OPTICAL-RECORDING OF THE RAT PIRIFORM CORTEX ACTIVITY

The piriform cortex (PCx) is a phylogenetically old brain structure wh ich presents characteristics of a content-addressable memory. Taking i nto account its particular anatomo-functional organization, we hypothe sized that this cortex could behave rather as an assembly of different functional units than as a functionally homogeneous structure. This h ypothesis was tested by using both anatomical and functional approache s. Immunohistological and tracing experiments demonstrated that both t he connections of the PCx with the higher nervous centres, and its mon oaminergic and cholinergic modulatory afferents exhibited a heterogene ous distribution. Then, optical monitoring of its neuronal activity wi th a voltage-sensitive dye pointed out that the PCx is a functionally heterogeneous structure. Electrical stimulations of the olfactory bulb showed that the inhibitory processes which control the cortical respo nsiveness were not identical in all the PCx area. Two different functi onal areas at least could be distinguished: in the ventromedial PCx, t he afferent activity is privileged since the level of inhibition of di synaptic activation remained large during repetitive stimuli. Contrari ly, in the posterior PCx, the disynaptic activity remained unchanged i n response to successive stimulations and the responses of neighbourin g sites were statistically more synchronized than in its anterior part . Moreover, a late depolarization wave was significantly larger in the posterior PCx. These data are in good agreement with the results prov ided by computational models of the PCx. In the future, theoretical an d experimental investigations of this cortex will he useful for unders tanding olfactory information processing and as a model of brain funct ioning at the neocortical level as well. (C) 1997 Elsevier Science Ltd .