Hippocampal Kindling Leads to Motor Map Expansion

Altered Resting State Networks Before and After Temporal Lobe Epilepsy Surgery

OBJECTIVES

To explore the resting state networks (RSNs) alterations in patients with unilateral mesial temporal lobe epilepsy (mTLE) before and after successful surgery.

METHODS

Resting-state functional MRI and T1-weighted structural MRI were obtained in 37 mTLE patients who achieved seizure freedom after anterior temporal lobectomy. Patients were scanned before surgery and at two years after surgery. Twenty-eight age- and sex-matched healthy controls were scanned once. Functional connectivity (FC) changes within and between ten common RSNs before and after surgery, and FC changes between hippocampus and RSNs were explored.

RESULTS

Before surgery, decreased FC was found within visual network and basal ganglia network, while after surgery, FC within basal ganglia network further decreased but FC within sensorimotor network and dorsal attention network increased. Before surgery, between-network FC related to basal ganglia network, visual network and dorsal attention network decreased, while between-network FC related to default mode network increased. After surgery, between-network FC related to visual network and dorsal attention network significantly increased. In addition, before surgery, ipsilateral hippocampus showed decreased FC with visual network, basal ganglia network, sensorimotor network, default mode network and frontoparietal network, while contralateral rostral hippocampus showed increased FC with salience network. After surgery, no obvious FC changes were found between contralateral hippocampus and these RSNs.

CONCLUSION

MTLE patients showed significant RSNs alterations before and after surgery. Basal ganglia network showed progressive decline in functional connectivity. Successful surgery may lead to RSNs reorganization. These results provide preliminary evidence for postoperative functional remodeling at whole-brain-network level.

Bayesian vector autoregressive model for multi-subject effective connectivity inference using multi-modal neuroimaging data

In this article a multi-subject vector autoregressive (VAR) modeling approach was proposed for inference on effective connectivity based on resting-state functional MRI data. Their framework uses a Bayesian variable selection approach to allow for simultaneous inference on effective connectivity at both the subject- and group-level. Furthermore, it accounts for multi-modal data by integrating structural imaging information into the prior model, encouraging effective connectivity between structurally connected regions. They demonstrated through simulation studies that their approach resulted in improved inference on effective connectivity at both the subject- and group-level, compared with currently used methods. It was concluded by illustrating the method on temporal lobe epilepsy data, where resting-state functional MRI and structural MRI were used. Hum Brain Mapp 38:1311-1332, 2017. © 2016 Wiley Periodicals, Inc.

HCN channels segregate stimulation-evoked movement responses in neocortex and allow for coordinated forelimb movements in rodents

KEY POINTS

The present study tested whether HCN channels contribute to the organization of motor cortex and to skilled motor behaviour during a forelimb reaching task. Experimental reductions in HCN channel signalling increase the representation of complex multiple forelimb movements in motor cortex as assessed by intracortical microstimulation. Global HCN1^KO mice exhibit reduced reaching accuracy and atypical movements during a single-pellet reaching task relative to wild-type controls. Acute pharmacological inhibition of HCN channels in forelimb motor cortex decreases reaching accuracy and increases atypical movements during forelimb reaching.

ABSTRACT

The mechanisms by which distinct movements of a forelimb are generated from the same area of motor cortex have remained elusive. Here we examined a role for HCN channels, given their ability to alter synaptic integration, in the expression of forelimb movement responses during intracortical microstimulation (ICMS) and movements of the forelimb on a skilled reaching task. We used short-duration high-resolution ICMS to evoke forelimb movements following pharmacological (ZD7288), experimental (electrically induced cortical seizures) or genetic approaches that we confirmed with whole-cell patch clamp to substantially reduce I_h current. We observed significant increases in the number of multiple movement responses evoked at single sites in motor maps to all three experimental manipulations in rats or mice. Global HCN1 knockout mice were less successful and exhibited atypical movements on a skilled-motor learning task relative to wild-type controls. Furthermore, in reaching-proficient rats, reaching accuracy was reduced and forelimb movements were altered during infusion of ZD7288 within motor cortex. Thus, HCN channels play a critical role in the separation of overlapping movement responses and allow for successful reaching behaviours. These data provide a novel mechanism for the encoding of multiple movement responses within shared networks of motor cortex. This mechanism supports a viewpoint of primary motor cortex as a site of dynamic integration for behavioural output.

Rapid chain generation of interpostsynaptic functional LINKs can trigger seizure generation: Evidence for potential interconnections from pathology to behavior

The experimental finding that a paroxysmal depolarizing shift (PDS), an electrophysiological correlate of seizure activity, is a giant excitatory postsynaptic potential (EPSP) necessitates a mechanism for spatially summating several EPSPs at the level of the postsynaptic terminals (dendritic spines). In this context, we will examine reversible interpostsynaptic functional LINKs (IPLs), a proposed mechanism for inducing first-person virtual internal sensations of higher brain functions concurrent with triggering behavioral motor activity for possible pathological changes that may contribute to seizures. Pathological conditions can trigger a rapid chain generation and propagation of different forms of IPLs leading to seizure generation. A large number of observations made at different levels during both ictal and interictal periods are explained by this mechanism, including the tonic and clonic motor activity, different types of hallucinations, loss of consciousness, gradual worsening of cognitive abilities, a relationship with kindling (which uses an augmented stimulation protocol than that used for inducing long-term potentiation (LTP), which is an electrophysiological correlate of behavioral makers of internal sensation of memory), effect of a ketogenic diet on seizure prevention, dendritic spine loss in seizure disorders, neurodegenerative changes, and associated behavioral changes. The interconnectable nature of these findings is explained as loss of function states of a proposed normal functioning of the nervous system.

Delta-9-tetrahydrocannabinol (THC) affects forelimb motor map expression but has little effect on skilled and unskilled behavior

It has previously been shown in rats that acute administration of delta-9-tetrahydrocannabinol (THC) exerts a dose-dependent effect on simple locomotor activity, with low doses of THC causing hyper-locomotion and high doses causing hypo-locomotion. However the effect of acute THC administration on cortical movement representations (motor maps) and skilled learned movements is completely unknown. It is important to determine the effects of THC on motor maps and skilled learned behaviors because behaviors like driving place people at a heightened risk. Three doses of THC were used in the current study: 0.2mg/kg, 1.0mg/kg and 2.5mg/kg representing the approximate range of the low to high levels of available THC one would consume from recreational use of cannabis. Acute peripheral administration of THC to drug naïve rats resulted in dose-dependent alterations in motor map expression using high resolution short duration intracortical microstimulation (SD-ICMS). THC at 0.2mg/kg decreased movement thresholds and increased motor map size, while 1.0mg/kg had the opposite effect, and 2.5mg/kg had an even more dramatic effect. Deriving complex movement maps using long duration (LD)-ICMS at 1.0mg/kg resulted in fewer complex movements. Dosages of 1.0mg/kg and 2.5mg/kg THC reduced the number of reach attempts but did not affect percentage of success or the kinetics of reaching on the single pellet skilled reaching task. Rats that received 2.5mg/kg THC did show an increase in latency of forelimb removal on the bar task, while dose-dependent effects of THC on unskilled locomotor activity using the rotorod and horizontal ladder tasks were not observed. Rats may be employing compensatory strategies after receiving THC, which may account for the robust changes in motor map expression but moderate effects on behavior.

Pathological uncoupling between amplitude and connectivity of brain fluctuations in epilepsy

Amplitude and functional connectivity are two fundamental parameters for describing the spontaneous brain fluctuations. These two parameters present close coupling in physiological state, and present different alteration patterns in epilepsy revealed by functional MRI (fMRI). We hypothesized that the alteration of coupling between these two imaging parameters may be underpinned by specific pathological factors of epilepsy, and can be employed to improve the capability for epileptic focus detection. Forty-seven patients (26 left- and 21 right-sided) with mesial temporal lobe epilepsy (mTLE) and 32 healthy controls underwent resting-state fMRI scans. All patients were detected to have interictal epileptic discharges on simultaneous electroencephalograph (EEG) recordings. Amplitude-connectivity coupling was calculated by correlating amplitude and functional connectivity density of low-frequency brain fluctuations. We observed reduced amplitude-connectivity coupling associated with epileptic discharges in the mesial temporal regions in both groups of patients, and increased coupling associated with epilepsy durations in the posterior regions of the default-mode network in the right-sided patients. Moreover, we proposed a new index of amplitude subtracting connectivity, which elevated imaging contrast for differentiating the patients from the controls. The findings indicated that epileptic discharges and chronic damaging effect of epilepsy might both contribute to alterations of amplitude-connectivity coupling in different pivotal regions in mTLE. Investigation on imaging coupling provides synergistic approach for describing brain functional changing features in epilepsy.

Recent seizure activity alters motor organization in frontal lobe epilepsy as revealed by task-based fMRI

PURPOSE

Patients with frontal lobe epilepsy (FLE) commonly demonstrate motor impairments, suggesting that frontal lobe seizures affect motor function. However, the underlying mechanisms of these deficits are not known, nor has any study systematically examined motor organization in these patients. We therefore examined cortical motor organization in a group of adult patients with FLE, using task-based fMRI.

METHODS

Eleven right FLE patients, six left FLE patients, and ten control subjects underwent task-based fMRI. Two tasks were performed using the right and left hands separately, and both hands together. The first task was a finger-tapping task and the second task was a more complex coordination task. Functional MR data were compared between patient groups and controls. A laterality index of brain activation was also calculated between the epileptic and healthy hemisphere to determine hemispheric dominance during task performance to explore its relationship with a variety of patient-specific epilepsy factors.

RESULTS

Overall, right FLE patients demonstrated decreased BOLD activity in the epileptic hemisphere and increased BOLD activity in the healthy hemisphere compared to controls (p<0.05). The comparison of left FLE patients to controls provided less conclusive differences, possibly due to the low number of left FLE patients studied. Laterality indices of the coordination task were positively correlated to the number of months since the last seizure in both patient groups (right FLE: rs=0.779, left FLE: rs=0.943). Patients that had experienced a recent seizure relied more on the sensorimotor cortex of the healthy hemisphere during task performance, compared to those that were relatively seizure free (p<0.05).

SIGNIFICANCE

Patients with FLE exhibited changes in motor BOLD activity that was dependent on the duration of seizure freedom. These results demonstrate the presence of seizure-related alteration of cortical motor organization in FLE, which may underlie the motor deficits seen in these patients.

Functional connectivity of hippocampal networks in temporal lobe epilepsy

OBJECTIVE

Temporal lobe epilepsy (TLE) affects brain areas beyond the temporal lobes due to connections of the hippocampi and other temporal lobe structures. Using functional connectivity magnetic resonance imaging (MRI), we determined the changes of hippocampal networks in TLE to assess for a more complete distribution of abnormality.

METHODS

Regions of interest (ROIs) were defined in the right and left hippocampi in three groups of participants: left TLE (n = 13), right TLE (n = 11), and healthy controls (n = 16). Brain regions functionally connected to these ROIs were identified by correlating resting-state low-frequency functional MRI (fMRI) blood oxygenation level-dependent (BOLD) signal fluctuations. The grouped results were compared using independent sample t-test.

RESULTS

TLE was associated with increased hippocampal connectivity involving several key areas of the limbic network (temporal lobe, insula, thalamus), frontal lobes, angular gyrus, basal ganglia, brainstem, and cerebellum, along with reduced connectivity involving areas of the sensorimotor cortex (visual, somatosensory, auditory, primary motor) and the default mode network (precuneus). Left TLE had more marked connectivity changes than right TLE.

SIGNIFICANCE

The observed connectivity changes in TLE indicate dysfunctional networks that underlie widespread brain involvement in TLE. There are identifiable differences in the connectivity of the hippocampi between left and right TLE.

Development of motor maps in rats and their modulation by experience

While a substantial literature demonstrates the effect of differential experience on development of mammalian sensory cortices and plasticity of adult motor cortex, characterization of differential experience on the functional development of motor cortex is meager. We first determined when forelimb movement representations (motor maps) could be detected in rats during postnatal development and then whether their motor map expression could be altered with rearing in an enriched environment consisting of group housing and novel toys or skilled learning by training on the single pellet reaching task. All offspring had high-resolution intracortical microstimulation (ICMS)-derived motor maps using methodologies previously optimized for the adult rat. First, cortical GABA-mediated inhibition was depressed by bicuculline infusion directly into layer V of motor cortex and ICMS-responsive points were first reliably detected on postnatal day (PND) 13. Without relying on bicuculline disinhibition of cortex, motor maps emerged on PND 35 and then increased in size until PND 60 and had progressively lower movement thresholds. Second, environmental enrichment did not affect initial detection of responsive points and motor maps in non-bicuculline-treated pups on PND 35. However, motor maps were larger on PND 45 in enriched rat pups relative to pups in the standard housing condition. Rats in both conditions had similar map sizes on PNDs 60, 75, and 90. Third, reach training in rat pups resulted in an internal reorganization of the map in the hemisphere contralateral, but not ipsilateral, to the trained forelimb. The map reorganization was expressed as proportionately more distal (digit and wrist) representations on PND 45. Our data indicate that both environmental enrichment and skilled reach training experience can differentially modify expression of motor maps during development.

High frequency stimulation alters motor maps, impairs skilled reaching performance and is accompanied by an upregulation of specific GABA, glutamate and NMDA receptor subunits

High frequency stimulation (HFS) has the potential to interfere with learning and memory. HFS and motor skill training both lead to potentiation of the stimulated network and alter motor map expression. However, the extent to which HFS can interfere with the learning and performance of a skilled motor task and the resulting effect on the representation of movement has not been examined. Moreover, the molecular mechanisms associated with HFS and skilled motor training on the motor cortex are not known. We hypothesized that HFS would impair performance on a skilled reaching task, and would be associated with alterations in motor map expression and protein levels compared to non-stimulated and untrained controls. Long Evans Hooded rats were chronically implanted with stimulating and recording electrodes in the corpus callosum and frontal neocortex, respectively. High frequency theta burst stimulation or sham stimulation was applied once daily for 20 sessions. The rats were divided into five groups: control, HFS and assessed at 1 week post stimulation, HFS and assessed 3 weeks post stimulation, reach trained, and HFS and reach trained. A subset of rats from each group was assessed with either intracortical microstimulation (ICMS) to examine motor map expression or Western blot techniques to determine protein expression of several excitatory and inhibitory receptor subunits. Firstly, we found that HFS resulted in larger and reorganized motor maps, and lower movement thresholds compared to controls. This was associated with an up-regulation of the GABA(A)α1 and NR1 receptor subunits 3 weeks after the last stimulation session only. Stimulation affected skilled reaching performance in a subset of all stimulated rats. Rats that were poor performers had larger rostral forelimb areas, higher proximal and lower distal movement thresholds compared to rats that were good performers after stimulation. Reach training alone was associated with an up-regulation of GABA(A)α1, α2, GluR2, NR1 and NR2A compared to controls. HFS and reach-trained rats showed an up-regulation of GABA(A)α2 compared to stimulated rats that were not reach-trained. Therefore, we have shown that HFS induces significant plasticity in the motor cortex, and has the potential to disrupt performance on a skilled motor task.

Persistent enhancement of functional MRI responsiveness to sensory stimulation following repeated seizures

PURPOSE

Neural reorganization and interictal behavioral anomalies have been documented in people with epilepsy and in animal seizure models. Alterations in behavior could be due to somatosensory dysfunction. This study was designed to determine whether seizures can lead to changes in somatosensory representations and whether those changes are persistent.

METHODS

Twice-daily seizures were elicited by delivering 1 s of electrical stimulation through carbon fiber electrodes implanted in both the corpus callosum and sensorimotor neocortex of young adult male Long-Evans rats until a total of 20 seizures were elicited. Either 1-3 days or 3-5 weeks following the last seizure, functional magnetic resonance imaging (MRI) was used to image the brain during electrical stimulation of each forepaw independently.

KEY FINDINGS

Forepaw stimulation in control rats resulted in a focused and contralateral fMRI signal in the somatosensory neocortex. Rats that had repeated seizures had a 151% increase in the number of voxels activated in the contralateral hemisphere 1-3 days after the last seizure and a 166% increase at 3-5 weeks after the last seizure. The number of voxels activated in response to forepaw stimulation was positively correlated with the duration of the longest seizure experienced by each rat. The intensity of the activated voxels was not significantly increased at either time interval from the last seizure.

SIGNIFICANCE

The increased area of activation in somatosensory cortex, which is persistent at 3-5 weeks, is consistent with previous observations of larger motor maps following seizures. Seizure-induced changes in the functioning of sensory cortex may also contribute to interictal behavioral anomalies.

Visuomotor coordination and motor representation by human temporal lobe neurons

The division of cortical visual processing into distinct dorsal and ventral streams is a key concept in primate neuroscience [Goodale, M. A., & Milner, A. D. Separate visual pathways for perception and action. Trends in Neurosciences, 15, 20-25, 1992; Steele, G., Weller, R., & Cusick, C. Cortical connections of the caudal subdivision of the dorsolateral area (V4) in monkeys. Journal of Comparative Neurology, 306, 495-520, 1991]. The ventral stream is usually characterized as a "What" pathway, whereas the dorsal stream is implied in mediating spatial perception ("Where") and visually guided actions ("How"). A subpathway emerging from the dorsal stream and projecting to the medial-temporal lobe has been identified [Kravitz, D. J., Saleem, K. S., Baker, C. I., & Mishkin, M. A new neural framework for visuospatial processing. Nature Reviews Neuroscience, 12, 217-230, 2011; Cavada, C., & Goldman-Raiuc, P. S. Posterior parietal cortex in rhesus monkey: I. Parcellation of areas based on distinctive limbic and sensory cortico-cortical connections. Journal of Comparative Neurology, 287, 393-421, 1989]. The current article studies the coordination of visual information typically associated with the dorsal stream ("Where"), with planned movements, focusing on the temporal lobe. We recorded extracellular activity from 565 cells in the human medial-temporal and frontal lobes while 13 patients performed cued hand movements with visual feedback (visuomotor task), without feedback (motor task), or observed visual feedback without motor movement (visual-only task). We discovered two different neural populations in the human medial-temporal lobe. One consists of motor-like neurons representing hand position, speed or acceleration during the motor task but not during the visuomotor or visual tasks. The other is specific to the parahippocampal gyrus (an area known to process visual motion [Gur, M., & Snodderly, D. M. Direction selectivity in V1 of alert monkeys: Evidence for parallel pathways for motion processing. Journal of Physiology, 585, 383-400, 2007; Sato, N., & Nakamura, K. Visual response properties of neurons in the parahippocampal cortex of monkeys. Journal of Neurophysiology, 90, 876-886, 2003]) and encodes speed, acceleration, or direction of hand movements, but only during the visuomotor task: neither during visual-only nor during motor tasks. These findings suggest a functional basis for the anatomical subpathway between the dorsal stream and the medial-temporal lobe. Similar to the recent expansion of the motor control process into the sensory cortex [Matyas, F., Sreenivasan, V., Marbach, F., Wacongne, C., Barsy, B., Mateo, C., et al. Motor control by sensory cortex. Science, 330, 1240-1243, 2010], our findings render the human medial-temporal lobe an important junction in the process of planning and execution of motor acts whether internally or externally (visually) driven. Thus, the medial-temporal lobe might serve as an integration node between the two processing streams. Our findings thus shed new light on the brain mechanisms underlying visuomotor coordination which is a crucial capacity for everyday survival, whether it is identifying and picking up food, sliding a key into a lock, driving a vehicle, or escaping a predator.

A prolonged experimental febrile seizure results in motor map reorganization in adulthood

INTRODUCTION

Clinical studies have suggested that children experiencing a febrile seizure (FS) before the age of 1year have persistent deficits, but it is unknown whether these seizures lead to permanent cortical reorganization and alterations in function. A FS on the background of increased genetic seizure susceptibility may also lead to negative long-term consequences. Alterations in neocortical motor map expression provide a measure of neocortical reorganization and have been reported in both adults with frontal lobe epilepsy and following seizure induction in experimental models. The objectives of the present study were to determine whether (1) an infantile FS leads to changes to motor map expression in adulthood; (2) long-term cortical reorganization is a function of the age at FS or genetic seizure susceptibility; and (3) different levels of GABA(A) or glutamate receptor subunits or cation-chloride-co-transporters (CCCs) at the time of FS correlate with alterations to motor map expression.

MATERIALS AND METHODS

FSs were induced in postnatal day 10 (P10) or P14 Long-Evans (LE) rats or in P14 seizure-prone FAST rats by the administration of the bacterial endotoxin lipopolysaccharide (LPS) and a subconvulsant dose of kainic acid. Ten weeks later intracortical microstimulation was performed to generate motor maps of forelimb movement representations. Sensorimotor neocortex samples were also dissected from naïve P10 FAST and P10 and P14 LE pups for western blotting with antibodies against various GABA(A), NMDA, and AMPA receptor subunits and for CCCs.

RESULTS

Adult FAST rats had larger motor maps with lower stimulation thresholds after a FS at P14, while adult LE rats had significantly lower map stimulation thresholds but similar sized maps after a FS at P10 compared to controls. There were no differences in neocortical motor map size or stimulation thresholds in adult LE rats after a FS at P14. Both P10 LE and P14 FAST rats had significantly lower levels of the GABA(A) receptor α1 subunit, higher levels of the α2 subunit, and a higher NKCC1/KCC2 ratio in the sensorimotor cortex compared with the P14 LE rat. In addition, the P14 FAST rats had lower levels of the GluR2 and NR2A receptor subunits in the sensorimotor cortex compared with the P14 LE rats.

CONCLUSIONS

A single infantile FS can have long-term effects on neocortical reorganization in younger individuals and those with underlying seizure susceptibility. These changes may be related to an increased level of excitability in the neocortex of younger or genetically seizure-prone rats, as suggested by immaturity of their GABAergic and CCC systems. Given the high incidence of FSs in children, it will be important to gain a better understanding of how age and genetic seizure predisposition may contribute to the long-term sequelae of these events.

Larger cortical motor maps after seizures

Expansion of motor maps occurs in both clinical populations with epilepsy and in experimental models of epilepsy when the frontal lobes are involved. We have previously shown that the forelimb area of the motor cortex undergoes extensive enlargement after seizures, although the extent to which many movement representation areas are altered is not clear. Here we hypothesize that movement representations in addition to the forelimb area will be enlarged after cortical seizures. To test our hypotheses, Long Evans Hooded rats received 20 sessions of callosal (or sham) kindling, and then were subjected to intracortical microstimulation to map several movement representations including the jaw, neck, forelimb, hindlimb, trunk and tail. We found significantly larger total map areas of several movement representations, including movements that could be evoked more posterior than they are in control rats. We also show the presence of more multiple movement sites and lower movement thresholds in kindled rats, suggesting that movements not only overlap and share cortical territory after seizures, but become present in formerly non-responsive sites as they become detectable with our intracortical microstimulation methodology. In summary, several motor map areas become larger after seizures, which may contribute to the interictal motor disturbances that have been documented in patients with epilepsy.

Optimal parameters for microstimulation derived forelimb movement thresholds and motor maps in rats and mice

Intracortical microstimulation (ICMS) is a technique that was developed to derive movement representations (motor maps) of the motor cortex, and was originally used in cats and the capuchin monkey. In more modern experiments, ICMS has been used in rats and mice to assess and interpret plasticity of motor maps in response to experimental manipulation; however, a systematic determination of the optimal ICMS parameters necessary to derive baseline motor maps in rats and mice has not been published. In the present manuscript, we describe two experiments. We first determined the optimal stimulation frequency, pulse number, neocortical depth, and current polarity to achieve the minimum current intensity (movement threshold) to elicit forelimb movements in rats and mice. We show that experimentally naïve rats and mice differ on several of these ICMS parameters. In the second experiment, we measured movement thresholds and map size in states of enhanced neocortical inhibition by the administration of diazepam, as well as neocortical sensitization as the result of repeated seizures. We conclude that movement thresholds are inversely related to motor map size, and that treatments result in a widespread shift the balance between excitation and inhibition in motor neocortical layer 5 influences both movement thresholds and map size.

Impaired perceptual networks in temporal lobe epilepsy revealed by resting fMRI

Viewed as a neural network disorder, mesial temporal lobe epilepsy (mTLE) may cause widespread deficits in human brain functions. Impairments in cognitive functions such as memory and language have been well addressed, but perceptual deficits have only been considered in terms of behavioral data. Little imaging research on perceptual deficits in mTLE has been reported. The present study is expected to reveal impairments in the perceptual networks in patients with mTLE using fMRI. The fMRI-based independent component analysis (ICA) was applied to 33 patients with mTLE and 33 matched healthy controls. In light of the resting-state networks (RSNs) corresponding to the basal functions of visual, auditory, and sensorimotor systems, the ICA data of functional connectivity within these RSNs were compared between the patients and controls. Compared with the controls, the mTLE patients presented decreased functional connectivity within the regions of the auditory and sensorimotor networks, as well increased functional connectivity in the primary visual cortex and decreased functional connectivity in the bilateral MT+ areas of the visual network. Our neuroimaging results are in agreement with the previous findings that specific perceptual functions are impaired in mTLE. Furthermore, our findings in the visual network support the belief that the primary visual function is not impaired and that there may be deficits in the high-order visual function in mTLE. Our fMRI study may contribute to the understanding of neuropathophysiological mechanisms underlying perceptual impairments in mTLE.

Motor map expansion in the pilocarpine model of temporal lobe epilepsy is dependent on seizure severity and rat strain

Functional alterations in movement representations (motor maps) have been observed in some people with epilepsy and, under experimental control, electrically-kindled seizures in rats also result in persistently larger motor maps. To determine if a single event of status epilepticus and its latent consequences can affect motor map expression, we assessed forelimb motor maps in rats using the pilocarpine model of temporal lobe epilepsy. We examined both pilocarpine-induced seizures, and status epilepticus (SE) in two strains that differ in their propensity for epileptogenesis; Wistar and Long-Evans. Pilocarpine was administered intraperitoneally at dosages that resulted in equivalent proportions of seizures, SE, and survival in both strains. Rats from both strains were given saline injections as a control. Diazepam was administered to all rats to attenuate seizure activity and promote survival. All rats had high-resolution movement representations derived using standard intracortical microstimulation methodologies at 48 h, 1 week, or 3 weeks following treatment. Pilocarpine-induced seizures only gave rise to motor map enlargement in Wistar rats, which also showed interictal spiking, and only at 3 weeks post-treatment indicating altered motor map expression in this strain following a latent or maturational period. Pilocarpine-induced SE yielded larger motor maps at all time points in Wistar rats but only a transient (48 h) map expansion in Long-Evans rats. Our results demonstrate that seizures and SE induced by a convulsant agent alter the functional expression of motor maps that is dependent on seizure severity and a genetic (strain) predisposition to develop epileptiform events.

Functional brain mapping at 9.4T using a new MRI-compatible electrode chronically implanted in rats

There is a need for acute and chronic stimulation of the brain within the MRI for studies of epilepsy, as well as deep brain stimulation for movement and behavioral disorders. This work describes the production and characteristics of carbon fiber-based electrodes for acute and chronic stimulation in the brain. Increasing MRI field strengths are making it increasingly difficult to introduce foreign objects without a susceptibility artifact. We describe the production of, and the characteristics of carbon fiber-based electrodes. These are biocompatible and can be implanted for chronic studies. We show the use of these electrodes at 9.4T for studying functional activation. Data are presented showing regional connectivity. Activation not only occurs near the electrode, but at sites distant and often contralateral to the electrode. In addition, there were sites showing strong negative activation to stimulation both with direct stimulation and during a kindling-associated seizure.

Repeated seizures lead to altered skilled behaviour and are associated with more highly efficacious excitatory synapses

People with epilepsy have a high incidence of interictal behavioural problems that appear to be related to the location of their seizure focus. This study investigated a novel test of the hypotheses that repeated seizures result in behavioural deficits and altered performance during the interictal state, and that those behaviours are related to the presence of more highly efficacious excitatory synapses. We tested these hypotheses by first repeatedly eliciting seizures with electric current through indwelling electrodes in the corpus callosum at the level of the caudal forelimb area of sensorimotor neocortex in the rat. We then assessed learned skilled behaviours that primarily utilize the forelimbs on tasks that are sensitive to the functional integrity of that structure. We observed both behavioural deficits and altered kinematic performance in rats that experienced repeated neocortical seizures relative to an electrode-implanted control group. From a separate set of rats, tissue was prepared for quantification of thickness and excitatory synaptic subtypes from neocortical layer V. We observed significantly increased numbers of perforated synapses that make their connections directly onto the dendritic shaft at 3 weeks following the last seizure. Altered reaching behaviours are likely due to neural reorganization in the neocortex including more efficacious synapses.

Low-frequency stimulation reverses kindling-induced neocortical motor map expansion

Repeated application of low-frequency stimulation can interrupt the development and progression of seizures. Low-frequency stimulation applied to the corpus callosum can also induce long-term depression in the neocortex of awake freely moving rats as well as reduce the size of neocortical movement representations (motor maps). We have previously shown that seizures induced through electrical stimulation of the corpus callosum, amygdala or hippocampus can expand the topographical expression of neocortical motor maps. The purpose of the present study was to determine if low-frequency stimulation administered to the corpus callosum could reverse the expansion of neocortical motor maps induced by seizures propagating from the hippocampus. Adult Long-Evans hooded rats were electrically stimulated in the right ventral hippocampus, twice daily until 30 neocortical seizures were recorded. Subsequently, low-frequency stimulation was administered to the corpus callosum once daily for 20 sessions. High-resolution intracortical microstimulation was then utilized to derive forelimb-movement representations in the left (un-implanted) sensorimotor neocortex. Our results show that hippocampal seizures result in expanded motor maps and that subsequent low-frequency application can reduce the size of the expanded motor maps. Low-frequency stimulation may be an effective treatment for reversing seizure-induced reorganization of brain function.

Reduction of seizure thresholds following electrical stimulation of sensorimotor cortex is dependent on stimulation intensity and is not related to synaptic potentiation

Epilepsy is characterized as a chronic brain state with a very low seizure threshold, and the occurrence of repeated seizure activity. Currently, there is no animal model of induced epilepsy that allows for the exploration of the brain mechanisms underlying a low seizure threshold without the elicitation of seizures. In this study, we employed repeated application of different intensities of electrical stimulation in an attempt to reduce afterdischarge (seizure) thresholds without eliciting seizures. We utilized an in vivo model of neocortical activation via stimulation of the corpus callosum of the adult rat. The intensities were chosen to be subthreshold (20, 30, 40, 50 microA), near threshold (150 microA), and suprathreshold (250, 500 microA) relative to the mean initial afterdischarge threshold (ADT). We also examined changes in the evoked field responses of the transcallosal pathway to the sensorimotor cortex as a measure of synaptic efficacy. Our results indicated that stimulation at 50 microA was effective at reducing the ADT, while minimizing the number of seizures elicited. Stimulation at 150 microA resulted in the concomitant reduction of ADT and repeated seizures typical of most electrical kindling studies. Finally, the 500 microA group showed repeated seizures, but no reduction of afterdischarge threshold. These stimulation intensities (50 microA, 150 microA, 500 microA and 0 microA-control) can be used to independently determine the brain mechanisms responsible for 1) the acquisition of a low afterdischarge threshold independent of the reorganizing effect of repeated seizures, and 2) the elicitation of repeated seizures independent of stimulation induced reduction of afterdischarge threshold.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.