Anatomical evidence for enkephalin immunoreactive climbing fibres in the cerebellar cortex of the opossum.

Enkephalin immunoreactivity is present in the cerebellum of the adult opossum within axonal arbors that resemble mature climbing fibres. In the developing cerebellum, enkephalinergic axons form pericellular nests around the perikarya of Purkinje cells in a manner which resembles developing climbing fibres seen in Golgi impregnations. Serial electron micrographs of adult climbing fibres reveal elongate enkephalin immunoreactive profiles that contain synaptic vesicles and make contact with the thorns and shafts of Purkinje cell dendrites. These results suggest that a peptide, enkephalin or an enkephalin-like substance may mediate synaptic interactions between certain populations of climbing fibres and Purkinje cells in the cerebellum of the opossum. Enkephalin immunoreactive axonal arbors, present in the molecular layer, are localized in restricted areas of vermal lobules II-VIII and X. The intermediate cortex and hemispheres are devoid of enkephalinergic climbing fibres except in restricted areas of the paramedian lobule, paraflocculus and the flocculus. In an attempt to establish the origin of enkephalin axons in the cerebellum, a double labelling technique that combines retrograde labelling of cells with horseradish peroxidase and enkephalin immunohistochemistry has been employed. Enkephalin immunoreactive neurons within specific portions of the medial accessory olive are retrogradely labelled in this paradigm. The presence of enkephalin immunoreactivity in selected climbing fibres provides evidence for chemical heterogeneity within one of the major afferent systems to the cerebellum previously thought to be uniform in its transmitter content.