Farid Hamzei-Sichani

Gap junctions on hippocampal mossy fiber axons demonstrated by thin-section electron microscopy and freeze–fracture replica immunogold labeling

Gap junctions have been postulated to exist between the axons of excitatory cortical neurons based on electrophysiological, modeling, and dye-coupling data. Here, we provide ultrastructural evidence for axoaxonic gap junctions in dentate granule cells. Using combined confocal laser scanning microscopy, thin-section transmission electron microscopy, and grid-mapped freeze–fracture replica immunogold labeling, 10 close appositions revealing axoaxonic gap junctions (≈30–70 nm in diameter) were found between pairs of mossy fiber axons (≈100–200 nm in diameter) in the stratum lucidum of the CA3b field of the rat ventral hippocampus, and one axonal gap junction (≈100 connexons) was found on a mossy fiber axon in the CA3c field of the rat dorsal hippocampus. Immunogold labeling with two sizes of gold beads revealed that connexin36 was present in that axonal gap junction. These ultrastructural data support computer modeling and in vitro electrophysiological data suggesting that axoaxonic gap junctions play an important role in the generation of very fast (>70 Hz) network oscillations and in the hypersynchronous electrical activity of epilepsy.

Quantitative morphologic classification of layer 5 neurons from mouse primary visual cortex

The understanding of any neural circuit requires the identification and characterization of all its components. Morphologic classifications of neurons are, therefore, of central importance to neuroscience. We use a quantitative method to classify neurons from layer 5 of mouse primary visual cortex, based on multidimensional clustering. To reconstruct neurons, we used Golgi impregnations and biocytin injections, as well as DiOlistics, a novel technique of labeling neurons with lipophilic dyes. We performed computerized 3‐D reconstructions of 158 layer 5 cells to measure a series of morphologic variables. Principal component analysis and cluster analysis were used for the classification of cell types. Five major classes of cells were found: group 1 includes large pyramidal neurons with apical dendrites that reach layer 1 with an apical tuft; group 2 consists of short pyramidal neurons and large multipolar cells with “polarized” dendritic trees; group 3 is composed of less extensive pyramidal neurons; group 4 includes small cells; and group 5 includes another set of short pyramidal neurons in addition to “atypically oriented” cells. Our sample included a relatively homogeneous group of 27 neurons that project to the superior colliculus, which clustered mainly in group 1, thus supporting the validity of the classification. Cluster analysis of neuronal morphologies provides an objective method to quantitatively define different neuronal phenotypes and may serve as a basis for describing neocortical circuits. J. Comp. Neurol. 461:415–428, 2003.

Mixed Electrical–Chemical Synapses in Adult Rat Hippocampus are Primarily Glutamatergic and Coupled by Connexin-36

Dendrodendritic electrical signaling via gap junctions is now an accepted feature of neuronal communication in mammalian brain, whereas axodendritic and axosomatic gap junctions have rarely been described. We present ultrastructural, immunocytochemical, and dye-coupling evidence for “mixed” (electrical/chemical) synapses on both principal cells and interneurons in adult rat hippocampus. Thin-section electron microscopic images of small gap junction-like appositions were found at mossy fiber (MF) terminals on thorny excrescences of CA3 pyramidal neurons (CA3pyr), apparently forming glutamatergic mixed synapses. Lucifer Yellow injected into weakly fixed CA3pyr was detected in MF axons that contacted four injected CA3pyr, supporting gap junction-mediated coupling between those two types of principal cells. Freeze-fracture replica immunogold labeling revealed diverse sizes and morphologies of connexin-36-containing gap junctions throughout hippocampus. Of 20 immunogold-labeled gap junctions, seven were large (328–1140 connexons), three of which were consistent with electrical synapses between interneurons; but nine were at axon terminal synapses, three of which were immediately adjacent to distinctive glutamate receptor-containing postsynaptic densities, forming mixed glutamatergic synapses. Four others were adjacent to small clusters of immunogold-labeled 10-nm E-face intramembrane particles, apparently representing extrasynaptic glutamate receptor particles. Gap junctions also were on spines in stratum lucidum, stratum oriens, dentate gyrus, and hilus, on both interneurons and unidentified neurons. In addition, one putative GABAergic mixed synapse was found in thin-section images of a CA3pyr, but none were found by immunogold labeling, suggesting the rarity of GABAergic mixed synapses. Cx36-containing gap junctions throughout hippocampus suggest the possibility of reciprocal modulation of electrical and chemical signals in diverse hippocampal neurons.

Angioarchitectures and Hemodynamic Characteristics of Posterior Communicating Artery Aneurysms and Their Association with Rupture Status

The authors studied 313 posterior communicating artery aneurysms (145 ruptured, 168 unruptured) with image-based computational fluid dynamics. Aneurysms were classified into different angioarchitecture types depending on the location of the aneurysm with respect to parent artery bifurcation. Ruptured aneurysms had higher, more concentrated, and more oscillatory wall shear stress distributions; stronger and more concentrated inflow jets; and more complex and unstable flow patterns compared with unruptured aneurysms. They conclude that high-flow intrasaccular hemodynamic characteristics, commonly found in bifurcation-type angioarchitectures, are associated with the posterior communicating artery aneurysm rupture status. BACKGROUND AND PURPOSE: Intracranial aneurysms originating at the posterior communicating artery are known to have high rupture risk compared with other locations. We tested the hypothesis that different angioarchitectures (ie, branch point configuration) of posterior communicating artery aneurysms are associated with aneurysm hemodynamics, which in turn predisposes aneurysms to rupture. MATERIALS AND METHODS: A total of 313 posterior communicating artery aneurysms (145 ruptured, 168 unruptured) were studied with image-based computational fluid dynamics. Aneurysms were classified into different angioarchitecture types depending on the location of the aneurysm with respect to parent artery bifurcation. Hemodynamic characteristics were compared between ruptured and unruptured aneurysms, as well as among aneurysms with different angioarchitectures. RESULTS: Angioarchitecture was associated with rupture (P = .003). Ruptured aneurysms had higher, more concentrated, and more oscillatory wall shear stress distributions (maximum wall shear stress, P < .001; shear concentration index, P < .001; mean oscillatory shear index, P < .001), stronger and more concentrated inflow jets (represented as Q, P = .01; inflow concentration index, P < .001), and more complex and unstable flow patterns (vortex core length, P < .001; proper orthogonal decomposition entropy, P < .001) compared with unruptured aneurysms. These adverse conditions were more common in aneurysms with bifurcation-type angioarchitectures compared with those with lateral or sidewall angioarchitectures. Interestingly, ruptured aneurysms also had lower normalized mean wall shear stress (P = .02) and minimum wall shear stress (P = .002) than unruptured aneurysms. CONCLUSIONS: High-flow intrasaccular hemodynamic characteristics, commonly found in bifurcation-type angioarchitectures, are associated with the posterior communicating artery aneurysm rupture status. These characteristics include strong and concentrated inflow jets, concentrated regions of elevated wall shear stress, oscillatory wall shear stress, lower normalized wall shear stress, and complex and unstable flow patterns.

Differences in Hemodynamics and Rupture Rate of Aneurysms at the Bifurcation of the Basilar and Internal Carotid Arteries

BACKGROUND AND PURPOSE: Cerebral aneurysms in the posterior circulation are known to have a higher rupture risk than those in the anterior circulation. We sought to test the hypothesis that differences in hemodynamics can explain the difference in rupture rates. MATERIALS AND METHODS: A total of 117 aneurysms, 63 at the tip of the basilar artery (27 ruptured, 36 unruptured, rupture rate = 43%) and 54 at the bifurcation of the internal carotid artery (11 ruptured, 43 unruptured, rupture rate = 20%) were analyzed with image-based computational fluid dynamics. Several hemodynamic variables were compared among aneurysms at each location and between ruptured and unruptured aneurysms at each location. RESULTS: On average, aneurysms at the basilar tip had more concentrated inflow (P < .001), a larger inflow rate (P < .001), a larger maximum oscillatory shear index (P = .003), more complex flows (P = .033), and smaller areas under low wall shear stress (P < .001) than aneurysms at the bifurcation of the internal carotid artery. In general, ruptured aneurysms had larger inflow concentration (P = .02), larger shear concentration (P = .02), more complex flows (P < .001), and smaller minimum wall shear stress (P = .003) than unruptured aneurysms. CONCLUSIONS: High flow conditions, characterized by large and concentrated inflow jets, complex and oscillatory flow patterns, and wall shear stress distributions with focalized regions of high shear and large regions of low shear, are associated with aneurysm rupture, especially for basilar tip aneurysms. The higher flow conditions in basilar tip aneurysms could explain their increased rupture risk compared with internal carotid bifurcation aneurysms.

High-frequency brain networks undergo modular breakdown during epileptic seizures.

Cortical high‐frequency oscillations (HFOs; 100–500 Hz) play a critical role in the pathogenesis of epilepsy; however, whether they represent a true epileptogenic process remains largely unknown. HFOs have been recorded in the human cortex but their network dynamics during the transitional period from interictal to ictal phase remain largely unknown. We sought to determine the high‐frequency network dynamics of these oscillations in patients with epilepsy who were undergoing intracranial electroencephalographic recording for seizure localization.

Hemodynamic Characteristics of Ruptured and Unruptured Multiple Aneurysms at Mirror and Ipsilateral Locations

BACKGROUND AND PURPOSE: Different hemodynamic patterns have been associated with aneurysm rupture. The objective was to test whether hemodynamic characteristics of the ruptured aneurysm in patients with multiple aneurysms were different from those in unruptured aneurysms in the same patient. MATERIALS AND METHODS: Twenty-four mirror and 58 ipsilateral multiple aneurysms with 1 ruptured and the others unruptured were studied. Computational fluid dynamics models were created from 3D angiographies. Case-control studies of mirror and ipsilateral aneurysms were performed with paired Wilcoxon tests. RESULTS: In mirror pairs, the ruptured aneurysm had more oscillatory wall shear stress (P = .007) than the unruptured one and tended to be more elongated (higher aspect ratio), though this trend achieved only marginal significance (P = .03, 1-sided test). In ipsilateral aneurysms, ruptured aneurysms had larger maximum wall shear (P = .05), more concentrated (P < .001) and oscillatory wall shear stress (P < .001), stronger (P < .001) and more concentrated inflow jets (P < .001), larger maximum velocity (P < .001), and more complex flow patterns (P < .001) compared with unruptured aneurysms. Additionally, ruptured aneurysms were larger (P < .001) and more elongated (P < .001) and had wider necks (P < .001) and lower minimum wall shear stress (P < .001) than unruptured aneurysms. CONCLUSIONS: High wall shear stress oscillations and larger aspect ratios are associated with rupture in mirror aneurysms. Adverse flow conditions characterized by high and concentrated inflow jets; high, concentrated, and oscillatory wall shear stress; and strong, complex and unstable flow patterns are associated with rupture in ipsilateral multiple aneurysms. In multiple ipsilateral aneurysms, these unfavorable flow conditions are more likely to develop in larger, more elongated, more wide-necked, and more distal aneurysms.

Imaging Brain Slices

Brain slices are convenient preparations to study synapses, neurons, and neural circuits because, while they are easily accessed by experimental manipulations such as drug applications, intracellular recordings, and optical imaging, they preserve many of the essential functional properties of these circuits. In this chapter, we describe techniques of live brain-slice imaging used in our laboratory. We cover in detail experimental protocols and know-how acquired over the years about preparing neocortical and hippocampal slices and slice cultures, loading neurons with dyes or using biolistic transfection techniques, two-photon and second harmonic imaging, morphological reconstructions, and image processing and analysis. These techniques are used to study the functional or morphological dynamics of synaptic structures, including dendritic spines and axon terminals, and to characterize circuit connectivity and dynamics.

Dopamine drives left-hemispheric lateralization of neural networks during human speech

Although the concept of left‐hemispheric lateralization of neural processes during speech production has been known since the times of Broca, its physiological underpinnings still remain elusive. We sought to assess the modulatory influences of a major neurotransmitter, dopamine, on hemispheric lateralization during real‐life speaking using a multimodal analysis of functional MRI, intracranial EEG recordings, and large‐scale neural population simulations based on diffusion‐weighted MRI. We demonstrate that speech‐induced phasic dopamine release into the dorsal striatum and speech motor cortex exerts direct modulation of neuronal activity in these regions and drives left‐hemispheric lateralization of speech production network. Dopamine‐induced lateralization of functional activity and networks during speaking is not dependent on lateralization of structural nigro‐striatal and nigro‐motocortical pathways. Our findings provide the first mechanistic explanation for left‐hemispheric lateralization of human speech that is due to left‐lateralized dopaminergic modulation of brain activity and functional networks.

Development and internal validation of an aneurysm rupture probability model based on patient characteristics and aneurysm location, morphology, and hemodynamics

PurposeUnruptured cerebral aneurysms pose a dilemma for physicians who need to weigh the risk of a devastating subarachnoid hemorrhage against the risk of surgery or endovascular treatment and their complications when deciding on a treatment strategy. A prediction model could potentially support such treatment decisions. The aim of this study was to develop and internally validate a model for aneurysm rupture based on hemodynamic and geometric parameters, aneurysm location, and patient gender and age.MethodsCross-sectional data from 1061 patients were used for image-based computational fluid dynamics and shape characterization of 1631 aneurysms for training an aneurysm rupture probability model using logistic group Lasso regression. The model’s discrimination and calibration were internally validated based on the area under the curve (AUC) of the receiver operating characteristic and calibration plots.ResultsThe final model retained 11 hemodynamic and 12 morphological variables, aneurysm location, as well as patient age and gender. An adverse hemodynamic environment characterized by a higher maximum oscillatory shear index, higher kinetic energy and smaller low shear area as well as a more complex aneurysm shape, male gender and younger age were associated with an increased rupture risk. The corresponding AUC of the model was 0.86 (95% CI [0.85, 0.86], after correction for optimism 0.84).ConclusionThe model combining variables from various domains was able to discriminate between ruptured and unruptured aneurysms with an AUC of 86%. Internal validation indicated potential for the application of this model in clinical practice after evaluation with longitudinal data.