ORGANIZATION AND SYNAPTIC CONNECTIONS OF CHOLINERGIC FIBERS IN THE CAT SUPERIOR COLLICULUS

The cat superior colliculus (SC) receives a dense cholinergic input fr om three brainstem nuclei, the pedunculopontine tegmental nucleus, the lateral dorsal tegmental nucleus, and the parabigeminal nucleus (PBG) . The tegmental inputs project densely to the intermediate gray layer (IGL) and sparsely to the superficial layers. The PBG input probably p rojects only to the superficial layers. In the present study, the morp hology of choline acetyltransferase (ChAT)-immunoreactive axons and sy naptic endings in the superficial and deep layers of the SC was examin ed by light and electron microscopy to determine whether these choline rgic afferents form different types of synapses in the superficial and deep layers. Two types of fibers were found within the zonal (ZL) and upper superficial gray layers (SGL): small diameter fibers with few v aricosities and larger diameter fibers with numerous varicosities. Qua ntitative analysis demonstrated a bimodal distribution of axon diamete rs, with one peak at approximately 0.3-0.5 mum and the other at 0.9-1. 0 mum. On the other hand, ChAT-immunoreactive fibers in the IGL were a lmost all small and formed discrete patches within the IGL. Two types of ChAT-immunoreactive synaptic profiles were observed within the ZL a nd upper SGL using the electron microscope. The first type consisted o f small terminals containing predominantly round synaptic vesicles and forming asymmetric synaptic contacts, mostly on dendrites. The second type was comprised of varicose profiles that also contained round syn aptic vesicles. Their synaptic contacts were always symmetric in profi le. ChAT-immunoreactive terminals in the IGL patches contained round o r pleomorphic synaptic vesicles, and the postsynaptic densities varied from symmetric to asymmetric, including intermediate forms. However, no large varicose profiles were observed. This study suggests that cho linergic fibers include at least two different synaptic morphologies: small terminals with asymmetric thickenings and large varicose profile s with symmetric terminals. The large varicose profile in the superfic ial layers is absent in the IGL. This result suggests that the choline rgic inputs that innervate the superficial layers and the patches in t he IGL of the cat SC differ in their synaptic organization and possibl y also in their physiological actions. (C) 1993 Wiley-Liss, Inc.