Two major tachykinins, substance P and substance K, are localized to distinct subsets of amacrine cells in the anuran retina

Tachykinins are known to be major excitatory neuromodulators in neural systems, including the retina of many vertebrate species. Among them substance P has been detected in several species, including anurans. However, none of the other major tachykinins has been sought for in the anuran retina. The aim of this study was to identify another major tachykinin, substance K (neurokinin A) in the frog retina and compare its distribution to that of substance P. Since one of the synthetic pathways of the above mentioned tachykinins make it possible to produce both peptides from one large precursor RNA, we also investigated possible colocalization with double-label immunocytochemistry. Our results show that both substance P and substance K are localized to wide-field amacrine cells in the retina of a terrestrial frog (Pelobates fuscus). Preabsorption and double-label studies provided evidence that the populations of substance P- and substance K-immunoreactive cells are clearly distinct and their dendritic arbor do not overlap in the inner plexiform layer of the retina. These findings provide evidence for the presence of (i) a novel anatomically/neurochemically identifyable amacrine cell type and (ii) predict distinct synthetic pathways for substance P and substance K in the anuran retina.