Effects of olfactory peduncle sectioning on the single unit responses of olfactory bulb neurons to odor presentation in awake rabbits

Abstract

The influences of centrifugal inputs to the olfactory bulb were studied by recording singlecell responses evoked by olfactory stimuli in intact and peduncle-sectioned bulbs of awake freebreathing rabbits. Responses of intact animals were mainly characterized by a temporal reorganization of the single unit discharge - responsive second order neurons increased their firing activity during inspirations and were silent during expirations. This synchronization of firing discharge with respiration occurred in the absence of any significant change in the overall firing activity measured over intervals which included both the inspiratory and expiratory phases of the respiratory cycle. By contrast, neurons recorded in isolated olfactory bulbs exhibited either a significant increase or a decrease in firing activity during odor presentation, and, furthermore, the synchronization of these units to the respiratory cycle was markedly reduced compared with that in intact animals. Comparison of cell responsiveness between intact and isolated olfactory bulbs indicated that the lesion increased the number of odors which induced a response, but did not change the percentage of cells which failed to respond to any of the 5 odorants used in this study. The cell responsiveness increased for camphor and isoamyl acetate, and to a lesser extent for food odor. The results indicate that high order nervous structures exert a powerful inhibitory influence on the responses of olfactory bulb second-order neurons to odor stimuli. They also suggest that, in intact rabbits, centrifugal inputs play a role in the odor-induced synchronization of the single unit activity with respiration.