Night and day

摘要: How does the brain develop differently to support nocturnality in some mammals, but diurnality others? To answer this question, one might look suprachiasmatic nucleus (SCN), pacemaker of mammalian brain, which is required for all circadian biological and behavioral rhythms. Light arriving at retina entrains SCN daily light-dark cycle via retinohypothalamic tract (RHT). However, mammals studied thus far, whether nocturnal or diurnal, exhibits a rhythm increased activity during day decreased night. Therefore, structures downstream are likely determine species diurnal. From an evolutionary perspective, appears be primitive condition with having reemerged independently lineages. it unclear what mechanisms underlie development other phase preference. In adult Norway rats (Rattus norvegicus), nocturnal, RHT also projects ventral subparaventricular zone (vSPVZ), adjacent region that expresses in-phase pattern SCN-vSPVZ neuronal (in words, vSPVZ increase decrease together). contrast, Nile grass (Arvicanthis niloticus), diurnal closely related rats, anti-phase expressed increases as decreases, vice versa). We hypothesized these differences result part from weak absent RHTvSPVZ projection rats.