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# Why SSRI cause "syndrome of inappropriate ADH secretion"
Serotonin regulates territorial position in social hierarchy within mammals. Evolutionarily in bacteria from metabolic remnants of tryptophane, indoles, that were co-opted to signal "here is food, lets group up here" - territory and food - signals also involved in biofilm production and coordination. In social mammals that have a dominance hierarchy with a pecking order, dominant individuals are more extroverted and claim the territory more, and subordinate more introverted and take up less space in the territory. When individuals are elevated up the hierarchy their serotonin increases and they rexlexively claim more space, aand likewise when individuals are depressed down the hierarchy the serotonin decreases. Social mammals mark territory with urine. More dominant individuals produce more concentrated urine with a stronger smell - thus the hormone responsible for concentrating urine, antidiuretic hormone (also called vasopressin) is regulated by the same serotonin circuits as social status, originating in the dorsal raphe neurons that for vasopressin have axons to the hypothalamus, from where vasaopressin is excreted via the neuropituitary nerve. Attempts to artificially increase social status (which fails to reflect true status and thus faces the same external negative feedback and still has to conform territorially albeit without the reflex to do so) thus also cause dominant urine production. In the healthcare system this is called "syndrome of inappropriate ADH secretion", a term popular due to the popularity of SSRIs. "Inappropriate" ADH production can also happen from obscure cancers and such that can produce the hormone, lung cancers most famously, but this is distinct from the physiologically normal "dominant urine" response which may become abnormal if SSRI cause very high levels of serotonin there but it is still a normal and evolutionarily selected for reflex rather than a real pathology.