A specific harmaline-evoked increase in cerebellar 5'-nucleotidase activity.

This study examines harmaline-induced changes in 5'-nucleotidase (5'-ND) activity in cerebellar fractions from rats with an intact inferior olive (IO) or prior destruction of the IO by 3-acetylpyridine (3-AP) intoxication. Harmaline markedly increased 5'-ND activity in the crude homogenate (P less than 0.05) and P2 fraction (P less than 0.001) of cerebella from rats with an intact IO. This increase was absent in the P1, P3 and S3 fractions and it was abolished by 3-AP olivectomy. It was also absent in basal ganglia P2 fractions. Since harmaline produces rhythmic complex spike discharges of Purkinje cells by activating IO neurons [4, 18], these data suggest that climbing fiber activation per se increases 5'-ND activity in the P2 fraction. This raises the possibility that a climbing fiber-induced local increase in 5'-ND activity at parallel fiber-Purkinje cell synapses results in a local increase in adenosine concentration. This may account for climbing fiber-evoked suppression of simple spike activity [12, 13, 28].