1. Light →SCN: -direct via the retinohypothalamic (RHT) pathway -indirect via geniculohypothalamic (GHT) pathway

2. SCN: some morphological features
Parvocellular, paired structure, ~16-20,000 neurons in rodent, miniscule in man
Phenotypes: multiple potential transmitters
most express GABA (1993 proposal: SCN output is inhibitory)
Peptides:
vasoactive intestinal peptide (VIP) in cells in ventrolateral part; receives retinal input; forms part of the output projection
vasopressin (VP) in cells in dorsomedial part; forms part of the output projection
somatostatin (SS) in cells whose axons remain intrinsic to SCN

6. Lets consider the neural connections of SCN
A schematic outline from Ibata et al
Frontiers in Neuroendocrinology
20: , 1999

7. Notes: 1 -Based on immunocytochemical grounds, SCN can be subdivided into dorsomedial (shell) and ventrolateral (core) segments 2 -Retinal input is to the VIPergic neurons in the ventrolateral SCN 3 -output pathways arise from both VIPergic and vasopressinergic neurons in SCN 4 -most projections are local, to hypothalamic sites (exceptions: LGB, TPV)

11. SCN: techniques to define input-output pathways
Using retrograde and/or anterograde transport of suitable markers e.g. wheat germ agglutin (WGA)

14. SCN: techniques to define input-output pathways
Using retrograde and anterograde transport of suitable markers e.g. WGA
Using viral retrograde transneuronal traceing (pseudorabies)

15. Retrograde transneuronal labeling with pseudorabies virus (PRV) injected in the adrenal gland

16. SCN: techniques to define input-output pathways
Using retrograde and/or anterograde transport of suitable markers e.g. WGA
Using viral retrograde transneuronal tracing (PRV)
Using double label immunocytochemistry to define phenotype (peptides)

17. PRV (green) A-C: in PVN oxytocin (red) D-E: in SCN vasopressin (red) F: in SCN VIP (red)

18.  Anatomical and functional demonstration of a multisynaptic suprachiasmatic nucleus adrenal (cortex) pathway European Journal of Neuroscience 11: , 1999 RM Buijs, J Wortel, JJ van Heerikhuize, MGP Feenstra, GJ Ter Horst,HJ Romijn, A Kalsbeek

20. Suprachiasmatic nucleus (SCN)
Brain Res Jul 13;42(1):201-6.
Loss of a circadian adrenal corticosterone rhythm following suprachiasmatic lesions in the rat. Moore RY, Eichler VB.