The Human SCN in Health and Age-Related Neurological Disorders

The suprachiasmatic nucleus (SCN) is the primary unit of the mammalian circadian rhythm. The generation and coordination of many physiological, endocrine, and behavioural circadian rhythms are crucial (Morin and Blanchard 2001; Nakagawa and Okumura 2010). The suprachiasmatic nucleus (SCN) in humans, as well as in non-human model systems, acts as the neural basis for circadian rhythms. The disturbance of circadian behaviour, which has been clinically recorded, is linked to the participation of the SCN region (Haugh and Markesbery 1983; Schwartz et al. 1987). The decrease in the ability to keep track of circadian rhythms as one gets older has been linked to the deterioration of SCN neurons during the ageing process (Hofman and Swaab 1994; Mirmiran et al. 1992). The suprachiasmatic nucleus (SCN) is a compact anatomical region in the hypothalamus, positioned bilaterally above the optic nerve and close to the third ventricle. The suprachiasmatic nucleus (SCN) functions as the primary controller of the mammalian brain, coordinating and governing the hormonal and behavioural circadian and circannual rhythms (Swaab 2003). Damage to the suprachiasmatic nucleus (SCN) region, such as from tumours in the suprasellar pituitary or metastases, has been reported to decrease the synthesis of a crucial SCN peptide called arginine vasopressin (AVP) and disturb circadian rhythms (Borgers et al. 2011; Swaab 2003). Hence, it is crucial to employ immunocytochemical labelling methodologies utilising antibodies targeting arginine vasopressin (AVP), vasoactive intestinal polypeptide (VIP), or neurotensin to precisely ascertain the identity of this anatomical feature. Mai and his colleagues employed immunocytochemistry to categorise the human SCN into five primary subgroups (Mai et al. 1991). The region of the suprachiasmatic nucleus (SCN) that receives information from the retinohypothalamic tract (RHT) and is crucial for synchronisation is characterised by the existence of VIP neurons (Moore et al. 2002). AVP is found throughout the rest of the suprachiasmatic nucleus (SCN), whereas neurotensin is evenly distributed throughout the entire SCN. The suprachiasmatic nucleus (SCN) consists of fibres that contain somatostatin, thyrotropin-releasing hormone, galanin, preproenkephalin, delta-sleep-inducing peptide, leptin receptor, and hypocretin (Swaab 2003). Furthermore, the human suprachiasmatic nucleus (SCN) has specific sites that bind to vasoactive intestinal peptide (VIP), such as oestrogen receptor (ER) α and β, progesterone receptor (Kruijver and Swaab 2002), and melatonin receptors (MT) (Y.-H. Wu et al. 2013). Considering the existence of VIP in the SCN, it is unsurprising that peptide methionine amide (PHM), a peptide including histidine and methionine, is also present in the human SCN. The PHM and VIP genes are located on adjacent exons of a common prepro-VIP gene. The work conducted by Romijn et al. 1999 employed confocal laser scanning microscopy to identify a small fraction of neurons in the human SCN that showed co-localization with AVP and VIP. Following the application of microwave therapy for antigen retrieval, the detection of AVP and VIP staining exhibits enhanced responsiveness. The quantity of AVP-stained neurons exhibited a 70% rise, whereas the quantity of VIP-stained neurons showed a substantial increase of 280%. The neurons observed before microwave therapy were mainly localised in the central region of the SCN, whereas the neurons that became visible only after microwave treatment were situated in the peripheral area of the SCN. The results suggest that the AVP and VIP neurons in the central part of the SCN have a greater amount of peptide, maybe because they are more active than the peripheral neurons (Hofman and Swaab 1994).