A behavioral pharmacological study on CCK-8 related peptides in mice.

The effects of CCK-8, ceruletide, their non-sulfated forms and CCK-4 on locomotor activity and rearing in mice were examined. CCK-8 and ceruletide, but not their non-sulfated forms and CCK-4, significantly inhibited the behavioral parameters dose-dependently. The inhibitory effects of CCK-8 and ceruletide were similar, but ceruletide acted more slowly and its inhibitory effect continued much longer than CCK-8, suggesting a difference in stability with the chemical structure. The antagonistic effects of ceruletide on behavioral changes induced by DA agonists were also examined. Peripherally injected ceruletide antagonized both methylphenidate- and methamphetamine-induced hyperactivity in mice dose-dependently, whereas it had no dose-related antagonistic effect on methylphenidate-induced stereotyped behavior. Ceruletide also significantly inhibited apomorphine-induced hyperactivity when injected peripherally. However, no obvious dose-response relationship was observed in either intensity or duration of inhibitory action of ceruletide. These findings suggest that ceruletide does not interfere with dopaminergic transmission due to the blockade of postsynaptic DA receptors in the brain. In conclusion, peripheral injection of CCK-8 and ceruletide may affect directly or indirectly dopamine function producing behavioral changes that resemble those of neuroleptics in some respects.