Serotonergic mediation of orienting and defensive responses in zebrafish

Abstract

AbstractSerotonin (5-HT) is involved in arousal and defensive responses, both of which represent modulators of attention and orienting. Orienting responses (ORs), considered as a unit of attentional processing, are elicited by novel innocuous stimuli, differ from defense reflexes, as the latter are elicited by painful and threatening stimuli. Under situations of potential or distal threat, organisms can enter into a state of hypervigilance, in which innocuous stimuli can elicit defense reflexes instead of orienting responses. When zebrafish (Danio rerio) are exposed to a conspecific alarm substance (CAS), for example, innocuous visual stimuli elicit a defense reflex (DR). We treated zebrafish with either the serotonin synthesis inhibitorpara-chlorophenylalanine (pCPA) or with the serotonergic precursor 5-HTP, and exposed individuals to visual stimuli to elicit either ORs or DRs. pCPA blocked CAS-elicited OR-to-DR shift, as shown both by the shift from an approach to an escape response when the stimulus was turned “ON” and by the changes in directional focus. While pCPA did not alter ORs in animals which were not exposed to CAS, 5-HTP abolished those responses. However, 5-HTP also attenuated the CAS-elicited OR-to-DR shift, suggesting that the serotonergic tone is more important in regulating the general arousal levels in zebrafish.