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Acidosis in the gastrointestinal tract can be both a physiological and pathological condition. While gastric acid serves digestion and protection from pathogens, pathological acidosis is associated with defective acid containment, inflammation and ischaemia. The pH in the oesophagus, stomach and intestine is surveyed by an elaborate network of acid-sensing mechanisms to maintain homeostasis. Deviations from physiological values of extracellular pH (7.4) are monitored by multiple acid sensors expressed by epithelial cells and sensory neurones. Protons evoke multiple currents in primary afferent neurones, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Other molecular acid sensors comprise TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), epithelial Na(+) channels, two-pore domain K(+) (K₂(P)) channels, ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca²(+) channels and acid-sensitive G-protein-coupled receptors. Most of these acid sensors are expressed by primary sensory neurones, although to different degrees and in various combinations. As upregulation and overactivity of acid sensors appear to contribute to various forms of chronic inflammation and pain, acid-sensitive ion channels and receptors are also considered as targets for novel therapeutics., (© 2010 The Author. Journal compilation © 2010 Scandinavian Physiological Society.)

Background: Lafutidine is a histamine H2 receptor antagonist, the gastroprotective effect of which is related to its antisecretory activity and its ability to activate a sensory neuron-dependent mechanism of defence. The present study investigated whether intragastric administration of lafutidine (10 and 30 mg/kg) modifies vagal afferent signalling, mucosal injury, intragastric acidity and gastric emptying after gastric acid challenge., Methods: Adult rats were treated with vehicle, lafutidine (10 - 30 mg/kg) or cimetidine (10 mg/kg), and 30 min later their stomachs were exposed to exogenous HCl (0.25 M). During the period of 2 h post-HCl, intragastric pH, gastric volume, gastric acidity and extent of macroscopic gastric mucosal injury were determined and the activation of neurons in the brainstem was visualized by c-Fos immunocytochemistry., Results: Gastric acid challenge enhanced the expression of c-Fos in the nucleus tractus solitarii but caused only minimal damage to the gastric mucosa. Lafutidine reduced the HCl-evoked expression of c-Fos in the NTS and elevated the intragastric pH following intragastric administration of excess HCl. Further analysis showed that the gastroprotective effect of lafutidine against excess acid was delayed and went in parallel with facilitation of gastric emptying, measured indirectly via gastric volume changes, and a reduction of gastric acidity. The H2 receptor antagonist cimetidine had similar but weaker effects., Conclusion: These observations indicate that lafutidine inhibits the vagal afferent signalling of a gastric acid insult, which may reflect an inhibitory action on acid-induced gastric pain. The ability of lafutidine to decrease intragastric acidity following exposure to excess HCl cannot be explained by its antisecretory activity but appears to reflect dilution and/or emptying of the acid load into the duodenum. This profile of actions emphasizes the notion that H2 receptor antagonists can protect the gastric mucosa from acid injury independently of their ability to suppress gastric acid secretion.

Capsaicin-sensitive extrinsic afferent nerves have been demonstrated to release biologically active substances in the gastrointestinal (GI) tract. This fact may be useful for identifying sensory transmitter substances in isolated organ experiments. In the GI tract of animals neuropeptides like tachykinins and calcitonin gene-related peptide (CGRP) mediate specific excitatory and inhibitory effects of capsaicin; some evidence indicates a participation of purinergic mechanisms as well. The human gut (especially the circular musculature) is powerfully relaxed by capsaicin, and this effect seems to have a completely different transmitter background (nitric oxide (NO) and maybe VIP, neither of them of intrinsic neuronal origin). We propose that NO may be a sensory neurotransmitter. The "local efferent" (mediator-releasing) effect of extrinsic afferent neurons can also be demonstrated in vivo, both in animals and man. Yet, nearly normal motility of the small and large intestines (i.e., the most "autonomous" part of the GI tract) is maintained in animals with functionally inhibited capsaicin-sensitive nerves. The importance of this system in regulating GI movements may be exaggerated under pathopysiological conditions, first of all inflammation. The afferent function of capsaicin-sensitive nerves plays a role in sympathetic reflexes, such as the inhibition of GI motility after laparotomy or by peritoneal irritation.

The maintenance of gastrointestinal (GI) mucosal integrity depends on the rapid alarm of protective mechanisms in the face of pending injury. Two populations of extrinsic primary afferent neurons, vagal and spinal, subserve this goal through different mechanisms. These sensory neurons react to GI insults by triggering protective autonomic reflexes including the so-called cholinergic anti-inflammatory reflex. Spinal afferents, in addition, can initiate protective tissue reactions at the site of assault through release of calcitonin gene-related peptide (CGRP) from their peripheral endings. The protective responses triggered by sensory neurons comprise alterations in GI blood flow, secretion, and motility as well as modifications of immune function. This article focuses on significant advances that during the past couple of years have been made in identifying molecular nocisensors on afferent neurons and in dissecting the signaling mechanisms whereby afferent neurons govern inflammatory processes in the gut.

The maintenance of gastrointestinal mucosal integrity depends on the rapid alarm of protective mechanisms in the face of pending injury. To this end, the gastric mucosa is innervated by intrinsic sensory neurons and two populations of extrinsic sensory neurons: vagal and spinal afferents. Extrinsic afferent neurons constitute an emergency system that is called into operation when the gastrointestinal mucosa is endangered by noxious chemicals. The function of these chemoceptive afferents can selectively be manipulated and explored with the use of capsaicin which acts via a cation channel termed TRPV1. Many of the homeostatic actions of spinal afferents are brought about by transmitter release from their peripheral endings. When stimulated by noxious chemicals, these afferents enhance gastrointestinal blood flow and activate hyperaemia-dependent and hyperaemia-independent mechanisms of protection and repair. In the rodent foregut these local regulatory roles of sensory neurons are mediated by calcitonin gene-related peptide and nitric oxide. The pathophysiological potential of the neural emergency system is best portrayed by the gastric hyperaemic response to acid back-diffusion, which is governed by spinal afferent nerve fibres. This mechanism limits damage to the surface of the mucosa and creates favourable conditions for rapid restitution and healing of the wounded mucosa. Other extrinsic afferent neurons, particularly in the vagus nerve, subserve gastrointestinal homeostasis by signalling noxious events in the foregut to the central nervous system and eliciting autonomic, emotional-affective and neuroendocrine reactions. Under conditions of inflammation and injury, chemoceptive afferents are sensitized to peripheral stimuli and in this functional state contribute to the hyperalgesia associated with functional dyspepsia and irritable bowel syndrome. Thus, if GI pain is to be treated by sensory neuron-directed drugs it needs to be considered that these drugs do not inhibit nociception at the expense of GI mucosal vulnerability.

It is still unknown which receptors of peripheral sensory pathways encode and integrate an acid-induced nociceptive event in the gastric mucosa. The transient receptor potential vanilloid receptor 1 (TRPV1) and the acid-sensing ion channel 3 (ASIC3) are two nociception-related receptors. Here we investigated (i) to what extent these receptors are distributed in stomach-innervating neurons of dorsal root and nodose ganglia, using immunohistochemistry and retrograde tracing, and (ii) whether their expression is altered in response to a noxious acid challenge of the stomach. We also explored the presence of TRPV1 in the gastric enteric nervous system because of its possible expression by intrinsic sensory neurons. Most stomach-innervating neurons in nodose ganglia were immunoreactive for TRPV1 (80%) and ASIC3 (75%), these results being similar in the dorsal root ganglia (71 and 82%). RT-PCR and Western blotting were performed up to 6 h after oral application of 0.5 m HCl to conscious rats. TRPV1 protein was increased in dorsal root but not in nodose ganglia whereas TRPV1 and ASIC3 mRNAs remained unchanged. TRPV1 mRNA was detected in longitudinal muscle-myenteric plexus preparations of control stomachs and was not altered by the acid challenge. Combined vagotomy and ganglionectomy abolished expression of TRPV1, indicating that it may derive from an extrinsic source. In summary, noxious acid challenge of the stomach increased TRPV1 protein in spinal but not vagal or intrinsic sensory afferents. The TRPV1 receptor may be a key molecule in the transduction of acid-induced nociception of the gastric mucosa and a mediator of visceral hypersensitivity.

Functional bowel disorders (FBDs) are defined by symptoms of gastrointestinal (GI) dysfunction, discomfort and pain in the absence of a demonstrable organic cause. Since the prevalence of FBDs, particularly functional dyspepsia and irritable bowel syndrome, can be as high as 20%, FBDs represent a significant burden in terms of direct healthcare and productivity costs. There is emerging evidence that the discomfort and pain experienced by many FBD patients is due to persistent hypersensitivity of primary afferent neurons, which may develop in response to infection, inflammation or other insults. This concept identifies vagal and spinal sensory neurons as important targets for novel therapies of GI hyperalgesia. Sensory neuron-specific targets can be grouped into three categories: receptors and sensors at the peripheral nerve terminals, ion channels relevant to nerve excitability and conduction and transmitter receptors. Particular therapeutic potential is attributed to targets that are selectively expressed by afferent neurons, such as the transient receptor potential channel TRPV1, acid-sensing ion channels and tetrodotoxin-resistant Na + channels.

Challenge of the rat gastric mucosa with 0.5 mol L(-1) HCl activates nitrergic neurons in the myenteric plexus as visualized by c-Fos immunohistochemistry. In the present study, we characterized the activated neurons more extensively by their chemical coding and investigated whether a neural pathway that involves capsaicin-sensitive extrinsic afferents and/or cholinergic neurons transmitting via nicotinic receptors contributes to the activation of myenteric neurons. In multiple labelling experiments, c-Fos was examined for co-localization with nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), enkephalin (ENK), gastrin-releasing peptide (GRP), substance P (SP), calbindin D-28k (CALB) and neurofilament 145 (NF 145). All c-Fos-positive neurons were immunoreactive for NOS, VIP, NPY and NF 145, but not for SP, ENK, GRP and CALB. Nerve fibres co-expressing NOS, VIP and NPY were predominantly found in the external muscle layer and in the muscularis mucosae but rarely in the mucosa. Pre-treatment with capsaicin or hexamethonium or a combination of both pre-treatments reduced HCl-induced c-Fos expression by 54, 66 and 63%, respectively. Acid challenge of the stomach, therefore, leads to activation of presumably inhibitory motor neurons responsible for muscle relaxation. Activation of these neurons is partly mediated by capsaicin-sensitive afferents and involves ganglionic transmission via nicotinic receptors.

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.

A dense network of extrinsic and intrinsic sensory neurons supplies the gastrointestinal tract. Intrinsic sensory neurons provide the enteric nervous system with the kind of information that this brain of the gut requires for its autonomic control of digestion, whereas extrinsic afferents notify the brain about processes that are relevant to energy and fluid homeostasis and the sensation of discomfort and pain. The sensory repertoire of afferent neurons is extended by their responsiveness to mediators released from enteroendocrine and immune cells, which act like "taste buds" of the gut and serve as interface between the gastrointestinal lumen and the sensory nerve terminals in the lamina propria of the mucosa. Functional bowel disorders such as non-ulcer dyspepsia and irritable bowel syndrome are characterized by abdominal discomfort or pain in the absence of an identifiable organic cause. It is hypothesized with good reason that infection, inflammation or trauma causes sensory pathways to undergo profound phenotypic and functional alterations that outlast the acute insult. The pertinent changes involve an exaggerated sensitivity of the peripheral afferent nerve fibres as well as a distorted processing and representation of the incoming information in the brain. This concept identifies a number of receptors and ion channels that are selectively expressed by primary afferent neurons as important molecular targets at which to aim novel therapies for functional bowel disorders.

An intricate surveillance network consisting of enteroendocrine cells, immune cells and sensory nerve fibres monitors the luminal and interstitial environment in the alimentary canal. Functional bowel disorders are characterized by persistent alterations in digestive regulation and gastrointestinal discomfort and pain. Visceral hyperalgesia may arise from an exaggerated sensitivity of peripheral afferent nerve fibres and/or a distorted processing and representation of gut signals in the brain. Novel strategies to treat these sensory bowel disorders are therefore targeted at primary afferent nerve fibres. These neurons express a number of molecular traits including transmitters, receptors and ion channels that are specific to them and whose number and/or behaviour may be altered in chronic visceral pain. The targets under consideration comprise vanilloid receptor ion channels, acid-sensing ion channels, sensory neuron-specific Na(+) channels, P2X(3) purinoceptors, 5-hydroxytryptamine (5-HT), 5-HT(3) and 5-HT(4) receptors, cholecystokinin CCK(1) receptors, bradykinin and prostaglandin receptors, glutamate receptors, tachykinin and calcitonin gene-related peptide receptors as well as peripheral opioid and cannabinoid receptors. The utility of sensory neuron-targeting drugs in functional bowel disorders will critically depend on the compounds' selectivity of action for afferent versus enteric or central neurons.

Noxious challenge of the rat gastric mucosa by hydrochloric acid (HCl) is signaled to the nucleus tractus solitarii (NTS) and area postrema (AP). This study examined the participation of glutamate and tachykinins in the medullary transmission process. Activation of neurons was visualized by in situ hybridization autoradiography of c-fos messenger RNA (mRNA) 45 min after intragastric (IG) administration of 0.5 M HCl or saline. IG HCl caused many neurons in the NTS and some neurons in the AP to express c-fos mRNA. The NMDA glutamate receptor antagonist MK-801 (2 mg/kg), the NK(1) tachykinin receptor antagonist GR-205,171 (3 mg/kg) and the NK(2) receptor antagonist SR-144,190 (0.1 mg/kg) failed to significantly reduce the NTS response to IG HCl, whereas the triple combination of MK-801, GR-205,171 and SR-144,190 inhibited it by 45--50%. Only in rats that had been preexposed IG to HCl 48 h before the experiment was MK-801 alone able to depress the NTS response to IG HCl. In contrast, the c-fos mRNA response in the AP was significantly augmented by MK-801, an action that was prevented by coadministration of GR-205,171 plus SR-144,190. Inhibition of neuronal nitric oxide synthase with 7-nitroindazole (45 mg/kg) was without effect on the IG HCl-evoked c-fos mRNA expression in the NTS and AP. Our data show that glutamate acting via NMDA receptors and tachykinins acting via NK(1) and NK(2) receptors cooperate in the vagal afferent input from the acid-threatened stomach to the NTS and participate in the processing of afferent input to the AP in a different and complex manner. These opposing interactions in the AP and NTS and the increase in NMDA receptor function in the NTS after a gastric acid insult are likely to have a bearing on the neuropharmacology of dyspepsia.

Background: Stimulation of sensory nerves with capsaicin regulates ion transport in the small intestine in animal experiments., Aim: To investigate whether sensory nerves that are stimulated by capsaicin administration influence fluid and electrolyte absorption in the human jejunum in vivo., Method: Intestinal perfusion studies were performed in 12 healthy subjects using a four lumen tube with a proximal occlusion balloon and a plasma-like electrolyte solution. After an initial control period, 5 (n = 3), 10 (n = 8), or 50 (n = 1) micrograms/ml capsaicin was added to the perfusate, and this was followed by a final control period. Rates of absorption of water, sodium, potassium, chloride, and bicarbonate were determined in a 30 cm segment of jejunum using a non-absorbable volume marker., Results: At all three concentrations of capsaicin there were no significant changes in water and electrolyte absorption as compared with control periods. Two subjects who received 10 micrograms/ml and the subject receiving 50 micrograms/ml experienced crampy abdominal pain., Conclusion: The results do not support the hypothesis that capsaicin sensitive afferent nerves are involved in the physiological regulation of net absorption or secretion across the human jejunal mucosa. Chemical stimulation of these nerves, however, gives rise to abdominal pain.

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

1. This study examined the effect of interleukin-1 beta (IL-1 beta) on the capsaicin-induced increase in cutaneous blood flow of anaesthetized rats as measured by laser Doppler flowmetry. 2. The substances were administered by intraplantar subcutaneous injection of 10 microliters-volumes, saline being injected into one hindpaw and IL-1 beta into the other. 3. IL-1 beta (0.5-500 pg) was without effect on blood flow on its own but dose-dependently enhanced the hyperaemic response to intraplantar capsaicin (0.3 microgram) up to 180% (P < 0.05) of the response seen in saline-treated paws. 4. Il-1 beta-(163-171), a fragment devoid of proinflammatory activity, failed to enhance capsaicin-induced hyperaemia when given at a dose of 50 pg. 5. Indomethacin (10 mg kg-1, i.p.) did not alter the capsaicin-induced vasodilatation but prevented IL-1 beta (50 pg) from augmenting the hyperaemic response to capsaicin. 6. The hyperaemia evoked by intraplantar calcitonin gene-related peptide (0.038-3.8 ng) was not altered by IL-1 beta (50 pg). 7. These data indicate that IL-1 beta enhances the cutaneous hyperaemic response to afferent nerve stimulation with capsaicin in a prostaglandin-dependent manner. This proinflammatory action of the cytokine appears to arise from sensitization of afferent nerve endings.

Capsaicin-sensitive afferent neurons control blood flow via release of peptide transmitters and formation of nitric oxide (NO). The present study examined whether capsaicin-sensitive afferent neurons and NO interact in the control of hemostasis. Afferent nerve ablation by pretreating rats with a neurotoxic dose of capsaicin (125 mg/kg) led to a 26% reduction of the time of bleeding from punctured small mesenteric arteries in pentobarbital-anesthetized animals. Blockade of NO formation by NG-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg) attenuated the bleeding time in capsaicin-pretreated rats but had no effect in vehicle-pretreated rats. Platelet aggregation induced by ADP was significantly augmented by 12% in capsaicin-pretreated rats. L-NAME did not alter platelet aggregation in vehicle-pretreated rats but enhanced it in capsaicin-pretreated animals. The prothrombin and partial thromboplastin time and the plasma levels of fibrinogen and antithrombin III remained unchanged by capsaicin or L-NAME, whereas the thrombin time was reduced in capsaicin-pretreated rats. These data indicate that capsaicin-sensitive afferent neurons play an inhibitory role in platelet aggregation and hemostasis, a function in which they interact with the NO system.

Capsaicin sensitive afferent neurones have previously been reported to play a part in gastric mucosal protection. The aim of this study was to investigate whether these nociceptive neurones strengthen mucosal defence against injury or promote rapid repair of the damaged mucosa, or both. This hypothesis was examined in anaesthetised rats whose stomachs were perfused with ethanol (25 or 50% in saline, wt/wt) for 30 minutes. The gastric mucosa was inspected 0 and 180 minutes after ethanol had been given at the macroscopic, light, and scanning electron microscopic level. Rapid repair of the ethanol injured gastric mucosa (reduction of deep injury, partial re-epithelialisation of the denuded surface) took place in rats anaesthetised with phenobarbital, but not in those anaesthetised with urethane. Afferent nerve ablation as a result of treating rats with a neurotoxic dose of capsaicin before the experiment significantly aggravated ethanol induced damage as shown by an increase in the area and depth of mucosal erosions. Rapid repair of the injured mucosa, however, as seen in rats anesthetised with phenobarbital 180 minutes after ethanol was given, was similar in capsaicin and vehicle pretreated animals. Ablation of capsaicin sensitive afferent neurones was verified by a depletion of calcitonin gene related peptide from the gastric corpus wall. These findings indicate that nociceptive neurones control mechanisms of defence against acute injury but are not required for rapid repair of injured mucosa.

The gastric mucosa, in particular submucosal blood vessels, are innervated by afferent neurons containing neuropeptides such as calcitonin gene-related peptide. Stimulation of sensory neurons innervating the gastric mucosa increases submucosal blood flow. Since sensory neurons supplying the stomach are of dual origin from nodose and dorsal root ganglia, we examined the effect of selective ablation of either the vagal or spinal sensory innervation to the upper gastrointestinal tract on the increase in gastric mucosal blood flow in response to acid back diffusion into the gastric mucosa. Perineural application of capsaicin to the celiac/superior mesenteric ganglia, but not to the vagus nerves, significantly inhibited by 53% the hyperemic response to acid back diffusion. Tissue levels of immunoreactive calcitonin gene-related peptide in the gastric corpus were significantly reduced (by 73%) by periceliac capsaicin treatment, but unaffected by perivagal capsaicin treatment. These data suggest that spinal capsaicin-sensitive afferents containing calcitonin gene-related peptide immunoreactivity are involved in mediating increases in gastric mucosal blood flow. This increase in gastric mucosal blood flow mediated by sensory neurons may act as a protective mechanism against mucosal injury, similar to responses seen in other tissues such as skin.

Stimulation of somatic or visceral nociceptors causes changes in gastrointestinal motor activity and blood pressure. The present study examined the possible participation of capsaicin-sensitive afferent and noradrenergic efferent neurons in the blood pressure and gastric motor responses to laparotomy and intraperitoneal injection of capsaicin or hydrochloric acid in the rat. Gastric motor activity was measured by recording the intragastric pressure of phenobarbital-anaesthetized rats via an oesophageal catheter. Laparotomy as well as intraperitoneal injection of capsaicin (33 and 330 microM) or hydrochloric acid (30 mM) caused a transient reduction of gastric motor activity stimulated by intravenous infusion of bombesin (200 pmol/min) and a brief fall of blood pressure (depressor effect). The depressor effect of laparotomy was followed by prolonged hypertension. Defunctionalization of capsaicin-sensitive afferent neurons by systemic pretreatment of rats with capsaicin (0.4 mmol/kg) prevented the depressor effect and gastric motor inhibition elicited by laparotomy, intraperitoneal capsaicin (33 microM) or intraperitoneal hydrochloric acid (30 mM). However, the effects of 330 microM capsaicin on blood pressure and gastric motility were only partially reduced by capsaicin pretreatment. Blockade of noradrenergic sympathetic neurons by pretreating rats with guanethidine (0.225 mmol/kg) prevented the gastric motor inhibition and depressor effects of laparotomy and intraperitoneal injection of hydrochloric acid (30 mM). The inhibition of gastric motility caused by capsaicin (33 and 330 microM) was only partially reduced by guanethidine pretreatment. The secondary hypertension following the depressor effect of intraperitoneal capsaicin or hydrochloric acid was enhanced in guanethidine-pretreated rats whereas the prolonged hypertension induced by laparotomy was left unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Disruption of the gastric mucosal barrier is quickly followed by an increase in gastric mucosal blood flow, which is thought to be a defensive reaction to prevent further injury. This study examined how this increase in blood flow is brought about. When the stomach of urethane-anesthetized rats was perfused with 0.15N HCl, disruption of the gastric mucosal barrier with 15% ethanol increased the disappearance of acid from the gastric lumen and enhanced gastric mucosal blood flow. This increase in blood flow was blocked by local arterial infusion of tetrodotoxin (60 ng/min) to the stomach and by chemical ablation of capsaicin-sensitive sensory neurons. Inhibition of the blood flow increase was associated with exaggeration of gross and histological injury to the mucosa. IV injection of atropine (0.2 mg/kg) or pyrilamine (2 mg/kg) did not affect blood flow increase in response to barrier disruption, whereas morphine injection (2 mg/kg) inhibited it. The current findings show that the increase in gastric mucosal blood flow that follows disruption of the gastric mucosal barrier in the presence of acid is mediated by sensory neurons that seem to monitor acid back-diffusion and in turn signal for a protective increase in blood flow.

Sensory nerve stimulation by intragastric capsaicin (160 microM) prevents ethanol injury to the gastric mucosa and facilitates gastric mucosal blood flow (GMBF). The present study examined whether the capsaicin-induced increase in GMBF accounts for mucosal protection. Gastric perfusion of capsaicin (160 microM) in urethan-anesthetized rats did not change blood pressure but significantly enhanced GMBF as measured by hydrogen gas clearance. The same increase in GMBF was seen when capsaicin was administered together with an injurious concentration of ethanol (25%). GMBF was facilitated by capsaicin in a dose-related manner (10-640 microM), and the dose-dependent vasodilatation was significantly correlated with a dose-dependent reduction of gross damage to the mucosa. Histology showed that capsaicin prevented deep but not superficial mucosal damage. The vasodilator and protective effects of capsaicin resulted from stimulation of sensory neurons and propagation of nerve activity, since they were blocked after ablation of capsaicin-sensitive neurons or local intra-arterial infusion of tetrodotoxin. This and the finding of a limited access of intragastric capsaicin to the gastric wall indicates that the vasodilator and protective effects of capsaicin are mediated by a neural reflex. It is concluded that facilitation of GMBF is the major mechanism of sensory nerve-mediated prevention of gastric mucosal injury.

In this article, we have presented evidence that vagal capsaicin-sensitive afferent fibers are involved in the regulation of gastric mucosal and motor function. Gastric acid secretion stimulated by gastric distension, histamine and central injection of TRH analog are all partly dependent on vagal capsaicin-sensitive afferent mechanisms. It is possible that as vagal efferent activity releases histamine, the common final pathway is the reduction in the response to histamine. At present, it is unclear as to the mechanism by which capsaicin-sensitive afferents are involved in the secretory response to histamine. With regard to the gastric acid and mucosal blood flow responses to TRH, it is not clear whether the sensory neurons represent a component of the efferent pathway that is activated by TRH or whether their role is to set the sensitivity of, or exert feedback control on this efferent pathway. As perineural capsaicin application decreases peptide content in the peripheral terminal fields of sensory neurons and these peptides may produce local effector functions within the tissue, it is possible that alterations in the gastric responses to TRH result from a decrease in the local effector functions of vagal neurons. From the experiments on electrical stimulation of the vagus nerve, it is evident that antidromic stimulation of vagal afferents can stimulate gastric mucosal blood flow, although under these experimental conditions there was no evidence for a capsaicin-sensitive stimulation of gastric acid secretion. The physiological relevance of this stimulation of gastric mucosal blood flow is at present unclear, but it is possible that physiological stimuli, such as distension or nutrients, may stimulate afferents and signal for an increase in gastric mucosal blood flow. In addition, pathophysiological or noxious stimulation of vagal afferents may also signal for an increase in gastric mucosal blood flow and may play a role in the response of the mucosa to injury.

The present findings have revealed a new aspect of how mechanisms of gastric mucosal resistance to injury are called into effect and are coordinated by the nervous system. Capsaicin-sensitive sensory neurons in the stomach play a physiological role in monitoring acid influx into the superficial mucosa. Once activated, they strengthen gastric mucosal defense against deep injury, with a key process in this respect being an increase in blood flow through the gastric mucosa. This concept opens up completely new perspectives in the physiology and pathophysiology of the gastric mucosa if we consider that the long-term integrity of the gastric mucosa may be under the subtle control of acid-sensitive sensory neurons and that, vice versa, improper functioning of these neutral control mechanisms may predispose to gastric ulcer disease. The present observations also indicate that some of the peptides contained in gastric sensory nerve endings might fulfill a transmitter or mediator role in controlling gastric mucosal blood flow and integrity. Whereas substance P and neurokinin A are unlikely to play a role in the regulation of gastric mucosal blood flow, there is severalfold evidence that CGRP is very important in this respect. This peptide, which in the rat gastric mucosa originates exclusively from spinal sensory neurons, is released upon stimulation of sensory nerve endings and is extremely potent in facilitating gastric mucosal blood flow and in protecting the mucosa from injurious factors. Selective ablation of spinal sensory neurons containing CGRP weakens the resistance of the gastric mucosa against acid injury, which is most likely due to inhibition of protective vasodilator reflexes. We now aim at providing direct pharmacological evidence that antagonism of endogenously released CGRP results in similar pathophysiological consequences as ablation of capsaicin-sensitive sensory neurons.

The exceptional selectivity with which capsaicin acts on a population of peptide-containing thin primary afferent neurons has made this drug an important tool with which to investigate the neuroanatomical, neurochemical and functional implications of these neurons. As a consequence, the use of capsaicin has enabled a substantial furthering of our understanding of the physiological and pathophysiological roles of thin primary sensory neurons. With appropriate controls, both the acute excitatory and long-term neurotoxic actions of capsaicin can be utilized in these studies but it is important to know the advantages and disadvantages and the limitations of each of the different experimental approaches. Table 1 is a brief checklist of the caveats that should be considered and that have been dealt with in this article.

Laparotomy significantly attenuates ethanol-induced gastric mucosal lesions in the rat. The effects of sensory denervation by capsaicin, indomethacin, atropine, guanethidine, and hexamethonium on laparotomy-induced protection were studied in the rat. Gastric mucosal injury was induced by the intragastric instillation of 1 mL of 75% ethanol. The laparotomy-induced protection against ethanol injury was abolished by sensory denervation by capsaicin (total dose, 125 mg/kg, SC) and also by pretreatment with indomethacin (5 mg/kg, SC). In contrast, pretreatment with atropine (0.5 mg/kg, IP), guanethidine (total dose, 20 mg/kg, SC), or hexamethonium (20 mg/kg, IP) had no significant effect on laparotomy-induced protection. These data indicate that capsaicin-sensitive sensory afferent neurons, but not cholinergic or adrenergic autonomic neurons, mediate laparotomy-induced protection against ethanol injury. The hypothesis is put forward that the protective response to laparotomy arises from a somatovisceral and/or viscerovisceral axon reflex of capsaicin-sensitive afferent neurons. Prostaglandins might play a mediator role in the activation by laparotomy of somatic and/or visceral branches of the afferent neurons.

Intragastric capsaicin protects against ethanol-induced gross mucosal lesion formation by stimulation of afferent nerve endings in the rat stomach. The aims of the present study were to examine histologically the protective effect of capsaicin and to test whether this effect is related to changes in mucosal eicosanoid formation and mucosal blood flow. Intragastric capsaicin (160 microM) significantly reduced gross mucosal lesion formation induced by 25% ethanol. Light microscopy revealed that the depth of erosions was attenuated likewise. However, capsaicin did not prevent ethanol from causing superficial damage to the mucosa as observed by light and scanning electron microscopy. The protective action of capsaicin against ethanol remained unchanged by a dose of indomethacin that reduced the ex vivo formation of prostaglandin E2 and 6-oxo-prostaglandin F1 alpha in the gastric mucosa by about 90%. Capsaicin alone did not affect the ex vivo formation of these prostaglandins and of leukotriene C4. Intragastric capsaicin (160 microM) enhanced gastric mucosal blood flow by 89% as measured by the hydrogen gas clearance technique. This effect was also observed when capsaicin was administered together with 25% ethanol. These data indicate that afferent nerve stimulation by intragastric capsaicin protects against deep mucosal damage in response to ethanol, an effect that seems related to an increase in mucosal blood flow but not to eicosanoid formation.

1 Rats were pretreated with capsaicin (50 mg/kg, s.c.) on the 2nd, 10th, or 20th day of life. Three months later immunoreactive substance P (I-SP) was determined in skin, sensory nerves and the central nervous system. Neurogenic plasma extravasation was also examined.2 Pretreatment at the age of 2 or 10 days resulted in a decrease (26 to 69%) of I-SP in skin, saphenous and vagus nerve, dorsal roots, dorsal half of the spinal cord, and medulla oblongata. The I-SP content of the ventral half of the spinal cord, of midbrain, hypothalamus, striatum, cortex, and cerebellum remained unchanged. Neurogenic plasma extravasation was inhibited by more than 80%.3 In contrast to this irreversible effect of capsaicin on newborn rats, pretreatment of 20 day old rats led to reversible depletion of I-SP and to reversible impairment of neurogenic plasma extravasation.4 Capsaicin pretreatment of adult rats caused a marked depletion of I-SP in the skin of the hind paw and an impairment of neurogenic plasma extravasation. A similar decrease of I-SP was seen after chronic denervation of the skin.5 Intra-arterial infusion of substance P (threshold dose 5 x 10(-13) mol/min) or physalaemin induced dose-dependent plasma extravasation. Somatostatin, vasoactive intestinal polypeptide, caerulein and the enkephalin-analogue FK 33-824 were ineffective in doses 100 fold higher.6 The results indicate that the action of capsaicin on substance P neurones is restricted to primary sensory neurones. Since in every case a decreased substance P content of the skin was associated with impaired neurogenic plasma extravasation, it is suggested that release of substance P is involved in neurogenic plasma extravasation.

Treatment of newborn rats with capsaicin (0.16 mmol/kg) is known to cause a permanent degeneration of certain, primarily unmyelinated, afferent neurons. In this study, experimentally induced gastric ulceration was investigated in adult rats treated with capsaicin as neonates. It was found that in capsaicin-treated animals the formation of gastric mucosal lesions in response to indomethacin, ethanol, or cysteamine was significantly enhanced as compared with vehicle-treated controls. The duodenal ulceration caused by cysteamine was not altered after capsaicin treatment. In further experiments the possible pathways involved in the effect of capsaicin treatment on gastric mucosal protection were explored. It was found that the capacity of gastric tissue to release prostaglandins E2 and I2 was unchanged after capsaicin treatment. Atropine, hexamethonium, cimetidine, or terbutaline all reduced gastric ulceration in response to indomethacin in both solvent- and capsaicin-treated rats but did not counteract the enhancement of the ulcerogenic effect of indomethacin in capsaicin-treated rats. Guanethidine enhanced the ulcerogenic effect of indomethacin in solvent-treated but not in capsaicin-treated animals. Ethanol-induced formation of gastric lesions remained unaltered by guanethidine in both solvent- and capsaicin-treated rats. These results indicate that capsaicin-sensitive afferent neurons are involved in gastric mucosal protection against ulcerogenic factors. The data further suggest that this type of gastric defense is primarily due to a local mechanism initiated by sensory nerve endings in the gastric mucosa.

Ablation of capsaicin-sensitive afferent neurons enhances experimentally induced ulceration in the rat gastric mucosa, which suggests that these neurons are involved in gastric mucosal protection. To provide direct evidence for such a function it was investigated whether stimulation of afferent nerve endings by the intragastric administration of capsaicin could counteract the ulcerogenic effect of 25% ethanol. Capsaicin (3.2-640 microM), administered together with ethanol, inhibited the development of haemorrhagic lesions in a concentration-dependent fashion but did not alter the ethanol-induced fall in the gastric potential difference. This suggests that capsaicin does not prevent ethanol from damaging gastric epithelial cells but can counteract the vascular lesions caused by ethanol. The anti-lesion effect of intragastric capsaicin was absent in adult rats which had been treated with a high dose of systemic capsaicin as neonates in order to achieve a permanent degeneration of unmyelinated afferent neurons. It would appear, therefore, that intragastric capsaicin reduces lesion formation by an action on afferent neurons. The protective effect of intragastric capsaicin was not altered following acute subdiaphragmatic vagotomy, acute removal of the coeliac-superior mesenteric ganglion complex, acute bilateral ligation of the adrenal glands, or pretreatment of the rats with atropine or guanethidine. These findings indicate that stimulation of afferent neurons by intragastric capsaicin affords protection of the rat gastric mucosa against ethanol-induced damage. As the autonomic nervous system is not involved gastroprotection appears to represent a local effector function of sensory nerve endings in the stomach.

Gastrointestinal propulsion (gastric emptying and gastrointestinal transit) of a test meal containing charcoal and 51Cr as well as defaecation in relation to food intake were studied in adult rats pretreated with capsaicin (0.40 mmol kg-1 S.C.) or its solvent one week before experimentation. In solvent-treated rats, gastrointestinal propulsion did not differ when the test meal was given to conscious animals or when the meal was administered under ether anaesthesia. In rats treated with capsaicin, gastrointestinal propulsion was unchanged as compared with solvent-treated rats when the test meal was administered to conscious animals. When, however, the meal was given under ether anaesthesia capsaicin-treated rats showed a delayed gastrointestinal propulsion as compared with solvent-treated rats. This delay in gastrointestinal propulsion was abolished by pretreatment of the animals with guanethidine (67 mumol kg-1 S.C.). Rats treated with capsaicin exhibited a prolonged sleeping time in response to ether as compared with solvent-treated rats. Defaecation in relation to food intake was not changed in rats treated with capsaicin as compared with solvent-treated animals. These results indicate that capsaicin-sensitive afferent neurones do not participate in the physiologic control of gastrointestinal propulsion. The delay in gastrointestinal propulsion observed in capsaicin-treated rats when fed under ether anaesthesia seems to involve the sympathetic nervous system. This finding suggests that secondary functional changes in the autonomic nervous system following capsaicin treatment can influence physiologic processes in which capsaicin-sensitive afferent neurones are not primarily implicated.