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Distribution of cholecystokinin‐like immunoreactivity in the rat main olfactory bulb

The anatomical localization of cholecystokinin‐like immunoreactivity (CCK‐I) within the rat main olfactory bulb was analyzed by using the‐peroxidase‐antiperoxidase immunocytochemical technique. Neurons or neuronal processes containing CCK‐I were localized within all laminae of the olfactory bulb except the olfactory nerve fiber layer. A large population of CCK‐I neurons, with morphology, size, and distribution corresponding to that of the middle and external tufted cells, was observed within a zone extending from the deep periglomerular region through the superficial one‐half to one‐third of the external plexiform layer. A smaller number of immunoreactive perikarya were found in the deep external plexiform layer, the glomerular layer, and rarely within the inner plexiform layer. These CCK‐I neurons appeared to correspond to internal tufted cells, periglomerular cells, and deep short‐axon cells, respectively. Dense CCK‐I staining of fibers and terminals was present within the internal plexiform layer and, less densely, within the neuropil of the granule cell layer. In addition, terminal‐like CCK‐I was localized within layer 1A of the anterior olfactory nucleus, the olfactory tubercle, and the most rostral piriform cortex. This observation provides corroboration for the identification of the principal CCK‐I neuron in the rat olfactory bulb as the centrally projecting middle tufted cell. The present results, demonstrating the localization of CCK‐I to both local circuit and projection neurons of the olfactory bulb and to terminal‐like puncta in the internal plexiform and granule cell layers, suggest that CCK may be significantly involved in olfactory processing at several levels.