Variability of position of the P2 glomerulus within a map of the mouse olfactory bulb

Olfactory receptor neurons (ORNs) that express a common odorant receptor molecule target specific glomeruli in the olfactory bulb. We systematically assessed the location of the olfactory glomeruli that receive input from ORNs expressing P2 receptors in the P2-internal ribosome entry site-tau-lacZ mouse. We present a new mapping method that includes an Internet-accessible computer program for generating two- and three-dimensional maps of the glomerular sheet in the olfactory bulbs of mice. Cylindrical coordinates were used to define glomerular location: The coordinates were given as the anteroposterior (AP) distance parallel to the long axis of the bulb (rostrocaudal; RC) and angular measurements with origin defined by the remnant ependymal layer in the center of the granule cell layer in the bulb. Using this method, we can apply rigorous statistical methods to give objective estimates of position and variability. At the 95% confidence interval, the lateral P2 glomerulus lies at coordinates 1,008 μm ± 306 μm AP × 146° ± 12°, and the medial P2 glomerulus lies at 1,828 μm ± 196 μm AP × 204° ± 8°. We estimate that these coordinates encompass a domain containing 29 and 37 of the 1,800 glomeruli (∼2%) for the lateral and medial glomeruli, respectively. Furthermore, the data reported here demonstrate that the rostrocaudal position of small P2 glomeruli is three times more variable than that of large glomeruli.