Unit responses changing with behavioral outcome in the olfactory bulb of unrestrained rats

The unit activity of 16 neurons, including 12 likely mitral cells, was recorded in one olfactory bulb of 5 unrestrained rats, together with contralateral multiunit activity, and respiratory rhythm. The hungry animals were stimulated either by food odor (F) learned as a signal for one available food pellet, or by isoamyl acetate (IA), presented randomly. Eighty-four sequences, each with one stimulation, were analyzed to determine how the odors modified unit, multiunit and respiratory activities. The stimulation could change the variance and/or mean of the unit discharge and its correlation with the respiration phase or frequency or with multiunit activity. Besides the IA sequences, F+, F= and F-situations had to be distinguished, when the pellet was eaten spontaneously, accepted if presented at mouth or refused actively. The neuron responses were reproducible, in a given situation, but their occurrence within a series of identical stimuli was unpredictable from the controlled or observed events. The positive mitral responses were more probable in the F+ and F= than in the F-sequences; they were then associated with multiunit and respiratory activation. Response criteria and neuron typology are discussed. The functional involvements of neuron variability and modulation with internal state are considered.