Vasotocin receptor antagonist Manning compound enhances fight, reduces flight in Betta splendens facing looming threat.

Defensive behaviors like fight, flight, or freeze are crucial for survival in predator-prey contexts, often triggered by stimuli such as a rapidly expanding dark disk (looming stimulus). While nonapeptides like oxytocin and vasotocin are well-established for their roles in social behavior and stress regulation, their specific involvement in modulating acute defensive strategies against a looming threat, particularly in fish, remains less explored. Understanding these neuroendocrine pathways could offer insights into stress coping mechanisms and behavioral control.

Researchers investigated the role of nonapeptides in male Betta splendens exposed to a 10-min looming stimulus, which simulates an approaching threat. Fish were treated with either the oxytocin receptor antagonist (L-368,899) or the vasotocin receptor antagonist Manning compound. The study assessed behavioral responses, including freezing, fight, and flight, along with the expression of conspicuous body stripes. Post-looming plasma cortisol levels were also measured using standard ELISA methods to evaluate the endocrine stress response.

Male Betta splendens displayed pronounced freezing behavior and the expression of conspicuous body stripes in response to the 10-min looming stimulus. Despite these marked behavioral reactions, no concomitant changes in plasma cortisol levels were recorded, suggesting a dissociation between acute behavioral and endocrine stress responses in this context. The oxytocin receptor antagonist L-368,899 did not affect any of the measured behavioral responses. In contrast, treatment with the vasotocin receptor antagonist Manning compound significantly altered defensive strategies. > Manning compound treatment enhanced fight responses and reduced flight responses, highlighting a specific role for the vasotocin system in modulating these behaviors. None of the nonapeptide treatments modulated post-looming plasma cortisol levels, further supporting the observed behavioral-endocrine dissociation.