Serotonin neurons, neuroplasticity, and homeostasis of neural tissue

Homeostasis is the process by which the internal milieu of the body is able to maintain equilibrium in the face ofconstant insults from the external world. Endocrine, immune, and vascular systems play pivotal roles inadjusting internal biochemical reactions to counteractassaults from the outside. Despite the vast accumulation ofdata over the last 50 years, a role for serotonin in brainhomeostasis has not been proposed. In this chapter I willreview the plasticity and anatomy of serotonergic neuronsin integrating external sensory and motor systems as wellas internal endocrine, glial and vascular signals with thevarious cellular elements comprising neural tissue. Steroids and neuropeptides have both been shown to alter the morphology of serotonergic neurons. In turn, alterations inserotonin levels in the adult brain can change themorphology of its target cells. A pivotal role for serotonin inthe homeostasis of neural tissue is consistent with thefunction of serotonin throughout evolution and explains thelarge number of biological systems, behavioral activities, and clinical diseases associated with serotonergic neurons.