OLFACTORY-BULB REPETITIVE STIMULATIONS REVEAL NONHOMOGENEOUS DISTRIBUTION OF THE INHIBITORY PROCESSES IN THE RAT PIRIFORM CORTEX

Optical signals were recorded in the in vivo rat piriform cortex in re sponse to a burst of seven electrical stimulations (100 ms interval) d elivered in the olfactory bulb. Based on the recorded responses, three types of signal could be identified according to the relative amplitu de of their monosynaptic and disynaptic components. The disynaptic com ponent had a larger (type 1) or an equal amplitude (type 2) compared w ith the monosynaptic one, Type 3 exhibited only the monosynaptic compo nent. Type 1 represented 96% of the first response. The second respons e was characterized by an increase in type 3 signals (39%). The remain ing type 1 signals were lower in amplitude when compared with the firs t response. The responses to the last five stimulations did not differ from one another but were different from the first two (type 1, 74%; type 2, 7.8%; type 3, 18.2% on average). The spatial distribution of t hese three types of signal was analysed by dividing the piriform corte x into several areas. These areas were not homogeneous in the percenta ge of each signal type: the percentage of type 3 signals was highest ( similar to 30%) in the area near the lateral olfactory tract and <10% in the most posterodorsal area. Thus the level of inhibition remained high in some piriform areas whereas it decreased rapidly in others, su ggesting that the inhibitory processes were not homogeneously distribu ted in the whole piriform cortex. Functional implications are discusse d.