Central injections of capsaicin cause antidiuresis mediated through neurokinin-1 receptors in rat hypothalamus and vasopressin release

Species differences in tachykinin receptor distribution: further evidence that the substance P (NK1) receptor predominates in human brain

Marked species differences in the distribution of central tachykinin receptors are reported but uncertainty remains about the ability of available ligands to detect NK2 and NK3 receptors in human brain. We compared the distribution of NK1, NK2, and NK3 receptors in sections from rodent, primate, and human brain using the 125I-labeled ligands substance P (SP) for the NK1 receptor, neurokinin A (NKA) for the NK2 receptor, and neurokinin B (NKB) and eledoisin for NK3 receptors. Duration of exposure to autoradiographic film was from 7 days for [125I]SP up to 90 days for the other ligands. High levels of specific [125I]SP binding were seen throughout the brains of all species studied. Specific [125I]NKA binding was detected in brains from neonatal rat, and to a lesser level in adult rat, gerbil, and guinea pig; it was not detected in monkey or human brain, but was present in circular muscle of human duodenum, confirming that this ligand binds to human NK2 receptors under our experimental conditions. Specific [125I]NKB and [125I]eledoisin binding was widespread in brain sections from rats, gerbils, and guinea pigs, and very low levels were also detected in marmoset, squirrel monkey, and rhesus monkey brain after prolonged (up to 90 days) exposure. We failed to identify specific eledoisin binding in human brain, even after prolonged exposures. These findings demonstrate that the NK1 receptor is the predominant tachykinin receptor expressed in primate and human brain, but that low levels of NK3 receptor are present in nonhuman, primate brain.

Role of neuropeptide receptor systems in vanilloid VR1 receptor-mediated gastric acid secretion in rat brain

Previously, we reported that the injection of capsaicin into the lateral cerebroventricle (i.c.v.) stimulated gastric acid secretion via vanilloid VR1 receptors and the vagal cholinergic pathways in anesthetized rats. In the present study, we investigated the involvement of receptor systems for neurokinin A, calcitonin gene-related peptide (CGRP) and glutamate in the vanilloid VR1 receptor-mediated response. The i.c.v. injection of neurokinin A (30 nmol) stimulated gastric acid secretion in the presence of cis-2-(diphenylmethyl)-N-[(2-iodophenyl)methyl]-1-azabicyclo[2.2.2]octan-3-amine oxalate (L-703606, a tachykinin NK1 receptor antagonist, 30 nmol) and the effect was inhibited by cyclo[Gln-Trp-Phe-Gly-Leu-Met] (L-659877, a tachykinin NK2 receptor antagonist, 30 nmol); the values were 145.9 +/- 32.3 and 21.1 +/- 16.6 microEq HCl per 120 min, respectively. The value in the control group was 14.3 +/- 3.8 microEq HCl. The tachykinin NK2 receptor-mediated secretion was inhibited by i.c.v. injections of antagonists of the CGRP1 receptor (human CGRP fragment 8-37, 15 nmol) and non-N-methyl-D-aspartate (non-NMDA)-type glutamate receptor (6-cyano-7-nitroquinoxaline-2,3-dione, 10.9 nmol); the values were 30.8+/-29.8 and 5.7+/-16.9 microEq HCl, respectively. Gastric acid secretion induced by the i.c.v. injection of 30 nmol capsaicin (178.4 +/- 34.0 microEq HCl) was inhibited by antagonists of tachykinin NK2 (23.7 +/- 6.2) and CGRP1 (21.2 +/- 8.5), but not tachykinin NK1 (181.4 +/- 37.0), receptors. The gastric acid secretion induced by capsaicin was decreased by the i.c.v. pre-injection of low doses of neurokinin A or CGRP, which alone had no effect on the secretion. These findings suggest the involvement of tachykinin NK2, CGRP and non-NMDA receptor systems in the vanilloid VR1 receptor-mediated regulation of gastric acid secretion in the rat brain regions close to the lateral cerebroventricle.

Stimulatory effect of centrally injected capsaicin, an agonist of vanilloid receptors, on gastric acid secretion in rats

Capsaicin, the main pungent ingredient in chilli peppers, acts through specific vanilloid receptors on sensory neurons. The vanilloid receptors have been localized in the brain. We describe here a stimulatory effect of centrally injected capsaicin on gastric acid secretion in urethane-anesthetized rats. Injection of capsaicin (10-30 nmol per rat) into the lateral cerebroventricle markedly stimulated the secretion. Injection of capsazepine (30 nmol) or ruthenium red (30 nmol), antagonists for vanilloid receptors, into the lateral cerebroventricle inhibited the secretion induced by capsaicin, although these antagonists alone significantly stimulated the secretion. Injection of capsaicin into the fourth cerebroventricle also stimulated gastric acid secretion. The effects of centrally injected capsaicin into the lateral and fourth cerebroventricle were mediated via the vagus cholinergic nerve, because the effects were abolished by bilateral vagotomy at the cervical level. The present findings showed that central injection of capsaicin stimulated gastric acid secretion, via vanilloid receptors in the central nervous system (CNS), and through vagus nerve mechanisms in the perfused stomach of urethane-anesthetized rats.

Fos activation and upregulation of nicotinamide adenine dinucleotide phosphate diaphorase in the rat pituitary by acute capsaicin injection

In this study, we examined the effect of acute capsaicin injection on nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d, a histochemical index of nitric oxide, NO, synthase) and Fos expression in the rat pituitary gland. Compared with vehicle, capsaicin significantly activated Fos expression in the anterior and intermediate lobes. In addition, capsaicin-treated rats showed a significant upregulation of NADPH-d in the anterior and neural lobes. Pretreatment of the animals with a specific NO synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME), significantly attenuated the capsaicin-induced Fos expression in the anterior and intermediate lobes. These observations suggest that NO is a key regulator of the acute effect of capsaicin on the pituitary gland.