Stimulatory effects of centrally injected kappa-opioid receptor agonists on gastric acid secretion in urethane-anesthetized rats

Capsaicin- and anandamide-induced gastric acid secretion via vanilloid receptor type 1 (TRPV1) in rat brain

The activation of transient receptor potential vanilloid receptor 1 (TRPV1) by capsaicin in rat brain stimulates gastric acid secretion via tachykinin NK2 receptors and the vagus cholinergic nerve, but the involvement of other receptor systems has not been elucidated. We investigated the role of the glutamate and gamma-amino-butyric acid (GABA) receptor systems on the capsaicin response. Gastric acid secretion stimulated by the injection of capsaicin (30 nmol) into the lateral cerebroventricle (i.c.v.) was inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, an antagonist of non-N-methyl-D-aspartate (non-NMDA) receptors, 10.9 nmol, i.c.v.) and bicuculline (a GABA(A) receptor antagonist, 222 microg kg(-1) 10 min(-1), i.v. infusion). Secretion stimulated by the injection of capsaicin (50 nmol) into the fourth cerebroventricle was inhibited by CNQX and bicuculline. I.c.v. injection of anandamide (an endogenous ligand of TRPV1 and cannabinoid receptors, 30 and 100 nmol) stimulated gastric acid secretion, and the response was inhibited by an antagonist of TRPV1 and in the capsaicin-treated rats, but not by an antagonist of cannabinoid receptors. In conclusion, the TRPV1 system, which is activated by capsaicin and anandamide, is preferentially coupled with non-NMDA and GABA(A) receptor systems in the brain and stimulates gastric acid secretion in rats.

Calcitonin gene-related peptide-induced suppression of luteinizing hormone pulses in the rat: the role of endogenous opioid peptides

Calcitonin gene-related peptide (CGRP) is involved in a variety of stress responses in the rat. Central administration of CGRP activates the hypothalamo-pituitary-adrenal axis resulting in increased corticosterone secretion. We have previously shown that central CGRP suppresses the gonadotrophin-releasing hormone (GnRH) pulse generator, specifically LH pulses. Endogenous opioid peptides (EOPs) have been shown to play an important role in stress-induced suppression of the reproductive axis. The aim of the present study was to test the hypothesis that EOPs mediate CGRP-induced suppression of pulsatile LH secretion. Ovariectomized rats were implanted with intracerebroventricular (i.c.v.) and i.v. cannulae. Intravenous administration of the opioid antagonist naloxone (250 microg) completely blocked the suppression of LH pulses induced by 1.5 microg i.c.v. CGRP and significantly attenuated the suppression of pulsatile LH secretion induced by 5 microg i.c.v. CGRP. Furthermore, intravenous administration of naloxone was found to immediately restore normal LH pulse frequency in animals treated 90 min earlier with 1.5 microg i.c.v. CGRP. Co-administration (i.c.v.) of CGRP (1.5 microg) with the mu and kappa opioid receptor-specific antagonists naloxone (10 microg) and norbinaltorphimine (5 microg), respectively, blocked the CGRP-induced suppression of LH pulses, whilst i.c.v. co-administration of CGRP (1.5 microg) with the delta opioid receptor-specific antagonist naltrindole (5 microg) did not. These data provide evidence that EOPs play a pivotal role in mediating the inhibitory effects of CGRP on pulsatile LH secretion in the rat. They also suggest that the mu and kappa, but not the delta, opioid receptors may be responsible for mediating the effects of CGRP on LH pulses.

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.

Involvement of glutamate and gamma-amino-butyric acid receptor systems on gastric acid secretion induced by activation of kappa-opioid receptors in the central nervous system in rats

1. Various neurotransmitters in the brain regulate gastric acid secretion. Previously, we reported that the central injection of kappa-opioid receptor agonists stimulated this secretion in rats. Although the existence of kappa(1)-kappa(3)-opioid receptor subtypes has been proposed, the character is not defined. We investigated the interactions between kappa-opioid receptor subtypes and glutamate, gamma-amino-butyric acid (GABA) or 5-hydroxy tryptamine (5-HT) receptors in the rat brain. 2. Gastric acid secretion induced by the injection of U69593 (8.41 nmol, a putative kappa(1)-opioid receptor agonist) into the lateral cerebroventricle was completely inhibited by the central injection of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10.9 nmol, an antagonist for non-N-methyl-D-aspartate (non-NMDA) receptors) and by bicuculline infusion (222 micro g kg(-1) per 10 min, i.v., GABA(A) receptor antagonist). The secretion induced by bremazocine (8.52 nmol, a putative kappa(2)-opioid receptor agonist) was inhibited by bicuculline infusion, but not by CNQX. The secretion induced by naloxone benzoylhydrazone (224 nmol, a putative kappa(3)-opioid receptor agonist) was slightly and partially inhibited by CNQX and bicuculline. 3. Treatment with CNQX and bicuculline inhibited gastric acid secretion induced by the injection of dynorphin A-(1-17) into the lateral, but not the fourth, cerebroventricle. Antagonists for NMDA, GABA(B) and 5-HT(2/1C) receptors did not inhibit the secretions by kappa-opioid receptor agonists. 4. In rat brain regions close to the lateral cerebroventricle, kappa-opioid receptor systems (kappa(1)>kappa(3)>>kappa(2)) are regulated by the non-NMDA type of glutamate receptor system, and kappa(1)- and kappa(2)-opioid receptor systems are regulated by the GABA(A) receptor system. The present findings show pharmacological evidence for kappa-opioid receptor subtypes that regulate gastric acid secretion in the rat brain.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Effect of mitragynine, derived from Thai folk medicine, on gastric acid secretion through opioid receptor in anesthetized rats

Mitragynine, an indole alkaloid from Thai folk medicine Mitragyna speciosa, exerts agonistic effects on opioid receptors. Gastric acid secretion is proposed to be regulated by opioid receptors in the central nervous system (CNS). Previously, we reported the dual roles (inhibition via micro-opioid receptors and stimulation via kappa-opioid receptors) of the opioid system in the central control of gastric acid secretion. We investigated whether mitragynine affects gastric acid secretion via opioid receptors in the CNS. Injection of mitragynine (30 microg) alone into the lateral cerebroventricle did not have a significant effect on basal gastric acid secretion in the perfused stomach of anesthetized rats. Injection of mitragynine (3-30 microg) into the fourth cerebroventricle, like morphine, inhibited 2-deoxy-D-glucose-stimulated gastric acid secretion. The inhibitory effect of mitragynine (30 microg) was reversed by naloxone (100 microg). These results suggest that mitragynine has a morphine-like action on gastric acid secretion in the CNS.

Gastric acid secretion stimulated by centrally injected nociceptin in urethane-anesthetized rats

Nociceptin is a preferred endogenous ligand for the orphan opioid receptor-like 1 (ORL1) receptor. Central administration of nociceptin showed various pharmacological effects on analgesia, cardiovascular and renal responses, food intake, and so on. In the present study, we investigated the effect of nociceptin injected into the central nervous system (CNS) on gastric acid secretion in the perfused stomach of urethane-anesthetized rats. Injection of nociceptin (0.55-5.52 nmol per rat) into the fourth cerebroventricle stimulated gastric acid secretion and the secretion was inhibited in atropine-treated (1 mg/kg, i.v.) and vagotomized rats. The secretion induced by nociceptin (1.65 nmol) was not inhibited by the central injection of naloxone (275 nmol, a non-selective antagonist of opioid receptors). The secretion was significantly inhibited by the central injection of [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin-(1-13)-NH(2) ([F/G]nociceptin-(1-13), 0.21 nmol, an antagonist of ORL1 receptor), although [F/G]nociceptin-(1-13) alone at higher doses (2.10 and 7.31 nmol) markedly stimulated gastric acid secretion. In the 0-40 min period, the secretion induced by nociceptin was inhibited at least partially by CompB (68.8 nmol, a nonpeptidic antagonist of ORL1 receptor). Injection of nociceptin (5.52 nmol) into the lateral cerebroventricle also stimulated the secretion. Injection of nociceptin did not modify gastric acid secretion stimulated by 2-deoxy-D-glucose (200 mg/kg, i.v.). In conclusion, nociceptin injected into the CNS stimulated gastric acid secretion in rats via the ORL1 receptors and through mechanisms involving the vagus nerve.

Gastric secretion

The influence of central and peripheral stimuli on gastric acid secretion is mediated via activation of histaminergic, gastrinergic, and cholinergic pathways coupled to intracellular second-messenger systems that determine the trafficking and activity of H+ K+-ATPase, the proton pump of the parietal cell. Histamine, released from enterochromaffin-like cells stimulates the parietal cell directly via H-2 receptors coupled to generation of cAMP. Gastrin, acting via cholecystokinin-2 receptors on enterochromaffin-like cells coupled to an increase in intracellular calcium, stimulates the parietal cell indirectly by activating histidine decarboxylase, releasing histamine, and inducing enterochromaffin-like cell hypertrophy and hyperplasia. Acetylcholine, released from gastric postganglionic intramural neurons, stimulates the parietal cell directly via M-3 receptors coupled to intracellular calcium release and calcium entry. The second-messenger systems activated in the parietal cell converge on H+ K+-ATPase that catalyzes the exchange of luminal K+ for cytoplasmic H+ and is responsible for gastric luminal acidification. The main inhibitor of acid secretion is somatostatin which, acting via sst2 receptors, exerts a tonic inhibitory influence on parietal, enterochromaffin-like, and gastrin cells. Acute infection with Helicobacter pylori results in hypochlorhydria, whereas chronic infection may be associated with either hypo- or hyperchlorhydria. Although prostaglandins are thought to play a physiologic role in the regulation of acid secretion and maintenance of gastric mucosal integrity, the precise roles of cyclooxygenase-1 and cyclooxygenase-2 in these processes still eludes us.

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.

Gastric acid secretion by central injection of dynorphin A-(1-17), an endogenous ligand of kappa-opioid receptor, in urethane-anesthetized rats

Gastric acid secretion has been proposed to be regulated by opioid receptors in the central nervous system (CNS). Previously, we reported that central injection of synthetic agonists of kappa-opioid receptors stimulated gastric acid secretion in rats, and the secretion by the agonists was inhibited by norbinaltorphimine (an antagonist of kappa-opioid receptor). In the present study, we investigated the effect of dynorphin A-(1-17), an endogenous ligand of kappa-opioid receptor on the gastric acid secretion in the perfused stomach of urethane-anesthetized rats. Injection of dynorphin A-(1-17) (0.1-1 microg per rat) into the lateral cerebroventricle (LV) stimulated the secretion in a dose-dependent manner. The effect of dynorphin A-(1-17) was almost completely inhibited by the LV injection of norbinaltorphimine (10 microg) and in vagotomized rats. Although some studies of dynorphin A-(1-17) after central injection showed non-opioid effects such as the involvement of N-methyl-D-aspartate (NMDA) receptor, the effect of dynorphin A-(1-17) was not inhibited by a selective antagonist of the NMDA receptor ((+/-)-3-(2-carboxypiperazin-4-yl)-1-propylphosphonic acid, 10 microg). The LV injection of naloxone benzoylhydrazone (a kappa3-opioid receptor agonist, 100 microg) also stimulated the secretion in norbinaltorphimine-sensitive manner. These findings showed that both an endogenous ligand dynorphin A-(1-17) and a synthetic kappa3-opioid receptor agonist stimulated gastric acid secretion via kappa-opioid receptors in the CNS of rats in vivo.