ABSTRACT
Research into circadian effects of melatonin in mammals has focused on melatonin injections in a very limited number of rodent species. The magnitude of the phase shifts obtained with acute melatonin injections, for example, a 5-h delay, may be a reflection of the dose used since phase shifts within the range of 1 h would be sufficient to entrain these animals. Irrespective of whether it is ultimately demonstrated that endogenous melatonin plays a natural, physiological role as an internal zeitgeber, exogenous melatonin's pharmacological property of inducing synchronicity makes it a therapeutic tool which has great potential for ameliorating a number of circadian based disorders. However, lesions to the suprachiasmatic nuclei (SCN) which abolish circadian rhythmicity of locomotor rhythms, prevent entrainment, suggesting that the SCN are pivotal sites for the exogenous melatonin entrainment mechanism. Melatonin taken orally at a dose of 2.5 to 5 mg per human gives a supraphysiological plasma level for several hours.
