Biology of depression
（重定向自Monoamine Hypothesis）
![Depression may be related to the same brain mechanisms that control the cycles of sleep and wakefulness.](/Images/godic/202501/28/Biological_clock_human4529.PNG")
![Illustration of the major elements in a prototypical synapse. Synapses are gaps between nerve cells. These cells convert their electrical impulses into bursts of chemical relayers, called neurotransmitters, which travel across the synapses to receptors on adjacent cells, triggering electrical impulses to travel down the latter cells.](/Images/godic/202501/28/SynapseSchematic_en.svg4529.png")
![Monoamine receptors affect phospholipase C and adenylyl cyclase inside of the cell. Green arrows means stimulation and red arrows inhibition. Serotonin receptors are blue, norepinephrine orange, and dopamine yellow. Phospholipase C and adenylyl cyclase start a signaling cascade which turn on or off genes in the cell.
Many variations of the monoamine hypothesis involve the neurotransmitter, serotonin, regulated by the serotonin transporter, which assists the modulation of feelings and behavior such as anxiety, anger, appetite, sexuality, sleep, mood, etc. People with depression may have differences in serotonin transporter gene length.[24] People with both alleles that are long are less likely to become depressed, while people with one short and one long or two short alleles are more likely to develop depression.[25]
The 5HT-3 receptor is associated with gastrointestinal adverse effects and has no relationship to the other monoamine receptors.](/Images/godic/202501/28/Monoamine_receptor_effects4529.png")
Scientific studies have found that numerous brain areas show altered activity in patients suffering from depression, and this has encouraged advocates of various theories that seek to identify a biochemical origin of the disease, as opposed to theories that emphasize psychological or situational causes. Several theories concerning the biologically based cause of depression have been suggested over the years, of which the most prominent and widely researched is the monoamine hypothesis.