The influence of the vagally induced rebound contractions on the non-adrenergic, non-cholinergic (NANC) inhibitory motility of the rabbit stomach and the role of prostaglandins

Acute effects of vagus nerve stimulation parameters on gastric motility assessed with magnetic resonance imaging

BACKGROUND

Vagus nerve stimulation (VNS) is an emerging bioelectronic therapy for regulating food intake and controlling gastric motility. However, the effects of different VNS parameters and polarity on postprandial gastric motility remain incompletely characterized.

METHODS

In anesthetized rats (N = 3), we applied monophasic electrical stimuli to the left cervical vagus and recorded compound nerve action potential (CNAP) as a measure of nerve response. We evaluated to what extent afferent or efferent pathway could be selectively activated by monophasic VNS. In a different group of rats (N = 13), we fed each rat a gadolinium-labeled meal and scanned the rat stomach with oral contrast-enhanced magnetic resonance imaging (MRI) while the rat was anesthetized. We evaluated the antral and pyloric motility as a function of pulse amplitude (0.13, 0.25, 0.5, 1 mA), width (0.13, 0.25, 0.5 ms), frequency (5, 10 Hz), and polarity of VNS.

KEY RESULTS

Monophasic VNS activated efferent and afferent pathways with about 67% and 82% selectivity, respectively. Primarily afferent VNS increased antral motility across a wide range of parameters. Primarily efferent VNS induced a significant decrease in antral motility as the stimulus intensity increased (R = -.93, P < .05 for 5 Hz, R = -.85, P < .05 for 10 Hz). The VNS with either polarity tended to promote pyloric motility to a greater extent given increasing stimulus intensity.

CONCLUSIONS AND INFERENCES

Monophasic VNS biased toward the afferent pathway is potentially more effective for facilitating occlusive contractions than that biased toward the efferent pathway.

Analysis of the excitatory motor response evoked by nicotinic and muscarinic blockade of ovine small bowel

It has been reported that the administration of anticholinergic drugs evokes inhibitory and excitatory responses, but the precise character of the latter has not yet been defined. This study was thus devoted to analyzing its occurrence following various doses of hexamethonium (Hx) and atropine (At) administration in the course of different phases of the small-intestinal migrating motor complex (MMC) in fasted and non-fasted sheep and to further characterize the excitatory responses in comparison with individual phases of the MMC. Two basic types of excitatory response were found. In the course of chronic experiments, various doses of Hx and At evoked rebound excitation (RE, i.e., irregular contractions or spike bursts evoked in response to the anticholinergic drug) alternating with phase 3-like activity (not the organized phase 3 of the MMC or its parts). The intensity of these changes varied and was related to the drug dose. Thus intense and non-intense RE activity were distinguished. In non-fasted sheep, these alterations were slightly less pronounced than in fasted animals. When the drug was given during phase 1 of the MMC, RE did not occur or was greatly reduced and its arrival was delayed. Hx triggered RE mostly in the duodenum, while the action of At was most effective in the jejunum. It is concluded that Hx and At initially hamper small-intestinal motility and just after that evoke a secondary stimulatory response, i.e., phase 3-like activity and RE of different intensity, duration, and repeatability in fasted and non-fasted sheep. These stimulatory effects may resemble unorganized phases of the MMC.

Area postrema glutamate receptors mediate respiratory and gastric responses in the rabbit

The role of NMDA and non-NMDA receptors of the area postrema (AP) in the control of respiration and gastric motility was investigated in anaesthetized rabbits using microinjections (10-20 nl) of specific agonists or antagonists. NMDA (20 mM) or AMPA (10 mM) caused excitatory effects on respiration and gastric relaxation. Selective blockade of NMDA or non-NMDA receptors, respectively with D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 10 mM) and 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX; 5 mM), decreased respiratory frequency and increased gastric tone. Both these effects were more marked following non-NMDA receptor blockade and were prevented by vagotomy. These findings show that NMDA and non-NMDA receptors are present on AP neurones and have a role in the tonic control of respiration and gastric motility.

Pattern of nonadrenergic, noncholinergic responses during short- or long-lasting electrical stimulation in guinea-pig ileum

1. The pattern of responses of longitudinally oriented guinea pig ileum organ bath preparations was studied during short- (1-5 sec) or long-lasting (20 sec) electrical field stimulation (EFS, 0.8 msec, 40 V, 1-20 Hz). 2. In the presence of phentolamine (5 microM), propranolol (5 microM), and atropine (3 microM), the EFS elicited nonadrenergic, noncholinergic (NANC), tetrodotoxin (0.3 microM)-sensitive responses. 3. The 1-sec EFS evoked relaxation. The response to 5-sec EFS consisted of relaxation followed by twitch, whereas relaxation, twitch and tonic contraction characterized the NANC response to 20-sec EFS. The maximum relaxation was observed at 10-Hz short- or long-lasting EFS. 4. Both N-G-nitro-L-arginine (L-NNA, 0.1-0.5 mM) and apamin (1-5 microM) concentration dependently inhibited the relaxation of the NANC response to 10-Hz 20-sec EFS. During L-NNA treatment, the twitch and the tonic contractions were increased. The inhibitory effect of L-NNA was reversed by L-arginine (0.1-0.5 mM) but not by D-arginine. Sodium nitroprusside (1-10 microM) was without effect. 5. AP 13.2 ACOH (0.1 microM), a blocker of Substance P receptors, inhibited the twitch and the tonic contractions. The contractions were decreased after desensitization of purinoceptors by ATP and in the presence of the GABA(A) receptor antagonist bicuculline (30 microM). 6. Depending on the EFS duration, a subsequent occurrence of relaxation and contractions characterized the NANC responses. It seems that relaxation is mediated by nitric oxide whereas Substance P and ATP are involved in the maintenance of the twitch and the tonic contractions. Nitric oxide appears to exert an inhibitory effect on the excitatory transmitters, whereas purinergic mechanism(s) could modulate the nitric oxide-dependent relaxation.

Contribution of acetylcholine, vasoactive intestinal polypeptide and nitric oxide to CNS-evoked vagal gastric relaxation in the rat

Several in vitro models of gastric relaxation have elucidated a role of nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) in non-adrenergic, non-cholinergic (NANC) vagally mediated gastric relaxation. However, these models do not necessarily mimic the events leading to gastric relaxation in the whole animal. We have recently described a vagally mediated gastric relaxation evoked by micro-injection of substance P (SP) into the nucleus raphe obscurus (NRO). The present study was performed to elucidate whether this CNS-stimulated in vivo gastric relaxation involved acetylcholine, NO and VIP. Atropine (1 mg kg-1 i.v.), reduces both the rapid nadir and sustained gastric relaxation evoked by SP in the NRO, and the residual responses are abolished by NG-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 10 mg kg-1 i.v.), an NO synthase inhibitor. Blockade of NO synthase alone is not sufficient to abolish the effect of SP into the NRO on intragastric pressure. A VIP antagonist, [p-chloro-D-Phe6, Leu17]VIP (32 micrograms i.v.) alone, or with the addition of L-NAME, does not affect the nadir of the gastric relaxation in response to SP microinjected into the NRO; however, both antagonists reduce the CNS-evoked sustained intragastric pressure relaxation. We conclude that, in CNS-evoked gastric relaxation, inhibition of cholinergic pathways is potentially important for both the rapid nadir and sustained gastric relaxation, and both NO and VIP contribute to sustained gastric relaxation.

Vagal control of nitric oxide and vasoactive intestinal polypeptide release in the regulation of gastric relaxation in rat

1. Gastric motility and neurotransmitter release in response to vagal stimulation were studied using a vascularly isolated perfused rat stomach. Gastric motor responses were recorded by a strain gauge force transducer implanted on the proximal stomach. 2. Electrical stimulation of vagal trunk (0.5-20 Hz) produced a triphasic response which was composed of a rapid transient relaxation (first phase) followed by a phasic contraction (second phase) and a delayed prolonged relaxation (third phase). Maximum responses of the first, second and third phase were observed at 2.5, 5 and 10 Hz, respectively. Intra-arterial infusion of tetrodotoxin (0.1 microM) or hexamethonium (100 microM) completely abolished the triphasic response. 3. The nitric oxide (NO) biosynthesis inhibitor NG-nitro-L-arginine (L-NNA; 100 microM) significantly antagonized the rapid relaxation but had no effect on the delayed relaxation, while vasoactive intestinal polypeptide (VIP) antagonist (1 microM) significantly reduced the delayed relaxation without affecting the rapid relaxation. 4. In response to vagal stimulation, NO production ([3H]citrulline formation in gastric tissue preloaded with [3H]arginine) was maximum at 2.5 Hz, whereas VIP release into the venous effluent was largest at 10 Hz. Hexamethonium abolished vagal-stimulated NO production and VIP release. L-NNA had no effect on VIP release in response to vagal stimulation. 5. The nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperizinium (DMPP; 100 microM) also caused a triphasic response similar to that observed with vagal stimulation and produced a significant increase in VIP and NO formation. DMPP-evoked VIP release was not affected by L-NNA. Similarly, DMPP-evoked NO production was not antagonized by VIP antagonist. 6. These results suggest that vagus nerve stimulation evokes NO and VIP release via nicotinic synapses which cause different modes of relaxation of the stomach. There is no interaction between NO and VIP release in response to vagal stimulation.

Nitric oxide formation is involved in vagal inhibition of the stomach of the trout (Salmo gairdneri)

Stimulation of the vagus nerves to isolated stomach preparations from trout (Salmo gairdneri) caused both excitation and inhibition of smooth muscle activity. Excitatory responses were partly reduced by atropine, suggesting that cholinergic neurons are involved. The atropine-resistant excitation could not readily be ascribed to a 'rebound' from preceding inhibition, to the production of prostaglandins, or to the activation of either adrenergic or capsaicin-sensitive fibres. Inhibitory responses were reduced by the nicotinic antagonist d-tubocurarine and more profoundly reduced by an inhibitor of nitric oxide synthase, NG-nitro-L-arginine. The latter effect was reversed by excess L-arginine. The nitric oxide donors sodium nitroprusside and nitroglycerine caused inhibition of the muscle. It is concluded that the vagal inhibitory postganglionic neurons are nitergic in this tissue.

Gastric relaxation in response to chemical stimulation of the area postrema in the rabbit

Microinjections of DL-homocysteic acid into the area postrema (AP) of anesthetized rabbits provoked gastric relaxations associated with small changes in blood pressure and marked excitatory effects on respiration. Both gastric and cardiovascular effects failed to occur after bilateral vagotomy. Comparable gastric relaxations were induced before and after treatment with atropine or atropine and guanethidine. The AP appears to play a role in gastric motility via vagus nerves and nonadrenergic noncholinergic intramural inhibitory neurons.

Role of nitric oxide and vasoactive intestinal polypeptide in vagally mediated relaxation of the gastric corpus in the anaesthetized ferret

The roles of VIP and NO in vagally mediated relaxations of the gastric corpus were investigated in the anaesthetized ferret. Intracorpus pressure was recorded manometrically during electrical stimulation of the cervical vagus nerve in three groups of animals: one control group (n = 6), one group treated with an inhibitor of NO synthesis (NG-nitro-L-arginine methyl ester (L-NAME), 1.6 mg/kg); and a third group which had been immunized, prior to the experiment, with a VIP-thyroglobulin conjugate (25 nmol equivalent) in Freund's complete adjuvant. In control animals, following treatment with atropine (100 micrograms/kg), vagal stimulation resulted in a frequency dependent fall in intracorpus pressure with the maximum response at 5 Hz of 2.2 +/- 0.3 cm H2O. Two components of the response could be observed: an initial rapid fall over the first 10 s of stimulation followed by a slower decline over the remainder of the stimulation period. In animals treated with L-NAME (n = 6) the initial rapid response was significantly reduced at all frequencies of stimulation (P < 0.05 - P < 0.005, Mann-Whitney U-test) leaving only the slower second component. In immunized animals (n = 6) the initial rapid response to vagal stimulation was not different from control but the slower second component was significantly reduced at 1 Hz (P < 0.005). We conclude that the response to vagal stimulation appears to consist of two components which can be differentiated using L-NAME and autoimmunization to VIP.