GENERATION AND ENTRAINMENT OF CIRCADIAN-RHYTHMS

1. The present brief review examines some of the new developments in t he area of circadian rhythm research. 2, The discovery of the mouse cl ock and m-per genes and their similarity to other clock genes like per and tim has provided new insight into the control of rhythms in verte brates, In mice, these genes are expressed in the site of the biologic al clock, the suprachiasmatic nucleus (SCN), and so will now become a focus of research into the generation of rhythmicity, 3, Because SCN c ells expressing endogenous rhythms have a periodicity different from 2 4 h, there must be mechanisms in place to reset the rhythms on a daily basis. This is achieved in mammals by retinal light perception and ne ural transmission through several discrete pathways to the SCN, 4, The nature of the neurotransmitters involved in this transfer of environm ental information to the timing system is controversial and may even v ary between similar species but, in the rat, there is compelling evide nce that a serotonergic pathway is pre eminent in mediating the effect s of light. How the re-setting is achieved at the cellular level is no t known.