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Table 1.3 Evidence for cell and core subdivisions in the SCN
Rhythms in AVP and VIP release in organotypic culture show phase
dissociation.
Commissural connections between SCNs of each cerebral hemisphere maintain
a shell-shell, core-core organization.
97
Light-induced Fos expression occurs in core.
SCN efferent projections are predominantly core- or shell specific.
Per 1 rhythms in LD occur only in the shell.
Behavioral forced desynchronization produces loss of synchrony of core-shell
Per 1 and BMAL 1 expression.
Circadian rhythmicity appears to develop sooner in the shell than in the core,
whereas the photoperiodic response may develop sooner in the core.
Core VIP neurons are necessary for shell synchronization and neuronal
coherence.
Circadian rhythms in intracellular calcium differ in timing between core and
shell in SCN and are dissociated by TTX.
brains. Second, the core, and particularly the VIP neuron group, is critical to
SCN function (Refs.
86,99,148,149
).
4. AFFERENT AND EFFERENT CONNECTIONS
4.1. Intrinsic and commissural connections
The organization of SCN connections is summarized in
Fig. 1.5
. Three
methods have been useful in studying intrinsic and commissural connec-
tions. The first is the Golgi method. In his Golgi material, van den Pol
72
observed axons emerging from both perikarya and dendrites of SCN neu-
rons. Occasional axons extended across to the opposite SCN. Within the
SCN of origin, axons had numerous branches and varicosities. We have
made similar observations in our material (J.P. Card and R.Y. Moore,
unpublished). In one instance, an axon originating from the cell body of
a ventral SCN neuron formed 64 collateral branches within 100
m
mof
the cell of origin. Some of the collaterals that could be followed for at least
short distances had numerous axonal varicosities. We would conclude from
the Golgi material that there is an extensive intrinsic plexus derived from
SCN neurons.
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