The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 13

The location of neurons immunoreactive for AVP, VIP, GRP, CAR,
ENK, SS, SP, and NT is shown diagrammatically in Fig. 1.3 . AVP neurons
occupy the rostral pole of the SCN and extend to the caudal pole. Through-
out most of the rostrocaudal extent of the nucleus, AVP perikarya lie in the
shell. The AVP axonal plexus is evident throughout the shell and extends
dorsally into the PSCN and SPVZ. Within the core, VIP perikarya are ven-
tral, in part, to the GRP perikarya, and some VIP perikarya extend into the
dorsal chiasm as small pockets of neurons and neuropil. The GRP perikarya
are generally situated more dorsal and lateral than the VIP perikarya, but
there is substantial overlap. The VIP and GRP populations make up 25%
and 14% of the SCN, respectively. CAR-containing perikarya are an
unusual group. First described by Ref. 137 , these neurons are located in
the dorsal and lateral portion of the SCN but extend beyond the borders
of the SCN into the adjacent PSCN-anterior hypothalamic area. CAR neu-
rons make up 14% of the nucleus, and we interpret the entire group to be
part of the shell. All of the remaining peptide phenotypes are included in the
shell. The shell contains 57% of SCN neurons and the core 43%.
A calculation of subdivision volumes indicates that the shell volume is
0.0022 þ 0.0004 mm 3 and the core 0.0014 þ 0.0002 mm 3

(R.Y. Moore

and J.C. Speh, unpublished).

3.4. SCN organization in the human brain
The human exhibits interesting differences from the rat and mouse. 88 The
AVP þ and VIP þ neurons are located in exactly the same areas as in the
mouse and rat. Mai 89 reports GRP þ neurons in the core but does not note
their relative number compared to VIP þ neurons. In addition to the VIP þ
and AVP þ populations, there is a substantial number of neuropeptide
Y (NPY þ ) neurons in the core with a sparse-associated axonal plexus
and a large population of NT þ neurons throughout the nucleus
( Fig. 1.4 ) and also with a dense plexus. 90 Thus, although the core-shell con-
figuration of AVP and VIP neurons is maintained, the human SCN is more
complex than that of rodents and the macaque monkey. 90

3.5. The subdivision view of SCN organization
The hypothesis that the SCN is organized into two anatomical and func-
tional subdivisions has been a subject of some controversy since it was
offered as a general principle. 78 This seems somewhat unusual as hypotha-
lamic nuclei frequently have subdivisions (e.g., the paraventricular nucleus
and the ventromedial nucleus; Ref. 91 ) . The arguments against the