Neuronal populations of the anterior olfactory nucleus

Understanding the cellular components of neural circuits is an essential step in discerning regional function. The anterior olfactory nucleus (AON) is an important station in the olfactory pathway as it is reciprocally connected the ipsi- and contralateral olfactory bulb (OB) and piriform cortex (PC). Furthermore, the AON's simple-two layered organization makes it a model for cortical function. The present work used targeted whole-cell patch clamping to investigate the morphological and electrophysiological properties of two main neuronal populations - excitatory projection neurons and inhibitory interneurons. Projection neurons were identified by retrograde fluorescent tracers placed into either the OB or PC. Inhibitory experiments used transgenic mice, in which gamma-aminobutyric acid (GABA)-containing cells were labeled with green fluorescent protein (GFP). GABAergic inhibitory cells were further identified as containing one or more of seven molecular markers including three calcium-binding proteins (calbindin, calretinin, parvalbumin) or four neuropeptides (somatostatin, vasoactive intestinal peptide, neuropeptide Y, cholecystokinin). Results indicate that the AON contains two classes of excitatory cells, superficial and deep pyramidal neurons, and at least five classes of inhibitory cells -fast-spiking multipolar, regular-spiking multipolar, superficial neurogliaform, deep neurogliaform, and horizontal neurons. Correlates for these cl.asses were identified in the PC, hippocampus, and neocortex. Well-established connections between cell classes in neocortex were used to create a putative wiring diagram for the AON. These studies provide the first examination of the cellular components of this simple cortical system.

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