Catecholamine innervation of the piriform cortex: a tracing and immunohistochemical study in the rat.

In order to determine the origin of the catecholamine innervation of the rat piriform cortex (PC), we combined retrograde transport of the B subunit of the cholera toxin (CTb) with tyrosine hydroxylase (TH) immunohistochemistry. A substantial number of CTb retrogradely labeled cells was found in the parabrachial pigmented, paranigral and interfascicular nuclei of the ventral tegmental area and the dorsal part of the locus coeruleus, whereas nearly no labeling was noted in the substantia nigra. Following TH immunohistochemistry on the same sections, most if not all of the CTb labeled cells were also TH immunoreactive. Occasional double-labeled cells were also observed in the anterior part of the raphe dorsal nucleus. As visualized with dopamine beta-hydroxylase, dopamine or TH immunohistochemistry, the noradrenaline fibers were homogeneously distributed whereas the dopamine fibers showed rostro-caudal and latero-medial differences. The distribution of TH fibers overlapped both patterns. Our report suggests that the heterogeneous distribution of the DA fibers could support a differential centrifugal modulation of the olfactory information processing throughout the PC.