Figure 4. A schematic drawing of a 5-HT terminal showing two forms of 5-HT release: vesicular release induced by depolarization is Ca++ dependent, and cytoplasmic release occurs through the transporter protein by exchange diffusion and is driven by the Na+ gradient. Monoamine oxidase-A (MAO-A) metabolizes intracellular 5-HT, and MAO-B metabolizes extracellular 5-HT. Note that the vesicular 5-HT is available for release from the cytoplasm by the transporter mechanism. The release is modified by receptors. The 5-HT1B/D autoreceptor inhibit the release of vesicles, while serotonin specific reuptake inhibitors (SSRI) can block the release from the transporter. CLORG: clorgyline; MDMA: methylenedioxymethamphetamine (ecstasy); PCA: para-chloroamphetamine; PCPA: para-chlorophenylalanine; TRY: tryptophan.