Sonia Gasparini - US grants

[unreadable] DESCRIPTION (provided by applicant): The general objectives of this research proposal are to establish the mechanisms that control the dendritic processing of incoming synaptic information. In most CNS neurons, incoming synaptic inputs are widely distributed across dendritic arborizations that are both morphologically and electrically complicated and it is in these dendrites that tens of thousands of excitatory and inhibitory synaptic inputs are blended together to generate a coherent output response. In hippocampal CA1 pyramidal neurons, 85% of excitatory synaptic input is received by radial oblique dendrites. These are relatively short, small diameter secondary branches off the main dendrite trunk whose morphology suggests they might provide a favorable site for highly non-linear forms of synaptic processing. At present very little is known about the active properties of these dendrites or of the properties of the synapses that are formed on them. We propose to test the central hypotheses that: specific properties of oblique dendrites and the synapses formed on them provide CA1 neurons with multiple modes of processing synaptic input. The key players in determining the form of synaptic integration in these cells should be both the spatio-temporal aspects of the input itself and the availability of the voltage-gated ion channels within the obliques. We have designed experiments using a variety of dendritic whole-cell patch-clamp and advanced optical recording techniques to determine 1) the types and properties of voltage-gated ion channels located in these branches, 2) the properties of the synaptic inputs to the branches 3) precisely how synaptic input and active channels interact to shape integration within the branches and 4) how physiologically-relevant channel modulation can produce different forms of synaptic processing, all the while trying to relate the findings to the naturally occurring functional states of the hippocampus. The information produced by these experiments should provide us with a greater understanding of how information processing proceeds in central neurons and therefore a more fundamental understanding of both normal and pathological brain functioning. [unreadable] [unreadable]

DESCRIPTION (provided by applicant): The formation and consolidation of memories requires bidirectional communication between the hippocampus and the neocortex via the entorhinal cortex (EC). EC layer V neurons are the main target of the processed output from the hippocampus and in turn project to cortical regions;these neurons are thus likely to play an important role in the formation and consolidation of memories. In addition, they have prominent apical dendrites that extend and branch in complex tufts in the EC superficial layers, where axons from different cortical areas are known to make synapses on EC layer II and III neurons. Our preliminary data show that these tufts carry many spines. The presence of these spines, combined with their proximity to axons emerging from cortical areas, suggests that these axons may form glutamatergic synaptic contacts on the distal dendrites of layer V neurons. The long-term goal of this research is to provide new insights with respect to the processing of memories by clarifying how entorhinal layer V neurons integrate the synaptic input they receive from the hippocampus with other inputs to generate their output to the neocortex. Two-photon Ca2+ imaging, glutamate uncaging and electrophysiological techniques will be employed to characterize different aspects of dendritic integration in layer V neurons of the rat entorhinal cortex. The project will focus on three specific aims: (1) to test the prediction that the distal apical dendrites of EC layer V neurons are activated by glutamatergic inputs whose features differ from those of the proximal hippocampal synapses;(2) to test the prediction that proximal and distal compartments can communicate through back-propagating action potentials (bAPs);(3) to test the prediction that the distal dendrites of EC layer V neurons can initiate dendritic spikes that alter the entorhinal output. The information acquired about the computational capabilities specific to the dendrites of these neurons will shed light on signal processing during normal and pathological neuronal activity. The resultant improvement in our understanding of how information is transmitted and stored in this critical area of the brain may eventually guide therapeutic strategies for disorders in which this area is particularly vulnerable, such as Alzheimer's disease, schizophrenia and epilepsy. PUBLIC HEALTH RELEVANCE: The formation and consolidation of memories requires bidirectional communication between the hippocampus and the neocortex via the entorhinal cortex. Two-photon Ca2+ imaging, glutamate uncaging and electrophysiological techniques will be used to characterize different aspects of dendritic integration in layer V neurons of the rat entorhinal cortex. The resultant improvement in our understanding of how information is transmitted and stored in this critical area of the brain may eventually lead to therapeutic strategies for disorders in which brain functions are compromised, such as epilepsy, Alzheimer's disease and schizophrenia.

Abstract The goals for the Multiphoton Microscopy Core Facility are to make advancements on projects that were initiated during the first two phases of the COBRE program and to expand the use of the facility to investigators that require multiphoton microscopy for novel applications, but have limited experience with these techniques. COBRE investigators, in collaboration with other faculty at the Neuroscience Center of Excellence (NCE) have used our multiphoton imaging capabilities to make discoveries in hair cell synapses and plasticity of intrinsic and synaptic properties of dendrites in the hippocampus. The pioneering multiphoton imaging equipment and its applications attracted collaborations that included scientists from outside the Center and University, including Tulane University Medical School. These diverse projects illustrate the applications that can be considered with organized collaboration and core infrastructure. While extending the availability of multiphoton imaging to more researchers in the greater New Orleans area, ultimately the core will attract sufficient new funding, which, in addition to the institutional commitment of the School of Medicine, will make the core self-sufficient by the end of Phase III funding. The three specific aims of this core are: 1) To provide the infrastructure to sustain existing grant projects that listed the core as essential equipment in their NIH applications; 2) To utilize the multiphoton facility to implement new research projects; enable collaborations between investigators; create new imaging applications in neuroscience; and help investigators obtain preliminary data for individual grant applications; 3) To develop new research strategies by continuing to develop novel capabilities including optogenetics and in vivo imaging.

Area CA 1 of the hippocampus plays a key role in learning and memory. CA 1 pyramidal neurons receive two major input streams, one from the entorhinal cortex that mediates sensory information about the external world, and another from area CA3 that mediates retrieval of previously stored representations. The neuromodulator acetylcholine (ACh) is thought to gate encoding and retrieval. Our preliminary results indicate that ACh, by activating the Ca2+-activated nonspecific cation current ICAN and modulating other ion channels, dramatically affects the intrinsic response to a triangular current ramp simulating the excitatory input, thus levels of ACh may regulate the pattern of place field firing by shifting the peak to later positions in ramp. This shift could cause the coding to shift from prospective, representing upcoming locations, to retrospective, representing recently visited locations, which may favor encoding over retrieval. Our preliminary data also shows that ACh can induce sustained firing; since novelty increases ACh levels, this persistent firing may facilitate the synaptic plasticity required to form new place fields. Moreover, we find that there are both intrinsic and synaptic mechanisms that endow CA 1 pyramidal neurons with low pass filtering capabilities specific to the Schaffer collateral (SC) inputs from CA3, and that these capabilities are switched off by ACh. In Aim 1, we test the hypothesis that cholinergic modulation converts the shape of the rate response to a triangular current ramp from decelerating to accelerating in a dose-dependent fashion, with a concomitant shift in the peak. In Aim 2, we test the hypotheses that intrinsic properties of CA 1 neurons mediate low pass filtering of SC inputs, with a critical additional component provided by parvalbumin-positive (PV+) basket cells, and that cholinergic modulation removes the low pass filtering, such that CA 1 can follow higher input frequencies from CA3. This work seeks to elucidate the neural bases of complex behaviors and mental illnesses. Better understanding of the mechanisms underlying cholinergic modulation of neuronal ensembles may lead not only to an improved understanding of information processing important in learning and memory, but eventually to improved therapies for cognitive disorders.