Rhythms in a diurnal brain

Abstract

The neural mechanisms governing circadian rhythms generate patterns of behavior and physiology that are very different in diurnal and nocturnal species. Here we review data bearing on the issue of where and how in the brain these differences might be generated. Molecular data from several species now confirm that the central circadian clock, located in the suprachiasmatic nucleus (SCN), is coupled to the light – dark cycle in the same manner in nocturnal and diurnal species, indicating that the fundamental differences arise from mechanisms coupling the clock to effector systems. Major differences in this coupling become apparent only when one steps beyond the SCN to look at brain regions that directly or indirectly receive input from it. This review focuses on our work on brain regions and cell populations to which the SCN projects in the diurnal species Arvicanthis niloticus (Nile grass rats). We have found rhythms in the numbers of cells containing cFos, or PER1, in a number of these regions, and the patterns of these rhythms are always different from those seen in nocturnal laboratory rats. In some areas these rhythms are simply inverted in the two species, but in other extra-SCN regions the phase of the rhythms in these two species differs in less extreme ways. Taken together, these data suggest that there is no single simple switch that causes some animals to be nocturnal and others to be diurnal. Rather, the differences likely emerge through a variety of mechanisms operating within and downstream of the cells to which the SCN projects.