Introduction Serotonin (5-hydroxytryptamine, 5-HT) was isolated and characterized during the late 1930s through the 1950s. Since then, serotonin has been shown to play a key role in a range of behaviors and processes, including sensory gating and processing, behavioral inhibition, appetite, aggression, sleep, mood, and neuroendocrine secretion (Anderson, 2005) . The serotonin neuron system stretches abundant branches from a limited number of cells in the brain stem and widely and densely

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... s the brain (Takeuchi, 1988). Therefore, the serotonin neuron system is considered to be "the total control system". During development of the brain, the serotonin neuron system is not only essential for formation and maintenance of synapses (Lauder, 1990) , but also is affected by a variety of environmental factors. These findings are crucial for understanding the pathogenesis of many developmental disorders. This paper reviews the involvement of the serotonin neuron system in neurotransmission in autism spectrum disorders. Anatomical characteristics of the serotonin neuron system The distribution of the serotonin neuron system overwhelms that of other neuron systems (Nieuwenhuys, 1985; Takeuchi, 1988). The serotonin neuron system is a slow synaptic potential system with a long time lapse, similar to the physiology of noradrenalin and dopamine neuron systems. In this review, we examine the central nervous system (CNS) in monkeys (macaca fuscata) describing the anatomical characteristics of the serotonin neuron system, in order to understand the diversity of functions of serotonin in the brain and the pathogenesis of developmental disorders. Cell bodies of serotonin neurons are localized in the brain stem The cell bodies of serotonin neurons are localized from the caudal part of the midbrain red nucleus to the decussation of the pyramid of the medulla. This pattern is similar among species. Approximately 65% of the cell bodies are localized near the raphe nuclei group at the center of the brain stem, including the dorsal raphe nucleus (B7), superior central nucleus (B8), pontine raphe nucleus (B5), raphe magnus nucleus (B3), obscurus raphe nucleus (B2), and raphe pallidus nucleus (B1) from the rostral side, while the remaining cell bodies are localized at other sites. Serotonin neurons number between 25-50,000 in rodents and >150,000 in primates (monkeys), and the B7 group accounts for a large proportion of the serotonin neurons (Nieuwenhuys, 1985; Takeuchi, 1988). www.intechopen.com Autism -A Neurodevelopmental Journey from Genes to Behaviour 92 Serotonin fiber distribution Axons and dendrites of serotonin neurons are distributed more densely and widely than previously predicted in the central nervous system. The density and area of their distribution exceed those of the noradrenalin neuron system. The distribution of serotonergic axons and dendrites differs markedly between species. This tendency is most remarkable in the neocortex. The distribution pattern of serotonin fibers in the motor, sensory, and visual cortices of rodents is similar, whereas it differs markedly in the neocortex in primates. These distribution patterns of serotonin fibers are presumed to be dependent on the functions of the neocortex. In general, serotonin fibers are distributed more densely in the granular cell layer in the input system of the cortices than in the pyramidal cell layer in the output system, and the density of serotonin distribution is much higher in the primary visual cortex than in the primary motor cortex in primates. On the other hand, noradrenalin fibers are distributed more densely in the pyramidal cell layer in the output system and the distribution is complementary to that of serotonin fibers, i.e. "mutually exclusive distribution pattern" (Nieuwenhuys, 1985; Takeuchi, 1988).