Table 1.

Pathways and signals involved in clock-to-clock communication.

Integration	SCN	CNS Clocks	Peripheral Clocks
Intra-cellular	TTL oscillations: cAMP Ca2+ kinase/phosphatase	TTL oscillations: cAMP Ca2+ kinase/phosphatase	TTL oscillations: NAD(P)+/NAD(P)H AMP/ATP Redox balance
Inter-cellular	Core-shell coupling: VIP, GABA, GRP, SP, CALR, ENK, glutamate	Synaptic connections, AVP, GABA	Metabolites (glucose, fatty acids) EVs
Inter-tissue	Photic input (via RHT): Glutamate, PACAP, SP Output to CNS clocks: AVP, GABA, DS	Input from SCN: AVP, GABA, DS Neuroendocrine output: HRF	Insulin, glucagon, GCs, ghrelin, leptin, etc.
Systemic	Behaviour (sleep/wake, food intake) SNS/PNS	SNS/PNS Melatonin PH	GCs FDRS

Body’s clocks communicate with each other to process, integrate and translate environmental signals to adapt physiology and behaviour to the external time. The table shows some of the well-known pathways and signals that are involved in clock-to-clock synchronization at intracellular, intercellular, inter-tissue and systemic levels. TTL: transcriptional translational loop, VIP: vasointestinal polypeptide, GRP: gastrin-releasing peptide, SP: substance P, CALR: calretinin-releasing peptide, ENK: met-enkephalin, RHT: Retino-hypothalamic tract, PACAP: pituitary adenylyl cyclase activating peptide, CNS: Central nervous system, AVP: vasopressin, DS: diffusible signals, SNS: sympathetic nervous system, PNS: parasympathetic nervous system, HRF: hypothalamic releasing factors, PH: pituitary hormones, GCs: glucocorticoids, EVs: extracellular vesicles, FDRS: food-dependent resetting signals (insulin, glucagon, ghrelin, leptin, oxyntomodulin, etc), cAMP: cyclic-Adenosine monophosphate, GABA: gamma-aminobutyic acid, NAD: nicotinamide adenine dinucleotide, AMP/ATP: Adenosine monophosphate/Adenosine triphosphate.