- Raphe nuclei
- A cluster of nuclei running along the midline of the brainstem from the medulla to the midbrain that constitutes the primary source of serotonergic innervation of the forebrain. The dorsal raphe nucleus projects extensively to the cortex, hippocampus, amygdala, basal ganglia, and thalamus; the median raphe nucleus projects primarily to the hippocampus and septum. These diffuse projections allow serotonin to modulate broad regions of the forebrain, regulating mood, anxiety, cognitive flexibility, and the consolidation of emotional memories.
- Monoamine hypothesis of depression
- The proposal that depression results from insufficient activity of monoamine neurotransmitters — particularly serotonin and/or norepinephrine — at central synapses. Formulated in the 1960s following the clinical observation that drugs increasing monoamine availability (monoamine oxidase inhibitors, tricyclic antidepressants) relieved depression, while reserpine (which depletes monoamines) could induce depressive symptoms. The hypothesis is an important historical framework but an inadequate complete account: it does not explain the therapeutic delay of antidepressants, the modest effect sizes of SSRIs, or depression in patients with normal monoamine metabolite levels.
- Selective serotonin reuptake inhibitors (SSRIs)
- Drugs (including fluoxetine, sertraline, escitalopram) that selectively block the serotonin transporter (SERT), increasing the concentration of serotonin in synaptic clefts throughout the brain. They have a more favourable side-effect profile than earlier antidepressants and are first-line treatment for depression, generalised anxiety disorder, OCD, PTSD, and panic disorder. The delayed therapeutic onset — clinical improvement typically requires 2–6 weeks despite immediate effects on serotonin reuptake — suggests that downstream adaptations in receptor sensitivity and neuroplasticity, rather than acute reuptake inhibition, mediate the therapeutic effect.
- 5-HT receptor diversity
- Serotonin acts on at least 14 receptor subtypes grouped into seven families (5-HT1 through 5-HT7). With the exception of 5-HT3 (an ionotropic receptor), all are metabotropic G-protein-coupled receptors. Key subtypes include: 5-HT1A (autoreceptors in the raphe and postsynaptic in the hippocampus and amygdala — targets of anxiolytics and implicated in the therapeutic action of antidepressants); 5-HT2A (cortical and limbic — involved in psychosis, and the target of antipsychotic antagonism; also mediates the effects of classical psychedelic drugs); 5-HT2C (hypothalamus and limbic — regulates appetite; its blockade by some antipsychotics contributes to weight gain).
- Serotonin and impulsivity
- A consistent body of animal and human research links reduced central serotonergic function to heightened impulsivity and reactive aggression. Studies in non-human primates showed that low cerebrospinal fluid levels of the serotonin metabolite 5-HIAA predicted impulsive aggression and early death. Tryptophan depletion (acutely reducing serotonin synthesis) increases impulsive choice on laboratory tasks in humans. Low serotonin function has been proposed as a biological substrate for the impulsivity dimensions of borderline personality disorder, antisocial personality disorder, and certain presentations of suicidality.