Effects of cholecystokinin tetra and octa peptides on locomotor activity in mice

Intraperitoneal injection of cholecystokinin octapeptide (10 and 100 micrograms/kg, i.p.), but not the tetra-peptide (1 mg/kg, i.p.), into mice significantly reduced spontaneous locomotor activity in a dose-dependent manner. The increase in locomotor activity induced by methamphetamine (1 mg/kg, i.p.) and thyrotropin-releasing hormone (5 mg/kg, i.p.) was significantly reduced by cholecystokinin peptides. However, the inhibitory effects of these peptides differed. Neither the tetra nor octa peptide influenced the increased locomotor activity induced by nomifensine (5 mg/kg, i.p.), apomorphine (3 mg/kg, i.p.) or scopolamine (2 mg/kg, i.p.). Thus, these cholecystokinin peptides seem to selectively antagonize increased locomotor activity via the presynaptic dopaminergic system.