Study of Two Olfactory Stimuli on Electrophysiological Responses, Behavioral Patterns, and Olfactory Receptor Gene Expression in Megalobrama amblycephala

Social odor and alarm odor are key chemical signals for fish to perceive the environment and regulate behavior. The olfactory system underlies the sensitivity and behavioral response differences of fish to different odors, a process that initiates with the specific recognition and signal transduction of olfactory receptors. The molecular and behavioral mechanisms of this process are one of the core contents for analyzing chemical communication in fish. To explore the recognition patterns of the fish olfactory system towards such cues, we assessed the sensitivity and behavioral preference of the M. amblycephala to these odorants were assessed using electro-olfactogram recordings and behavioral assays. Furthermore, quantitative real-time PCR (RT-qPCR) was employed to compare the expression levels of 10 representative olfactory receptors (ORs) in the olfactory epithelial, olfactory bulb and brain. EOG results demonstrated that EOG amplitudes increased exponentially in response to both odorants across concentrations, indicating a clear dose-response relationship. Behavioral tests have shown that M. amblycephala exhibits adaptive responses to ammonium chloride (without significant attraction or avoidance) and displays avoidance behavior towards conspecific blood. Following ammonium chloride stimulation, the expression levels of Beta-2, Epsilon-10 and Delta-58 was significantly up-regulated in the olfactory epithelium. Beta-2 was significantly up-regulated in the olfactory bulb, and Beta-2, Beta-9, Beta-11, Epsilon-10 and Epsilon-13 was significantly up-regulated in the brain (P<0.05). Following blood stimulation, the expression of Beta-2, Epsilon-10 and Delta-58 was significantly up-regulated in the olfactory epithelial, while Beta-9, Beta-10, Epsilon-10 and Delta-58 were significantly up-regulated in the olfactory bulb, and Beta-2, Beta-11 and Epsilon-13 were significantly up-regulated in the brain (P<0.05). This study developed an integrated underwater system for recording olfactory-electrophysiological responses and behavioral behaviors in fish. At both electrophysiological and behavioral levels, M. amblycephala was demonstrated sensitivity to social odorants and alarm stimuli and exhibited significant avoidance responses. These findings provide a foundational basis for further investigation into the mechanisms of olfactory recognition in fish.