Effects of prolyl-leucyl-glycinamide and cyclo(leucyl-glycine) on morphine-induced antinociception and brain mu, delta and kappa opiate receptors.

The effects of prolyl-leucyl-glycinamide and cyclo (leucyl-glycine) on morphine-induced antinociception in mice and on in vitro binding of 3H-ligands for opiate receptor subtypes (mu, delta and kappa) in the mouse brain homogenate were determined. Subcutaneous administration of either of the above peptides (1, 2, and 4 mg/kg) 10 min prior to the injection of morphine did not affect morphine-induced antinociception as evidenced by the identical ED50 values of morphine in vehicle and peptide treated groups. The binding of 3H-dihydromorphine and 3H-naloxone (mu receptors), 3H-D-Ala2-D-Leu5-enkephalin (delta receptors), and 3H-ethylketocyclazocine (kappa receptors) to opiate receptors in the mouse brain homogenate was also unaffected by both the peptides over a large concentration range. It is concluded that these peptides do not interact with brain opiate receptors.