Serotonin (5-HT) plays an important role in sensitization of defensive reflexes in Aplysia and is also involved in several aspects of arousal, such as the control of locomotion and of cardiovascular tone. In the preceding paper, we showed that tail-nerve shock, a noxious stimulus that readily induces sensitization, increases the firing rate of a large number of serotonergic neurons throughout the CNS. However, the functional consequences of such an increase in serotonergic tone are still poorly understood. In this study, we examined this question by using the 5-HT precursor 5-hydroxytryptophan (5-HTP) to specifically increase 5-HT release in the CNS. Increased tonic 5-HT release after 5-HTP treatment was manifested by facilitation of sensorimotor (SN-MN) synapses, increased firing rate of serotonergic neurons in the pedal and abdominal ganglia, and enhanced 5-HT release evoked by tail-nerve shock. When 5-HTP was administered to freely moving animals, it produced a strong arousal response characterized by increased locomotion and heart rate, which was reminiscent of the defensive arousal reaction triggered by noxious stimulation such as tail-shock. In contrast, 5-HTP actually inhibited the tail-induced siphon-withdrawal reflex. It is possible that 5-HT-induced facilitation of SN-MN synapses was counteracted by inhibition of polysynaptic reflex pathways between SNs and MNs, resulting in transient behavioral inhibition of the reflex, which could favor escape locomotion and/or respiration shortly after an aversive stimulus. We conclude that a major function associated 1 with the activation of the Aplysia serotonergic system evoked by noxious stimuli is the triggering of a defensive arousal response. It is known that tail-shock-induced serotonergic activation contributes to 1 memory encoding at least in part by facilitating SN-MN synapses. However, this effect in isolation might not be sufficient for the behavioral expression of sensitization.
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