Faculty Bibliography

Transmitter release at chemical synapses is triggered by high calcium concentration microprofiles at the presynaptic cytosol. Such microprofiles, generated by the opening of voltage-dependent calcium channels at the presynaptic plasma membrane, have been defined as calcium concentration microdomains. Using total internal reflection fluorescent microscopy in conjunction with calcium and vesicular release indicator dyes, we have directly visualized the close apposition of calcium concentration microdomains and synaptic release sites at single synaptic terminals from the CNS from rat cerebellar mossy fiber and squid optic lobe. These findings demonstrate the close apposition of calcium entry and release sites and the dynamics of such site locations over time. Kinetic analysis shows that vesicles can be released via two distinct mechanisms: full-fusion and kiss-and-run. Calcium triggers vesicular motion toward the membrane, and the speed of such movement is calcium concentration-dependent. Moreover, the immediately available vesicular pool represents molecularly trapped vesicles that can be located at a larger distance from the plasma membrane than the field illuminated by total internal reflection fluorescent microscopy

Substance P (SP) is known to be a peptide that facilitates epileptic activity of principal cells in the hippocampus. Paradoxically, in other models, it was found to be protective against seizures by activating substance P receptor (SPR)-expressing interneurons. Thus, these cells appear to play an important role in the generation and regulation of epileptic seizures. The number, distribution, morphological features and input characteristics of SPR-immunoreactive cells were analyzed in surgically removed hippocampi of 28 temporal lobe epileptic patients and eight control hippocampi in order to examine their changes in epileptic tissues. SPR is expressed in a subset of inhibitory cells in the control human hippocampus, they are multipolar interneurons with smooth dendrites, present in all hippocampal subfields. This cell population is considerably different from SPR-positive cells of the rat hippocampus. The CA1 (cornu Ammonis subfield 1) region was chosen for the detailed morphological analysis of the SPR-immunoreactive cells because of its extreme vulnerability in epilepsy. The presence of various neurochemical markers identifies functionally distinct interneuron types, such as those responsible for perisomatic, dendritic or interneuron-selective inhibition. We found considerable colocalization of SPR with calbindin but not with parvalbumin, calretinin, cholecystokinin and somatostatin, therefore we suppose that SPR-positive cells participate mainly in dendritic inhibition. In the non-sclerotic CA1 region they are mainly preserved, whereas their number is decreased in the sclerotic cases. In the epileptic samples their morphology is considerably altered, they possessed more dendritic branches, which often became beaded. Analyses of synaptic coverage revealed that the ratio of symmetric synaptic input of SPR-immunoreactive cells has increased in epileptic samples. Our results suggest that SPR-positive cells are preserved while principal cells are present in the CA1 region, but show reactive changes in epilepsy including intense branching and growth of their dendritic arborization

The development of the mammalian brain is dependent on extensive neuronal migration. Mutations in mice and humans that affect neuronal migration result in abnormal lamination of brain structures with associated behavioral deficits. Here, we report the identification of a hyperactive N-ethyl-N-nitrosourea (ENU)-induced mouse mutant with abnormalities in the laminar architecture of the hippocampus and cortex, accompanied by impaired neuronal migration. We show that the causative mutation lies in the guanosine triphosphate (GTP) binding pocket of alpha-1 tubulin (Tuba1) and affects tubulin heterodimer formation. Phenotypic similarity with existing mouse models of lissencephaly led us to screen a cohort of patients with developmental brain anomalies. We identified two patients with de novo mutations in TUBA3, the human homolog of Tuba1. This study demonstrates the utility of ENU mutagenesis in the mouse as a means to discover the basis of human neurodevelopmental disorders

Direction-selective neurons in primary visual cortex have small receptive fields that encode the motions of local features. These motions often differ from the motion of the object to which they belong and must therefore be integrated elsewhere. A candidate site for this integration is visual cortical area MT (V5), in which cells with large receptive fields compute the motion of patterns. Previous studies of motion integration in MT have used stimuli that fill the receptive field, and thus do not test whether motion information is really integrated across this whole area. For each MT neuron, we identified two regions ('patches') within the receptive field that were approximately equally effective in driving responses. We then measured responses to plaids whose component gratings overlapped within a patch, and compared them with responses to the same component gratings presented in separate patches. Cells that were selective for the direction of motion of the whole pattern when the gratings overlapped lost this selectivity when the gratings were separated and became selective instead for the direction of motion of the individual components. If MT cells simply pooled all of the inputs that endow them with a receptive field, they would encode all of the motions in the receptive field as belonging to a single object. Our results indicate instead that critical elements of the computations underlying pattern-direction selectivity in MT are done locally, on a scale smaller than the whole receptive field

Research investigating the pathophysiology of Parkinson's disease (PD) mostly focuses on basal ganglia dysfunction. However, the main output from the basal ganglia is via the thalamus, and corticothalamic feedback constitutes the primary source of synapses in the thalamus. We therefore focus on the thalamocortical interplay. During the surgical intervention in six patients, local field potentials (LFPs) were recorded from pallidal-recipient thalamic nuclei VA and VLa. Simultaneously, EEG was recorded from several sites on the scalp. The highest thalamocortical coherence was found in the theta frequency band (4-9 Hz) with a mean peak frequency of 7.5 Hz. The magnitude of thalamocortical theta coherence was comparable to the magnitude of EEG coherence between scalp electrode pairs. Thalamocortical theta coherence reached 70% and was maximal with frontal scalp sites on both hemispheres. In the 13-20 Hz beta frequency band, maximal coherence was comparatively low but localized on the scalp ipsilateral to the site of thalamic LFP recording. The high thalamocortical coherence underlines the importance of thalamic function for the genesis of scalp EEG. We discuss the PD pathophysiology within the framework of dysrhythmic thalamocortical interplay, which has important consequences for the choice of therapeutic strategy in patients with severe forms of PD

Even in the absence of sensory stimulation, the neocortex shows complex spontaneous activity patterns, often consisting of alternating 'DOWN' states of generalized neural silence and 'UP' states of massive, persistent network activity. To investigate how this spontaneous activity propagates through neuronal assemblies in vivo, we simultaneously recorded populations of 50-200 cortical neurons in layer V of anesthetized and awake rats. Each neuron displayed a virtually unique spike pattern during UP states, with diversity seen amongst both putative pyramidal cells and interneurons, reflecting a complex but stereotypically organized sequential spread of activation through local cortical networks. Spike timing was most precise during the first approximately 100 ms after UP state onset, and decayed as UP states progressed. A subset of UP states propagated as traveling waves, but waves passing a given point in either direction initiated similar local sequences, suggesting local networks as the substrate of sequential firing patterns. A search for repeating motifs indicated that their occurrence and structure was predictable from neurons' individual latencies to UP state onset. We suggest that these stereotyped patterns arise from the interplay of intrinsic cellular conductances and local circuit properties