Diverse roles in cellular functions have been ascribed to nitric oxide (NO), and its involvement in induction of long-term depression in cerebellar Purkinje cells has been demonstrated. Manipulations of NO concentration or its synthesis in cerebellar tissues therefore provide a means for investigating roles of NO in cerebellar functions at both cellular and behavioral levels. We tested adaptive control of locomotion to perturbation in cats, and found that this form of motor learning was abolished by application of either an inhibitor of NO synthase or a scavenger of NO to the cerebellar cortical locomotion area. This finding supports the view that NO in the cerebellum plays a key role in motor learning.In the cerebellum, long-term depression (LTD) is persistent reduction of transmission efficacy at synapses from parallel fibers to Purkinje cells, which occurs when the parallel fibers are activated in conjunction with climbing fibers converging onto the same Purkinje cells (1, 2). It has been reported that stimulation of the white matter of the cerebellum enhances nitric oxide (NO) concentration in the molecular layer (3), and that NO activates soluble guanylate cyclase, leading to an increase in the level of cyclic GMP (4, 5). When combined with parallel fiber stimulation, NO donors or cyclic GMP induce LTD-like phenomena in slice preparations (3, 6). LTD and these LTD-like phenomena are abolished by N G -monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthase, or hemoglobin, a scavenger of NO (3,[7][8][9]. In a recent study, when NO was photolytically released inside Purkinje cells in conjunction with postsynaptic depolarization, LTD was induced as a manner of parallel fiber activation plus depolarization (10). Thus, NO has been proposed to play an important role in signal transduction processes underlying LTD.LTD has been thought to be a cellular basis of motor learning (1). To establish a causal relationship between LTD and actual motor learning, it is prerequisite that a blockade of a biochemical step that is unique to LTD impairs motor learning. As has been reported (11), exposure of one forelimb to a belt velocity higher than that of the other limbs during locomotion of a cat on a treadmill provides an effective means for testing motor learning. Thereby the coordination of limb movements was disturbed, and climbing fiber responses were induced at high probability in Purkinje cells of the cerebellar vermis (12). After a number of steps in the perturbed locomotion, the cats regained interlimb coordination and stability. In view of the findings that the adaptation takes place in decerebrate cats (11), that interlimb coordination is seriously impaired in spinal cats (13,14), and that mechanical lesions or cooling of the cerebellum induces disturbances of locomotion (15-18), the cerebellum is the most likely site responsible for this adaptation. In this study, we investigated the involvement of NO in the adaptation of locomotion by direct localized application of either an inhibitor of NO synthase o...