Cholecystokinin (CCK)-8-immunoreactivity in the piriform cortex of the rat with special reference to its fine...

Characteristic features of cholecystokinin-8 (CCK-8) containing neuronal structures in the rat piriform cortex were examined by the immunocytochemistry at the light and electron microscopic levels. CCK-immunoreactive (CCKI) neurons were divided into at least four distinct morphological classes; bipolar, bitufted, multipolar and semilunar types. Each type of CCKI cells as well as CCKI axon terminals were studied by the electron microscopy in the layer II and III where most of them were located. Bipolar and bitufted types of CCKI cells made synaptic contacts with both non-CCKI and CCKI axon terminals with the former predominance. A total of 200 synaptic contacts formed by CCKI axon terminals were examined by random section analysis. Over 98% of CCKI axon terminals formed symmetrical synapses. In several cases, non-immunoreactive postsynaptic targets were identified; the pyramidal cells and small/medium sized multipolar cells in the layer II and III, and semilunar cells in the layer II. Additionally, the axonal initial segments of non-immunoreactive pyramidal cells were occasionally made symmetrical contacts with CCKI axon terminals. These findings suggest that CCK afferents exert monosynaptic influences on both projecting neurons and interneurons in the piriform cortex, and are thereby involved in the control of cortical neuronal activities.