Tyrosine hydroxylase immunoreactive neurons and fibers in the olfactory system of the hamster

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Abstract
Tyrosine hydroxylaselike immunoreactivity is associated with several different classes of neurons in the main olfactory bulb of the hamster. Most of the tyrosine hydroxylaselike immunoreactive (THLI) neurons are located in the glomerular layer and in the outer two‐thirds of the external plexiform layer. The majority of THLI neurons in the glomerular layer have somal sizes and dendritic features which correspond to those of external tufted cells. A small population of THLI periglomerular cells also is present. The majority of THLI neurons in the external plexiform layer have the morphological characteristics of middle tufted cells. A small population of THLI neurons which appear to be Van Gehuchten cells is distributed throughout the external plexiform layer and in the mitral body layer. Small populations of THLI deep short‐axon cells are distributed throughout the mitral body, internal plexiform, and granule cell layers. Tyrosine hydroxylase‐positive fibers of central origin also are observed in the main olfactory bulb. They may originate from THLI neurons in the anterior olfactory nucleus, ventral hippocampal rudiment, dorsal peduncular cortex, septal and basal portions of the vertical limb and nucleus of the horizontal limb of the diagonal band, lateral and dorsal hypothalamus, ventral tegmental area and substantia nigra, dorsal and median raphe nuclei, and locus ceruleus. The frontal neocortex, infralimbic and anterior cingulate cortex, and medial amygdaloid nucleus also contain THLI neurons.In contrast to the widespread distribution of tyrosine hydroxylaselike immunoreactivity in the main olfactory bulb, substance P‐like immunoreactivity is restricted to external tufted cells and to centrifugal afferents which may originate in the dorsal and median raphe nuclei. Both types of immunoreactivity appear to be present in centrifugal afferents to the accessory olfactory bulb, but THLI neurons are extremely rare and no neurons with substance P‐like immunoreactivity have been observed in the accessory olfactory bulb. Intraventricular injections of colchicine which were expected to enhance the labeling of neuronal somata did not alter the relative incidences of labeling among the various classes of neurons in the olfactory bulbs.These findings provide immunocytochemical evidence that populations of superficially situated tufted cells in the main olfactory bulb are functionally distinct from the deeper‐lying output neurons and from output neurons in the accessory olfactory bulb, that populations of catecholami‐nergic neurons are present within several olfactory and related cortical structures in addition to the olfactory bulbs, that the medical septum‐diagonal band complex also contains catecholaminergic neurons, and that a substantial population of neurons in the dorsal raphe nucleus is capable of vatechol synthesis.

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