Serotonin

The 'low serotonin causes depression' formulation — derived from the monoamine hypothesis — is an oversimplification that has been substantially revised. The evidence for and against includes: SSRIs (which increase serotonin availability) are effective antidepressants in many patients, supporting a serotonergic role; but tryptophan depletion (which acutely reduces serotonin synthesis) does not reliably cause depression in healthy volunteers, and direct measurements of serotonin metabolites in depressed patients have not consistently shown deficiency. A 2022 umbrella review by Moncrieff et al. challenged the serotonin theory as the primary account of depression, though this was contested on methodological grounds. The current consensus is that serotonin dysregulation is one factor — important but not the sole or sufficient cause — in the heterogeneous biology of depression.

SSRIs inhibit the serotonin transporter within hours, immediately increasing synaptic serotonin. However, this triggers compensatory changes that initially limit the effect: autoreceptors (primarily 5-HT1A) on the raphe neurons detect the increased serotonin and reduce the firing rate of the raphe nucleus, partially offsetting the reuptake block. Over 2–4 weeks, these autoreceptors desensitise (downregulate), allowing the raphe to resume normal firing and the net synaptic serotonin increase to take effect. This delayed desensitisation corresponds roughly to the onset of therapeutic action. Some researchers also point to downstream effects on neuroplasticity — including BDNF expression and hippocampal neurogenesis — as the more fundamental mechanism of antidepressant action.

The relationship between serotonin and anxiety is complex and depends on which receptor subtype is considered. 5-HT1A receptors in the raphe and amygdala appear to buffer threat responses; drugs acting as partial agonists at 5-HT1A (such as buspirone) are used as anxiolytics. SSRIs are highly effective treatments for most anxiety disorders, including GAD, social anxiety disorder, OCD, panic disorder, and PTSD, even though they acutely increase serotonin and can cause a transient increase in anxiety at the start of treatment. The anxiolytic effect appears to emerge from adaptive changes in 5-HT1A sensitivity and amygdala reactivity that develop over weeks. The apparent paradox — more serotonin causing short-term anxiety but long-term anxiolysis — reflects the complexity of 14 different receptor subtypes with sometimes opposing functions.

Serotonin is involved in sleep-wake regulation in two ways. First, serotonergic raphe neurons are most active during wakefulness, less active during slow-wave sleep, and virtually silent during REM sleep — a pattern consistent with a role in maintaining wakefulness. Second, serotonin is the immediate precursor to melatonin: in the pineal gland, serotonin is converted to melatonin via two enzymatic steps, and melatonin production rises sharply in darkness to signal the onset of the sleep period. This precursor relationship means that tryptophan (the dietary precursor to serotonin) availability affects melatonin synthesis, and that agents affecting central serotonin can alter circadian timing and sleep architecture.