Terminal field of cholecystokinin‐8‐like immunoreactive projection neurons of the rat main olfactory bulb

The terminal field of cholecystokinin-8 (CCK)-like immunoreactive (CCK-IR) tufted cells in the rat main olfactory bulb was examined by means of immunohistochemistry combined with either an anterograde tracer or a degeneration method. CCK immunostaining was carried out in animals in which Phaseolus vulgaris agglutinin (PHA) had been injected into the main olfactory bulb. Pairs of adjacent sections were processed for CCK and PHA immunostaining, respectively. Dense CCK-IR terminallike staining was noted in layer Ia of the anterior olfactory nucleus and lateral part of the olfactory tubercle; weaker staining was also observed in the transitional area between the anterior olfactory nucleus and the piriform cortex, in the medial part of the olfactory tubercle, and in the cortical amygdaloid nucleus. The CCK-IR staining was limited to the area containing PHA-labeled terminals and was diminished in these sites after unilateral olfactory bulbectomy. Immuno-electron microscopic analysis showed that CCK-IR profiles in such regions made asymmetric synaptic contacts, mainly with dendritic spines. These results suggest that CCK-IR tufted cells project mainly to the anterior olfactory nucleus and lateral part of the olfactory tubercle, and act mainly via axospinous synapses.