Brainstem origin of serotonin- and enkephalin-immunoreactive afferents to the opossum's cerebellum

Previous studies have described the distribution of serotonin- and enkephalin-immunoreactive elements in the posterior lobe vermis of the opossum's cerebellum. In the present study we have used a double labeling paradigm which combines the retrograde transport of horseradish peroxidase (HRP) with serotonin and enkephalin immunohistochemistry to determine the brainstem origin of serotoninergic and enkephalinergic neurons that project to the opossum's cerebellar cortex. Subsequent to HRP injections into the posterior lobe vermis, widespread areas of the medulla and pons were found to contain retrogradely labeled neurons. Serotonin-immunoreactive somata are present primarily in the raphe nuclei and the adjacent reticular formation. Enkephalinergic neurons were numerous in the raphe nuclei, medial accessory olive, gigantocellular reticular formation, locus coeruleus, and the nucleus of the trapezoid body. However, serotoninergic neurons that project to the cerebellum were located only in the medullary pyramids and the reticular formation adjacent to the raphe. Double-labeled enkephalinergic neurons were located 1) within the medullary pyramids, 2) throughout the extent of the caudal medial accessory olive, 3) in the rostral subnucleus a of the medial accessory olive, 4) in the nucleus reticularis gigantocellularis pars ventralis, 5) in the nucleus reticularis lateralis, and 6) in the nucleus reticularis ventralis lateral to the inferior olivary complex. These results indicate that although neurons containing serotonin and enkephalin immunoreactivity may be present in some of the same pontine and medullary nuclei, those serotoninergic and enkephalinergic neurons that project to the cerebellum are present primarily in restricted and spatially separate regions of the caudal medulla.