Nonapeptide modulation of looming-induced responses in male Betta splendens

The looming stimulus is a widely used paradigm for investigating defensive behaviours in prey-predator contexts, as it simulates an approaching threat through the rapid expansion of a dark disk. Such stimuli typically evoke fight, flight, or freeze responses. In the present study, the role of the nonapeptides oxytocin and vasotocin, both known for their roles in social behaviour and stress regulation, in the behavioural and endocrine response to a looming stimulus in the Siamese fighting fish Betta splendens was assessed. Fish displayed pronounced freezing behaviour and the expression of conspicuous body stripes in response to a 10-min looming stimulus. Despite the marked behavioural reaction, no concomitant changes in plasma cortisol levels were recorded. The oxytocin receptor antagonist (L-368,899) did not affect the behavioural response, whereas treatment with the vasotocin receptor antagonist Manning compound enhanced fight and reduced flight responses. None of the nonapeptides modulated post-looming plasma cortisol levels. These findings provide new insights into the expression and regulation of stripe patterns in B. splendens, a phenomenon previously noted in aquaculture and behavioural studies but not quantitatively described, and highlight the involvement of the vasotocin system in the modulation of defensive strategies.