1. Neighboring mitral cells in the rat olfactory bulb have been previously shown to give similar response profiles to a series of odorants. We now analyze their temporal patterns of activity before and during stimulation to evaluate to what extent soma proximity may act on their temporal correlation and to what extent olfactory stimulation may force two close cells to fire with similar patterns. 2. In anesthetized adult rats, we recorded simultaneously the extracellular single-unit activities of two mitral cells with the use of twin micropipettes with tips separated by < 40-mu-m. These activities were recorded before and during stimulation by a series of five odorants. 3. Activities were classified into nine types according to their temporal pattern along the respiratory cycle. These types comprised nonrhythmic patterns and rhythmic ones, the latter being simple or complex. A phase parameter was also calculated to compare the positions of maximal activity within respiratory cycles of pairs of cells that had rhythmic activities. 4. All analyses were made by comparing data from pairs of close cells with data from pairs of control cells obtained by pairing each first cell of all recorded pairs with the second units of all other pairs. Results reveal a probability of similar activity types significantly higher in pairs of close cells than in control ones before stimulation. Odorant stimulation enhances this difference by reducing the probability of similar associations in control pairs. Close cells present similar patterns as frequently before, as during, stimulation; however, the probability of double nonrhythmic firings decreases whereas the probability of double rhythmic ones increases. 5. Rhythmic firings are phase locked with the transition period between inspiration and expiration. As shown by the phase parameter, stimulation enhances the synchronization of rhythmic activities in pairs of close cells but does not affect it in control cells. Last, patterns that tum into rhythmic ones under stimulation gain a better synchronization in close than in control cells. 6. A substantial proportion of complementary patterns, which are a burst in one cell paired with a pause in the other unit, exists, and this proportion is similar for close and control cells. These patterns, which might be the result of different granular and/or centrifugal influences, seem not to be specific of close cells. 7. Our results reveal that close cell activities present a better temporal coherence than independent cells and suggest that close pairs of mitral cells are under common influences even before stimulation. This may be the result of a common glomerular and/or granular connectivity.
