ABSTRACT
Clusters of serotonin neurons are scattered throughout the brainstem, mostly towards the midline in the raphe nuclei. Projections into the spinal cord that terminate in the dorsal horn are important in pain sensation by reducing nociceptor input into the spinothalamic tract. Other serotonergic spinal cord axons synapse with preganglionic autonomic neurons. Serotonin synthesis is matched to the firing frequency of the neuron. The precursor for serotonin is the amino acid tryptophan. The plasma concentration of tryptophan, which varies according to dietary intake, can alter brain serotonin levels. There are numerous subtypes of serotonin receptor, all except one are metabotropic. Oxidative deamination of serotonin by monoamine oxidase yields its principle metabolite, 5-hydroxy-indoleacetic acid. Decarboxylation of 5-hydroxytryptophan by l-aromatic amino acid decarboxylase gives serotonin, also referred to as 5-hydroxytryptamine (5-HT), which is an indolamine. Presynaptic autoreceptors of the 5-HT1A subtype, inhibit the release of serotonin.
