Synaptic 5'-nucleotidase is transient and indicative of climbing fiber plasticity during the postnatal development of rat cerebellum

The transient appearance of 5'-nucleotidase, an adenosine-producing ecto-enzyme, was studied during specific stages of postnatal synaptogenesis in the rat cerebellum. For ultrastructural detection of 5'-nucleotidase activity, an enzyme-cytochemical technique was used. Between postnatal days 4 and 6, enzymatic reaction product was present in the synaptic clefts of climbing fibers containing the perisomatic spines, apical cones and emerging dendrites of Purkinje cells (CF-PC synapses). Labeled parallel fiber synapses were observed on dendritic shafts of cerebellar interneurons. At postnatal days 9 and 12, enzyme-positive parallel fiber terminals were in addition numerous on the spines of peripheral Purkinje branchlets, and gradually disappeared thereafter. Between postnatal days 8 and 15, labeling of perisomatic CF-PC contacts persisted. In contrast, climbing fiber synapses on Purkinje dendrites were only occasionally labeled. Between postnatal days 18 and 21, synaptic reaction product was restricted to mossy fibers. At the same time, association of 5'-nucleotidase with glial profiles was prominent throughout the cerebellar layers. In adult cerebellum (from 24 days onwards) all synapses were devoid of enzymatic activity. Throughout development, basket, stellate and Golgi cell synapses were devoid of enzymatic activity. We conclude that 5'-nucleotidase is present in excitatory cerebellar synapses during part of their generation period. The transient nature of this phenomenon suggests that 5'-nucleotidase may serve as a novel, cytochemical marker for a specific state of synaptic maturation, and in particular for climbing fiber plasticity. A role of 5'-nucleotidase in purinergic neuromodulation and cellular contact formation could be significant in these processes.