Claus Bruehl

Neuronal Hyperexcitability and Reduction of GABAA-Receptor Expression in the Surround of Cerebral Photothrombosis:

Changes of neuronal excitability and γ-aminobutyric acid (GABAA)-receptor expression were studied in the surround of photothrombotic infarcts, which were produced in the sensorimotor cortex of the rat by using the rose bengal technique. In a first series of experiments, multiunit recordings were performed on anesthetized animals 2–3 mm lateral from the lesion. Mean discharge frequency was considerably higher in recordings from lesioned animals (>100 Hz in the first postlesional week) compared with control animals (mean, 15 Hz). These alterations were already present after 1 day but were most pronounced 3 to 7 days after lesion induction. Thereafter the hyperexcitability declined again, although it remained visible up to 4 months. In a second series of experiments, the GABAA-receptor expression was studied autoradiographically. This revealed a reduction of GABAA receptors in widespread brain areas ipsilateral to the lesion. The reduction was most pronounced in the first days after lesion induction and declined with longer intervals. It is concluded that cortical infarction due to photothrombosis leads to a long-lasting and widespread reduction of GABAA-receptor expression in the surround of the lesion, which is associated with an increased neuronal excitability. Such alterations may be responsible for epileptic seizures that can be observed in some patients after stroke and may contribute to neurologic deficits after stroke.

Electrophysiological transcortical diaschisis after middle cerebral artery occlusion (MCAO) in rats

Remote changes in brain function following stroke are called diaschisis. These remote effects may contribute to the neurological deficit following brain infarction; in addition they may lead to post-stroke epilepsy and affect functional recovery. In the present study we addressed the question of whether an increase in excitability can be observed contralateral to middle cerebral artery (MCA) infarction. Permanent occlusion of the middle cerebral artery (MCAO) was induced experimentally in rats with an intraluminal silicon-coated filament. Seven days later, brain excitability was tested with extracellulare recording techniques in neocortical coronal brain slices using a paired-pulse stimulus protocol. In rats with MCAO, excitability was increased in the neocortex contralateral to the infarction compared with the control group. These alterations extended through wide parts of the contralateral neocortex. The study demonstrates that MCAO causes transcallosal electrophysiological diaschisis. Together with results obtained previously with photothrombotic cortical lesions, it can be concluded that these remote effects are not due to characteristics of the individual lesion model, but are common consequences of brain lesions.

Uncoupling of Blood Flow and Metabolism in Focal Epilepsy

Summary: Purpose: Interictal measurements of cerebral blood flow are less helpful in localizing epileptic foci than are measurements of brain metabolism. This may be related to an uncoupling of blood flow and metabolism. In this study, brain metabolism and blood flow were compared in an acute experimental model of focal interictal epilepsy.

Dynamic changes of focal hypometabolism in relation to epileptic activity.

The interictal hypometabolism in patients with focal epilepsy is usually regarded as stationary. In this study we investigated to which extent the hypometabolism may depend on the activity of the epileptic focus. In focal penicillin-induced epilepsy in rats the epileptic focus is hypermetabolic. This focus is accompanied by hypometabolism in widespread areas of adjacent cerebral cortex. The experiments revealed that these metabolic alterations are transient. Data from a patient experiencing a focal seizure during PET scanning gave similar results. They showed that the transition from interictal to ictal activity was accompanied by the development of hypermetabolic epileptic focus and the dynamic enlargement of the surrounding hypometabolism. Both, the experimental and clinical data provide evidence that the cerebral hypometabolism may vary in size depending on the activity of the epileptic focus. It is hypothesized that in human PET studies the large interictal hypometabolism may prevent the identification of hyperactive interictal epileptic foci due to the partial volume effects resulting from the limited spatial resolution of PET cameras.

Enlargement of cortical vibrissa representation in the surround of an ischemic cortical lesion.

It has been shown that cortical lesions are associated with an increase of excitability in surrounding brain regions, and with a downregulation of GABA(A) receptors. In the present study we investigated whether this increased excitability affects the cortical map of inputs represented in areas surrounding the lesioned brain area. Focal lesions with a diameter of 2-2.5 mm were induced photochemically in the hindlimb area at the border of the primary somatosensory cortex of the rat. One week after lesioning, the cortical representation of the B3 vibrissa was studied using 14C-deoxyglucose (DG) autoradiography. In all animals mechanical stimulation of the B3 vibrissa produced a column-shaped DG-labeling in the somatosensory cortex, corresponding to the B3-barrel with a maximum of the glucose uptake in layer IV. In control animals without cortical lesions (n=6), stimulation increased the glucose uptake rate by 50.8+/-10.5% in layer IV. In lesioned animals (n=6) maximum DG-uptake in layer IV (54.8+/-8.6%) did not differ significantly from that in controls. However, as compared to control animals, lesioned animals showed also increased glucose uptake within the activated column in layers II/II (51.+/-11.1%, lesioned animals; 31.8+/-11.2%, controls; P<0.05, lesioned vs. control) and V (47.5+/-5.8%, lesioned animals, 28.8+/-10.5%, controls; P<0.05, lesioned vs. control). The diameter of the metabolically activated B3-barrel area of layer IV was expanded from 461.8+/-77.6 microm in control animals to 785.5+/-103.6 microm; P<0.01) in lesioned animals. Lesioned animals also showed expansion of the activated area in layers II/III (890.4+/-134.8 microm, lesioned animals; 430.6+/-95.1 microm, controls; P<0.01) and layer V (1117.5+/-163.6 microm, lesioned animals; 648.7+/-114.1 microm, controls; P<0.01). The depth profile of the activation columns showed a maximum in layer IV in control animals, which was expanded towards layers II/III and layer V in lesioned animals. It is concluded that cortical lesions alter the representational area of neighboring afferent inputs through disinhibition or unmasking of pre-existing silent or ineffectual intracortical synapses. The present observations raise the possibility that the brain supports recovery from lesions by decreasing GABAergic inhibition, thereby facilitating a remapping of the cortical representation in neighboring brain areas.

Coupling of cortical and thalamic metabolism in experimentally induced visual and somatosensory focal epilepsy

Focal epileptic activity induces widespread metabolic disturbances beyond the area of the electroencephalographically detectable focus. In order to find out whether the metabolic coupling between the epileptic focus and other brain regions depends on the localization of the focus, two groups of rats with epileptic foci at different sites were investigated. In the first group acute epileptic activity was induced by application of penicillin to the secondary visual cortex (Oc2), and in the second group to the primary somatosensory cortex (Par1). Metabolism was analyzed using the [14C]deoxyglucose autoradiographic method. In both groups of animals, hypermetabolism in the area of the focus and in specific functionally coupled thalamic nuclei was observed. Focal epileptic activity in the secondary visual cortex induced significant hypometabolism in remote ipsilateral cortical areas. In rats with epileptic foci in the primary somatosensory cortex hypometabolism in extrafocal ipsilateral cortical areas was less prominent. These findings provide further support for the integral involvement of the thalamus in modulating metabolism in remote cortical brain regions during focal epileptic activity. The extent of metabolic alterations may depend on the site of the epileptic focus and the connectivity of the recruited thalamic nuclei.

Enhancement of whole cell calcium currents following transient MCAO

Cerebral infarctions have been shown to cause widespread changes of neuronal excitability in non-infarcted tissue. Calcium currents are major determinants of neuronal behavior, and pathological modulation of Ca(2+)-channels is known to lead to altered excitability states in a variety of paradigms. In the present study we addressed the question to what extent whole cell calcium currents are altered after middle cerebral artery occlusion (MCAO) in both the ipsi- and contralateral sensory cortex. Transient middle cerebral artery occlusion was induced for 1 h in rats using the intraluminal thread model. After 7 or 28 days survival, whole cell patch clamp studies were carried out on freshly isolated neurons of the ipsi- and contralateral sensory cortex, and high voltage activated (HVA) calcium currents were examined. In lesioned animals, we found a significant increase of calcium current amplitude and maximal conductance in the sensory cortex contralateral to the infarcts. This was paralleled by a prominent positive shift of the potential of half-maximal activation (V(h,a)) in these cells. Changes were long-lasting and at least stable for the following 28 days. These alterations were present in animals with lesions of moderate size, but not in those with massive infarction, and only in the cortex contralateral to the lesion. Following cortical infarctions, changes of calcium current properties are selectively observed in neurons contralateral to the lesion. At the behavioral level, compensatory mechanisms involving the unaffected hemisphere may induce this alteration of calcium current properties.

Metabolic and electrophysiological alterations in an animal model of neocortical neuronal migration disorder.

Cortical migration disorders are a major cause for intractable epilepsy syndromes. High resolution MRI and PET are increasingly capable to identify cortical dysgenesis. In this study we used the rat freeze lesion model to investigate cortical morphological and functional changes in adult rats after induction of a cortical freeze lesion at postnatal day (p) 0. Autoradiographic measurements of basic cortical [14C]deoxyglucose metabolism showed a significant reduction up to 1 mm lateral to the lesion but no remote changes. Electrophysiological in vitro recordings revealed a significant reduction in the amplitude of stimulus-evoked field potential responses recorded lateral to the lesion as compared to medial recording sites. Our data provide further evidence that spatially restricted developmental alterations of cortical morphology cause functional changes in surrounding and histologically normal areas that need to be considered for a better understanding of the resulting pathophysiology.

Effects of tetanus toxin on functional inhibition after injection in separate cortical areas in rat

Tetanus Toxin is widely used as a model of chronic focal epilepsy and is assumed to act by blocking neurotransmitter release with high selectivity for inhibitory synapses. However, the exact mechanisms are not fully understood, since, e.g., GABA release is only temporarily decreased although epileptiform activity persists pointing towards a change in the interplay of excitation and inhibition. Furthermore there have been reports about different effects of tetanus toxin after injection in separate brain areas. Therefore, we investigated the functional inhibition after injecting tetanus toxin either in the motor or sensory cortex of adult rats by using a paired-pulse paradigm as a measure of excitatory and inhibitory drive. Tetanus toxin injection into the motor cortex (n=10) induced a marked, long-lasting reduction in inhibition which was highly significant in most parts of the injected cortical area. Injections into the sensory cortex, however, showed less marked changes in inhibition which were more widespread and significant only in 3 of 14 animals injected. These results give further evidence for a prominent effect of tetanus toxin on functional inhibition and strengthen the idea of a differential effect in separate cortical areas. They may be accounted for by the different cytoarchitecture of cortical areas with variable inhibitory and excitatory intracortical connections.