Enhanced nerve-stimulated muscarinic and neurokinin contractions of ileum from streptozotocin guinea-pigs.

Diabetes mellitus can lead to neuropathy of enteric neurons, resulting in abnormal gut motility. These studies investigated voltage-dependent contributions of muscarinic M3 receptor activation by acetylcholine and neurokinin NK1 receptor activation by neurokinins to nerve-stimulated contractions of longitudinal ileal strips from STZ guinea-pigs, a type 1 diabetic model with insulin deficiency, but mild hyperglycaemia. Contractions to bethanechol, substance P methyl ester, and nerve stimulation were greater in diabetic as compared to control ileum. The muscarinic M3 receptor antagonist 4-DAMP at lower voltages and the neurokinin NK1 receptor antagonist SR140333 at higher voltages, but not the neurokinin NK1 receptor antagonist CP-96,345, were more effective at inhibiting nerve-stimulated immediate peak contractions and total areas of contraction of ileum from diabetic as compared to control animals. For diabetic ileum, voltage-dependent increases in the areas of nerve-stimulated contraction were observed in the presence of 4-DAMP and CP-96,345 but not SR140333. At low voltages only, nerve-stimulated release of acetylcholine was greater from diabetic as compared to control ileum. Fluorescence intensity of tachykinin-like immunoreactivity was increased in ileal myenteric ganglia from diabetic as compared to control animals. In diabetic guinea-pigs, stronger ileal nerve-stimulated contractions reflected increased release of acetylcholine at lower voltages and tachykinins at higher voltages, as well as increased sensitivity of smooth muscle M3 and NK1 receptors to acetylcholine and tachykinins. Hypoinsulinaemia may be a primary contributor to intestinal motility dysfunction in type 1 diabetes mellitus.

Presynaptic modulation of cholinergic and non-cholinergic fast synaptic transmission in the myenteric plexus of guinea pig ileum.

Abstract These studies investigated receptors modulating release of mediators of fast excitatory postsynaptic potentials (fEPSPs) in guinea pig ileum myenteric plexus using electrophysiological methods. Fast EPSPs inhibited by >95% by hexamethonium (100 micromol L(-1)) were cholinergic; mixed fEPSPs were inhibited <95% by hexamethonium. Non-cholinergic fEPSPs were studied in the presence of hexamethonium. The alpha2-adrenergic receptor agonist UK 14304 inhibited cholinergic (maximum inhibition = 76%, EC(50) = 18 nmol L(-1)), mixed (81%, 21 nmol L(-1)) and non-cholinergic (76%, 44 nmol L(-1)) fEPSPs equally. The 5-HT(1) receptor agonist 5-carboxamidotryptamine inhibited cholinergic, mixed and non-cholinergic fEPSPs equally. Renzapride, increased non-cholinergic (33%) less than mixed (97%, 13 micromol L(-1)) fEPSPs. Renzapride inhibited the purely cholinergic fEPSPs (-29%) but potentiated the cholinergic component of mixed fEPSPs (39%). Prucalopride potentiated all fEPSPs equally (30-33%). 5-HT (0.1 micromol L(-1)) induced potentiation of cholinergic (75%), mixed (97%) and non-cholinergic (84%) fEPSPs was not statistically different. The potentiating effects of renzapride and 5-HT on fEPSPs were inhibited by the 5-HT(4) receptor antagonist, SB 204070 (10 nmol L(-1)). Renzapride (0.3 micromol L(-1)) blocked 5-HT-induced increases in cholinergic fEPSPs. alpha2-Adrenergic and 5-HT(1) receptors mediate inhibition of transmitter release from cholinergic and mixed terminals. 5-HT and prucalopride, acting at 5-HT(4) receptors, facilitate all fEPSPs; renzapride facilitates the cholinergic and non-cholinergic components of mixed fEPSPs but not purely cholinergic fEPSPs. Cholinergic synapses may express few 5-HT(4) receptors or a renzapride-insensitive 5-HT(4) receptor isoform.

Multiple mechanisms of fast excitatory synaptic transmission in the enteric nervous system.

The enteric nervous system (ENS) can control gastrointestinal function independent of direct connections with the central nervous system. Enteric nerves perform this important function using multiple mechanisms of excitatory neurotransmission in enteric ganglia. Fast excitatory synaptic transmission in the autonomic nervous system (ANS) is largely mediated by acetylcholine (ACh) acting at nicotinic cholinergic receptors but in the ENS there are noncholinergic fast excitatory neurotransmitters. There are two broad types of neurons in the ENS: S neurons and AH neurons. S neurons are interneurons and motoneurons while AH neurons are sensory neurons. Three subsets of S neurons in the myenteric plexus can be distinguished on the basis of the neurotransmitters producing fast excitatory postsynaptic potentials (fEPSPs) in each subset. In one subset, fEPSPs are mediated solely by ACh acting at nicotinic cholinergic receptors. In a second subset of S neurons, ATP acting at P2X purine receptors and ACh contribute to the fEPSP while in a third subset, fEPSPs are mediated by 5-hydroxytryptamine (5-HT) acting at 5-HT(3) receptors and ACh. Some AH neurons also receive fast excitatory synaptic input. The fEPSPs recorded from AH neurons are mediated ACh and also by glutamate acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. Multiple mechanisms of fast excitatory synaptic transmission in the ENS are likely to contribute to its capacity to regulate complex gastrointestinal functions.

Analysis of fast synaptic pathways in myenteric plexus of guinea pig ileum.

Most fast excitatory postsynaptic potentials (fEPSPs) recorded from guinea pig ileum myenteric plexus are mediated by acetylcholine acting at nicotinic receptors and ATP acting at P2X receptors. These studies examine length and polarity of projection of neurons releasing mediators of fEPSPs. Under ketamine-xylazine anesthesia, animals were sham treated or myenteric pathways were interrupted. After severed axons degenerated, fEPSPs were recorded at the operated site using conventional, intracellular electrophysiological methods and were classified as nicotinic or mixed on the basis of sensitivity to hexamethonium. Cholinergic and noncholinergic fEPSPs were recorded from small, operated segments, suggesting that some neurons have projections between adjacent ganglia. The mean amplitudes of nicotinic and mixed fEPSPs were reduced after circumferential and descending pathways degenerated. The proportion of nicotinic vs. mixed fEPSPs recorded from tissues lacking descending projections was greater than that recorded from sham-treated tissues, suggesting that fibers releasing noncholinergic mediators project aborally. Descending projections communicate with neurons in ganglia at least three rows aboral to their origin. The data suggest that fast noncholinergic neurotransmission could contribute to hexamethonium-resistant descending inhibition during the peristaltic reflex.

Purinergic fast excitatory postsynaptic potentials in myenteric neurons of guinea pig: distribution and pharmacology.

BACKGROUND & AIMS: Adenosine triphosphate (ATP) acting at P2 receptors mediates some fast excitatory postsynaptic potentials (fEPSPs) in myenteric neurons of guinea pig ileum. The present studies investigate the distribution of purinergic fEPSPs along the length of the gut and characterize the P2-receptor subtype mediating fEPSPs. METHODS: Conventional intracellular electrophysiological methods were used to record from myenteric neurons in vitro. RESULTS: At a membrane potential of -97 +/- 1 mV, the amplitude (25 +/- 1 mV; n = 307) of fEPSPs was similar along the gut. Hexamethonium (100 micromol/L) inhibited fEPSPs in the gastric corpus by 98% +/- 1% (n = 31) and in the duodenum, ileum, taenia coli, proximal colon, and distal colon by 42%-55%. In the presence of hexamethonium, suramin (100 micromol/L) or the P2X antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 10 micromol/L) reduced the control fEPSP amplitude in the duodenum, ileum, taenia coli, proximal colon, and distal colon by 71%-84%. The pharmacology of the purinergic fEPSPs was investigated in detail in the ileum. Noncholinergic fEPSPs were concentration-dependently (1-30 micromol/L) inhibited by PPADS (50%-inhibitory concentration, 3 micromol/L). In addition, alpha,beta-methylene 5'-adenosine triphosphate (1 micromol/L) also reduced purinergic fEPSPs. CONCLUSIONS: Fast EPSPs mediated in part through P2X receptors are prominent in myenteric neurons along the small and large intestines but are rare in the gastric corpus.

Gastric ECL-cell hyperplasia produces enhanced basal and stimulated gastric acid output but not gastric erosion formation in the rat.

1. The purpose of this study was to examine the change in gastric acid output and gastric erosion formation produced by inducing gastric enterochromaffin-like (ECL) cell hyperplasia in female rats. 2. Rats were treated with vehicle or ranitidine (1,200 mumol/kg/day x 4 wks) administered via SC Alzet minipumps. Experiments were performed 24 hours after removing the minipump, when the inhibitory effect of ranitidine on gastric acid secretion had been lost. 3. Basal gastric acid secretion was 7-fold higher in chronic ranitidine animals than in sham control. 4. Both total and net gastric acid secretions stimulated by carbachol/pentagastrin infusion or histamine injection were significantly higher in the chronic ranitidine animals than in controls. 5. By contrast, intracisternal injection of the chemical vagal stimulant RX77368 (100 ng) resulted in no net increase in acid output of recovered ranitidine-pretreated group. 6. No significant changes in gastric erosions produced experimentally by cold exposure plus restraint or indomethacin pretreatment were noted in recovered chronic ranitidine animals compared to sham controls. 7. These findings suggest that achlorhydria-induced ECL cell hyperplasia augments both basal and stimulated gastric acid secretory function. The histamine results implicate an enhanced parietal cell mass, upregulation of H2 receptors, and/or second-messenger events at the parietal cell as the mechanism for the enhanced gastric secretory response.

Gastric antisecretory effect of serotonin: quantitation of release and site of action.

Despite many reports that serotonin (5-HT) inhibits gastric acid output, the role and mechanism of action of endogenous 5-HT to modulate gastric secretion remain unclear. Vagal stimulation enhanced the basal rate of 5-HT release into both the gastric lumen (600%) and the portal circulation (265%) of the rat. The peak rate of 5-HT release into the portal circulation was 1,000-fold higher that luminal release (12 micrograms/min and 1.2 ng/min, respectively). To elucidate site(s) of action of 5-HT to inhibit acid secretion, several approaches were taken. Intraluminal perfusion of exogenous 5-HT to encompass enhanced levels seen after vagal stimulation did not reduce gastric acid output. In contrast, administration of systemic 5-HT, which raised portal venous 5-HT to similar levels as vagal stimulation, had a marked antisecretory effect. Chemical or surgical ablation of enteric or sympathetic nerves innervating the stomach did not attenuate the inhibitory effect of exogenous 5-HT on gastric acid output. The antisecretory effect of systemic 5-HT was insensitive to pretreatment with piroxicam, doxantrazole, close gastric intra-arterial sodium nitroprusside, somatostatin monoclonal antibody, or bilateral adrenalectomy. The results suggest that 5-HT is released from endogenous stores into the portal circulation in sufficient quantities after vagal stimulation to alter gastric physiology and that its action is independent of the autonomic nervous system, gastric mucosal prostaglandins or somatostatin, mucosal mast cell or adrenal constituents, or changes in gastric mucosal blood flow.

8-OH-DPAT stimulates gastric acid secretion through a vagal-independent, adrenal-mediated mechanism.

Serotonin (5-hydroxytryptamine, 5-HT) is a neuroendocrine component of the gastrointestinal tract. 5-HT1A receptors exist both in the brain and have been demonstrated autoradiographically in high density in the rat stomach. However, the physiologic role of 5-HT1A receptors in modulating gastric function is not known. The effect of the selective 5-HT1A receptor agonist, (+/-)-8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT), on gastric acid secretory function was compared to 5-HT in acute, urethane-anesthetized gastric-fistulated rats during pentagastrin infusion. 5-HT inhibited, but 8-OH-DPAT stimulated, gastric acid secretion in a dose-dependent manner. Bilateral cervical vagotomy or celiac ganglionectomy did not reverse the effect of 8-OH-DPAT on acid secretion. However, the enhancement of acid by 8-OH-DPAT was attenuated by acute adrenalectomy or close intra-arterial administration of spiperone, but not idazoxan. Thus, the data suggest that the selective 5-HT1A receptor agonist 8-OH-DPAT may augment gastric secretory function via an adrenal-dependent mechanism.

Serotonin inhibits gastric acid secretion through a 5-hydroxytryptamine1-like receptor in the rat.

5-Hydroxytryptamine (serotonin, 5-HT) is released from the gastrointestinal tract by vagal stimulation. This biogenic amine produces many alterations in gastric functional parameters, including inhibition of gastric acid secretion. This study was designed to characterize the 5-HT receptor subtype modulating gastric acid secretion. In urethane-anesthetized acute gastric fistula rats, systemic 5-HT (3.5 mumol/kg i.v.) inhibited acid output stimulated by pentagastrin infusion by 58%. Close gastric intra-arterial (i.a.) injection of methysergide, methiothepin, metergoline or spiperone but not tropisetron, renzapride or ritanserin was effective in reversing 5-HT-induced inhibition of acid secretion. Close i.a. administration of the potent 5-HT1A/1B antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine displayed partial agonist properties in this model, but did not antagonize 5-HT-induced inhibition of acid output. In a study of 5-HT agonists given close i.a. to the gastric circulation, 5-HT (0.88 mumol/kg) inhibited acid secretion by 48%. A 3-fold higher dose (2.6 mumol/kg) of the general 5-HT1 agonist, 5-carboxamidotryptamine (5-CT), was needed to inhibit acid secretion significantly. In contrast, neither the selective 5-HT1A agonist (+-)-8-hydroxy-2-(n-dipropylamino)-tetralin nor 5-HT2 agonist (+-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (0.88 or 2.6 mumol/kg) attenuated gastric acid secretion. Thus, the data suggest that the site mediating inhibition of acid secretion by exogenous 5-HT belongs to the 5-HT1 family, but may not be of the 5-HT1A subtype.

Intracisternal neutral endopeptidase-24.11 inhibitors produce inhibition in gastric acid output: independence from opiate, bombesin, or neurotensin-mediated mechanisms.

Intracisternal (ic) injection of the neutral endopeptidase-24.11 inhibitor phosphoramidon (1-100 nmol) produced a dose-dependent inhibition of gastric acid secretion in 2-h pylorus-ligated rats. The response resulted from a reduction in acid concentration and volume. Likewise, ic injection of another neutral endopeptidase-24.11 inhibitor Zincov (200 nmol) produced a 63% inhibition in gastric acid output. In contrast, neither intravenous injection of phosphoramidon (100 nmol) nor ic injection of the aminopeptidase inhibitor amastatin (100 nmol) produced any change in gastric acid secretion. The inhibitory effect of ic phosphoramidon (10 nmol) was not reversed by a dose of naloxone sufficient to antagonize the acid inhibitory effects of ic [D-Ala2-D-met5]enkephalinamide (8.5 nmol). Moreover, phosphoramidon-induced inhibition of acid was not reduced by the centrally effective bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 or by reserpine pretreatment at a dose effective to antagonize ic neurotensin-induced inhibition in acid secretion. These results suggest that an endogenous neutral endopeptidase-24.11 sensitive substrate may act in the brain to inhibit gastric acid output by mechanisms independent of CNS opiate, bombesin or neurotensin activity.

Fluoxetine pretreatment potentiates intracisternal TRH analogue-stimulated gastric acid secretion in rats.

Central injection of TRH or its metabolically stable analogue RX 77368 has been demonstrated to produce a vagal-dependent stimulation in gastric acid secretion. Accumulating evidence exists regarding the interaction of serotonin (5HT) with TRH containing neuronal systems. This study was performed to assess the effect of pretreatment with the 5HT uptake inhibitor fluoxetine on the TRH analogue-induced gastric acid secretory response. Systemic fluoxetine (30 mumol/kg, i.v.) produced a 43-85% increase in the intracisternal RX 77368 (78-780 pmol)-induced gastric acid output, while not affecting the basal acid response. The acid response to a lower dose of RX 77368 (26 pmol) was not altered. In addition, intracisternal fluoxetine (180 nmol) produced a 71% augmentation of the acid secretory response of i.c. RX 77368 (260 pmol). Intracisternal injection of lower doses (60, 120 nmol), or intravenous injection of 180 nmol of fluoxetine was ineffective in altering the intracisternal RX 77368-induced acid response. Pretreatment with the noradrenergic or dopaminergic uptake inhibitor desipramine or GBR 12909 did not alter the RX 77368-stimulated gastric acid secretory response. The results show that fluoxetine pretreatment potentiates the effect of intracisternal RX 77368 on acid secretion. The effect appears to be impulse dependent, and central sites of action are involved. The data suggest an interaction of synaptic serotonin with a RX 77368-elicited event (activation of TRH receptors, second messenger systems and/or firing of the motor vagus) results in potentiation of the RX 77368-induced gastric response.