Tools for tachykinin and neuropeptide research: a conference report

Substance P and related tachykinins induce receptor-mediated hydrolysis of polyphosphoinositides in the rat anterior pituitary

In the present study characterization of phosphatidylinositol 4,5-bisphosphate-specific phospholipase C (PIP2-PLC) activity and receptor-mediated hydrolysis of PIP2 in rat anterior pituitary membranes were investigated. Incubation of the membrane fraction of anterior pituitary homogenate with [3H]inositol-labeled PIP2 in the presence of calcium increased the concentration of the water-soluble degradation product inositol trisphosphate (IP3) in a time-dependent manner. PIP2-PLC in the rat anterior pituitary had a pH optimum at 5.5 and a requirement for cations. Ca2+ and Mg2+ could activate the enzyme. Activity was maximal at a total magnesium concentration of 1 mM and at a free Ca2+ concentration of 100 microM. The addition of the detergent Triton X-100 (0.05% w/v) to the membrane fraction resulted in a 50% decrease of PIP2-PLC activity, whereas the presence of sodium deoxycholate (1 mg/ml) in the membrane fraction increased the PIP2-PLC activity by 100%. The tachykinins substance P, 8-Tyr-substance P, physalaemin, neurokinin A, eledoisin, kassinin and neurokinin B induced receptor-mediated breakdown of [3H]inositol-labeled PIP2 in the membrane fraction in a concentration-dependent manner, but with different potencies. The tachykinins displayed the following rank order of potencies: substance P greater than 8-Tyr-substance P greater than physalaemin greater than neurokinin A greater than eledoisin greater than kassinin greater than neurokinin B, which is consistent with the involvement of a NK-1 receptor. Combined treatment of anterior pituitary membranes by substance P and thyrotropin-releasing hormone (TRH) resulted in an additional increase in PIP2-PLC activity compared to stimulation with TRH alone.

Role of the N-terminal arginine in the histamine-releasing activity of substance P, bradykinin and related peptides

A series of substance P (SP)- and bradykinin (BK)-related peptides have been compared for their histamine-releasing activities on rat peritoneal mast cells. Some of these peptides only differed in the N-acetylation of the N-terminal arginine residue or by the removal of charged residue at the N-terminal. The aim was to examine the role of the N-terminal positive charges in the histamine-releasing activity of compounds that are selective for the SP receptor (named NK-1) or for the B2 type bradykinin receptor. Only compounds with positive charges at the N-terminal caused non-cytotoxic histamine release from rat mast cells. It is suggested that SP- and BK-related peptides caused histamine release by a mechanism which appeared to be non-specific and not related to the activation of mast cell NK-1 or B2 receptors, respectively. Our results show that NK-1 agonists or B2 antagonists devoid of histamine-releasing activity, which could be of potential use in the clinic, can be obtained by removing the positively charged N-terminal aminoacids or by N-acetylation of the N-terminal arginine.

Characterization of neurokinin effects and receptor selectivity in human isolated bronchi

Sensitive afferent nerves and the neurokinins they release upon activation are considered to be important in controlling bronchomotor tone. Human isolated bronchi respond to neurokinin A (NKA), substance P (SP), and neurokinin B (NKB) with dose-dependent contractions. The order of potency of the three natural neurokinins is NKA greater than SP greater than NKB, suggesting the presence of NK-2 receptors. To further characterize the neurokinin receptors in human bronchi, we used selective agonists for each receptor type (i.e., NK-1, NK-2, and NK-3). In fact, NK-1 selective compounds, [Pro9]SP(1-11) sulfone and [beta-ala4,Sar9]SP(4-11) sulfone, did not induce significant contractions up to 10(-5) M. Similarly, the selective agonist for the NK-3 receptor, [MePhe7]NKB(4-10), was almost inactive. However, the NK-2 selective fragment [Nle]NKA(4-10) was a potent stimulant. The negative log of the peptide concentration that caused 50% of maximal effect (pD2) was 6.99 for NKA and 6.12 for [Nle10]NKA(4-10). Removal of the epithelium significantly enhanced the contractile responses to the three neurokinins and also to the NK-2 selective agonist. Phosphoramidon, an enkephalinase inhibitor, was more potent than epithelium removal in enhancing the contractile responses to these agonists. However, epithelium removal and phosphoramidon did not increase the weak responses to the NK-1 and NK-3 selective compounds. In the presence of phosphoramidon, removal of the epithelium slightly enhanced the contractile responses to NKA and [Nle]NKA(4-10) but not to SP and NKB.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding and internalization of a iodinated substance P analog by cultured anterior pituitary cells

Substance P binding to cultured anterior pituitary cells was characterized using Bolton-Hunter iodinated substance P as a ligand. At 0 degrees C, the interaction of the ligand and the cellular surface binding sites was found to be specific, rapid and reversible. Scatchard and Hill analysis of specific binding revealed a single class of non-interacting binding sites with a high affinity (KD = 0.48 nM) and a moderate density of binding sites (Bmax = 1187 binding sites/cell). At 37 degrees C a NaOH-soluble intracellular ligand pool was observed in addition to a surface-bound ligand pool released by a low pH buffer. Thus, substance P seems to be internalized after binding to cellular surface binding sites by means of receptor-mediated endocytosis. The internalization was rapid and could be blocked by colchicine (20 microM), an inhibitor of microtubuli assembly. Following internalization, intracellular degradation of the ligand could be demonstrated. Leupeptin (100 microM), an inhibitor of certain lysosomal enzymes could inhibit the cellular degradation of the added ligand, but had only a moderate influence on internalization. These results demonstrate that substance P after binding to a surface-localized receptor on its pituitary target cells is internalized and subsequently degraded by lysosomal enzymes.

Localization of substance P binding sites in submucous plexus of guinea pig ileum, using whole-mount autoradiography

Whole mounts of guinea pig ileum submucosa were incubated with radiolabeled tachykinins, and binding sites were visualized using autoradiography. Very dense specific binding for [125I]-Bolton-Hunter substance P (BHSP) was observed over ganglia of the submucous plexus, with weaker binding over internodal strands. Dense specific binding was also seen over occasional strands of circular muscle, with weak binding over clumps of mucosa. Although very weak binding was seen over some large blood vessels, no binding was associated with smaller blood vessels. Localization of binding was absent in whole-mounts coincubated with 1 microM substance P, used to define nonspecific binding. Localization of BHSP-specific binding was also abolished in whole-mounts coincubated with 1 nM substance P, but not with 1 nM neurokinin B, suggesting that binding was probably to an NK-1 tachykinin receptor. In whole-mounts incubated in [125I]-iodohistidyl neurokinin A (INKA) or [125I]-Bolton-Hunter neurokinin B (BHNKB), no specific binding over ganglia was observed. These binding sites for BHSP are probably identical with the neuronal substance P receptors mediating mucosal ion transport.