Locomotor rhythms induced by methylphenidate in suprachiasmatic nuclei-lesioned rats.

A robust locomotor activity rhythm with a circadian period appeared by chronic treatment with methylphenidate or methamphetamine in the suprachiasmatic nuclei (SCN)-lesioned rats whose circadian rhythms had been abolished. The appearance of the rhythmicity was accompanied by an increase in the activity level. However, significant circadian periods were detected only in rats whose locomotor activity increased more than 200% of the pretreatment levels. An increase in the activity level is necessary to generate and/or express the circadian locomotor rhythm in SCN-lesioned rats and there seems to be a threshold.