Diana L. Pettit - US grants

DESCRIPTION (provided by applicant): Kainate receptors (KARs) regulate neuronal excitability, and their global activation induces seizures and neuronal toxicity similar to that seen in epilepsy. Examination of KARs in synaptic function has largely focused on presynaptic KARs that appear to modulate neurotransmitter release. However, there is also clear evidence that KARs are located on the postsynaptic dendritic membranes of many neurons, and their role in synaptic physiology is unclear. KAR currents decay more slowly than AMPA currents, a property that may allow KARs to modulate the spread and integration of dendritic signals. Recent modeling of KAR function in response to afferent firing suggested that KAR activation could produce a tonic depolarization and increase neuronal excitability even at relatively low firing frequencies. This feature of KARs may confer a unique role in synaptic integration as well as in the timing and frequency of action potentials. While this is an intriguing hypothesis, it has not been directly tested. Our underlying hypothesis, based in part upon our preliminary data, is that postsynaptic KARs modulate firing frequency and integration of dendritic signaling in a cell-dependent manner. A direct test of this hypothesis will require examination of postsynaptic KARs in isolation from presynaptic KARs, with subcellular temporal and spatial resolution similar to that of synaptic activation. We will test the hypothesis that postsynaptic KARs are targeted to different dendritic regions of inhibitory and excitatory neurons in a cell-specific manner, and that postsynaptic dendritic KARs activation increases spike frequency in spontaneously firing inhibitory. Finally, we will test the hypothesis that postsynaptic dendritic KARs increase dendritic excitability and backpropagation of action potentials in hippocampal slice neurons. These experiments will be carried out using local photolysis, electrophysiology, and confocal imaging. These experiments will provide valuable new information about the role of postsynaptic, dendritic KARs in synaptic signaling and cell excitability.

[unreadable] DESCRIPTION (provided by applicant): N-methyl-D-aspartate receptor (NMDAR) activation can result in both long and short-term plasticity, promote cell survival, initiate cell death, and is also critical for normal synaptogenesis during development. A number of studies suggest that the consequences of NMDAR activation can vary widely depending on receptor localization, temporal characteristics, and size of the signal (Bito et al., 1996; Fields et al., 1997; Hardingham et al., 1999; Chawla and Bading, 2001; Hardingham et al., 2001a, b). The focus of this study is the physiological role of extrasynaptic vs. synaptic NMDARs. Cultured neuron studies have suggested that NMDARs can exist as synaptic NR1/NR2A heteromers and extrasynaptic NR1/NR2B heteromers. These two receptor types may be coupled to very different cellular processes, with calcium entry through extrasynaptic NMDARs activating cell death mechanisms and LTD rather than LTP (Lu et al., 2001; Hardingham et al., 2002). Although these experiments have provided valuable clues about possible spatial and functional segregation of extrasynaptic NMDARs in neurons, the existence and physiological relevance of these receptors in intact tissue remains unclear. Our preliminary results, which are significantly different from culture measurements, suggest that approximately 40% of the NMDAR population is extrasynaptic in acute hippocampal slice dendrites. This indicates that there is a large pool of extrasynaptic receptors available for activation during periods of high presynaptic activity. Our underlying hypothesis is that extrasynaptic NMDARs participate in neuronal interactions under pathological and physiologically relevant conditions. This hypothesis will be tested by determining the size of the extrasynaptic NMDAR pool, subunit composition, and developmental expression in acute hippocampal brain slices. We will determine the conditions under which extrasynaptic receptors can participate in transmission and their role in the expression of long-term potentiation or depression. [unreadable] [unreadable]

DESCRIPTION (provided by applicant): The task of combining sensory signals to form a coherent olfactory representation falls mainly on the piriform cortex (PC). Studies have shown that odor identity is represented as select but spatially dispersed neuronal subgroups in the PC. How these ensembles are generated is still a matter of conjecture. A key step to elucidate the mechanisms that establish these codes is to understand how the PC is set up to read and integrate incoming olfactory bulb (OB) signals. These computational capabilities are determined in large part by the functional connections PC neuronal components make with each other. Of particular interest are synaptic connections made by local interneurons onto pyramidal cells, as these circuits have been shown to be important for tuning pyramidal cells to odor-related inputs from the OB. To date, all studies that have examined PC intracortical circuitry have been purely anatomical and as such have not assessed functional synapses. To assay functional inhibitory circuitry, we focally uncaged glutamate over PC interneurons and recorded the resulting evoked inhibitory postsynaptic currents (IPSCs) in pyramidal cells. We then used IPSC charge as our measure for connective strength. This method allowed us to sample a large pool of unique inhibitory connections onto a single and population of pyramidal cells spread over a wide PC area. Because of this technical advantage, we have found, for the first time, a computationally significant spatial organization to PC circuitry. We found that the relative location of an interneuron to a pyramidal cell dictates connective strength. Interneurons located caudal to a pyramidal cell are more likely to inhibit its spike output than interneurons at more rostral regions. Consequently, OB excitatory inputs that activate mostly caudal microcircuits are less likely to elicit spiking in a pyramidal cell than inputs activating mostly rostral microcircuits. In addition, we have found that pyramidal cells in caudal PC regions receive 3-fold greater inhibition than pyramidal cells located in comparatively rostral areas. Thus, the strength of inhibitory connectivity onto a pyramidal cell is not only determined by interneuron location, but also by the location of the pyramidal cell itself along the PC rostro-caudal axis. We hope to further understand the significance of this rostro-caudal asymmetry by elucidating the cellular and circuit basis for such differential inhibition over PC space. PUBLIC HEALTH RELEVANCE: Findings from this proposal will not only reveal fundamental principles involved in cortical olfactory coding, but will also provide significant insight into clinical issues related to neurodegenerative disease. For instance, imaging studies have revealed that schizophrenic patients display significant dysfunctions in cortical regions involved in processing olfactory stimuli (Moberg et al., 1999;Schneider et al., 2007). Severe deficits in olfactory discrimination and recognition are early warning symptoms of patients with Parkinson's disease (Mesholam et al., 1998;Kranick and Duda, 2008). Alzheimer's disease, Huntington's chorea, alcoholic Korsakoff's syndrome, Pick's disease, all have characteristic olfactory dysfunctions and neuropathology (Mesholam et al., 1998). Thus, from a public health standpoint, elucidation of the underpinnings olfactory network coding holds much promise in understanding neurodegenerative disorders.