Demonstration of a neurokinin A receptor subtype in transfected fibroblasts

The mammalian tachykinin receptors

The tachykinins (TKs) are a family of small peptides which share the common C-terminal sequence Phe-X-Gly-Leu-MetNH2. Three peptides of this family, substance P, neurokinin A and neurokinin B, have an established role as neurotransmitters in mammals. 2. Three receptors for TKs have been cloned: they are G-protein coupled receptors with seven putative transmembrane spanning segments and have been termed NK1 (substance P-preferring), NK2 (neurokinin A-preferring) and NK3 (neurokinin B-preferring). 3. Synthetic agonists are available to selectively stimulate only one receptor, while natural TKs can act as full agonist at each one of the three receptors, albeit at different concentrations. 4. A number of potent and selective antagonists, both peptide and nonpeptide in nature, have recently been developed. 5. The introduction of these ligands has revealed an unforeseen pharmacological heterogeneity of NK1, NK2 and NK3 receptors which appears largely, if not exclusively, linked to the existence of species homologues of the three receptors.

Amastatin interferes with the antagonist properties of MEN 10,208 in the rabbit pulmonary artery but not in the hamster trachea

The tachykinin peptide agonists neurokinin A and [beta Ala8]neurokinin A-(4-10), and the NK2 tachykinin receptor-selective antagonists MEN 10,208, MEN 10,207, MEN 10,282, MEN 10,376 and R396 were assayed in the isolated rabbit pulmonary artery and isolated hamster trachea in the absence and in the presence of the aminopeptidase inhibitor amastatin (10 microM for 30 min). The affinity of MEN 10,208 in the rabbit pulmonary artery was markedly reduced in the presence of amastatin (pKB values from 7.47 to 5.94), while it was unchanged in the hamster trachea. Neither neurokinin A, [beta Ala8]neurokinin A-(4-10), nor the other antagonists were affected by pretreatment with amastatin in either bioassay. The results obtained in the rabbit pulmonary artery show that MEN 10,208 is degraded by local amastatin-sensitive enzymes (possibly aminopeptidase M), which may convert the linear octapeptide MEN 10,208 to the heptapeptide MEN 10,207 by removing the N-terminal Thr from the amino acid sequence of MEN 10,208. The present results are discussed in relation to a previously reported heterogeneity between NK2 receptors of the rabbit and bovine species, and show amastatin to be a new tool for the classification of tachykinin receptors with peptide ligands.

NK-1 tachykinin receptor in rat and guinea pig brains: pharmacological and autoradiographical evidence for a species difference

The affinities of a large variety of peptide or nonpeptide tachykinin analogues were determined on membranes from rat and guinea pig brains using the selective NK-1 radioligand 3H-[Pro9]SP. Nonpeptide antagonists clearly revealed a species difference; (+/-)CP-96,345 was more potent in the guinea pig, while RP 67580 was found to be a better competitor of 3H-[Pro9]SP binding to rat brain membranes. This was confirmed on brain slices by autoradiography. Numerous brain structures were analyzed by optical densitometry. From these data, a heterogeneity of NK-1 binding sites among different structures can be excluded in both rat and guinea pig brains.

Heterogeneity of tachykinin NK-2 receptors in rabbit, guinea-pig and human smooth muscles

Recent evidence indicates that the tachykinin NK-2 receptor is heterogenous (subtypes/species variants) and the existence of NK-2A (or 'non-classical') and NK-2B (or 'classical') forms of the NK-2 receptor in mammalian tissues has been proposed. In this study we have compared the affinities of 7 linear octa- and heptapeptide derivatives of neurokinin A (4-10) and that of two cyclic hexapeptides endowed with selective NK-2 receptor antagonist properties in 5 mammalian smooth muscle preparations previously characterized as expressing the NK-2A receptor subtype (rabbit pulmonary artery and bronchus, guinea-pig bronchus, human ileum and colon) and 2 preparations previously characterized as expressing the NK-2B receptor subtype (rat vas deferens and hamster trachea). The results of this comparative study reinforce the concept of two broad categories of preparations expressing pharmacologically distinguishable forms of the tachykinin NK-2 receptor and suggest the possibility of a further heterogeneity within the previously defined NK-2A receptor subtype.