neurokinin-a and vofopitant

Although the tachykinins substance P (SP) and neurokinin A (NKA) are coreleased from primary afferent nociceptors and act via neurokinin (NK) receptors, their differential effects in vivo are not known. Despite pharmacological evidence that NKA preferentially binds NK-2 receptors, this receptor is not found in spinal cord neurons. Thus, in the present studies, we compared the extent to which SP and NKA contribute to spinal nociceptive processing via the NK-1 receptor. We found that SP and NKA induce NK-1 receptor internalization with identical dose dependence and induce increases in intracellular calcium at the same concentrations, suggesting that SP and NKA equally activate the NK-1 receptor. We found, however, that the selective NK-1 receptor antagonist GR 205171 blocked NKA but not SP-induced NK-1 receptor internalization in the rat spinal cord in vivo and in embryonic day 19 rat spinal neurons in vitro. Using this selectivity of GR 205171 for NKA-induced NK-1 receptor activation, we examined the relative contribution of SP and NKA to noxious stimulus-induced activation of spinal NK-1 receptors. We estimate that NKA contributes to at least 50% of the NK-1 receptor activation in lamina I. Under inflammatory conditions, all noxious stimulus-induced NK-1 receptor internalization in deep dorsal horn neurons was blocked by GR 205171, suggesting that it is entirely NKA-mediated. Substance P-mediated NK-1 receptor internalization was focused at the site of termination of stimulated nociceptors but NKA also activated NK-1 receptors at more distant sites. We conclude that NKA not only targets the NK-1 receptor but may be a predominant pronociceptive primary afferent neurotransmitter.

Topics: Animals; Calcium; Cells, Cultured; Dose-Response Relationship, Drug; Intracellular Fluid; Male; Neurokinin A; Neurokinin-1 Receptor Antagonists; Neurons; Pain Measurement; Piperidines; Rats; Rats, Sprague-Dawley; Receptors, Neurokinin-1; Signal Transduction; Spinal Cord; Substance P; Tetrazoles

Binding experiments performed with [(125)I]-NKA allowed us to demonstrate the presence of "septide-sensitive" specific binding sites on membranes from rat CHO cells transfected with the NK(1) receptor cDNA (CHO-rat-NK1 cells), human astrocytoma U373 MG, or mouse cortical astrocytes, cells which express NK(1) but neither NK(2) nor NK(3) receptors. In all cases, [(125)I]-NKA was specifically bound with high affinity (2 to 5 nM) to a single population of sites. In the three preparations, pharmacological characteristics of [(125)I]-NKA binding sites were notably different from those of classical NK(1) binding sites selectively labelled with [(125)I]-BHSP. Indeed, the endogenous tachykinins NKA, NPK, and NKB and the septide-like compounds such as septide, SP(6-11), ALIE-124, [Apa(9-10)]SP, or [Lys(5)]NKA(4-10) had a much higher affinity for [(125)I]-NKA than [(125)I]-BHSP binding sites. Interestingly, differences were also found in the ratio of B(max) values for [(125)I]-NKA and [(125)I]-BHSP specific bindings from one tissue to another. These latter observations suggest that these two types of NK(1) binding sites are present on distinct NK(1) receptor isoforms (or conformers). Finally, while several tachykinins and tachykinin-related compounds stimulated cAMP formation or increased inositol phosphate accumulation in CHO-rat-NK1 cells, these compounds only increased the accumulation of inositol phosphates in the two other preparations.

Topics: Animals; Binding Sites; CHO Cells; Cricetinae; Humans; Indoles; Iodine Radioisotopes; Isoindoles; Mice; Neurokinin A; Neurokinin-1 Receptor Antagonists; Peptide Fragments; Piperidines; Protein Isoforms; Pyrrolidonecarboxylic Acid; Radioligand Assay; Rats; Receptors, Neurokinin-1; Substance P; Tetrazoles; Tumor Cells, Cultured