Dear Michael,

Serotonin seems to be everywhere, in fact, it is hard to find a function that will not be modulated by serotonin: motor systems, sensory systems (vision, hearing, taste, smell, touch, pain), central and peripheral nervous systems, mood, breathing, blood pressure, gut motility,walking, talking, thinking, learning pretty much most functions are modulated by serotonin. It may be that only in some very periheral tissues, like skin or heart muscle (?), may not directly respond to serotonin? It is involved in synaptic as well as non-synaptic transmission and humoral signalling. This ubiqutous modulatory activity tells us that serotonin is in evolutionary terms is a very old neurotransmitter/modulator. Indeed even very simple organisms use serotonin in their signalling.

I look forward to hearing from others on this interesting subject.

best wishes,

Refik