Olfactory subsystems associated with the necklace glomeruli in rodents

The necklace glomeruli are a loosely defined group of glomeruli encircling the caudal main olfactory bulb in rodents. Initially defined by the expression of various immunohistochemical markers, they are now better understood in the context of the specialized chemosensory neurons of the main olfactory epithelium and Grueneberg ganglion that innervate them. It has become clear that the necklace region of the rodent main olfactory bulb is composed of multiple distinct groups of glomeruli, defined at least in part by their afferent inputs. In this review, we will explore the necklace glomeruli and the chemosensory neurons that innervate them.

Unique molecular markers for GC-D-expressing olfactory sensory neurons and chemosensory neurons of the Grueneberg ganglion

ABSTRACTThe main olfactory bulb (MOB) is differentiated into subregions based on their innervation by molecularly distinct chemosensory neurons. For example, olfactory sensory neurons (OSNs) that employ a cGMP-mediated transduction cascade – guanylyl-cyclase D-expressing (GC-D+) OSNs of the main olfactory epithelium (MOE) and chemosensory neurons of the Grueneberg ganglion (GGNs) – project to distinct groups of “necklace” glomeruli encircling the caudal MOB. To better understand the unique functionality and neural circuitry of the necklace glomeruli and their associated sensory neurons, we sought to identify additional molecular markers that would differentiate GC-D+ OSNs and GGNs as well as their target glomeruli. We found in mouse that GC-D+ OSNs, but not other MOE OSNs or GGNs, express the neuropeptide CART (cocaine- and amphetamine-regulated transcript). Both GC-D+ OSNs and GGNs, but not other MOE OSNs, express the Ca2+/calmodulin-dependent phosphodiesterase Pde1a, which is immunolocalized throughout the dendrites, somata and axons of these neurons. Stronger Pde1a immunolabeling in necklace glomeruli innervated by GGNs than in those innervated by GC-D+ OSNs suggests either greater Pde1a expression in individual GGNs than in GC-D+ OSNs or a difference in sensory neuron innervation density between the two types of necklace glomeruli. Together, the unique molecular signatures of GC-D+ OSNs, GGNs and their MOB targets offer important tools for understanding the processing of chemosensory information by olfactory subsystems associated with the necklace glomeruli.