Response similarity to odors in olfactory bulb output cells presumed to be connected to the same glomerulus: electrophysiological study using simultaneous single-unit recordings

N. Buonviso and M. A. Chaput Universite Claude Bernard, Centre National de la Recherche Scientifique, Villeurbanne, France. 1. The glomeruli of the olfactory bulb are discrete anatomic structures in which the terminals of receptor cell axons make extensive contacts with the primary dendrites of the mitral and tufted output cells. In mammals, each mitral and deep tufted (M/T) cell possesses a single primary dendrite and sends it toward the glomerulus situated just in front of its somata. 2. We tested the hypothesis that the glomeruli, which appear to form anatomic units, could act to some extent as functional units. A unitary functioning implies that the M/T cells connected to the same glomerulus will more often display similar responses to odorants than cells having no common glomerular relationships, including cells related to adjacent glomeruli. 3. In anesthetized adult rats, we recorded the extracellular single-unit responses of pairs of M/T cells to a series of five odorants. Recordings were performed with the use of twin microelectrodes whose tips were separated either by less than 40 or by 150-200 microns. Because of the olfactory bulb organization, we assumed that the close cells, recorded at a distance less than 40 microns, were more often connected to the same glomerulus, whereas the distant cells, recorded at a distance of 150-200 microns, were more often connected to adjacent glomeruli. 4. Stimulus-evoked changes in firing rate were classified as either excitatory (+), suppressive (-), or null (0) responses. The collection of response types of a given cell to the 5 odorants composed its response profile. Response profiles were used to compare the responsiveness within close and within distant cell pairs with that observed within control pairs of cells.