ULTRASTRUCTURAL FEATURES OF THE CHOLINE ACETYLTRANSFERASE-CONTAINING NEURONS AND RELATIONSHIPS WITH NIGRAL DOPAMINERGIC AND CORTICAL AFFERENT PATHWAYS IN THE RAT STRIATUM

The aim of this study was first to specify the morphology and neuronal environment of the large cholinergic neurons, and second to determine the distribution and mode of termination of the corticostriatal and d opaminergic inputs on these neurons in the rat striatum. Immunocytoche mical procedures with a monoclonal antibody against choline acetyltran sferase, Golgi staining and standard electron microscopic techniques w ere used to specify the ultrastructural features of the putatively cho linergic classical large neurons. The large/choline acetyltransferase- positive neurons are characterized by a voluminous, eccentric, and dee ply indented nucleus leaving a large cytoplasmic area, and by the pres ence of an abundant granular endoplasmic reticulum and of many polysom es and free ribosomes. Serial ultrathin sectioning further indicated t he presence of nematosomes or nucleolus-like bodies within the nucleus and the cytoplasm of the large neurons. In addition, these neurons we re found to be in direct apposition with up to four surrounding neuron s showing features typical of medium-sized spiny neurons. These data s upport the view that the putatively cholinergic neurons may have an in tense metabolic activity and may be involved in striatal clusters. Whe n choline acetyltransferase immunostaining was coupled with the identi fication of degenerating corticostriatal afferents after lesion of the cerebral cortex, degenerating terminals were seen to form synapses of an asymmetrical type on distal labelled dendrites, but these contacts were very rare. On the other hand, nigrostriatal dopaminergic axons, identified by means of either the degeneration method or tyrosine hydr oxylase immunostaining, were often found to run directly for long dist ances around the choline acetyltransferase-positive cell bodies. Occas ionally, dopaminergic terminals formed possible symmetrical synapses o n choline acetyltransferase-positive cell bodies or proximal dendrites . These data provide evidence that the putatively cholinergic neurons are directly contacted by corticostriatal and dopaminergic nigrostriat al afferents. The respective positions and nature of the two types of contacts further provide morphological support for the hypothesis that postsynaptic interactions may occur between the corticostriatal and d opaminergic nigrostriatal afferents at the level of the cholinergic ne urons.