Maria Blatow

In Vivo Labeling of Parvalbumin-Positive Interneurons and Analysis of Electrical Coupling in Identified Neurons

GABAergic interneurons can pace the activity of principal cells and are thus critically involved in the generation of oscillatory and synchronous network activity. The specific role of various GABAergic subpopulations, however, has remained elusive. This is in part attributable to the scarcity of certain GABAergic neurons and the difficulty of identifying them in slices obtained from brain regions in which anatomical structures are not readily recognizable in the live preparation. To facilitate the functional analysis of GABAergic interneurons, we generated transgenic mice in which the enhanced green fluorescent protein (EGFP) was specifically expressed in parvalbumin-positive neurons. The high fidelity of expression obtained using bacterial artificial chromosome transgenes resulted in EGFP-labeled neurons in nearly all brain regions known to contain parvalbumin-expressing neurons. Immunocytochemical analysis showed that EGFP expression was primarily restricted to parvalbumin-positive cells. In addition to cell body labeling, EGFP expression was high enough in many neurons to enable the visualization of dendritic structures. With the help of these mice, we investigated the presence of electrical coupling between parvalbumin-positive cells in brain slices obtained from young and adult animals. In dentate gyrus basket cells, electrical coupling was found in slices from young [postnatal day 14 (P14)] and adult (P28 and P42) animals, but both strength and incidence of coupling decreased during development. However, electrical coupling between parvalbumin-positive multipolar cells in layer II/III of the neocortex remains unaltered during development. Yet another developmental profile of electrical coupling was found between layer II/III parvalbumin-positive cells and excitatory principal cells. Between these neurons, electrical coupling was found at P14 but not at P28. The results indicate that the presence and strength of electrical coupling is developmentally regulated with respect to brain area and cell type.

A Novel Network of Multipolar Bursting Interneurons Generates Theta Frequency Oscillations in Neocortex

GABAergic interneurons can phase the output of principal cells, giving rise to oscillatory activity in different frequency bands. Here we describe a new subtype of GABAergic interneuron, the multipolar bursting (MB) cell in the mouse neocortex. MB cells are parvalbumin positive but differ from fast-spiking multipolar (FS) cells in their morphological, neurochemical, and physiological properties. MB cells are reciprocally connected with layer 2/3 pyramidal cells and are coupled with each other by chemical and electrical synapses. MB cells innervate FS cells but not vice versa. MB to MB cell as well as MB to pyramidal cell synapses exhibit paired-pulse facilitation. Carbachol selectively induced synchronized theta frequency oscillations in MB cells. Synchrony required both gap junction coupling and GABAergic chemical transmission, but not excitatory glutamatergic input. Hence, MB cells form a distinct inhibitory network, which upon cholinergic drive can generate rhythmic and synchronous theta frequency activity, providing temporal coordination of pyramidal cell output.

Ca2+ Buffer Saturation Underlies Paired Pulse Facilitation in Calbindin-D28k-Containing Terminals

Ca2+ buffer saturation was proposed as a mechanism of paired pulse facilitation (PPF). However, whether it operates under native conditions remained unclear. Here we show that saturation of the endogenous fast Ca2+ buffer calbindin-D28k (CB) plays a major role in PPF at CB-containing synapses. Paired recordings from synaptically connected interneurons and pyramidal neurons in the mouse neocortex revealed that dialysis increased the amplitude of the first response and decreased PPF. Loading the presynaptic terminals with BAPTA or CB rescued the effect of the CB washout. We extended the study to the CB-positive facilitating excitatory mossy fiber-CA3 pyramidal cell synapse. The effects of different extracellular Ca2+ concentrations and of EGTA indicated that PPF in CB-containing terminals depended on Ca2+ influx rather than on the initial release probability. Experiments in CB knockout mice confirmed that buffer saturation is a novel basic presynaptic mechanism for activity-dependent control of synaptic gain.

fMRI reflects functional connectivity of human somatosensory cortex.

Unilateral sensory stimulation reliably elicits contralateral somatotopic activation of primary (SI) and secondary (SII) somatosensory cortex. There is an ongoing debate about the occurrence and nature of concomitant ipsilateral SI and SII activation. Here we used functional magnetic resonance imaging (fMRI) in healthy human subjects with unilateral tactile stimulation of fingers and lips, to compare somatosensory activation patterns from distal and proximal body parts. We hypothesized that fMRI in humans should reflect the functional connectivity of somatosensory cortex as predicted by animal studies. We show that both unilateral finger and lip stimulations activate contra- and ipsilateral SI and SII cortices with high detection frequency. Correlations of BOLD-signals to the applied hemodynamic reference function were significantly higher in contralateral as compared to ipsilateral SI and SII cortices for both finger and lip stimulation, reflecting strong contribution of contralateral thalamocortical input. Furthermore, BOLD-signal correlations were higher in SI than in SII activations on the contralateral but not on the ipsilateral side. While these asymmetries within and across hemispheres were consistent for finger and lip stimulations, indicating analogous underlying organizing principles, they were less prominent for lip stimulation. Somatotopic organization was detected in SI but not in SII representations of fingers and lips. These results qualitatively and quantitatively support the prevalent concepts of anatomical and functional connectivity in the somatosensory system and therefore may allow interpretation of sensory evoked fMRI signals in terms of normal human brain function. Thus, the assessment of human somatosensory function with fMRI may permit in the future investigations of pathological conditions.

Two Calretinin-Positive GABAergic Cell Types in Layer 2/3 of the Mouse Neocortex Provide Different Forms of Inhibition

Calretinin (CR)-positive GABAergic (gamma-aminobutyric acidergic) interneurons have been suggested to target preferentially other GABAergic cells in the neocortex. To systematically study this cell population in the cortex, we generated transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the CR promoter and characterized EGFP/CR-positive cells at the cellular and network level. Based on anatomical and electrophysiological characteristics, 2 types of EGFP/CR-positive cells could be distinguished that we termed bipolar (BCR) and multipolar (MCR) CR cells. Both cell types share the feature of preferential interneuron targeting but differ in most other characteristics, including firing pattern, biochemical markers, neurite arborization, and synaptic plasticity. Like many other GABAergic interneurons, BCR cells but not MCR cells exhibit restricted cell type-specific gap junction coupling. Notably, MCR cells are electrically coupled in an asymmetric fashion with GABAergic interneurons of another subtype, the parvalbumin-positive multipolar bursting (MB) cells. Most importantly, the strength of electrical coupling between MCR and MB cells underlies their synchronous activation during carbachol-induced oscillations.

Diagnostic benefits of presurgical fMRI in patients with brain tumours in the primary sensorimotor cortex

ObjectivesReliable imaging of eloquent tumour-adjacent brain areas is necessary for planning function-preserving neurosurgery. This study evaluates the potential diagnostic benefits of presurgical functional magnetic resonance imaging (fMRI) in comparison to a detailed analysis of morphological MRI data.MethodsStandardised preoperative functional and structural neuroimaging was performed on 77 patients with rolandic mass lesions at 1.5 Tesla. The central region of both hemispheres was allocated using six morphological and three functional landmarks.ResultsfMRI enabled localisation of the motor hand area in 76/77 patients, which was significantly superior to analysis of structural MRI (confident localisation of motor hand area in 66/77 patients; p < 0.002). FMRI provided additional diagnostic information in 96% (tongue representation) and 97% (foot representation) of patients. FMRI-based presurgical risk assessment correlated in 88% with a positive postoperative clinical outcome.ConclusionRoutine presurgical FMRI allows for superior assessment of the spatial relationship between brain tumour and motor cortex compared with a very detailed analysis of structural 3D MRI, thus significantly facilitating the preoperative risk-benefit assessment and function-preserving surgery. The additional imaging time seems justified. FMRI has the potential to reduce postoperative morbidity and therefore hospitalisation time.

Global activation of primary motor cortex during voluntary movements in man.

Unilateral voluntary movements are accompanied by robust activation of contralateral primary motor cortex (M1) in a somatotopic fashion. Occasionally, coactivation of M1 (M1-CoA) ipsilateral to the movement was described. In a study with brain tumor patients, we consistently observed additional somatotopic M1-CoAs and hypothesized that they might represent a basic feature of movement execution. To test this hypothesis, we used BOLD functional magnetic resonance imaging in healthy subjects and show that unilateral voluntary movements of the fingers or toes go along not only with contralateral M1 activation, but also with ipsilateral M1-CoA of the respective homotopic representation and bilateral M1-CoA of different heterotopic representations not directly involved in the executed movement. Moreover, bilateral M1-CoA of heterotopic representations was observed in tongue movements. All M1-CoAs respected the correct somatotopy; however, their Euclidean coordinates were shifted and resembled to those obtained for imagined movements rather than for actual movements. BOLD signal intensities and correlations to the applied hemodynamic reference function were lower in M1-CoAs as compared to the M1 activations driving the movement but did not differ between homo- and heterotopic M1-CoAs. Thus, we propose that specific unilateral voluntary movements are accompanied by a global activation of primary motor areas, reflecting an overall increase in neuronal activity and unraveling the fundamental principle of distributed processing in M1. Executive motor function may rely on a balance of inhibitory and excitatory neuronal activity, where actual movement would result from a shift towards excitation.

DTI of commissural fibers in patients with Chiari II-malformation.

Chiari II-malformation is a complex congenital deformity of the brain which is frequently associated with hydrocephalus. Abnormalities of the corpus callosum are known to occur in the majority of patients. The objective of the present study was to study the microstructure of the corpus callosum (CC) and the anterior commissure (AC) to differentiate between different mechanisms of damage to these structures. We investigated 6 patients with Chiari II-malformation and 6 well-matched healthy volunteers employing T1-weighted 3D imaging and diffusion tensor imaging (DTI) to determine the fractional anisotropy (FA) and cross-sectional area of the CC and AC, as well as with neuropsychological testing. Four patients showed hydrocephalus, two patients had callosal dysplasia and four had a hypoplastic CC. The callosal FA in the patients was significantly reduced which was less pronounced for the genu alone. The area of CC was also reduced in Chiari II-patients. There was a strong correlation between the size and FA of the CC in the patients. In contrast, the thickness of the AC was significantly increased and was associated with higher FA in the patients. In psychological tests all patients showed reduced verbal memory; all but one patient showed reduced IQ as well as impaired visuo-spatial performance, indicating deficits in tasks requiring parieto-occipital integration. The existence of callosal dysplasia in two patients, the diminished FA reduction in the genu and the correlation of the cross-sectional area and FA in the patients point to a developmental white matter damage beside that exerted by hydrocephalus alone.

Clinical functional MRI of sensorimotor cortex using passive motor and sensory stimulation at 3 tesla

To establish a passive motor paradigm for clinical functional MRI (fMRI) that could be beneficial for patients with motor or attention deficits who are not able to perform active motor tasks.

Leftward Lateralization of Auditory Cortex Underlies Holistic Sound Perception in Williams Syndrome

Background Individuals with the rare genetic disorder Williams-Beuren syndrome (WS) are known for their characteristic auditory phenotype including strong affinity to music and sounds. In this work we attempted to pinpoint a neural substrate for the characteristic musicality in WS individuals by studying the structure-function relationship of their auditory cortex. Since WS subjects had only minor musical training due to psychomotor constraints we hypothesized that any changes compared to the control group would reflect the contribution of genetic factors to auditory processing and musicality. Methodology/Principal Findings Using psychoacoustics, magnetoencephalography and magnetic resonance imaging, we show that WS individuals exhibit extreme and almost exclusive holistic sound perception, which stands in marked contrast to the even distribution of this trait in the general population. Functionally, this was reflected by increased amplitudes of left auditory evoked fields. On the structural level, volume of the left auditory cortex was 2.2-fold increased in WS subjects as compared to control subjects. Equivalent volumes of the auditory cortex have been previously reported for professional musicians. Conclusions/Significance There has been an ongoing debate in the neuroscience community as to whether increased gray matter of the auditory cortex in musicians is attributable to the amount of training or innate disposition. In this study musical education of WS subjects was negligible and control subjects were carefully matched for this parameter. Therefore our results not only unravel the neural substrate for this particular auditory phenotype, but in addition propose WS as a unique genetic model for training-independent auditory system properties.