to escape foot shocks and inclination to explore new environments were dependent on active serotonin transporters during the neonatal period. Human serotonin can also act as a growth factor directly. Liver damage increases cellular expression of 5-HT2A and 5-HT2B receptors, mediating liver compensatory regrowth (see ) Serotonin present in the blood then stimulates cellular growth to repair liver damage. 5HT2B receptors also activate osteocytes, which build up bone However, serotonin also inhibits osteoblasts, through 5-HT1B receptors. Cardiovascular growth factor Serotonin, in addition, evokes endothelial nitric oxide synthase activation and stimulates, through a 5-HT1B receptor-mediated mechanism, the phosphorylation of p44/p42 mitogen-activated protein kinase activation in bovine aortic endothelial cell cultures. In blood, serotonin is collected from plasma by platelets, which store it. It is thus active wherever platelets bind in damaged tissue, as a vasoconstrictor to stop bleeding, and also as a fibrocyte mitotic (growth factor), to aid healing. Skin Serotonin is also produced by Merkel cells which are part of the somatosensory system. Lungs Pulmonary neuroendocrine cells are specialized epithelial cells that occur as solitary cells or as clusters called neuroepithelial bodies in the lung. Pulmonary neuroendocrine cells are also known as Kulchitsky cells or K cells. Pharmacology Several classes of drugs target the 5-HT system, including some antidepressants, antipsychotics, anxiolytics, antiemetics, and antimigraine drugs, as well as, the psychedelic drugs and empathogens. Mechanism of action At rest, serotonin is stored within the vesicles of presynaptic neurons. When stimulated by nerve impulses, serotonin is released as a neurotransmitter into the synapse, reversibly binding to the postsynaptic receptor to induce a nerve impulse on the postsynaptic neuron. Serotonin can also bind to auto-receptors on the presynaptic neuron to regulate the synthesis and release of serotonin. Normally serotonin is taken back into the presynaptic neuron to stop its action, then reused or broken down by monoamine oxidase. Psychedelic drugs The serotonergic psychedelic drugs psilocin/psilocybin, DMT, mescaline, psychedelic mushroom and LSD are agonists, primarily at 5HT2A/2C receptors. The empathogen-entactogen MDMA releases serotonin from synaptic vesicles of neurons. Antidepressants Drugs that alter serotonin levels are used in treating depression, generalized anxiety disorder, and social phobia. Monoamine oxidase inhibitors (MAOIs) prevent the breakdown of monoamine neurotransmitters (including serotonin), and therefore increase concentrations of the neurotransmitter in the brain. MAOI therapy is associated with many adverse drug reactions, and patients are at risk of hypertensive emergency triggered by foods with high tyramine content, and certain drugs.