"Interneurons" in the olfactory bulb revisited.

The main olfactory bulbs (MOBs) are now one of the most interesting parts of the brain in at least two points; the first station of the olfaction as an excellent model for understanding the neural mechanisms of sensory information processing and one of the most prominent sites whose interneurons are generated continuously in the postnatal and adult periods. Here we point out some new aspects of the MOB organization focusing on the following 4 issues: (1) there might be both axon-bearing and anaxonic periglomerular cells (PG cells), (2) most parvalbumin positive medium-sized neurons in the external plexiform layer as well as a few nitric oxide synthase positive PG cells and calretinin positive granule cells are anaxonic but display dendritic hot spots with characteristics of axon initial segments, (3) some of so-called "short-axon cells" project to the higher olfactory related regions and thus should be regarded as "nonprincipal projection neurons" and (4) tyrosine hydroxylase positive GABAergic (DA-GABAergic) juxtaglomerular neurons (JG neurons) are a particular type of JG neurons as a main source of the interglomerular connection, forming an intrabulbar association system.