ABSTRACT
Clusters of serotonin neurons (designated B1-B9) are scattered throughout the brainstem, mostly towards the midline in the raphe nuclei. Projections into the spinal cord that terminate in the dorsal horn reduce nociceptor input into the spinothalamic tract. Forward projections run into the medial forebrain bundle to go to the hypothalamus, amygdala, striatum, thalamus, hippocampus, and neocortex (Figure 1). Some of these serotonergic neurons may be involved in the expression of anxiety. They are inhibited by GABAergic neurons, enhancing the activity of which reduces anxiety. Several potent anxiolytic drugs are 5-HT receptor ligands. Serotonin transmission is modulated by social status; that is, by how dominant an animal is within its social group. This may underlie the effect of serotonin on mood. Deficits are associated with depression and increased risk of suicide. Serotonin is also involved in sleep, satiety, and regulation of CSF secretion and cerebral blood flow.
