How does the brain develop differently to support nocturnality in some mammals, but diurnality in others? To answer this question, one might look to the suprachiasmatic nucleus (SCN), the pacemaker of the mammalian brain, which is required for all circadian biological and behavioral rhythms. Light arriving at the retina entrains the SCN to the daily light-dark cycle via the retinohypothalamic tract (RHT). However, in all mammals studied thus far, whether nocturnal or diurnal, the SCN exhibits a rhythm of increased activity during the day and decreased activity at night. Therefore, structures downstream of the SCN are likely to determine whether a species is nocturnal or diurnal. From an evolutionary perspective, nocturnality appears to be the primitive condition in mammals, with diurnality having reemerged independently in some lineages. However, it is unclear what mechanisms underlie the development of one or the other circadian phase preference. In adult Norway rats (Rattus norvegicus), which are nocturnal, the RHT also projects to the ventral subparaventricular zone (vSPVZ), an adjacent region that expresses an in-phase pattern of SCN-vSPVZ neuronal activity (in other words, activity in the SCN and vSPVZ increase and decrease together). In contrast, in adult Nile grass rats (Arvicanthis niloticus), a diurnal species that is closely related to Norway rats, an anti-phase pattern of SCN-vSPVZ neuronal activity is expressed (in other words, activity in the SCN increases as activity in the vSPVZ decreases, and vice versa). We hypothesized that these species differences in activity pattern result in part from a weak or absent RHT-to-vSPVZ projection in grass rats. Using a developmental comparative approach, we assessed differences in behavior, hypothalamic activity, and RHT and SCN connectivity to the vSPVZ between these two species. We report that a robust retina-to-vSPVZ projection develops in Norway rats around the end of the second postnatal week when nocturnal wakefulness and the in-phase pattern of SCN-vSPVZ activity emerge. In grass rats, however, such a projection does not develop and the emergence of the anti-phase SCN-vSPVZ activity pattern during the second postnatal week is accompanied by increased diurnal wakefulness. When considered within the context of previously published reports on RHT projections in a variety of other nocturnal and diurnal species, our current findings suggest that how and when the retina connects to the hypothalamus differentially shapes brain and behavior to produce animals that occupy opposing temporal niches.

Made available in DSpace on 2014-12-17T15:36:32Z (GMT). No. of bitstreams: 1 ExpeditoSNJ.pdf: 2182833 bytes, checksum: 1d14b6b7f5a8b99dccd3ec09fd418133 (MD5) Previous issue date: 2007-12-07 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this study, two circadian related centres, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) were evaluated in respect to their cytoarchitecture, retinal afferents and chemical content of major cells and axon terminals with a tract tracer and immunohistochemical techniques in the rock cavy (Kerodon rupestris), a Brazilian caviidae rodent species. The rock cavy SCN is innervated in its ventral portion by terminals from the predominantly contralateral retina. It also contains neurophisin and vasoactive intestinal polypeptide immunoreactive cell bodies and neuropeptide Y and enkephalin immunopositive fibres and terminals and is marked by intense GFAP immunoreactivity. The IGL receives a predominantly contralateral retinal projection, contains neuropeptide Y and nitric oxide synthase producing neurons and enkephalin immunopositive terminals and is characterized by dense GFAP immunoreactivity. This is the first report examining the neural circadian system in a crepuscular rodent species for which circadian properties have been described. The results are discussed comparing with what has been described for other species and in the context of the functional significance of these centres / O sistema de temporiza??o circadiana compreende um conjunto de estruturas neurais diferenciadas, que incluem um marcapasso central, o qual produz ritmicidade na aus?ncia de est?mulos externos; vias de entrada, incluindo as afer?ncias retininas, que permitem a sincroniza??o dos ritmos aos ciclos ambientais; e vias de sa?da, que conectam o marca-passo aos efetores comportamentais. Entre os componentes centrais do sistema de temporiza??o circadiana de mam?feros, destaca-se o n?cleo supraquiasm?tico (NSQ) do hipot?lamo, at? o presente o ?nico marca-passo circadiano formalmente comprovado, e o folheto intergeniculado (FIG) do complexo geniculado lateral do t?lamo, que atua como modulador do marca-passo. Neste estudo, estes dois centros foram avaliados com rela??o a sua citoarquitetura, padr?o de inerva??o retiniana e conte?do neuroqu?mico de c?lulas e terminais ax?nicos, usando um tra?ador neural e t?cnicas imuno-histoqu?micas no moc? (Kerodon rupestris), um roedor nativo do Nordeste Brasileiro, cuja atividade locomotora exibe um padr?o circadiano predominantemente crepuscular. O NSQ do moc? ? inervado em sua por??o ventrolateral por terminais da retina predominantemente contralateral. O NSQ ? dotado de neur?nios contendo neurofisina (NPH) e polipept?deo intestinal vasoativo (VIP) e as prote?nas ligantes de c?lcio calbindina (CB) e cal-retinina (CR), terminais imunorreativos a neuropept?deo Y (NPY) e encefalina (ENK) e ? tamb?m marcado por imunorreatividade ? prote?na ac?dica fibrilar glial (GFAP). O FIG recebe uma proje??o retiniana predominantemente contralateral, cont?m neur?nios produtores de NPY, CB e CR, terminais positivos para ENK e ? marcado por imunorreatividade a GFAP. Este ? o primeiro trabalho a examinar o sistema de temporiza??o circadiana (STC) numa esp?cie de roedor crepuscular, para o qual as propriedades formais dos ritmos circadianos tenham sido descritas. Os resultados s?o comparados com resultados previamente descritos em outras esp?cies diurnas e noturnas e discutidos no seu contexto funcional