Serotonergic mediation of orienting and defensive responses in zebrafish

Abstract

Serotonin (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 (DR)s, as the latter are elicited by painful and threatening stimuli. When zebrafish (Danio rerio) are exposed to a conspecific alarm substance (CAS), visual stimuli elicit a DR instead of ORs, suggesting a state of hypervigilance. CAS elicited an 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. Both pCPA and 5-HTP attenuated the CAS-elicited OR-to-DR shift. While pCPA did not alter ORs in animals which were not exposed to CAS, 5-HTP abolished those responses, suggesting that the serotonergic tone is important in regulating the general arousal levels in zebrafish.