Comparison of the binding of radiolabelled neurokinin A and eledoisin in rat cortex synaptic membranes.

1. The binding of the 125I-Bolton Hunter (BH) conjugates of neurokinin A and eledoisin to synaptic plasma membranes prepared from rat cerebral cortex was investigated. 2. Saturation analyses indicated that both radioligands labelled a similar number of binding sites, but [125I]BH-eledoisin had a 7 fold higher affinity than [125I]BH-neurokinin A. 3. An identical pharmacological profile was apparent for both radioligands and tachykinin peptides inhibited the binding in the order: neurokinin B greater than BH-eledoisin greater than kassinin greater than L-363,851, eledoisin greater than substance P, neurokinin A greater than physalaemin greater than DiMeC7 greater than substance P methylester, indicating a profile consistent with the NK3-subtype of tachykinin receptors. 4. The binding of [125I]BH-neurokinin A and [125I]BH-eledoisin was equally sensitive to inhibition by the guanosine triphosphate (GTP) analogue, guanyly-5'-(beta-gamma-imido) diphosphate. 5. These results indicate that [125I]BH-neurokinin A and [125I]BH-eledoisin appear to label a common site in rat cerebral cortex synaptic plasma membranes with the characteristics of an NK3-receptor, and thus [125I]BH-neurokinin A is not a selective radioligand for the NK2-receptor.