Circadian Integration of Glutamatergic Signals by Little SAAS in Novel Suprachiasmatic Circuits

Abstract

BackgroundNeuropeptides are critical integrative elements within the central circadian clock in the suprachiasmatic nucleus (SCN), where they mediate both cell-to-cell synchronization and phase adjustments that cause light entrainment. Forward peptidomics identified little SAAS, derived from the proSAAS prohormone, among novel SCN peptides, but its role in the SCN is poorly understood.Methodology/Principal FindingsLittle SAAS localization and co-expression with established SCN neuropeptides were evaluated by immunohistochemistry using highly specific antisera and stereological analysis. Functional context was assessed relative to c-FOS induction in light-stimulated animals and on neuronal circadian rhythms in glutamate-stimulated brain slices. We found that little SAAS-expressing neurons comprise the third most abundant neuropeptidergic class (16.4%) with unusual functional circuit contexts. Little SAAS is localized within the densely retinorecipient central SCN of both rat and mouse, but not the retinohypothalamic tract (RHT). Some little SAAS colocalizes with vasoactive intestinal polypeptide (VIP) or gastrin-releasing peptide (GRP), known mediators of light signals, but not arginine vasopressin (AVP). Nearly 50% of little SAAS neurons express c-FOS in response to light exposure in early night. Blockade of signals that relay light information, via NMDA receptors or VIP- and GRP-cognate receptors, has no effect on phase delays of circadian rhythms induced by little SAAS.Conclusions/SignificanceLittle SAAS relays signals downstream of light/glutamatergic signaling from eye to SCN, and independent of VIP and GRP action. These findings suggest that little SAAS forms a third SCN neuropeptidergic system, processing light information and activating phase-shifts within novel circuits of the central circadian clock.

Single-Cell Transcriptional Analysis Reveals Novel Neuronal Phenotypes and Interaction Networks Involved in the Central Circadian Clock

“…Similarly, intracellular signaling genes including Mapk3, Camk2b , and Prkaca (Zhu et al, 2012) and the neuropeptide signaling gene Pcsk1n , which codes for the precursor molecule of the peptide little-SAAS, were upregulated as well. Since little-SAAS has been reported to be involved in intercellular coordination within the SCN (Atkins et al, 2010), this upregulated behavior suggests that Group 4 plays a synchronizing role in the SCN. Similar to Group 3, these neurons did not show any clear spatial organization (Figure 6B).…”

Collective timekeeping among cells of the master circadian clock

“…These subgroups have distinct developmental patterns (Antle, et al 2005b) and are thought to represent distinct subclasses with minimal overlap in neuropeptide expression. However, as the index of SCN peptides grows, it is becoming apparent that SCN neurons can co-express different peptides that may contribute to their functional activity (e.g., Atkins, et al 2010; Drouyer, et al 2010; Geoghegan and Carter 2008; Hundahl, et al 2012; Lee, et al 2015). Although the shell-core scheme of the SCN network continues to be a convenient construct, it will likely morph to become more sophisticated as understanding of SCN circuitry increases (Morin 2007, 2012).…”

“…A recent study demonstrated that SCN network function is regulated by Neuromedin S, which is an SCN peptide produced by both AVP and VIP neurons (Lee et al 2015). Also, a forward peptidomics screen identified little SAS as a novel peptide produced in the SCN core that relays photic signals independent of VIP- or GRP-dependent signaling (Atkins et al 2010). Another interesting development is that intra-SCN glutamatergic signaling may play a role in coupling the left and right SCN (Michel, et al 2013), and yet glutamate has not been detected in the SCN (Strecker et al 1997).…”

Disruption of gene expression rhythms in mice lacking secretory vesicle proteins IA-2 and IA-2β

“…Exposure of mice to low-intensity constant light lengthens the circadian period in rodents while exposure to brighter light leads to arrhythmicity. Alterations in retinohypothalamic signaling, perhaps through altered release of glutamate, pituitary adenylyl cyclase-activating peptide, or little SAAS, could contribute to disrupted SCN function (2,12). To test the functional integrity of the retinohypothalamic tract and SCN responsiveness to lighting information, groups of animals of each genotype were exposed to light for 1 h beginning at CT17, and brains were collected at CT18.…”