Modulation of neuronal network activity in the primary motor cortex

In the present study I investigated the mechanisms of modulation of neuronal network activity in rat primary motor cortex using pharmacological manipulations employing the in vitro brain slice technique. Preparation of the brain slice in sucrose-based aCSF produced slices with low viability. Introducing the neuroprotectants N-acetyl-cysteine, taurine and aminoguanidine to the preparatory method saw viability of slices increase significantly. Co-application of low dose kainic acid and carbachol consistently generated beta oscillatory activity in M1. Analyses indicated that network activity in M1 relied on the involvement of GABAA receptors. Dose-response experiments performed in M1 showed that beta activity can be modulated by benzodiazepine site ligands. Low doses of positive allosteric modulators consistently desynchronised beta oscillatory activity, a mechanism that may be driven by a1-subunit containing GABAA receptors. Higher doses increased the power of beta oscillatory activity. Whole-cell recordings in M1 uncovered three interneuronal subtypes regularly encountered in M1; Fast-spiking, regular-spiking non-Pyramidal and low threshold spiking. With the paradoxical effects of positive allosteric modulators in mind, subsequent voltage-clamp recordings in FS cells revealed a constitutively active tonic inhibitory current that could be modulated by zolpidem in two different ways. Low dose zolpidem increased the tonic inhibitory current in FS cells, consistent with the desynchronisation of network oscillatory activity seen at this concentration. High dose zolpidem decreased the inhibitory tonic current seen in FS cells, coinciding with an increase in oscillatory power. These studies indicate a fundamental role for a tonic inhibitory current in the modulation of network activity. Furthermore, desynchronisation of beta activity in M1 decreased as viability of the in vitro brain slice increased, suggesting that the extent of desynchronisation is dependent upon the pathophysiological state of the network. This indicates that low dose zolpidem could be used as a therapeutic agent specifically for the desynchronisation of pathological oscillations in oscillopathies such as Parkinson’s disease.

Characterising the central mechanisms of sensory modulation in human swallowing motor cortex

Objective: Pharyngeal stimulation can induce remarkable increases in the excitability of swallowing motor cortex, which is associated with short-term improvements in swallowing behaviour in dysphagic stroke patients. However, the mechanism by which this input induces cortical change remains unclear. Our aims were to explore the stimulus-induced facilitation of the cortico-bulbar projections to swallowing musculature and examine how input from the pharynx interacts with swallowing motor cortex. Methods: In 8 healthy subjects, a transcranial magnetic stimulation (TMS) paired-pulse investigation was performed comprising a single conditioning electrical pharyngeal stimulus (pulse width 0.2 ms, 240 V) followed by cortical TMS at inter-stimulus intervals (ISI) of 10-100 ms. Pharyngeal sensory evoked potentials (PSEP) were also measured over the vertex. In 6 subjects whole-brain magnetoencephalography (MEG) was further acquired following pharyngeal stimulation. Results: TMS evoked pharyngeal motor evoked potentials were facilitated by the pharyngeal stimulus at ISI between 50 and 80 ms (Δ mean increase: 47±6%, P<0.05). This correlated with the peak latency of the P1 component of the PSEP (mean 79.6±8.5 ms). MEG confirmed that the equivalent P1 peak activities were localised to caudolateral sensory and motor cortices (BA 4, 1, 2). Conclusions: Facilitation of the cortico-bulbar pathway to pharyngeal stimulation relates to coincident afferent input to sensorimotor cortex. Significance: These findings have mechanistic importance on how pharyngeal stimulation may increase motor excitability and provide guidance on temporal windows for future manipulations of swallowing motor cortex. © 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

Pharmacologically induced and stimulus evoked rhythmic neuronal oscillatory activity in the primary motor cortex in vitro

Parkinson's disease (PD) is associated with enhanced synchronization of neuronal network activity in the beta (15-30 Hz) frequency band across several nuclei of the basal ganglia (BG). Deep brain stimulation of the subthalamic nucleus (STN) appears to reduce this pathological oscillation, thereby alleviating PD symptoms. However, direct stimulation of primary motor cortex (M1) has recently been shown to be effective in reducing symptoms in PD, suggesting a role for cortex in patterning pathological rhythms. Here, we examine the properties of M1 network oscillations in coronal slices taken from rat brain. Oscillations in the high beta frequency range (layer 5, 27.8 +/- 1.1 Hz, n=6) were elicited by co-application of the glutamate receptor agonist kainic acid (400 nM) and muscarinic receptor agonist carbachol (50 mu M). Dual extracellular recordings, local application of tetrodotoxin and recordings in M1 micro-sections indicate that the activity originates within deep layers V/VI. Beta oscillations were unaffected by specific AMPA receptor blockade, abolished by the GABA type A receptor (GABAAR) antagonist picrotoxin and the gap-junction blocker carbenoxolone, and modulated by pentobarbital and zolpidem indicating dependence on networks of GABAergic interneurons and electrical coupling. High frequency stimulation (HFS) at 125 Hz in superficial layers, designed to mimic transdural/transcranial stimulation, generated gamma oscillations in layers 11 and V (incidence 95%, 69.2 +/- 7.3 Hz, n=17) with very fast oscillatory components (VFO; 100-250 Hz). Stimulation at 4 Hz, however, preferentially promoted theta activity (incidence 62.5%, 5.1 +/- 0.6 Hz, n=15) that effected strong amplitude modulation of ongoing beta activity. Stimulation at 20 Hz evoked mixed theta and gamma responses. These data suggest that within M1, evoked theta, gamma and fast oscillations may coexist with and in some cases modulate pharmacologically induced beta oscillations.

Functional localisation of human sensory-motor cortex using magnetoencephalography

The 19 channel Neuromagnetometer system in the Clinical Neurophysiology Unit at Aston University is a multi-channel system, unique in the United Kingdom. A bite bar head localisation and MRI co-registration strategy which enabled accurate and reproducible localisation of MEG data into cortical space was developed. This afforded the opportunity to study magnetic fields of the human cortex generated by stimulation of peripheral nerve, by stimulation of visceral sensory receptors and by those evoked through voluntary finger movement. Initially, a study of sensory-motor evoked data was performed in a healthy control population. The techniques developed were then applied to patients who were to undergo neurosurgical intervention for the treatment of epilepsy and I or space occupying lesions. This enabled both validation of the effective accuracy of source localisation using MEG as well as to determine the clinical value of MEG in presurgical assessment of functional localisation in human cortex. The studies in this thesis have demonstrated that MEG can repeatedly and reliably locate sources contained within a single gyrus and thus potentially differentiate between disparate gyral activation. This ability is critical in the clinical application of any functional imaging technique; which is yet to be fully validated by any other 'non-invasive' functional imaging methodology. The technique was also applied to the study of visceral sensory representation in the cortex which yielded important data about the multiple cortical representation of visceral sensory function.

Beta frequency neuronal network activity in the primary motor cortex

In this study I investigated the mechanisms of neuronal network oscillatory activity in rat M1 using pharmacological manipulations and electrical stimulation protocols, employing the in vitro brain slice technique in rat and magnetoencephalography (MEG) in man. Co-application of kainic acid and carbachol generated in vitro beta oscillatory activity in all layers in M1. Analyses indicated that oscillations originated from deep layers and indicated significant involvement of GABAA receptors and gap junctions. A modulatory role of GABAB, NMDA, and dopamine receptors was also evident. Intracellular recordings from fast-spiking (FS) GABAergic inhibitory cells revealed phase-locked action potentials (APs) on every beta cycle. Glutamatergic excitatory regular-spiking (RS) and intrinsically-bursting (IB) cells both received phase locked inhibitory postsynaptic potentials, but did not fire APs on every cycle, suggesting the dynamic involvement of different pools of neurones in the overall population oscillations. Stimulation evoked activity at high frequency (HFS; 125Hz) evoked gamma oscillations and reduced ongoing beta activity. 20Hz stimulation promoted theta or gamma oscillations whilst 4Hz stimulation enhanced beta power at theta frequency. I also investigated the modulation of pathological slow wave (theta and beta) oscillatory activity using magnetoencephalography. Abnormal activity was suppressed by sub-sedative doses of GABAA receptor modulator zolpidem and the observed desynchronising effect correlated well with improved sensorimotor function. These studies indicate a fundamental role for inhibitory neuronal networks in the patterning beta activity and suggest that cortical HFS in PD re-patterns abnormally enhanced M1 network activity by modulating the activity of FS cells. Furthermore, pathological oscillation may be common to many neuropathologies and may be an important future therapeutic target.

Oscillatory beta activity mediates neuroplastic effects of motor cortex stimulation in humans

Continuous theta burst stimulation (cTBS) is a repetitive transcranial magnetic stimulation protocol that can inhibithumanmotor cortex (M1) excitability and impair movement for ≤1 h. While offering valuable insights into brain function and potential therapeutic benefits, these neuroplastic effects are highly variable between individuals. The source of this variability, and the electrophysiological mechanisms underlying the inhibitory after-effects, are largely unknown. In this regard, oscillatory activity at beta frequency (15-35 Hz) is of particular interest as it is elevated in motor disorders such as Parkinson's disease and modulated during the generation of movements. Here, we used a source-level magnetoencephalography approach to investigate the hypothesis that the presence of neuroplastic effects following cTBS is associated with concurrent changes in oscillatory M1 beta activity. M1 cortices were localized with a synthetic aperture magnetometry beamforming analysis of visually cued index finger movements. Virtual electrode analysis was used to reconstruct the spontaneous and movement-related oscillatory activity in bilateral M1 cortices, before and from 10 to 45 min after cTBS. We demonstrate that 40 s of cTBS applied over left M1 reduced corticospinal excitability in the right index finger of 8/16 participants. In these responder participants only, cTBS increased the power of the spontaneous beta oscillations in stimulated M1 and delayed reaction times in the contralateral index finger. No further changes were observed in the latency or power of movement-related beta oscillations. These data provide insights into the electrophysiological mechanisms underlying cTBS-mediated impairment of motor function and demonstrate the association between spontaneous oscillatory beta activity in M1 and the inhibition of motor function. © 2013 the authors.

Inter-individual variability in optimal current direction for transcranial magnetic stimulation of the motor cortex

We evaluated inter-individual variability in optimal current direction for biphasic transcranial magnetic stimulation (TMS) of the motor cortex. Motor threshold for first dorsal interosseus was detected visually at eight coil orientations in 45° increments. Each participant (n = 13) completed two experimental sessions. One participant with low test–retest correlation (Pearson's r < 0.5) was excluded. In four subjects, visual detection of motor threshold was compared to EMG detection; motor thresholds were very similar and highly correlated (0.94–0.99). Similar with previous studies, stimulation in the majority of participants was most effective when the first current pulse flowed towards postero-lateral in the brain. However, in four participants, the optimal coil orientation deviated from this pattern. A principal component analysis using all eight orientations suggests that in our sample the optimal orientation of current direction was normally distributed around the postero-lateral orientation with a range of 63° (S.D. = 13.70°). Whenever the intensity of stimulation at the target site is calculated as a percentage from the motor threshold, in order to minimize intensity and side-effects it may be worthwhile to check whether rotating the coil 45° from the traditional posterior–lateral orientation decreases motor threshold.

Spike firing and IPSPs in layer V pyramidal neurons during beta oscillations in rat primary motor cortex (M1) in vitro

Beta frequency oscillations (10-35 Hz) in motor regions of cerebral cortex play an important role in stabilising and suppressing unwanted movements, and become intensified during the pathological akinesia of Parkinson's Disease. We have used a cortical slice preparation of rat brain, combined with concurrent intracellular and field recordings from the primary motor cortex (M1), to explore the cellular basis of the persistent beta frequency (27-30 Hz) oscillations manifest in local field potentials (LFP) in layers II and V of M1 produced by continuous perfusion of kainic acid (100 nM) and carbachol (5 µM). Spontaneous depolarizing GABA-ergic IPSPs in layer V cells, intracellularly dialyzed with KCl and IEM1460 (to block glutamatergic EPSCs), were recorded at -80 mV. IPSPs showed a highly significant (P< 0.01) beta frequency component, which was highly significantly coherent with both the Layer II and V LFP oscillation (which were in antiphase to each other). Both IPSPs and the LFP beta oscillations were abolished by the GABAA antagonist bicuculline. Layer V cells at rest fired spontaneous action potentials at sub-beta frequencies (mean of 7.1+1.2 Hz; n = 27) which were phase-locked to the layer V LFP beta oscillation, preceding the peak of the LFP beta oscillation by some 20 ms. We propose that M1 beta oscillations, in common with other oscillations in other brain regions, can arise from synchronous hyperpolarization of pyramidal cells driven by synaptic inputs from a GABA-ergic interneuronal network (or networks) entrained by recurrent excitation derived from pyramidal cells. This mechanism plays an important role in both the physiology and pathophysiology of control of voluntary movement generation.

Laminar and cellular distribution of AMPA, kainate, and NMDA receptor subunits in monkey sensory-motor cortex

In situ hybridization histochemistry and immunocytochemistry were used to examine lamina- and cell-specific expression of glutamate receptor (GluR) mRNAs and polypeptide subunits in motor and somatosensory cortex of macaque monkeys. Radioactive complementary RNA (cRNA) probes were prepared from cDNAs specific for α-amino-3-hydroxy-5-methylisoxozolepropionate (AMPA)/kainate (GluR1-GluR4), kainate (GluR5-GluR7), and N-methylD-aspartate (NMDA; NR1, NR2A-NR2D) receptor subunits. AMPA/kainate and NR1, NR2A, and NR2B receptor transcripts show higher expression than other transcripts. All transcripts show lamina-specific patterns of distribution. GluR2 and GluR4 mRNAs show higher expression than do GluR1 and GluR3 mRNAs. GluR6 transcript expression is higher than that of GluR5 and GluR7. NR1 mRNA expression is much higher than that of NR2 mRNAs. NR2C subunit expression is very low except for a very distinct band of high expression in layer IV of area 3b. Immunocytochemistry, using subunit-specific antisera and double labeling for calbindin, parvalbumin, or α type II Ca2+/calmodulin-dependent protein kinase (CaMKII-α), allowed identification of cell types expressing different subunit genes. GluR1 and GluR5/6/7 immunoreactivity is found in both pyramidal cells and gamma-amino butyric acid (GABA) cells; GluR2/3 immunoreactivity is preferentially found in pyramidal cells, whereas GluR4 immunoreactivity is largely restricted to GABA cells; NMDA receptor subunit immunoreactivity is far greater in excitatory cells than in GABA cells. The density of expression of AMPA/kainate, kainate, and NMDA receptor subunit mRNAs differed within and across the architectonic fields of sensory-motor cortex. This finding and the lamina- and cell-specific patterns of expression suggest assembly of functional receptors from different arrangements of available subunits in specific neuronal populations.

Motor imagery in Parkinson's disease: A PET study

We used positron emission tomography (PET) with O-15-labelled water to record patterns of cerebral activation in six patients with Parkinson's disease (PD), studied when clinically off and after turning on as a result of dopaminergic stimulation. They were asked to imagine a Finger opposition movement performed with their right hand. externally paced at a rate of 1 Hz. Trials alternating between motor imagery and rest were measured. A pilot study of three age-matched controls was also performed. We chose the task as a robust method of activating the supplementary motor area (SMA), defects of which have been reported in PD. The PD patients showed normal de-rees of activation of the SMA (proper) when both off and on. Significant activation with imagining movement also occurred in the ipsilateral inferior parietal cortex (both off and when on) and ipsilateral premotor cortex (when off only). The patients showed significantly greater activation of the rostral anterior cingulate and significantly less activation of the left lingual gyrus and precuneus when performing the task on compared with their performance when off. PD patients when imagining movement and off showed less activation of several sites including the right dorsolateral prefrontal cortex (DLPFC) when compared to the controls performing the same task. No significant differences from controls were present when the patients imagined when on. Our results are consistent with other studies showing deficits of pre-SMA function in PD with preserved function of the SMA proper. In addition to the areas of reduced activation (anterior cingulate, DLPFC), there were also sites of activation (ipsilateral premotor and inferior parietal cortex) previously reported as locations of compensatory overactivity for PD patients performing similar tasks. Both failure of activation and compensatory changes a-re likely to contribute to the motor deficit in PD. (C) 2001 Movement Disorder Society.

5 Hz repetitive TMS increases anticipatory motor activity in the human cortex

In the present study, we analyzed how high-frequency repetitive transcranial magnetic stimulation (rTMS) of the primary motor hand area (M1-Hand) shapes anticipatory motor activity in frontal areas as indexed by the contingent negative variation (CNV). Eight right-handed volunteers received real or sham 5 Hz rTMS at an intensity of 90% resting motorthreshold (1500 stimuli per session). Real but not sham rTMS to left M1-Hand induced a site-specific increase in amplitude of the late component of the CNV at the electrode C3 overlaying the site of stimulation. The increase in pre-movement activity in the stimulated cortex may reflect an increase in facilitatory drive from connected motor areas, enhanced responsiveness of the stimulated cortex to these inputs or both. (c) 2005 Elsevier Ireland Ltd. All rights reserved.

Movement-related potentials in Parkinson's disease - Motor imagery and movement preparation

Movement-related potentials (MRPs) associated with voluntary movements reflect cortical activity associated with processes Of movement preparation and movement execution. Early-stage pre-movement activity is reduced in amplitude in Parkinson's disease. However it is unclear whether this neurophysiological deficit relates to preparatory or execution-related activity, since previous studies have not been able to separate different functional components of MRPs. Motor imagery is thought to involve mainly processes of movement preparation, with reduced involvement of end-stage movement execution-related processes. Therefore, MRP components relating to movement preparation and execution may be examined separately by comparing MRPs associated with imagined and actual movements. In this study, MRPs were recorded from 14 subjects with Parkinson's disease and 10 age-matched control subjects while they performed a sequential button-pressing task, and while they imagined performance of the same task. Early-stage pre-movement activity was present in both Parkinson's disease patients and control subjects when they imagined movement, but was reduced in amplitude compared with that for actual movement. Movement execution-related components, arising predominantly from the primary motor cortex, were relatively unaffected in Parkinson's disease subjects. However motor preparatory processes, probably involving the supplementary motor area, were reduced in amplitude overall and abnormally prolonged, Indicating impaired termination following the motor response. Further this impaired termination of preparatory-phase activity was observed only in patients with more severe parkinsonian symptoms, and not in early-stage Parkinson's disease.

Representation of motor habit in a sequence of repetitive reach and grasp movements performed by macaque monkeys: evidence for a contribution of the dorsolateral prefrontal cortex.

In the context of an autologous cell transplantation study, a unilateral biopsy of cortical tissue was surgically performed from the right dorsolateral prefrontal cortex (dlPFC) in two intact adult macaque monkeys (dlPFC lesioned group), together with the implantation of a chronic chamber providing access to the left motor cortex. Three other monkeys were subjected to the same chronic chamber implantation, but without dlPFC biopsy (control group). All monkeys were initially trained to perform sequential manual dexterity tasks, requiring precision grip. The motor performance and the prehension's sequence (temporal order to grasp pellets from different spatial locations) were analysed for each hand. Following the surgery, transient and moderate deficits of manual dexterity per se occurred in both groups, indicating that they were not due to the dlPFC lesion (most likely related to the recording chamber implantation and/or general anaesthesia/medication). In contrast, changes of motor habit were observed for the sequential order of grasping in the two monkeys with dlPFC lesion only. The changes were more prominent in the monkey subjected to the largest lesion, supporting the notion of a specific effect of the dlPFC lesion on the motor habit of the monkeys. These observations are reminiscent of previous studies using conditional tasks with delay that have proposed a specialization of the dlPFC for visuo-spatial working memory, except that this is in a different context of "free-will", non-conditional manual dexterity task, without a component of working memory.

Interregional compensatory mechanisms of motor functioning in progressing preclinical neurodegeneration.

Understanding brain reserve in preclinical stages of neurodegenerative disorders allows determination of which brain regions contribute to normal functioning despite accelerated neuronal loss. Besides the recruitment of additional regions, a reorganisation and shift of relevance between normally engaged regions are a suggested key mechanism. Thus, network analysis methods seem critical for investigation of changes in directed causal interactions between such candidate brain regions. To identify core compensatory regions, fifteen preclinical patients carrying the genetic mutation leading to Huntington's disease and twelve controls underwent fMRI scanning. They accomplished an auditory paced finger sequence tapping task, which challenged cognitive as well as executive aspects of motor functioning by varying speed and complexity of movements. To investigate causal interactions among brain regions a single Dynamic Causal Model (DCM) was constructed and fitted to the data from each subject. The DCM parameters were analysed using statistical methods to assess group differences in connectivity, and the relationship between connectivity patterns and predicted years to clinical onset was assessed in gene carriers. In preclinical patients, we found indications for neural reserve mechanisms predominantly driven by bilateral dorsal premotor cortex, which increasingly activated superior parietal cortices the closer individuals were to estimated clinical onset. This compensatory mechanism was restricted to complex movements characterised by high cognitive demand. Additionally, we identified task-induced connectivity changes in both groups of subjects towards pre- and caudal supplementary motor areas, which were linked to either faster or more complex task conditions. Interestingly, coupling of dorsal premotor cortex and supplementary motor area was more negative in controls compared to gene mutation carriers. Furthermore, changes in the connectivity pattern of gene carriers allowed prediction of the years to estimated disease onset in individuals. Our study characterises the connectivity pattern of core cortical regions maintaining motor function in relation to varying task demand. We identified connections of bilateral dorsal premotor cortex as critical for compensation as well as task-dependent recruitment of pre- and caudal supplementary motor area. The latter finding nicely mirrors a previously published general linear model-based analysis of the same data. Such knowledge about disease specific inter-regional effective connectivity may help identify foci for interventions based on transcranial magnetic stimulation designed to stimulate functioning and also to predict their impact on other regions in motor-associated networks.