During olfactory discrimination, odors often activate clusters of olfactory bulb glomeruli and mitral/tufted (M/T) cells that together could signal the odor "category" (e.g., banana/pear or fruity), whereas activation of a single glomerulus and its M/T cells likely signals a specific odor "note" (e.g., carbon chain length). This proposal focuses on synaptic interactions that may contribute to multiglomerular coding of odor categories. Differences in M/T cell anatomical and response properties suggest different subtypes signal different aspects of the odor category. Our goal is to determine how synaptic interactions of local interneurons with different M/T cells subtypes in the external plexiform layer (EPL) could make this possible. Using patch recording and biocytin labeling methods, we have found that mouse EPL interneurons exhibit several subsets of electrophysiological and morphological properties and that in two dimensions they bridge EPL space below adjacent glomeruli. The spontaneous activity and evoked responses of the interneurons are consistent with anatomical studies suggesting they are excited by and inhibit different M/T cell subtypes within different EPL sublaminae. Preliminary paired recordings show that some superficially located interneurons are indeed excited by superficial tufted cells. Our results indicate that the interneurons also receive inhibitory synaptic inputs, suggesting that they might form inhibitory networks similar to those seen in other cortical regions. We postulate that inhibition provided by the interneurons increases coherence of M/T cell spike output from glomerular clusters, which could signal the odor category and improve detection of weak odors within the category.
Aim 1 will test the hypothesis that EPL interneurons include several electrophysiologically and morphologically distinct subtypes or, alternatively, neurochemically distinct subtypes that have different distributions within the three EPL sublaminae.
Aim 2 will test the hypothesis that the excitatory synaptic inputs of EPL interneurons located in the superficial EPL are correlated with spike bursts of superficial tufted cells, whereas the excitatory synaptic inputs of interneurons located in deeper sublaminae are correlated with spike bursts of multiple M/T cell subtypes. We will also determine if the excitatory synaptic inputs are monosynaptically derived from the M/T cells.
Aim 3 will test the hypothesis that superficially-located EPL interneurons receive excitatory and inhibitory synaptic inputs related to clusters of adjacent glomeruli, and that the inhibitory inputs are from other EPL interneurons rather than from granule cells. As part of Aim 3, we will also determine if the fast excitatory and inhibitory responses are mediated by AMPA and GABAA receptors, respectively. The results will reveal if different EPL interneuron subtypes and M/T cell subtypes together define spatial domains for sensory encoding, which allow the M/T cells to act in parallel to transmit information about odors that activate clusters of glomeruli having overlapping odor sensitivities.
The sense of smell is affected early during the progression of many neurological diseases, most notably idiopathic Parkinson's disease and Alzheimer's disease (Hawkes 2003). In Parkinson's disease, olfactory deficits occur in 70-90% of patients (Double et al. 2003). The reasons for the observed decrease in olfactory sensitivity are poorly understood, but they appear to be localized to the central olfactory pathway, not to the nose. Recent evidence indicates that the EPL of the olfactory bulb is one of the earliest sites of cellular pathology in Parkinson's disease (Del Tredici et al. 2002;Hoogland et al. 2003). The cell types of this layer have not been characterized using modern, quantitative anatomical methods or with electrophysiological recording methods. The contributions of the cells in this layer with regard to olfactory discrimination are therefore unknown. The results of the proposed studies will provide a foundation for understanding changes that may occur in the synaptic networks of the EPL during Parkinson's and other diseases that affect the sense of smell.
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