Serotonergic signaling

Serotonergic signaling constitutes a highly pervasive neuromodulatory system that regulates mood, appetite, sleep-wake cycles, pain perception, cognition, and gastrointestinal motility. Serotonin (5-HT) is synthesized from L-tryptophan via tryptophan hydroxylase (TPH), the rate-limiting enzyme, of which TPH1 is peripheral and TPH2 is central, with approximately 90% of the body's serotonin produced in the gut (Walther et al., 2003; Gershon, 2013). Central serotonergic neurons cluster in the raphe nuclei, projecting broadly throughout the forebrain, cerebellum, and spinal cord, with the dorsal raphe innervating cortical and limbic regions and the median raphe targeting hippocampus and septum (Jacobs & Azmitia, 1992). Serotonin operates predominantly through volume transmission, diffusing beyond synapses to modulate large neuronal populations over extended timescales (Bunin & Wightman, 1998). Signaling is terminated by the serotonin transporter (SERT)-mediated reuptake and MAO degradation (Murphy et al., 2004).

Serotonin signals through seven receptor families (5-HT1 through 5-HT7) comprising at least 14 GPCRs, plus the ionotropic 5-HT3 receptor (Hoyer et al., 2002). 5-HT1 receptors (particularly 1A) couple to Gαi/o, inhibiting cAMP, and function as both postsynaptic receptors and presynaptic autoreceptors that inhibit raphe firing via negative feedback (Albert & Vahid-Ansari, 2019). 5-HT2 receptors (2A, 2B, 2C) couple to Gαq, activating phospholipase C and increasing intracellular calcium; 5-HT2A mediates the effects of psychedelics (González-Maeso & Sealfon, 2009). 5-HT3 receptors are cation channels mediating fast depolarization, relevant to emesis (Thompson & Lummis, 2007). 5-HT4, 5-HT6, and 5-HT7 couple to Gαs, stimulating cAMP and influencing cognition and circadian rhythms (Bockaert et al., 2006).

Functionally, serotonin promotes satiety (5-HT2C), modulates mood and stress resilience, regulates wakefulness and suppresses REM sleep, and modulates pain transmission (Monti, 2011; Halford et al., 2007). Gut-derived serotonin regulates motility and secretion, with 5-HT4 promoting peristalsis and 5-HT3 mediating emetic reflexes (Gershon, 2013). Serotonin is also critical for neurodevelopment, influencing neuronal migration and circuit formation (Gaspar et al., 2003). Pathologically, reduced serotonergic tone underlies major depression and anxiety, treated with SSRIs and SNRIs (Krishnan & Nestler, 2008; Cipriani et al., 2018). OCD and bulimia also respond to SSRIs (Pigott & Seay, 1999). Migraine involves serotonergic dysregulation, treated with 5-HT1B/1D agonists (triptans) (Goadsby et al., 2002). Serotonin syndrome results from excessive serotonergic activity, typically from drug interactions (Boyer & Shannon, 2005). Atypical antipsychotics act via 5-HT2A/D2 antagonism (Meltzer et al., 2003). Therapeutically, 5-HT3 antagonists (e.g., ondansetron) are antiemetics, 5-HT4 agonists treat constipation, and 5-HT2A agonists like psilocybin are under investigation for treatment-resistant depression (Carhart-Harris et al., 2016). In conclusion, serotonergic signaling, through its diverse receptor families and extensive projections, regulates mood, appetite, sleep, pain, and gut function, with dysfunction underpinning depression, migraine, and other disorders, making it a major therapeutic target.

References

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