 component of the median nerve-evoked somatosensory-evoked potential (MN-SSEP), which is probably generated in the superficial cortical layers of area 3b. SSEP components reflecting neuronal activity in deeper cortical layers (N20 component) or subcortical regions (P14 component) remained constant. PAS-induced enhancement of P25 amplitude displayed topographical specificity both for the recording (MN-SSEP versus tibial nerve-SSEP) and the stimulation (magnetic stimulation targeting somatosensory versus motor cortex) arrangements. Modulation of P25 amplitude was confined to a narrow range of interstimulus intervals (ISIs) between the MN pulse and the TMS pulse, and the sign of the modulation changed with ISIs differing by only 15 ms. The function describing the ISI dependence of PAS effects on somatosensory cortex resembled one previously observed in motor cortex, shifted by approximately 7 ms. The findings suggest a simple model of modulation of excitability in human primary somatosensory cortex, possibly by mechanisms related to the spike-timing-dependent plasticity of neuronal synapses located in upper cortical layers.

Corticotrophin-releasing hormone decreases synaptic transmission in rat sensorimotor cortex in vivo

Corticotrophin-releasing hormone is a key regulator of the mammalian stress response. Although its actions on behavior are well documented, the actions of corticotrophin-releasing hormone in cortical neuronal systems are poorly understood. In the present experiments, adult male Sprague-Dawley rats were anesthetized and field excitatory post-synaptic potential recordings were made from sensorimotor cortex layer II/III and layer V cells. Infusions of corticotrophin-releasing hormone (100 ng/nl) directly into the sensorimotor cortex produced a significant depression of the initial excitatory component of evoked responses that could be prevented by prior administration of a corticotrophin-releasing hormone antagonist. Although requiring the activation of corticotrophin-releasing hormone receptors, the depression was also dependent upon N-methyl-D-aspartate receptor activity and could be blocked by the competitive N-methyl-D-aspartate antagonist -3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonate. These findings demonstrate that corticotrophin-releasing hormone has a novel depressant-like action in sensorimotor cortex in vivo that may play a role in modulating motor activity during periods of stress.

The effect of short-duration bursts of high-frequency, low-intensity transcranial magnetic stimulation on the human motor cortex

OBJECTIVE

To explore the effect of applying a short burst of high-frequency repetitive transcranial magnetic stimulation (rTMS) to the human motor cortex as a preparatory investigation before attempting theta burst stimulation in humans.

METHODS

Five or 15 pulses of 50 Hz rTMS were given at 50-80% active motor threshold (AMT). The time course of changes in motor-evoked potential (MEP) size and short interval intracortical inhibition (SICI) were evaluated from 20 to 300 ms after the end of each burst in the relaxed first dorsal interosseous muscle of 15 healthy volunteers.

RESULTS

No subjects noted any adverse effects. MEPs were enhanced and SICIs were reduced at 20 ms after a burst of either 5 or 15 pulses at 70 or 80% AMT, but not at 50% AMT. Subsequent experiments used a 5 pulse burst at 80% AMT. The threshold for producing SICI increased from 60 to 80% AMT when tested 10 or 20 ms after the end of the burst. MEPs were enhanced for 100 ms, whereas SICI was reduced for 200-300 ms.

CONCLUSIONS

A short burst of low-intensity 50 Hz rTMS over the hand motor area transiently increases MEP amplitude with a longer lasting decrease in SICI.

SIGNIFICANCE

This means that it may be possible in future experiments to apply theta burst conditioning safely to the human cortex.

Synaptic basis for developmental plasticity in somatosensory cortex

Sensory experience drives plasticity of the body map in developing and adult somatosensory cortex, but the synaptic mechanisms underlying such plasticity are not well understood. Recently, several mechanisms that are likely to contribute to map plasticity have been directly observed in response to altered experience in vivo. These mechanisms include long-term potentiation and long-term depression at specific excitatory synapses, competition between lemniscal (barrel) and non-lemniscal (septal) processing streams, and regulation of the number of inhibitory synapses.

N-methyl-d-aspartate receptor-independent long-term depression and depotentiation in the sensorimotor cortex of the freely moving rat

Bidirectional modifications in synaptic efficacy are central components in recent models of cortical learning and memory, and we previously demonstrated both long-term synaptic potentiation (LTP) and long-term synaptic depression (LTD) in the neocortex of the unanaesthetized adult rat. Here, we have examined the effects of N-methyl-D-aspartate receptor (NMDAR) blockade on the induction of LTD, LTP, and depotentiation of field potentials evoked in sensorimotor cortex by stimulation of the white matter in the adult, freely moving rat. High frequency (300 Hz) stimulation (HFS) was used to induce LTP and prolonged, low-frequency (1 Hz) stimulation was used to induce either depotentiation or LTD. LTD was expressed as a reduction in the amplitude of the short and long-latency field potential components, while depotentiation was expressed as a decrease in the amplitude of a previously enhanced late component. Under NMDAR blockade, HFS failed to induce LTP and instead produced a depression effect similar to LTD. Following washout of the drug, HFS induced a normal LTP effect. Unlike LTP, LTD and depotentiation were found to be NMDAR-independent in the neocortex of the freely moving rat.