Effect of intracerebroventricular administration of two molecular forms of sulfated CCK octapeptide on anxiety-like behavior in the zebrafish danio rerio

Cholecystokinin octapeptide with sulfate (CCK−8s) regulates feeding behavior and psychomotor activity. In rodents and goldfish, intracerebroventricular (ICV) injection of CCK−8s decreases food intake and also induces anxiety-like behavior. The zebrafish has several merits for investigating the psychophysiological roles of neuropeptides. However, little is known about the brain localization of CCK and the behavioral action of CCK−8s in this species. Here we investigated the brain localization of CCK-like immunoreactivity and found that it was distributed throughout the brain. As CCK-like immunoreactivity was particularly evident in the ventral habenular nucleus, the interpeduncular nucleus and superior raphe, we subsequently examined the effect of zebrafish (zf) CCK−8s on psychomotor control. Since the zebrafish possesses two molecular forms of zfCCK−8s (zfCCKA−8s and zfCCKB−8s), two synthetic peptides were administered intracerebroventricularly at 1, 5 and 10 pmol g⁻¹ body weight (BW). As the zebrafish shows a greater preference for the lower area of a tank than for to the upper area, we used this preference for assessment of anxiety-like behavior. ICV administration of zfCCKA−8 s or zfCCKB−8s at 10 pmol g⁻¹ BW significantly shortened the time spent in the upper area. The actions of these peptides mimicked that of the central-type benzodiazepine receptor inverse agonist FG-7142 (an anxiogenic agent) at 10 pmol g⁻¹ BW. The anxiogenic-like action of the two peptides was attenuated by treatment with the CCK receptor antagonist proglumide at 200 pmol g⁻¹ BW. These results indicate that zfCCKA−8s and zfCCKB−8s potently induce anxiety-like behavior via the CCK receptor-signaling pathway in the zebrafish brain.