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Description:
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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 . |