Endogenous histamine stimulates ischemically sensitive abdominal visceral afferents through H1 receptors

A randomized, double-blind, placebo-controlled study to assess efficacy of mirtazapine for the treatment of diarrhea predominant irritable bowel syndrome

BACKGROUND

Ample evidence indicates the efficacy of serotonin type 3 (5-HT3) receptor antagonists in the treatment of patients with diarrhea-predominant irritable bowel syndrome (IBS-D). Mirtazapine is an atypical antidepressant with a well-known 5-HT3 receptor antagonist property. This study, therefore, was undertaken to investigate whether compared to placebo, mirtazapine would be efficacious and safe in the treatment of patients with IBS-D.

METHODS

From November 2019 until July 2020, 67 patients meeting Rome IV criteria for IBS-D were randomized in a double-blind fashion into either the mirtazapine treatment group (n = 34) or the placebo treatment group (n = 33). Patients started with mirtazapine 15 mg/day at bedtime for one-week; after which the dose was increased to 30 mg/day for an additional 7-week. Outcomes included changes in the total IBS symptom severity score (IBS-SSS), Hospital anxiety and depression scale score (HADS), and IBS Quality of Life. Additionally, changes in the diary-based symptoms scores including pain, urgency of defecation, bloating, stool frequency, and stool consistency based on the 7-point Bristol Stool Form Scale (BSFS), and a number of days per week with pain, urgency, diarrhea, or bloating, once during the 1-week run-in period, and once during the last week of treatment were recorded.

RESULTS

All analyses were performed on an Intention-to-Treat (ITT) analysis data set. The results showed compared to placebo, mirtazapine is more efficacious in decreasing the severity of IBS symptoms (P-value = 0.002). Further, at the end of the treatment period, all diary-derived symptoms except bloating showed significantly more improvement in the mirtazapine-treated subjects compared to the placebo-treated subjects. While was well-tolerated, mirtazapine also significantly improved the patients' quality of life (P-value = 0.04) and anxiety symptoms (P-value = 0.005).

CONCLUSIONS

Overall, mirtazapine seems to have a potential benefit in the treatment of patients with IBS-D, particularly those with concomitant psychological symptoms. However, further studies are warranted to determine whether these findings are replicated.

TRIAL REGISTRATION

Trial registration: Registration number at Iranian Registry of Clinical Trials: IRCT20120215009014N311 . Registration date: 2019-10-21.

Targeting Histamine Receptors in Irritable Bowel Syndrome: A Critical Appraisal

Irritable bowel syndrome is a group of functional gastrointestinal disorders with not yet fully clarified etiology. Recent evidence suggesting that mast cells may play a central role in the pathogenesis of irritable bowel syndrome paves the way for agents targeting histamine receptors as a potential therapeutic option in clinical treatment. In this review, the role of histamine and histamine receptors is debated. Moreover, the clinical evidence of anti-histamine therapeutics in irritable bowel syndrome is discussed.

Mast cells drive mesenteric afferent signalling during acute intestinal ischaemia

Acute intestinal ischaemia stimulates visceral afferent nerves but the mechanisms responsible for this excitation are not fully understood. Mast cells may participate in this process as they are known to signal to mesenteric afferents during intestinal anaphylaxis and contribute to early inflammation and neuronal damage in response to cerebral ischaemia. We therefore hypothesised that mast cells are early responders to acute intestinal ischaemia and their activation initiates rapid signalling to the CNS via the excitation of mesenteric afferents. Primary afferent firing was recorded from a mesenteric nerve bundle supplying a segment of jejunum in anaesthetized adult rats. Acute focal ischaemia was produced by clamping theme senteric vessels for 8 min, and reperfusion followed removal of the vessel clip. Two episodes of ischaemia–reperfusion (I–R) separated by a 30 min interval were performed. Drugs or their vehicles were administered 10 min before the 2nd I–R episode. Ischaemia caused a reproducible, intense and biphasic afferent firing that was temporally dissociated from the concomitantly triggered complex pattern of intestinal motor activity. The L-type calcium channel blocker, nifedipine, significantly attenuated this afferent firing by a mechanism independent of its action on intestinal tone. Ischaemia-induced afferent firing was also abrogated by the mast cell stabilizer, doxantrazole, and the H1 histamine receptor antagonist, pyrilamine. In contrast, the nicotinic receptor antagonist, hexamethonium, and the N-type calcium channel toxin, ω-conotoxin GVIA, each reduced the ischaemia-evoked motor inhibition but not the concurrent afferent discharge. Similarly, the cyclooxygenase inhibitor, naproxen, had no effect on the ischaemic afferent response but reduced the intestinal tone shortly from the onset of ischaemia to the early period of reperfusion. These data support a critical role for mast cell-derived histamine in the direct chemoexcitation of mesenteric afferents during acute intestinal ischaemia, whereas enteric reflex mechanisms and cyclooxygenase products contribute primarily to ischaemia-induced changes in intestinal motility. Therefore, targeting mast cells may provide benefits in patients with abdominal pain resulting from an ischaemic insult to the gastrointestinal tract.

Post-infectious irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common disorder associated with abdominal pain or discomfort and altered bowel habits. The majority of patients describe an insidious onset of symptoms; however, a subset report a fairly precise time of onset following an attack of acute gastroenteritis. Typically, the potential acute infectious symptoms, such as fever and vomiting, resolve after several days, but abdominal discomfort, bloating, and diarrhea persist. Although the underlying mechanism of post-infectious IBS (PI-IBS) has not been established, ongoing inflammation appears to play a role, with an increase in serotonin-containing enterochromaffin cells, T lymphocytes, mast cells, proinflammatory cytokines, and intestinal permeability. Psychiatric comorbidities are less common in PI-IBS, compared with IBS patients in general; however, the prevalence of psychological disorders is still higher compared with that in the general population and is associated with a poorer prognosis. Overall, patients with PI-IBS have a slightly improved prognosis compared with those with IBS without an infectious onset.

Interactions between histamine and bradykinin in stimulation of ischaemically sensitive cardiac afferents in felines

Cardiac spinal afferents are activated during myocardial ischaemia. Our previous studies have shown that during ischaemia, histamine and bradykinin (BK) stimulate cardiac spinal afferents. Because the two mediators are released together during ischaemia, the present study examined the interactions between these two mediators with respect to their influence on ischaemically sensitive cardiac afferents. Single-unit cardiac afferent activity was recorded from the left sympathetic chain or rami communicantes (T(2)-T(5)) in anaesthetized cats. Fifty-five ischaemically sensitive cardiac afferents (conduction velocity (CV) = 0.2-5.6 m s(-1), 8 Adelta- and 47 C-fibres) were identified. Administration of histamine (10 microg kg(-1)) and BK (1 microg) in combination into the left atrium (LA) caused an additive response in 16 afferents compared with administration of either BK or histamine alone (2.62 +/- 0.39 versus 1.67 +/- 0.20 versus 1.24 +/- 0.23 impulses s(-1) (imp s(-1)), BK + histamine versus BK versus histamine). To further evaluate interactions between these mediators, we observed that injection of histamine (10 microg kg(-1), LA) 4 min after the administration of BK (1 microg, LA) induced a significantly larger cardiac afferent response than the response to histamine before BK (1.24 +/- 0.23 versus 1.96 +/- 0.39 imp s(-1), before versus after, n = 10). In contrast, six other afferents responded reproducibly to repeated injections of histamine (10 microg kg(-1), LA) in the absence of BK. BK sensitization of the afferent response to histamine lasted for less than 10 min. Cyclooxygenase blockade with indomethacin (5 mg kg(-1), i.v.) abolished BK sensitization of the response to histamine (1.09 +/- 0.11 versus 1.11 +/- 0.10 imp s(-1), n = 10). Conversely, the response of most (7/9) cardiac afferents to repeat application of BK (1 microg, LA) 4 min after histamine (10 microg kg(-1), LA) was attenuated compared with the BK response before histamine (1.84 +/- 0.25 versus 1.31 +/- 0.18 imp s(-1), before versus after, P < 0.05). Repeat BK (1 microg, LA) induced a consistent response in five other afferents in the absence of histamine. Thus, BK interacts with histamine, and together they cause a larger response than either one alone. BK sensitizes cardiac afferents responding to histamine in a time-dependent fashion, and the BK sensitization effect is dependent on an intact cyclooxygenase pathway. Conversely, histamine reduces the response of most afferents to BK.

The role of mast cells in the pathogenesis of pain in chronic pancreatitis

BACKGROUND

The biological basis of pain in chronic pancreatitis is poorly understood. Mast cells have been implicated in the pathogenesis of pain in other conditions. We hypothesized that mast cells play a role in the pain of chronic pancreatitis. We examined the association of pain with mast cells in autopsy specimens of patients with painful chronic pancreatitis. We explored our hypothesis further using an experimental model of trinitrobenzene sulfonic acid (TNBS) -induced chronic pancreatitis in both wild type (WT) and mast cell deficient mice (MCDM).

METHODS

Archival tissues with histological diagnoses of chronic pancreatitis were identified and clinical records reviewed for presence or absence of reported pain in humans. Mast cells were counted. The presence of pain was assessed using von Frey Filaments (VFF) to measure abdominal withdrawal responses in both WT and MCDM mice with and without chronic pancreatitis.

RESULTS

Humans with painful chronic pancreatitis demonstrated a 3.5-fold increase in pancreatic mast cells as compared with those with painless chronic pancreatitis.WT mice with chronic pancreatitis were significantly more sensitive as assessed by VFF pain testing of the abdomen when compared with MCDM.

CONCLUSION

Humans with painful chronic pancreatitis have an increased number of pancreatic mast cells as compared with those with painless chronic pancreatitis. MCDM are less sensitive to mechanical stimulation of the abdomen after induction of chronic pancreatitis as compared with WT. Mast cells may play an important role in the pathogenesis of pain in chronic pancreatitis.

Histamine Contributes to Ischemia-Related Activation of Cardiac Spinal Afferents: Role of H1 Receptors and PKC

Myocardial ischemia activates cardiac spinal afferents that transmit the nociceptive information leading to chest pain and elicit excitatory cardiovascular reflexes. Previous studies have shown that histamine is increased in coronary sinus blood during myocardial ischemia and that this autacoid stimulates abdominal visceral afferents. The present investigation evaluated the role of endogenous histamine in stimulation of ischemically sensitive cardiac spinal afferents. Nerve activity of single-unit cardiac afferents was recorded from the left sympathetic chain or rami communicans (T2–T5) in anesthetized cats. Sixty-four cardiac afferents were identified. Injection (5–30 μg/kg) of histamine into the left atrium (LA) stimulated 7 ischemically sensitive cardiac afferents resulting in a significant increase in their activity in a dose-dependent manner. Also, LA injection of histamine (10 μg/kg) stimulated 7 of 8 ischemically insensitive cardiac spinal afferents. Administrations of 2-(3-chlorophenyl)histamine (250 μg/kg, LA), a specific H1 receptor agonist and histamine (10 μg/kg, LA), stimulated 9 other ischemically sensitive cardiac afferents (0.48 ± 0.10 to 1.40 ± 0.20 imp/s). In contrast, dimaprit (500 μg/kg, LA), an H2 receptor agonist, stimulated only one of the 9 afferents and thus did not alter their overall activity (0.40 ± 0.09 to 0.54 ± 0.09 imp/s). ( R)α-Methyl-histamine (500 μg/kg, LA), an H3 receptor agonist, did not stimulate any of the 9 afferents. Pyrilamine (300 μg/kg, iv), a selective H1 receptor antagonist, attenuated the activity of 8 afferents during 5 min of ischemia from 3.32 ± 0.38 to 1.87 ± 0.28 imp/s and abolished the response of 9 other cardiac afferents to histamine. Finally, administration of PKC-(19–36) (30 μg/kg, iv), a selective inhibitor of protein kinase C, attenuated the response of 8 cardiac afferents to histamine by 32%. These data indicate that endogenous histamine contributes to activation of cardiac sympathetic afferents during myocardial ischemia through H1 receptors and that the action of histamine on these cardiac afferents is partially dependent on the intracellular PKC pathway.

Activated platelets contribute to stimulation of cardiac afferents during ischaemia in cats: role of 5-HT(3) receptors

Myocardial ischaemia activates blood platelets and cardiac sympathetic afferents, which mediate chest pain and cardiovascular reflex responses. We have demonstrated that activated platelets stimulate ischaemically sensitive cardiac sympathetic afferents. Platelets absorb and release 5-hydroxytryptamine (5-HT) when they are activated. In the present study we hypothesized that, by releasing 5-HT, activated platelets stimulate cardiac afferents during ischaemia through a 5-HT(3) receptor mechanism. Platelet-rich plasma (PRP) and platelet-poor plasma (PPP) were obtained from cats. Activation of platelets in PRP was induced by thrombin (5 units ml(-1)) or collagen (2 mg kg(-1)). Using high-performance liquid chromatography, we observed that the concentration of 5-HT was increased significantly in suspensions of platelets activated with thrombin (PRP+thrombin, 28 +/- 1.7 microM) or collagen (PRP+collagen, 27 +/- 2.5 microM) compared with suspensions of unactivated platelets (PRP+saline, 2.3 +/- 0.8 microM) and PPP. During myocardial ischaemia and reperfusion, tirofiban, a specific inhibitor of platelet glycoprotein (GP) IIb-IIIa receptors (100 microg kg(-1), I.V., followed by 5 microg kg(-1) min(-1)), significantly reduced the increase in the concentration of 5-HT in cardiac venous plasma from ischaemic region. Nerve activity of single-unit cardiac afferents was recorded from the left sympathetic chain (T2-T5) in anaesthetized cats. Eighty ischaemically sensitive and seven ischaemically insensitive cardiac afferents were identified. Tirofiban reduced the ischaemia-related increase in activity of seven cardiac sympathetic afferents by 50 %. Injection of 1.5 ml of PRP+collagen or PRP+thrombin into the left atrium (LA) increased activity of 16 cardiac afferents. Tropisetron (300 microg kg(-1), I.V.), a selective 5-HT(3) receptor antagonist, eliminated the afferent's responses to platelets activated with collagen or thrombin. Moreover, LA injection of 5-HT (20-40 microg kg(-1)) and PBG (100 microg kg(-1)), a 5-HT(3) receptor agonist, stimulated nine ischaemically sensitive cardiac sympathetic afferents, significantly increasing the activity of these afferents. However, injection of alpha-M-5-HT (100 microg kg(-1), LA), a 5-HT(2) receptor agonist, stimulated only two of the nine ischaemically sensitive cardiac afferents, and thus did not significantly alter impulse activity of this group of afferents. Both the 5-HT(1) (5-CT, 100 microg kg(-1), LA) and 5-HT(4) receptor agonists (SC53116, 100 microg kg(-1), LA) did not stimulate any of the nine afferents tested. Tropisetron (300 microg kg(-1), I.V.) also eliminated the response of seven ischaemically sensitive cardiac afferents to exogenous 5-HT and attenuated the ischaemia-related increase in activity of nine cardiac sympathetic afferents by 41 %. Conversely, LA injection of 5-HT (40 microg kg(-1)) did not stimulate any of seven ischaemically insensitive cardiac afferents, although this group of afferents consistently responded to bradykinin (3 microg, LA). These data indicate that during myocardial ischaemia the activated platelets stimulate cardiac sympathetic afferents, at least in part, through a 5-HT(3) receptor mechanism.

Role of spinal NMDA and non-NMDA receptors in the pressor reflex response to abdominal ischemia

Abdominal ischemia induces a pressor reflex caused mainly by C-fiber afferent stimulation. Because excitatory amino acids, such as glutamate, bind to N-methyl-D-aspartate (NMDA) and non-NMDA [dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)] receptors and serve as important spinal neurotransmitters, we hypothesized that both receptors play a role in the abdominal ischemia pressor reflex. In chloralose-anesthetized cats, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate did not alter the pressor reflex (33 +/- 9 to 33 +/- 7 mmHg, P > 0.05, n = 4), whereas AMPA receptor blockade with 4.0 mM 6-nitro-7-sulfamylbenzo(f)quinoxaline-2,3-dione significantly attenuated the reflex (29 +/- 5 to 16 +/- 4 mmHg, P < 0.05, n = 6). Because several studies suggest that anesthesia masks the effects of glutamatergic receptors, this experiment was repeated on decerebrate cats, and in this group, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate significantly altered the pressor reflex (36 +/- 3 to 25 +/- 4 mmHg, P < 0.05, n = 5). Our combined data suggest that spinal NMDA and AMPA receptors play a role in the abdominal ischemia pressor reflex.

Role of activated platelets in excitation of cardiac afferents during myocardial ischemia in cats

Myocardial ischemia activates cardiac spinal afferents that mediate chest pain and excitatory reflex cardiovascular responses. Platelets are activated during myocardial ischemia and release 5-hydroxytryptamine, which stimulates abdominal spinal afferents. This study investigated the role of activated platelets in excitation of cardiac spinal afferents during ischemia. Platelet-rich plasma (PRP) and platelet-poor plasma (PPP) were obtained from cats and incubated with collagen (2 mg/ml) or thrombin (5 U/ml). We observed reduction of platelets in PRP indicative of platelet activation by collagen and thrombin, respectively. Activity of single-unit, ischemia-sensitive cardiac spinal afferents was recorded from the left sympathetic chain in anesthetized cats. Injection of 1.5 ml PRP + collagen (activated platelets) into the left atrium (LA) stimulated 12 of 13 cardiac afferents. PRP + saline (nonactivated platelets, LA) and PPP + collagen did not alter activity of these afferents. PRP + thrombin (1.5 ml, LA) stimulated eight of nine other cardiac afferents, whereas PPP + thrombin did not stimulate any of the nine afferents. Antiplatelet immune serum (1 ml/kg iv) significantly decreased circulating platelets as well as neutrophils (polymorphonuclear leukocytes, PMNs) in eight other cats, and in each animal, attenuated the ischemia-related increase in activity of cardiac afferents. Conversely, responses of five separate cardiac afferents to ischemia were not diminished after treatment with anti-PMN immune serum when concentration of circulating platelets was maintained by infusion of donated PRP despite the decrease in circulating PMNs. These data indicate activated platelets stimulate ischemia-sensitive cardiac spinal afferents and contribute to activation of these afferents during ischemia.

Characterization of mechanosensitive splanchnic nerve afferent fibers innervating the rat stomach

Splanchnic nerve fibers innervating the stomach were studied in anesthetized rats; 997 fibers in the T(9) or T(10) dorsal roots were identified by electrical stimulation of the splanchnic nerve. Thirty-one fibers responded to gastric distension. Extrapolated response thresholds ranged between 0 and 53 mmHg; seven fibers had thresholds for response > or =30 mmHg. Thermo- and/or chemosensitivity was tested in 18 of the 31 fibers. Four of twelve fibers responded to intragastric perfusion of heated saline; none of eight fibers tested responded to perfusion of cold saline. Infusion of glucose, L-arginine, or potassium oleate produced no change in resting activity. Intragastric instillation of 12% glycerol or an inflammatory soup (bradykinin 10(-5) M, PGE(2) 10(-5) M, serotonin 10(-5) M, histamine 10(-5) M, and KCl 10(-3) M) and prior heat stimulation sensitized responses to distension. The results reveal the presence of low- and high-threshold mechanosensitive fibers in the splanchnic innervation of the stomach. These fibers have the ability to sensitize, and they likely contribute to pain and altered sensations that can arise from the stomach.

Role of cAMP in activation of ischemically sensitive abdominal visceral afferents

A number of metabolites produced during abdominal ischemia can stimulate and/or sensitize visceral afferents. The precise mechanisms whereby these metabolites act are uncertain. Other studies have shown that the adenylate cyclase-cAMP system may be involved in the activation of sensory neurons. Therefore, we hypothesized that cAMP contributes to the activation of ischemically sensitive abdominal visceral afferents. Single-unit activity of abdominal visceral C fibers was recorded from the right thoracic sympathetic chain in anesthetized cats before and during 7 min of abdominal ischemia. Forty-six percent of ischemically sensitive C fibers responded to intra-arterial injection of 8-bromo-cAMP (0.35-1. 0 mg/kg), an analog of cAMP, with responses during ischemia increasing from 0.50 +/- 0.06 to 0.84 +/- 0.08 impulses/s (P < 0.05, n = 11 C fibers). Conversely, an inhibitor of adenylate cyclase, 2', 5'-dideoxyadenosine (DDA; 0.1 mg/kg iv), attenuated ischemia-induced increase in activity of afferents from 0.66 +/- 0.10 to 0.34 +/- 0. 09 impulses/s (P < 0.05; n = 8). Furthermore, whereas exogenous PGE(2) (3-4 microg/kg ia) augmented the ischemia-induced increase in activity of afferents (P < 0.05, n = 10), treatment with DDA (0.1 mg/kg iv) substantially reduced the increase in discharge activity of afferents during ischemia, which was augmented by PGE(2) (1.45 +/- 0.24 vs. 0.70 +/- 0.09 impulses/s, -DDA vs. +DDA; P < 0.05) in six fibers. A time control group (n = 4), however, demonstrated similar increases in the activity of afferents with repeated administration of PGE(2). These data suggest that cAMP contributes to the activation of abdominal visceral afferents during ischemia, particularly to the action of PGs on activation and/or sensitization of these endings.

Sensitization of visceral afferents to bradykinin in rat jejunum in vitro

1. We have investigated the effects of inflammatory mediators on visceral afferent discharge and afferent responses to bradykinin (BK) in rat jejunum using a novel in vitro technique. 2. Prostaglandin E2 (1 microM) augmented responses to BK without affecting basal firing, while histamine (100 microM) and adenosine (100 microM) activated basal discharge and enhanced BK responses. In contrast, 5-HT (100 microM) increased basal discharge without influencing responses to BK. 3. Afferent discharge induced by histamine was inhibited by both H1 (pyrilamine) and H3 (thioperamide) but not H2 (ranitidine) receptor antagonists at 10 microM. In contrast, sensitization to BK induced by histamine was inhibited by ranitidine (10 microM). 4. Afferent discharge induced by adenosine was blocked by the A1 receptor antagonist DPCPX (10 microM) but remained unaffected by A2A receptor blockade with ZM241385 (10 microM). In contrast, sensitization of BK responses by adenosine was unaffected by both antagonists. Basal discharge and BK-induced responses were unaffected by the A3 receptor agonist IB-MECA (1 microM). While involvement of A2B receptors is not excluded, adenosine may activate afferent discharge through A1 receptors, while sensitization to BK could involve a receptor other than A1, A2A or A3, possibly the A2B receptor. 5. Inhibition of cyclo-oxygenase with naproxen (10 microM) prevented sensitization after histamine but not adenosine. 6. Sensitization was mimicked by dibutyryl cAMP. This occurred without changes in basal firing and was unaffected by naproxen. 7. In conclusion, afferent discharge induced by BK is augmented by histamine, adenosine and PGE2, but not by 5-HT. Evidence suggests that sensitization involves separate mechanisms from afferent activation. Sensitization may be mediated by increases in cAMP following direct activation by mediators at the nerve terminal or through indirect pathways such as the release of prostaglandins.

Interleukin-1beta sensitizes abdominal visceral afferents of cats to ischaemia and histamine

1. Activation of abdominal splanchnic visceral afferents during mesenteric ischaemia induces visceral pain and evokes excitatory cardiovascular responses. Previous studies have shown that interleukin-1beta (IL-1beta) concentration is increased locally in tissues during ischaemia and reperfusion. Local administration of IL-1beta sensitizes somatic afferents to mechanical, thermal and chemical stimulation. Therefore, we hypothesized that IL-1beta stimulates or sensitizes splanchnic visceral afferents to ischaemia and to the action of chemical stimuli such as histamine. 2. The concentration of IL-1beta in mesenteric lymph and portal venous plasma in anaesthetized cats was measured with an enzyme-linked immunosorbent assay before, during and after 10 min of abdominal ischaemia. The level of IL-1beta was significantly increased during ischaemia in lymph, but not in plasma. 3. Discharge activity of single-unit abdominal visceral C fibre afferents was measured from the right thoracic sympathetic chain. Ischaemically sensitive C fibre afferents were identified according to their response to 5-10 min of abdominal ischaemia. 4. Intra-arterial (i.a.) injection of a high dose of IL-1beta (500 ng kg-1), but not of a lower dose (i.e. 15, 50 or 150 ng kg-1), stimulated most (six of seven) abdominal visceral afferents. 5. IL-1beta (15 ng kg-1, i.a.) significantly enhanced the increased activity of 11 of 13 C fibre afferents during 10 min of ischaemia. Conversely, an IL-1 type I receptor antagonist (IL-1ra, 1.5 microg kg-1, i.a.) significantly attenuated the increased activity in six of seven other C fibre afferents during ischaemia. 6. IL-1beta (15 ng kg-1, i.a.) significantly augmented the responses of 13 of 16 ischaemically sensitive abdominal afferents to histamine (5-10 microg kg-1, i.a.). Conversely, IL-1ra (1.5 microg kg-1, i.a.) significantly attenuated the responses of five of six other C fibre afferents to histamine. 7. These data strongly suggest that stimulation of IL-1 type I receptors by IL-1beta produced during brief abdominal ischaemia contributes to activation of visceral afferents during ischaemia, at least in part, by sensitizing the afferent nerve endings to ischaemia. Our data also show that exogenous IL-1beta sensitizes visceral afferents to histamine.

Role of 5-HT3 receptors in activation of abdominal sympathetic C fibre afferents during ischaemia in cats

1. Activation of abdominal sympathetic afferents during ischaemia reflexly excites the cardiovascular system. We have shown previously that exogenous 5-hydroxytryptamine (5-HT, i.e. serotonin) stimulates abdominal sympathetic afferent nerve endings, and recently have documented increased concentrations of 5-HT in intestinal lymph and portal venous plasma during brief abdominal ischaemia. The present investigation evaluated the role of endogenously produced 5-HT in activation of ischaemically sensitive abdominal sympathetic afferents. 2. Nerve activity of single-unit C fibre afferents innervating duodenum, mesentery, pancreas, portal hepatis, bile duct, gall bladder and jejunum was recorded from the right thoracic sympathetic chain of anaesthetized cats. Ischaemically sensitive C fibre afferents were identified according to their response to 5-10 min of abdominal ischaemia. 3. Intra-arterial injection of 5-HT (20 microg kg-1) increased discharge activity of twelve afferents from 0. 23 +/- 0.05 to 0.96 +/- 0.09 impulses s-1 after an onset latency of 5.7 +/- 1.4 s. Also, 2-methylserotonin (100 microg kg-1, i.a.), a 5-HT3 receptor agonist, stimulated eleven of twelve afferents to significantly increase their discharge activity from 0.25 +/- 0.05 to 0.90 +/- 0.10 impulses s-1 after a latency of 3.3 +/- 0.4 s. Furthermore, intravenous injection of tropisetron (200 microg kg-1), a 5-HT3 receptor antagonist, significantly attenuated the increase in activity of twelve other C fibre afferents during 10 min of abdominal ischaemia from 1.62 +/- 0.18 to 0.94 +/- 0.22 impulses s-1, and eliminated the response of eleven other afferents to 5-HT. 4. Both the 5-HT2 receptor agonist, alpha-methylserotonin (100 microg kg-1, i.a.), and the 5-HT1 receptor agonist, 5-carboxamidotryptamine (100 microg kg-1, i.a.), did not alter the impulse activity of these twelve afferents (0.29 +/- 0.05 to 0.31 +/- 0.06, and 0.26 +/- 0.06 to 0.29 +/- 0.06 impulses s-1, respectively). 5. Treatment with indomethacin (5 mg kg-1, i.v.) in eight different cats did not alter the response of nine C fibre afferents to exogenous 5-HT (0.91 +/- 0. 17 vs. 1.19 +/- 0.25 impulses s-1, P > 0.05). 6. The results suggest that, during mesenteric ischaemia, endogenous 5-HT contributes to the activation of abdominal sympathetic afferents, mainly through direct stimulation of 5-HT3 receptors and that the action of 5-HT on these afferents appears to be independent of the cyclo-oxygenase pathway.