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Morphological Features and Neural Connections of the Suprachiasmatic Nucleus (SCN)

The suprachiasmatic nucleus (SCN), a parvocellular structure of ~16-20,000 neurons in rodents, plays a crucial role in circadian rhythms. It receives direct retinal input via the retinohypothalamic (RHT) pathway and indirect input through the geniculohypothalamic (GHT) pathway. Key neurotransmitters include GABA, vasoactive intestinal peptide (VIP), and vasopressin (VP). The SCN can be divided into dorsal and ventral segments, with significant local output projections. Techniques such as retrograde tracing and immunocytochemistry have been utilized to explore its intricate neural connections.

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Morphological Features and Neural Connections of the Suprachiasmatic Nucleus (SCN)

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  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

  3. Lets consider the neural connections of SCN A schematic outline from Ibata et al Frontiers in Neuroendocrinology 20: 241-268, 1999

  4. Notes:1 -Based on immunocytochemical grounds, SCN can be subdivided into dorsomedial (shell) and ventrolateral (core) segments2 -Retinal input is to the VIPergic neurons in the ventrolateral SCN3 -output pathways arise from both VIPergic and vasopressinergic neurons in SCN4 -most projections are local, to hypothalamic sites (exceptions: LGB, TPV)

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

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

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

  8. 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)

  9. PRV (green)A-C: in PVN oxytocin (red)D-E: in SCNvasopressin (red)F: in SCNVIP (red)

  10. Anatomical and functional demonstration of a multisynaptic suprachiasmatic nucleus adrenal (cortex) pathwayEuropean Journal of Neuroscience 11: 15351544, 1999RM Buijs, J Wortel, JJ van Heerikhuize, MGP Feenstra, GJ Ter Horst,HJ Romijn, A Kalsbeek

  11. Suprachiasmatic nucleus (SCN) • Brain Res. 1972 Jul 13;42(1):201-6. • Loss of a circadian adrenal corticosterone rhythm following suprachiasmatic lesions in the rat.Moore RY, Eichler VB.

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