We have recently synthesized a neurokinin B analog, [D-Pro2,D-Trp6,8,Nlel0]NB (DPDTNLE-NB), which shows selective competitive antagonism of neurokinin B with minimal agonist activity [1]. Our objective in this study was to use this antagonist and two substance P analog antagonists (Spantide and DPDT-SP) to systematically investigate neurokinin receptors in the guinea pig ileum.

We have studied the selectivity and competitiveness of three neurokinin antagonists and atropine against substance P, neurokinin A, and neurokinin B. DPDTNLE-NB, [D-Pro2, D-Trp6,8, Nle10]-neurokinin B is a competitive antagonist of neurokinin B (pA2=5.5), but not substance P or neurokinin A. DPDT-SP ([D-Pro2, Trp7,9]-substance P), competitively blocks substance P (pA2=6.9) and neurokinin B (pA2=6.8), but not neurokinin A. Spantide ([D-Arg1, D-Trp7,9, Leu11]-substance P) competitively blocks substance P (pA2=6.7) and at a log unit higher concentration blocks neurokinin A (pA2=5.8), but does not block neurokinin B. Atropine is a competitive antagonist of neurokinin B (pA2=9.0) at ten times the concentration needed to block acetylcholine (pA2=10.1), but does not inhibit the other neurokinins These results support the hypothesis of multiple neurokinin receptors in the guinea pig ileum and indicate that the site of neurokinin B, but not substance P or neurokinin A is predominantly on intramural neurons. This indirect stimulation appears to be dependent on the release of acetylcholine. Neurokinin B also has activity on smooth muscle receptors since the contractile response could not be completely antagonized by atropine. There appear to be two smooth muscle neurokinin receptors on the basis of results obtained with DPDT-SP and spantide, one predominantly responsive to substance P and the other to neurokinin A. Only spantide appeared to have any effect on the neurokinin A receptor and that was at a much higher concentration than that needed to block substance P.

Neuromedin K, a decapeptide isolated from porcine spinal cord and suggested to have tachykinin-like activity in vitro, produced reciprocal hind-limb scratching when injected intrathecally to mice. Neuromedin K was 20–60 times less potent in producing scratching (on a molar basis) than substance P, kassinin, eledoisin or physalaemin. The activity of neuromedin K was blocked by the substance P antagonist [D-Pro2D-Trp7,9]substance P at doses of antagonist which effectively blocked the activity of the other tachykinins. These data provide the first evidence for tachykinin-like activity of neuromedin K in the central nervous system.