- In an attempt to delineate the mechanism subserving the demonstrated central effects of the tripeptide, L-prolyl-L-leucyl-glycinamide (PLG) in mammals including humans, we developed a radioligand binding assay to characterize the binding of 3H-PLG to rat brain membranes. Equilibrium binding studies indicated that PLG binds to rat striatum with high affinity (KD = 4.69 +/- 0.50 nM), saturability (Bmax = 9.20 +/- 0.30 fmoles mg-1 protein) and reversibility. Kinetic data yielded a KD = 1.42 +/- 0.21 nM for rat striatum. Regional distribution profile of specific 3H-PLG binding revealed that the striatum has the highest density of PLG binding sites, followed by the hypothalamus and the cerebral cortex. Analogues of PLG compete for specific PLG binding in rat striatum with potencies parallelling their in vivo activities in behavioural systems. Our results support the existence of a unique class of putative peptide receptor sites specific for PLG mediating a spectrum of pharmacological effects.