The suprachiasmatic nucleus of the domestic chicken, Gallus domesticus



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Texas A&M University


The avian circadian system is composed of multiple inputs, oscillators and outputs. Among its oscillators is a hypothalamic structure presumed to be homologous to the primary circadian pacemaker in mammals, the suprachiasmatic nucleus (SCN). The SCN in avian species is poorly defined: two structures in the hypothalamus, the medial SCN (mSCN) and visual SCN (vSCN), have been referred to in the literature as the SCN. The present studies were designed to answer one central question: where is the avian homolog to the mammalian SCN? Uptake of 2-[14C]-deoxyglucose (2DG), an indicator of glucose metabolism, fluctuates in the mSCN and vSCN in both a daily and circadian manner. These data indicate a possible role in the circadian system for both the vSCN and the mSCN. Additionally, several visual structures display daily fluctuations of 2DG uptake, two of which exhibit circadian variations, supporting previous studies indicting circadian regulation of the visual system. Efferents and afferents of the mSCN and vSCN were identified and compared to those of rodents. While the mSCN bears a stronger resemblance to the rodent SCN in its efferent connections than the vSCN, afferents of both are comparable. The total number of mSCN and vSCN neuronal connections far exceeds that of the rodent SCN. A subset of these connections is strikingly similar to those of the rodent SCN, while others are found to connect these two nuclei to the visual system. These data further support the involvement of both the mSCN and vSCN in the circadian and visual systems. Suprachiasmatic organization was addressed using classical techniques. Though loosely similar in location to the mammalian SCN, the mSCN is cyto- and chemoarchitecturally different, while the vSCN bears more similarity to the mammalian SCN in this regard. A unique astrocytic bridge exists between the mSCN and vSCN, suggesting a role for astrocytes in the circadian system. Finally, the vSCN efferent to the medial nucleus of Edinger-Westphal was verified using a technique that may advance future studies of avian of circadian organization. The current data and the available literature were considered in the development of a working model of the avian SCN.