Endomorphins interact with the substance P (SP) aminoterminal SP sub(1-7) 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 SP sub(1-7) in the rat spinal cord. This site appeared very specific for SP sub(1-7) 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 SP sub(1-7) from this site. In the present work using a [ super(3)H]-labeled derivative of the heptapeptide we have identified and characterized [ super(3)H]-SP sub(1-7) binding in the rat ventral tegmental area (VTA). Similarly to the [ super(3)H]-SP sub(1-7) binding in the spinal cord the affinity of unlabeled SP sub(1-7) 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 mu - 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 SP sub(1-7) site was 4-5 times weaker than that for SP sub(1-7) 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 SP sub(1-7) binding in the rat VTA is distinct from the MOP receptor although it exhibits high affinity for EM-2.