Substance P analogs displace sigma binding differentially in the brain and spinal cord of the adult mouse

We have previously observed similarities in the behavioral effects produced by the NH₂-terminus of the undecapeptide substance P (SP) and by 1,3-di(2-tolyl)-guanidine (DTG) in the adult mouse. The present series of experiments indicate differences in the rank-order of potency of sigma ligands [DTG; haloperidol (HAL)], SP analogs [SP; SP(1-7); SP(5-11); [D-Pro², D-Phe⁷]-SP(1-7) (D-SP(1-7))] and miscellaneous compounds [morphine (MOR), naloxone (NAL)] at competing for [³H]-DTG binding sites in the mouse brain and spinal cord in vitro: Brain; DTG = HAL ≫ SP = MOR = NAL ≫ SP(1-7) ≫ D-SP(1-7) ≫ SP(5-11): Spinal cord; DTG = HAL ≫ SP(1-7) = MOR = NAL ≫ SP ≫ D-SP(1-7) = SP(5-11). The observed difference in the rank-order potencies of the displacing ligands at these same binding sites supports the notion of two distinct populations of sigma binding sites in these tissues in the adult mouse. Given the low (micromolar) potency of SP analogs at displacing [³H]-DTG binding in the present series of experiments, it is unlikely that the similar behavioral effects we have previously observed elicited by SP(1-7) and DTG in the adult mouse are a result of a direct action of SP(1-7) at the sigma binding site.