Some observations on the circadian rhythm of locomotor activity of mice after depletion of cerebral monoamines

Abstract

Male albino mice kept under an illumination cycle of 12 h light–12 h darkness with the period of light from 06.00–18.00 h, established a biphasic circadian rhythm of locomotor activity with peak levels of activity occurring at approximately 20.00–01.00 h and 06.00–08.00 h. An injection of alphamethyl-p-tyrosine methylester (H44/68, 250 mg/kg, i.p.) at 12.00 h abolished the first phase of activity but not the second. If the injection was given at 24.00 h, the level of activity was reduced initially, increased to a maximum at 08.00 h, and then maintained a lower value than the control value for a further 12–18 h. Treatment with p-chlorophenylalanine (p-CPA, 150 mg/kg, i.p., twice daily for 3 days) led to a single phase of continuous activity. Reserpine treatment resulted in an almost total inhibition of activity. The results obtained with H44/68 and p-CPA suggest that the initiation of the first phase of activity depends on an intact synthesis for the catecholamines whereas its termination may depend on a change in the metabolism of 5-hydroxytryptamine. The inhibition of locomotor activity following an injection of reserpine suggests that an unimpaired mechanism for the storage of the cerebral monoamines is essential for the maintenance of either phase of activity.