Synaptic Circuitry of the Mouse External Plexiform Layer

DIANNA BARTEL; LORENA RELA; LAWRENCE HSIEH; CHARLES A. GREER

Bonita Springs, FL

Congreso; AChemS 36th Annual Meeting; 2014

Association for Chemoreception Sciences

Odor information is significantly processed within external plexiform layer (EPL) local circuits. These circuits were first conceived solely as reciprocal dendrodendritic (D:D) synapses between projection neurons and granule cells. Recently, there is recognition of short axon cells that also shape odorant activity. To reassess EPL circuitry we focused on the location and synaptology of EPL D:D synapses with the longterm goal of a comprehensive analysis of all EPL synapses. Electron micrographs (EMs) showed that the density of symmetric excitatory synapses was 2X greater than asymmetric inhibitory synapses. Asymmetric synapses from mitral/tufted to granule cells were uniform in the EPL (9.3 synapses/100μm2) while symmetric synapses varied with higher density superficially versus deep in the EPL (5.5 vs 3.3 synapses/100μm2). Of interest, our initial analyses suggest that structural detail of individual pairs, including vesicle number, length of specialized membrane, etc. were highly correlated supporting the notion of dynamic equilibrium at D:D synapses. To pursue analyses at a molecular level and to begin subcellular localization of D:D synapses on projection neuron dendrites, we quantified the distribution of gephyrin, a postsynaptic scaffolding molecule at granule to mitral/tufted cell synapses. Gephyrin puncta, measuring 0.4μm2, were evenly distributed in the EPL, consistent with our EM analyses of symmetric synapses. We are now assessing spatial distribution of D:D synapses on labeled mitral cell secondary dendrites. Initial results suggest the number of D:D increases as dendrites taper toward their terminal end. Our data thus far imply that processing may vary based upon the number of D:D synapses/unit area as well as individual synaptological features.