Respiration and olfactory bulb unit activity in the unrestrained rat: Statements and reappraisals

The activity of 26 olfactory bulb units, including 19 mitral, 5 granular and 2 external plexiform cells, was recorded in unrestrained rats associating food odor stimuli/isoamyl acetate to a food reward/no reward. The respiratory activity was transduced from the intranasal air pressure and used as a time-base to analyze the unit discharge. The patterning of neuron activity was presented in histograms built from sequences of 30 successive cycles each resolved into 5 equal bins. 64 sequences were defined by the low or high respiratory frequency and by olfactory stimulation. In resting conditions, 15 (13 mitral) units displayed significant respiratory patterning, mainly characterized by the absolute and relative phases of the maximal and minimal activity in the cycle. Six typical groups of units could be defined accordingly. Increased respiratory frequency erased patterning, except in the 2 most typical units. The histograms from adjacent mitral cells showed that the various types were distributed as in a neuronal network with lateral recurrent inhibition, where noise was introduced at each inspiration. The data verified that the spatial and temporal distribution of the input activity elicited by the olfactory stimuli created local interferences, modifying the patterning of mitral activity. The odor-induced changes (R1 responses) were as consistent as the typology itself; they were selective and habituated rapidly. The transient R1 activity could also give rise to an R2 firing, atypical, regular and lasting, mainly when food odor elicited food intake. Possible functional interpretations of these phenomena are presented.