Circadian pacemakers in vertebrates.

The identification and isolation of circadian pacemaker cells is of critical importance to studies of circadian clocks at all phylogenetic levels. In the vertebrate classes, a few structures of diencephalic origin have been implicated as potential sites but for only two, the avian pineal and the mammalian suprachiasmatic nucleus (SCN), has a pacemaker role in addition to oscillatory behaviour been demonstrated by the transfer of pacemaker properties from one organism to another. Studies of the mammalian system in particular have benefited from the ability to restore circadian function using transplantation of tissue from the SCN and from the availability of a hamster period mutant, tau, that allows donor-derived and host-derived rhythms to be distinguished easily. Initial cross-genotype transplantation studies and the subsequent creation of circadian chimeras expressing two phenotypes simultaneously demonstrated the pacemaker capability of the SCN, and demonstrated the relative autonomy of this nucleus as a pacemaking structure. Despite an abundance of information regarding the anatomy, physiology and pharmacology of these nuclei, the identity of the pacemaker cells and their methods of communication with each other and the organism remain obscure. None the less, it is possible under certain conditions to create chimeras with two clocks that interact. The behaviour of these animals provides a unique opportunity to study the nature and timing of pacemaker communication.