Corey Flynn

Cortical stimulation improves skilled forelimb use following a focal ischemic infarct in the rat

Abstract Improving functional recovery following cerebral strokes in humans will likely involve augmenting brain plasticity. This study examined skilled forelimb behavior, neocortical evoked potentials, and movement thresholds to assess cortical electrical stimulation concurrent with rehabilitative forelimb usage following a focal ischemic insult. Adult rats were trained on a task that required skilled usage of both forelimbs. They then underwent an acute focal ischemic insult to the caudal forelimb area of sensorimotor cortex contralateral to their preferred forelimb. During the same procedure, they also received a stimulation electrode over the infarct area and two depth electrodes anterior to the lesion to record evoked potentials. One week following the surgery, rats received cortical stimulation during performance of the skilled task. Evoked potentials and movement thresholds were also determined. Functional assessment revealed that cortical stimulation resulted in superior performance compared to the no stimulation group, and this was initially due to a shift in forelimb preference. Cortical stimulation also resulted in enhanced evoked potentials and a reduction in the amount of current required to elicit a movement, in a stimulation frequency dependent manner. This study suggests that cortical stimulation, concurrent with rehabilitative training, results in better forelimb usage that may be due to augmented synaptic plasticity.

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.

FGF-2-induced cell proliferation stimulates anatomical, neurophysiological and functional recovery from neonatal motor cortex injury

Infant rats treated with basic fibroblast growth factor‐2 (FGF‐2) after postnatal day (P)10 motor cortical injury, show functional improvement in adulthood relative to those that do not receive FGF‐2. In this study we used a combination of behavioural, immunohistochemical, electrophysiological, electron microscopic and teratological approaches to investigate possible mechanisms by which FGF‐2 may influence functional recovery. We show that subcutaneous injections of FGF‐2 following bilateral lesions to the motor cortex at P10 in the rat leads to filling of the lesion area with migrating neuroblasts and cycling cells. We assessed the functionality of this tissue in adulthood, and show that cells from the filled region spontaneously fire and form synapses. Behavioural analysis shows enhanced motor performance in the FGF‐2‐treated lesion rats in comparison to vehicle‐treated lesion rats, and this improvement is reversed by removal of the tissue from the previously lesioned area or by blocking cortical regeneration by embryonic treatment with bromodeoxyuridine (BrdU). The results show that FGF‐2 stimulates filling of the lesion cavity with cells after neonatal motor cortex lesions, that the new tissue has anatomical and physiological properties similar to control tissue, and that the filled region supports motor behaviour.

Differential neuroplastic changes in neocortical movement representations and dendritic morphology in epilepsy-prone and epilepsy-resistant rat strains following high-frequency stimulation

The epileptogenic‐prone (FAST) and epileptogenic‐resistant (SLOW) rat strains have become a valuable tool for investigating the neurochemical and neurophysiological basis of epilepsy. This study examined the two strains with respect to their neocortical movement representations and cortical layer III pyramidal cell dendritic morphology in both control and potentiated conditions. FAST and SLOW rats received high‐frequency stimulation of the corpus callosum in order to induce long‐term polysynaptic potentiation of the transcallosal pathway to the sensorimotor neocortex. Baseline‐evoked potentials of this pathway were recorded in the left hemisphere before stimulation, and following 5, 10, 15 and 20 days of high‐frequency stimulation. All rats then underwent high‐resolution intracortical microstimulation (ICMS) in order to assess functional movement representations of the left caudal forelimb area of the sensorimotor cortex. Immediately following ICMS, the brains were stained with the Golgi–Cox method, and the length, branching and spine density of frontal and occipital neocortical layer III pyramidal neurons were measured. We observed that high‐frequency stimulation induced similar increases in polysynaptic potentiation in both rat strains; however, only the FAST strain showed an increase (doubling) in the size of their motor maps. We also observed decreases in dendritic length and branching in the FAST rats, and the opposite profile in the SLOW rats. The potentiated FAST rats also showed an increase in spine density. Our results suggest that differences in susceptibility to epileptogenesis may result in a differential response to stimulation‐induced plasticity.

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.

Motor maps, seizures, and behaviour.

Atypically organised motor maps have been described in some people with epilepsy and we have modelled this in rats. Our goal is to more fully understand the mechanisms responsible for seizure-induced functional brain reorganisation and to reverse their effects. Here we present an overview of the relationship between neocortical motor maps, seizures, and interictal behaviour. To begin we summarise the observations of atypical motor maps with epilepsy and in animal models following experimentally induced seizures. Our novel experiments have established that motor map expansion is linked to a functional alteration of motor behaviour. Evidence for some of the putative brain mechanisms responsible for motor map size is discussed. Our successes reversing seizure-induced map expansion by two different methods are also briefly reviewed. Lastly, unanswered questions for possible future experimentation are posed.

Seizures, but not lowered seizure thresholds, results in larger neocortical motor maps and concomitant disruptions in skilled motor behaviour

Kindling of the sensorimotor neocortex has been found to result in reorganization of the somatotopic map of movement representations as well as disruptions of skilled forelimb behaviours. It has been suggested that the repeated seizures induced during kindling altered motor maps, thereby disrupting the motor engram necessary for the production of skilled movements. However, kindling leads to neural changes other than those associated with repeated seizures, and the role of these comorbid effects is often overlooked. Our lab has developed a stimulation paradigm, which allows for the dissociation of the two main effects of kindling; repeated seizures and the reduction of afterdischarge (seizure) threshold. In the current study, we have utilized this paradigm to examine the effects of electrical stimulation on motor maps and skilled forelimb behaviour. We found that repeated seizures with no concomitant reduction of afterdischarge threshold resulted in large motor maps, as well as task specific deficits in skilled forelimb use and deficiencies in task acquisition. Rats that had reduced seizure thresholds and few seizures did not show alterations in map size or skilled forelimb use. These results suggest that movement disturbances following kindling are the result of repeated seizures, and not other stimulation-induced effects such as reduction of afterdischarge threshold. These results also corroborate the relationship between the integrity of movement representations and the ability to perform skilled motor tasks.

Zincergic innervation of the forebrain distinguishes epilepsy-prone from epilepsy-resistant rat strains.

Zinc is released from a subset of cerebral cortical neurons whereupon it exerts a powerful modulatory influence on excitatory and inhibitory neurotransmission. A number of studies have suggested that alterations in the regulation of zinc may contribute to the genesis of epilepsy. Here, we tested this hypothesis by examining the distribution of zinc-containing axon terminals in rats selectively bred for an innate susceptibility (FAST) or resistance (SLOW) to the development of kindling-induced seizures. Zinc was stained histochemically and levels of staining were quantitatively assessed. We found that the levels of synaptic zinc were significantly lower in the SLOW rats throughout the telencephalon. This relative reduction was most pronounced in limbic cortices where levels were less than 30% of FAST rats. These results suggest that innate differences in the homeostatic regulation of synaptic zinc, particularly in limbic cortices, may underlie differences in epileptogenicity.

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.

Neurophysiological properties of cells filling the neonatal medial prefrontal cortex lesion cavity.

Removal of the medial prefrontal cortex (mPFC) of the rat during the initial 7-12 days of life results in spontaneous filling of lesion cavity that is accompanied by recovery of cognitive and motor functions. To date, it remains uncertain whether tissue filling the lesion cavity is actually supporting the functional improvement. In the present study, we examined whether spontaneous neuronal activity could be recorded in adulthood from the tissue that fills the lesion cavity. We recorded EEG and multiunit activity in adulthood from the mPFC and the motor cortex of rats that had received neonatal mPFC lesions on post-natal day 10 (P10) or their non-lesioned littermate controls. We found similarities in both the firing pattern and firing rate of cells from the filled-in region compared to that of controls, although the power associated with peak frequencies in the delta, alpha, and beta range in the EEG recorded from the filled-in region was lower compared to controls. Overall, our results suggest that the cells found in the lesion cavity have similar neurophysiological properties to those found in normal tissue and thus should be capable of at least partially supporting the observed recovery of function.