[The role of serotonin in behavior modulation].

The central projection systems represent an expansive and important component of the brainstem reticular core which provide modulatory input into multiple target networks throughout the entire vertebrate neuraxis. Most of the afferent input into the cranial raphe originates within sensory uni- and polymodal, associative and limbic cortices suggesting that serotonin modulates preprocessed information. The serotonergic neurons discharge in a remarkably stable and tonic fashion during wakefulness. Some 5-HT neurons increase their discharge rate phasically in association with the activation of central rhythmic pattern generators involved in consummatory and grooming behaviour. In concert with enhancing motor functions, the serotonergic systems discretely deamplify sensory attentiveness and pain processing, thereby establishing an essential and protective filter mechanism against distracting and irritating noise effects of sensory afferent input level. In addition, serotonin restrains the latency to responding, i.e. impulsivity. These effects of serotonin are mediated by multiple receptor subtypes with distinct pre- and postsynaptic localisation and regional distribution pattern, acting via amplifying (5-HT2 receptors) or desamplifying (5-HT1 receptors) G-protein-dependent transduction mechanisms. The breakdown of these protective and adaptive functions of 5-HT in complex behaviour and in basic aspects of sensorimotor integration may have a pathogenetic role in disorders of impulse control (e.g. bulimia nervosa and OCD) which have been found to respond to high-dose, long-term treatment with selective serotonin reuptake inhibitors.