Abstract
Understanding the cellular components of neural circuits is an essential step in discerning regional function. The anterior olfactory nucleus (AON) is reciprocally connected to both the ipsi- and contralateral olfactory bulb (OB) and piriform cortex (PC), and, as a result, can broadly influence the central processing of odor information. While both the AON and PC are simple cortical structures, the regions differ in many ways including their general organization, internal wiring and synaptic connections with other brain areas. The present work used targeted whole-cell patch clamping to investigate the morphological and electrophysiological properties of the AON's two main neuronal populations: excitatory projection neurons and inhibitory interneurons. Retrograde fluorescent tracers placed into either the OB or PC identified projection neurons. Two classes were observed with different physiological signatures and locations (superficial and deep pyramidal neurons), suggesting the AON contains independent efferent channels. Transgenic mice in which GABA-containing cells expressed green fluorescent protein were used to assess inhibitory neurons. These 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). The proportion of GABAergic cells containing these markers varied across subregions reinforcing notions that the AON has local functional subunits. At least five classes of inhibitory cells were observed: fast-spiking multipolar, regular-spiking multipolar, superficial neurogliaform, deep neurogliaform, and horizontal neurons. While some of these cell types are similar to those reported in the PC and other cortical regions, the AON also has unique populations. These studies provide the first examination of the cellular components of this simple cortical system.
Highlights
The brain will only be understood once the function of its component circuits and their constituent neuronal elements are unraveled
Mice 18–25 days old were used for electrophysiology experiments and 2–3 month olds were used to in immunohistochemistry experiments that surveyed the neurophenotypes of GAD67-green fluorescent protein (GFP) cells
Despite its large size and central location in the olfactory circuit, relatively little is known about the neuronal elements that make up the anterior olfactory nucleus (AON)
Summary
The brain will only be understood once the function of its component circuits and their constituent neuronal elements are unraveled. Perhaps the simplest rubric is to separate neurons into two main groups: excitatory projection neurons, and inhibitory local circuit neurons. Each of these classes contains broadly different kinds of cells. Intensive examinations of the neocortex have revealed that excitatory cells can be divided into subtypes based upon their projection targets as well as their physiology and morphology (e.g., Kumar and Ohana, 2008; Brown and Hestrin, 2009; Groh et al, 2010). Inhibitory cells are much more varied and have been categorized on the basis of their morphology, molecular markers, and physiology (McBain and Fisahn, 2001; Markham et al, 2004; Hestrin and Galarreta, 2005; Ascoli et al, 2008). The studies of neocortex have prompted others to investigate whether species of neurons are conserved in other cortical areas including the hippocampus (Price et al, 2005; Klausberger and Somogyi, 2008) and piriform cortex (PC) (Young and Sun, 2009; Suzuki and Bekkers, 2010a,b, 2011; Bekkers and Suzuki, 2013)
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