RESPONSE SIMILARITY TO ODORS IN OLFACTORY-BULB OUTPUT CELLS PRESUMED TO BE CONNECTED TO THE SAME GLOMERULUS - ELECTROPHYSIOLOGICAL STUDY USING SIMULTANEOUS SINGLE-UNIT RECORDINGS

1. The glomeruli of the olfactory bulb are discrete anatomic structures in which the terminals of receptors 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 <40 or by 150-200 μm. Because of the olfactory bulb organization, we assumed that the close cells, recorded at a distance <40 μm, were more often connected to the same glomerulus, whereas the distant cells, recorded at a distance of 150-200 μm, 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. Response profiles of these control pairs were constructed by pairing the profile of each close cell with the profile of each distant cell. 5. Results first reveal a similar reactivity to odorants within close cell pairs. Indeed, only close cells displayed similar response profiles to the series of odorants. They responded significantly more often by paired excitations (++) or paired suppressions (--) than did control pairs. Furthermore, the probability of finding excitation paired with suppression was significantly lower in the close cell group than in the control group. 6. In contrast, pairs of distant cells showed an opposite reactivity to odorants. None of them displayed similar response profiles. Probabilities of observing a double excitation (++) or a double suppression (--) were significantly lower in this group than in the control one. Last, the probability of opposite (+-) responses was significantly higher than in the control group. 7. Our results suggest that the mitral cell layer responds during odor stimulations as foci of exicted cells surrounded by zones of suppressed cells. We discuss the possibility that these foci of excited cells correspond to the efferents from individual glomeruli.