Endomorphins interact with the substance P (SP) aminoterminal SP1–7 binding in the ventral tegmental area of the rat brain

We have recently identified a specific binding site for the tachykinin peptide substance P (SP) fragment SP1–7 in the rat spinal cord. This site appeared very specific for SP1–7 as the binding affinity of this compound highly exceeded those of other SP fragments. We also observed that endomorphin-2 (EM-2) exhibited high potency in displacing SP1–7 from this site. In the present work using a [3H]-labeled derivative of the heptapeptide we have identified and characterized [3H]-SP1–7 binding in the rat ventral tegmental area (VTA). Similarly to the [3H]-SP1–7 binding in the spinal cord the affinity of unlabeled SP1–7 to the specific site in VTA was significantly higher than those of other SP fragments. Further, the tachykinin receptor NK-1, NK-2 and NK-3 ligands showed no or negligible binding to the identified site. However, the μ-opioid peptide (MOP) receptor agonists DAMGO, EM-1 and EM-2 did, and significant difference was observed in the binding affinity between the two endomorphins. As recorded from displacement curves the affinity of EM-2 for the SP1–7 site was 4–5 times weaker than that for SP1–7 but about 5 times higher than that of EM-1. The opioid receptor antagonists naloxone and naloxonazine showed weak or negligible binding. It was concluded that the specific site identified for SP1–7 binding in the rat VTA is distinct from the MOP receptor although it exhibits high affinity for EM-2.