Fine structure of labelled axons in the cerebellar cortex and nuclei of rodents and primates after intraventricular infusions with tritiated serotonin

Summary

The cerebellar cortex and deep cerebellar nuclei in rats and rhesus monkey were studied after treatment with monoamine oxidase inhibitor and continuous intraventricular infusion with 10^-5 M serotonin-³H. Autoradiographs were prepared for light and electron microscopy. The cerebellum contained no labelled cells. Labelled unmyelinated axons arrive from the brain stem via the periventricular zones of the aqueduct and fourth ventricle. In the parafloccular cortex about 1 per cent of the mossy fibers are labelled, together with a small number of fine varicose axons in the molecular layer that run parallel to the folial axes (less than 0.1%). In the paravermal and vermal cortex there are few labelled fibers in the granular layer and a five-fold greater number of labelled axons in the molecular layer (about 0.5%). Apparently three systems of serotonin-containing axons are present in the cortex: mossy fibers, parallel fiber-like, and a diffuse system in granular and molecular layers. The fastigial (medial), interpositus, and dentate (lateral) nuclei, lateral vestibular and other vestibular nuclei all have numerous labelled axons. The dentate and interpositus nuclei receive labelled fibers which arrive through the superior cerebellar peduncle as well as from the periventricular area. Six morphologically different classes of labelled axon terminals have been differentiated. Class 1a, the mossy fiber rosettes, and class 1b, the CAT₂ axons, have small, round, clear synaptic vesicles and large granular vesicles (LGV); class 2 axons have a distinctive collection of round granular vesicles; class 3 boutons have numerous tubular profiles, a few containing dense dots, packed in a dark axoplasmic matrix; class 4 axons have tiny 250 Å granular vesicles, clear tubular profiles and occasional LGV; class 5 terminals have numerous LGV, both round and elongated, with clear round and tubular profiles; class 6 terminals have LGV, clear and granular synaptic vesicles and clear tubular profiles. All these axons have LGV 900 Å in diameter with 500–600 Å variably dense centers that do not fill the vesicle, and Gray's type 1 axodendritic or axasomatic synapses on postsynaptic locations in the cortex and nuclei. Labelled axons in the cortex end as mossy fibers upon granule cell dendrites in glomeruli (Class 1a) or upon dendrites of cortical interneurons, e.g. Golgi cells, basket and stellate cells, and not on Purkinje cells. In the deep nuclei, axosomatic terminations on the interneurons (Class 5) and synapses upon dendrites of large and small neurons are common. Experiments with intraventricular 5,6-dihydroxytryptamine confirm that axons that accumulate this serotonin analog are present in the granular and molecular layers of the cerebellar cortex. Results from horseradish peroxidase studies suggest that the raphe dorsalis, superior centralis, magnus, pontis and obscurus nuclei provide afferent axons to the dentate nuclei in both species and are probably some of the major contributors to the cerebellar serotonin system described here. Questions of specificity are discussed, and summary diagrams for the new cerebellar circuitry are provided. It is suggested that the raphe mossy fibers and their collaterals are specific afferents, as are the parallel axons, whereas the remaining diffuse system may constitute a neurohumoral system for setting and/or responding to general baseline activity of the neuropil.