Influence of beta-casomorphins on apomorphine-induced hyperlocomotion

Derivatives of beta-casomorphin Tyr-Pro-Phe-Pro-Gly and their des-Tyr1-derivatives were investigated on the model of apomorphine-induced hyperlocomotion (1 mg/kg = 3 microM/kg, IP). D-Pip4 CM 5 (5 nM) inhibited the apomorphine hypermotility completely, while D-Phe3 CM 5 (5 nM) and D-Pro4 CM 5 (5 nM) decreased it only to about 50%. The normal exploration was nearly completely inhibited by D-Pro4 CM 5 (40 nM), by D-Pip4 CM 5 (5 nM) depressed to 20%, and by D-Phe3 CM 5 (10 nM) to 35%. The maximum inhibition of apomorphine-induced hyperlocomotion by the des-Tyr-casomorphin derivatives was about 50%. The dose-response curves were U-shaped. The exploratory activity was not significantly influenced. The mode of action and the involvement of different neurotransmitter systems in the inhibitory effect of beta-casomorphin derivatives on apomorphine hyperlocomotion are discussed.