SIMILARITY OF GRANULAR-INDUCED INHIBITORY PERIODS IN PAIRS OF NEIGHBORING MITRAL TUFTED CELLS/

1. Neighboring mitral/tufted cells have been previously shown to prese nt temporal correlations of their firings related to the respiratory r hythm, particularly under odor stimulation. This occurs despite the ex istence of a powerful inhibitory control exerted by granule cells onto mitral/tufted cells. In the present study, we hypothesized that neigh boring mitral cells can present granular-induced inhibitory periods wi th similar latencies and durations and that such a similarity would pr eserve them from a possible suppression of their temporal correlations . 2. To test this hypothesis, we analyzed the latencies and durations of the inhibitory periods induced by granular activation in pairs of s imultaneously recorded neighboring mitral cells. The activation of gra nule cells was achieved by electrical stimulation of the different pat hways known to directly activate granule cells [lateral olfactory trac t (LOT), anterior limb of the anterior commissure (AC), and piriform c ortex (PC)]. Data from this group were compared with those of a contro l group composed of distant cells also recorded simultaneously. 3. Res ults first show that the latencies to onset of inhibition or to recove ry were more frequently similar in neighboring cells than in control c ells and that this similarity was enhanced by odor stimulation. Second , the probability that two cells exhibit similar inhibitory periods (i .e., similar latencies to both onset and to recovery) in response to e lectrical stimulation of LOT, AC, or PC was significantly higher in ne ighboring than in control cells. Third, only neighboring cells were fo und to present similar inhibitory periods in response to the stimulati on of all of the three structures. 4. Granular activation was also fou nd to modify the temporal patterns of individual mitral cells. However , although these patterns were not systematically modified similarly i n neighboring mitral cells, they remained perfectly synchronized with zero delay if they were already synchronous without electrical stimula tion. On the contrary, if patterns were spontaneously uncorrelated, el ectrical stimulation never produced a synchronization of their firings , even if their temporal relationships could be profoundly modified. 5 . These results show that neighboring mitral cells can receive granula r-induced inhibition with similar latencies and durations with a proba bility much higher than control cells. Such similarities allow neighbo ring mitral cells to preserve their temporal correlations despite the powerful inhibitory input from granule cells. Functional hypotheses ab out the role of the cortical feedback projections onto the bulb are di scussed.