THE ULTRASTRUCTURE OF THE OLIVARY PRETECTAL NUCLEUS IN RATS - A TRACING AND GABA IMMUNOHISTOCHEMICAL STUDY

The olivary pretectal nucleus is a primary visual centre, involved in the pupillary light reflex. In the present study an ultrastructural an alysis was made of the olivary pretectal nucleus by means of separate, anterograde and retrograde tracing techniques and immunohistochemistr y of gamma-aminobutyric acid. Large-projection neurons and two types o f gamma-aminobutyric acid-immunoreactive (GABA-ir) neurons are observe d in the olivary pretectal nucleus. The primary dendrites of the proje ction neurons have a dichotomous appearance, the secondary dendrites a multipolar appearance. At the ultrastructural level the projection ne urons have well-developed Golgi fields, abundant rough endoplasmic ret iculum and the nucleus is always heavily indented. Numerous small GABA -ir neurons and a few medium-sized GABA-ir neurons are found. The smal l GABA-ir neurons contain a few stacks of rough endoplasmic reticulum and the nucleus is oval-shaped. The medium-sized GABA-ir neurons have well-developed Golgi fields, a moderate number of rough endoplasmic re ticulum stacks and an indented nucleus. GABA-positive dendritic profil es containing vesicles also are observed. In the neuropil of the oliva ry prerectal nucleus, retinal terminals are found that contain round c lear vesicles and electron-lucent mitochondria. They make asymmetric s ynaptic contacts (Gray type I) with dendritic profiles and with profil es containing vesicles. Terminals originating from the contralateral o livary prerectal nucleus exhibit small, round clear vesicles, electron -dense mitochondria and make asymmetric synaptic contacts (Gray type I ) mainly with dendritic profiles. Two types of GABA-ir terminals were found. One type is incorporated in glomerulus-like arrangements, where as the other type is not. GABA-ir terminals contain pleomorphic vesicl es, electron-dense mitochondria and make symmetric synaptic contacts ( Gray type II). Retinal terminals, terminals originating from the contr alateral olivary prerectal nucleus and GABA-ir terminals are organized in glomerulus-like structures, in which dendrites of the large projec tion neurons form the central elements. Triadic arrangements are obser ved in these structures; a retinal terminal contacts a dendrite and a GABA-ir terminal and the GABA-ir terminal also contacts the dendrite. The complexity of the synaptic organization and the abundancy of inhib itory elements in the olivary pretectal nucleus suggest that the oliva ry pretectal nucleus is strongly involved in processing visual informa tion in the pupillary light reflex are.