INTRINSIC ASSOCIATION FIBER SYSTEM OF THE PIRIFORM CORTEX - A QUANTITATIVE STUDY BASED ON A CHOLERA-TOXIN-B SUBUNIT TRACING IN THE RAT

By using retrograde and anterograde transport of the B subunit of chol era toxin (CTb), we examined quantitatively the association fiber syst ems, i.e., the collaterals of pyramidal cell axons, that reciprocally connect both the rostral and the caudal parts of the piriform cortex ( PC). Well-defined CTb injections were obtained in layers Ib or II-III of the rostral and the caudal parts of the PC. Using precision countin g, we determined the proportion of cellular profiles in layers II and III that gave rise to association fibers and thus demonstrated a predo minance of rostrocaudal fibers over the caudorostral ones, Our data al so support a precise laminar organization of the PC in which the rostr ocaudal fibers originated mainly from layer II and the caudorostral fi bers primarily from layer III. Cholera toxin injections into layer Ib produced a peak of labeled profiles 2 mm from the site, indicating tha t a large proportion of the association fibers from layer II travel fo r at least 2 mm and then synapse in layer Ib. At either end of the PC, the association projections are concentrated laterally. The functiona l significance of these anatomical features is discussed with respect to olfactory processing, propagation of the activity within the PC, an d the possible role of intrinsic fibers in olfactory memory. (C) 1996 Wiley-Liss, Inc.