SEROTONERGIC MODULATION OF BEHAVIOR - A PHYLOGENETIC OVERVIEW

Serotonergic neurons are present in all phyla that possess nervous sys tems. In most of these phyla, serotonin modulates important behaviours , including feeding, sexual and aggressive behaviour. Serotonin exerts its effects by acting in three basic modes: as a classical neurotrans mitter, as a neuromodulator, or as a neurohormone. In a number of inve rtebrate species, the neural circuitry underlying the effects of serot onin has been well characterized, whereas in vertebrates, the mechanis ms by which serotonin affects behaviour are currently less fully under stood. The following review examines the role played by serotonin in t he generation and modulation of behaviour in successively more complex species, ranging from coelenterates to humans.