Abstract

Binding characteristics of 125I-labeled Bolton-Hunter substance P ([125I]BHSP), a radioactive analogue of substance P, were studied with mesencephalic primary cultures prepared from embryonic mouse brain. Nonspecific binding represented no more than 20% of the total binding observed on the cells. In contrast, significant specific binding--saturable, reversible, and temperature-dependent--was demonstrated. Scatchard analysis of concentration-dependent binding saturation indicates a single population of noninteracting sites with a high affinity (Kd = 169 pM). Substance P and different substance P analogues were tested for their competitive potencies with regard to [125I]BHSP binding. BHSP itself, substance P, (Tyr8)-substance P, and (nor-Leu11)-substance P strongly inhibited the binding. Good inhibition was also obtained with physalaemin and eledoisin, two peptides structurally related to substance P. When substance P C-terminal fragments were tested for their ability to compete with [125I]BHSP binding, a good relationship was found between competitive activity and peptide length. Regional distribution of [125I]BHSP binding sites was found using primary cultures obtained from different regions of embryonic mouse brain. Mesencephalic, hypothalamic, and striatal cultures had the highest [125I]BHSP binding capacities, whereas cortical, hippocampal, and cerebellar cells shared only little binding activity. Finally, when mesencephalic cells were grown under conditions impairing glial development, [125I]BHSP binding was not affected, demonstrating that binding sites are located on neuronal cells.