Substance P in the bovine retina: localization, identification, release, uptake and receptor analysis

Substance P-like immunoreactivity is restricted to at least three different cell types in the bovine retina. They are amacrine cells, displaced amacrine cells and interplexiform neurones. High performance liquid chromatography and radioimmunoassay have shown that the bovine retina substance P-like immunoreactive material cannot be distinguished from authentic substance P. Isolated bovine retina does not take up exogenous [3H]substance P as revealed by autoradiography. By increasing the external K+ concentration it was shown that the release of endogenous substance P from the retina was stimulated. This release is Ca2+ dependent. The binding of [3H]substance P to crude membrane preparations of bovine retina was saturable and revealed a single population of binding sites with a KD value of 0.32 nM and a Bmax of 2.45 pmol/g wet wt. The characteristics of the binding site suggest the presence of substance P receptors. It is suggested that substance P is a likely neurotransmitter in the bovine retina.