Cholecystokinin-like immunoreactivity in cat inferior colliculus. Light and ultrastructural study.

Cholecystokinin-like immunoreactive (CCK-LI) neurons are observed in the pericentral nucleus, the external nucleus and the dorsomedial part of the central nucleus of the cat's inferior colliculus. The largest number of CCK-LI cell bodies is observed in the caudal part of the pericentral nucleus. Using the electronmicroscopical examination we distinguish two different types of CCK-LI neurons. The first type CCK-LI neurons is characterized by a relatively large nucleus, surrounded by a dark rim of cytoplasm containing single cisterns of granular endoplasmic reticulum, few mitochondria and lysosomes. The second type CCK-LI neurons are smaller than the neurons of the first type. The nucleus is relatively larger while the cytoplasmic rim is quite thin. The CCK-LI immunoreactivity is also found in the large, medium-sized and small dendrites as well as in the dendritic spines. The CCK-LI axon terminals contain small round or pleomorphic vesicles. Sometimes they show a mixed population of clear and dense core vesicles. The immunoreactive terminal boutons establish typical asymmetrical synaptic contacts with dendritic spines, small or medium-sized dendrites. More rarely, the immunoreactive axon terminals perform symmetrical or intermediate synaptic contacts with proximal dendritic trunks or perikarya.