Faculty Bibliography

Physiological data suggest that in the CA1-CA3 hippocampal areas of rats, entorhinal cortical efferents directly influence the activity of interneurons, in addition to pyramidal cells. To verify this hypothesis, the following experiments were performed: 1) light microscopic double-immunostaining for parvalbumin and the anterograde tracer Phaseolus vulgaris-leucoagglutinin injected into the entorhinal cortex; 2) light and electron microscopic analysis of cleaved spectrin-immunostained (i.e., degenerating axons and boutons) hippocampal sections following entorhinal cortex lesion; and 3) an electron microscopic study of parvalbumin-immunostained hippocampal sections after entorhinal cortex lesion. The results demonstrate that in the stratum lacunosum-moleculare of the CA1 and CA3 regions, entorhinal cortical axons form asymmetric synaptic contacts on parvalbumin-containing dendritic shafts. In the stratum lacunosum-moleculare, parvalbumin-immunoreactive dendrites represent processes of GABAergic, inhibitory basket and chandelier cells; these interneurons innervate the perisomatic area and axon initial segments of pyramidal cells, respectively. A feed-forward activation of these neurons by the entorhinal input may explain the strong, short-latency inhibition of pyramidal cells

In the dentate gyrus, granule cells are ischemia-resistant, but at least five types of predominantly spiny hilar neurons are extremely vulnerable to ischemia. Many of the ischemia-sensitive subtypes of hilar neurons appear to be involved in: (i) the regulation of GABAergic inhibition in the dentate gyrus, and (ii) the generation of hilar neuronal synchrony. The present study examined functional consequences of ischemia-induced hilar neuronal loss on GABAergic inhibition of granule cells and hilar neuronal synchrony. Transient (15 min) forebrain ischemia was induced by a modification of the four-vessel-occlusion method producing a substantial hilar neuronal loss as demonstrated by the Gallyas silver stain method. Three months later, we have examined spontaneous and stimulus-evoked inhibitory postsynaptic currents mediated by both GABA(A) and GABA(B) receptors, and inhibitory bursts induced by 4-aminopyridine (50 microM) using whole-cell recordings in coronal brain slices maintained at 34-36 degree C in the presence of excitatory amino acid receptor blockers. Spontaneous dentate spikes reflecting hilar neuronal synchrony and synaptic responses evoked by perforant path stimulation were also recorded in vivo to assess synchrony and inhibition in the dentate gyrus. In spite of significant damage to several types of hilar neurons, there were no marked differences in the conductance, kinetics, and 4-aminopyridine-induced burst frequencies of synaptic GABA(A) and GABA(B) responses in granule cells. Furthermore, both paired-pulse inhibition and dentate spikes appeared to be normal in vivo. We conclude that there appears to be little impairment of GABAergic inhibition of granule cells or of hilar neuronal synchrony three months following a massive ischemic damage to spiny hilar neurons

Fast spiking interneurons in the CA1 area of the dorsal hippocampus were recorded from and filled with biocytin in anesthetized rats. The full extent of their dendrites and axonal arborizations as well as their calcium binding protein content were examined. Based on the spatial extent of axon collaterals, local circuit cells (basket and O-LM neurons) and long-range cells (bistratified, trilaminar, and backprojection neurons) could be distinguished. Basket cells were immunoreactive for parvalbumin and their axon collaterals were confined to the pyramidal layer. A single basket cell contacted more than 1500 pyramidal neurons and 60 other parvalbumin-positive interneurons. Commissural stimulation directly discharged basket cells, followed by an early and late IPSPs, indicating interneuronal inhibition of basket cells. The dendrites of another local circuit neuron (O-LM) were confined to stratum oriens and it had a small but high-density axonal terminal field in stratum lacunosum-moleculare. The fastest firing cell of all interneurons was a calbindin-immunoreactive bistratified neuron with axonal targets in stratum oriens and radiatum. Two neurons with their cell bodies in the alveus innervated the CA3 region (backprojection cells), in addition to rich axon collaterals in the CA1 region. The trilaminar interneuron had axon collaterals in strata radiatum, oriens and pyramidale with its dendrites confined to stratum oriens. Commissural stimulation evoked an early EPSP-IPSP-late depolarizing potential sequence in this cell. All interneurons formed symmetric synapses with their targets at the electron microscopic level. These findings indicate that interneurons with distinct axonal targets have differential functions in shaping the physiological patterns of the CA1 network

Epidemiological data and genetic studies indicate that certain forms of human epilepsy are inherited. Based on the similarity between the human and mouse genomes, mouse models of epilepsy could facilitate the discovery of genes associated with epilepsy syndromes. Here, we report an insertional murine mutation that inactivates a novel gene and results in whole body jerks, generalized clonic seizures, and epileptic brain activity in transgenic mice. The gene, named jerky, encodes a putative 41.7 kD protein displaying homology to a number of nuclear regulatory proteins, suggesting that perhaps the jerky protein is able to bind DNA

Network oscillations are postulated to be instrumental for synchronizing the activity of anatomically distributed populations of neurons. Results from recent studies on the physiology of cortical interneurons suggest that through their interconnectivity, they can maintain large-scale oscillations at various frequencies (4-12 Hz, 40-100 Hz and 200 Hz). We suggest that networks of inhibitory interneurons within the forebrain impose co-ordinated oscillatory 'contexts' for the 'content' carried by networks of principal cells. These oscillating inhibitory networks may provide the precise temporal structure necessary for ensembles of neurons to perform specific functions, including sensory binding and memory formation

CA3 pyramidal neurons were stained with biocytin during intracellular recording in rat hippocampus in vivo and reconstructed using a computer-based system. The in vivo CA3 neurons were characterized primarily according to their proximity to the hilus and secondarily with respect to the septotemporal location. Neurons measured in CA3a (n = 4), in CA3b (n = 4), and in posterior/ventral locations (n = 3) had the greatest dendritic lengths (19.8, 19.1, and 26.8 mm on average, respectively). Cells closer to the hilus showed much shorter dendritic lengths, averaging 10.4 mm for CA3c neurons (n = 4) and 11.6 mm for zone 3 neurons (n = 2). Half of the cells showed more than one major apical dendrite, and dendritic trees were highly variable even within CA3 subregions. The mean electronic length for these cell groups averaged between 0.30 lambda (CA3c) and 0.45 lambda (posterior/ventral), assuming a constant specific-membrane resistivity of 60 K omega-cm2. These CA3 neurons form a database of reconstructed neurons for further morphometric and electrical modelling studies. The large degree of variability between individual CA3 neurons indicates that both dendritic and electrical properties should be specifically calculated for each cell rather than assuming a 'typical' morphology

The effects of cerebral ischemia on calcium/calmodulin-dependent kinase II (CaM kinase II) were investigated using the rat four-vessel occlusion model. In agreement with previous results using rat or gerbil models of cerebral ischemia or a rabbit model of spinal cord ischemia, this report demonstrates that transient forebrain ischemia leads to a reduction in CaM kinase II activity within 5 min of occlusion onset. Loss of activity from the cytosol fractions of homogenates from the neocortex, striatum, and hippocampus correlated with a decrease in the amount of CaM kinase alpha and beta isoforms detected by immunoblotting. In contrast, there was an apparent increase in the amount of CaM kinase alpha and beta in the particulate fractions. The decrease in the amount of CaM kinase isoforms from the cytosol but not the particulate fractions was confirmed by autophosphorylation of CaM kinase II after denaturation and renaturation in situ of the blotted proteins. These results indicate that ischemia causes a rapid inhibition of CaM kinase II activity and a change in the partitioning of the enzyme between the cytosol and particulate fractions. CaM kinase II is a multifunctional protein kinase, and the loss of activity may play a critical role in initiating the changes leading to ischemia-induced cell death. To identify a structural basis for the decrease in enzyme activity, tryptic peptide maps of CaM kinase II phosphorylated in vitro were compared. Phosphopeptide maps of CaM kinase alpha from particulate fractions of control and ischemic samples revealed not only reduced incorporation of phosphate into the protein but also the absence of a limited number of peptides in the ischemic samples. This suggested that certain sites are inaccessible, possibly due to a conformational change, a covalent modification of CaM kinase II, or steric hindrance by an associated molecule. Verifying one of these possibilities should help to elucidate the mechanism of ischemia-induced modulation of CaM kinase II

1. This paper describes two novel population patterns in the dentate gyrus of the awake rat, termed type 1 and type 2 dentate spikes (DS1, DS2). Their cellular generation and spatial distribution were examined by simultaneous recording of field potentials and unit activity using multiple-site silicon probes and wire electrode arrays. 2. Dentate spikes were large amplitude (2-4 mV), short duration (< 30 ms) field potentials that occurred sparsely during behavioral immobility and slow-wave sleep. Current-source density analysis revealed large sinks in the outer (DS1) and middle (DS2) thirds of the dentate molecular layer, respectively. DS1 and DS2 had similar longitudinal, lateral, and interhemispheric synchrony. 3. Dentate spikes invariably were coupled to synchronous population bursts of putative hilar interneurons. CA3 pyramidal cells, on the other hand were suppressed during dentate spikes. 4. After bilateral removal of the entorhinal cortex, dentate spikes disappeared, whereas sharp wave-associated bursts, reflecting synchronous discharge of the CA3-CA1 network, increased several fold. 5. These physiological characteristics of the dentate spikes suggest that they are triggered by a population burst of layer II stellate cells of the lateral (DS1) and medial (DS2) entorhinal cortex. 6. We suggest that dentate spike-associated synchronized bursts of hilar-region interneurons provide a suppressive effect on the excitability of the CA3-CA1 network in the intact brain