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

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Abstract
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. 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. An identical pharmacological profile was apparent for both radioligands and tachykinin peptides inhibited the binding in the order: neurokinin B > BH‐eledoisin > kassinin > L‐363,851, eledoisin > substance P, neurokinin A > physalaemin > DiMeC7 > substance P methylester, indicating a profile consistent with the NK3‐subtype of tachykinin receptors. The binding of [125I]BH‐neurokinin A and [125I]BH‐eledoisin was equally sensitive to inhibition by the guanosine triphosphate (GTP) analogue, guanyly‐5′‐β‐γ‐imido) diphosphate. 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.