Cholinergic innervation of the superior colliculus in the cat

Abstract

The superficial and intermediate gray layers of the superior colliculus are heavily innervated by fibers that utilize the neurotransmitter acetylcholine. The distribution, ultrastructure, and sources of the cholinergic innervation of these layers have been examined in the cat by using a combination of immunocytochemical and axonal transport methods. Putative cholinergic fibers and cells were localized by means of a monoclonal antibody to choline acetyltransferase (ChAT).ChAT immunoreactive fibers are distributed throughout the depth of the superior colliculus, with particularly dense zones of innervation in the upper part of the superficial grey layer and in the intermediate grey layer. Within the superficial grey layer, the fibers form a continuous, dense band, whereas within the intermediate grey layer the fibers are arranged in clusters or patches. Although the patches are present throughout the rostrocaudal extent of the superior colliculus, they are most prominent in middle to caudal sections.The structure of the ChAT immunoreactive terminals was examined electron microscopically. The appearance of the terminals is similar in the superficial and intermediate grey layers. They contain closely packed, mostly round vesicles, and form contacts with medium‐sized dendrites that exhibit small, but prominent postsynaptic densities; a few of the terminals contact vesicle‐containing profiles.To identify the sources of the cholinergic input to the superior colliculus, injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA‐HRP) were made in the superior colliculus and the sections were processed to demonstrate both the retrograde transport of WGA‐HRP and ChAT immunoreactivity. Neurons containing both labels were found in the parabigeminal nucleus, and in the lateral dorsal and pedunculopontine tegmental nuclei of the pontomesencephalic reticular formation. Almost every cell in these nuclei that contained retrograde label was also immunoreactive for ChAT.The similarities between the laminar distributions of the ChAT terminals and the terminations of the pathway from the parabigeminal nucleus (Graybiel: Brain Res. 145:365–374, '78) support the view that the latter nucleus is a source of the cholinergic fibers in the superficial grey layer. The possibility that the pedunculopontine tegmental nucleus is a source of cholinergic fibers in the deep layers was tested by examining the distribution of labeled fibers following injections of WGA‐HRP into this region of the tegmentum. Patches of labeled terminals were found in the intermediate grey layer that resemble in distribution the patches of ChAT immunoreactive fibers in this layer. Only a sparse distribution of labeled terminals was found in the other layers. These results suggest that there are at least two distinct sources of the cholinergic innervation of the superior colliculus: the parabigeminal nucleus for the superficial grey layer and the pedunculopontine tegmental nucleus for the intermediate grey layer. Other potential sources of cholinergic input to the superior colliculus include ChAT immunoreactive neurons that were observed in the present study in the superficial layers of the superior colliculus itself and the cholinergic cells of the lateral dorsal tegmental nucleus that project to the superior colliculus.Since the pontomesencephalic reticular formation is known to have extensive connections with efferent pathways of the basal ganglia, in a final series of experiments the relationships between the cholinergic pathways from the tegmentum to the superior colliculus and the projections of substantia nigra pars reticulata were explored. Injections of WGA‐HRP were madeinto the substantia nigra pars reticulata and alternate sections were processed for either HRP histochemistry or ChAT immunocytochemistry. Within the superior colliculus, the nigrotectal terminals form patches that are approxi‐mately equal in number and aligned with the patches of ChAT immunoreac‐tive processes. Within the tegmentum, extensive overlap was found between the ChAT immunoreactive cells in the pedunculopontine tegmental nucleus and the terminal field of substantia nigra pars reticulata. These results suggest that there is a close association between the cholinergic innervation of the intermediate grey layer and the nigral outflow of the basal ganglia.