Effects And Mechanisms Of Nucleus Ambiguus On Gastric Motility In Rats

Numerous morphologic and physiological studies indicate that the vagal parasympathetic preganglionic neurons innervating the stomach are largely located in dorsal motor nucleus of the vagus (DMV) and partly in the nucleus ambiguus (NA). This suggests the NA is involved in the modulation of gastric functions. However, there are no unanimous standpoints about the effects of NA on the gastric motility up to now. To clarify the effects of excitation of NA on gastric motility, our research group electrically stimulated the NA in the anterior work and found that electrical stimulation of NA inhibited gastric motility significantly and the vagotomy beneath the diaphragm abolished the inhibitory response in rats. Was the result of electrical stimulation due to exciting neuron bodies or the passed fibres? To illuminate this question, we microinjected of L-Glutamate (L-Glu), a neuron body incitant, into NA to further investigate the effect of exciting NA on gastric motility in this paper. Our primary results showed that L-Glu microinjected into the right NA significantly inhibited gastric motility. Since the electrical stimulation of NA inhibited the gastric motility by the vagus nerves, is the vagal pathway involved in this response in rats either? Did microinjection of L-Glu into NA still inhibit gastric motility significantly in the state of stomach contracting strongly. Acetylcholine (ACh) is a neurotransmitter of modulating contraction of gastric smooth muscles. So we further explored the effect and the pathway of microinjection of L-Glu into NA on gastric motility in the state of stomach contracting strongly with intraperitoneal injection ACh.L-Glu is a major neurotransmitter of the mammalian central nervous system. It activates two families of receptors: the metabotropic and ionotropic receptors. The ionotropic glutamate receptors are nonselective cation permeable receptor channels that are classified in two subtypes according to their most selective agonist: N-methyl-D-aspartate (NMDA) and a-amino-3-hydroxy-5-methyl-isoxazole-4-proprionate (AMPA)/kainite (non-NMDA). The Glu-mediated excitability transfer among the central neurons is via the NMDA and AMPA receptors. L-Glu in the NA also regulates respiration, heart rate, oesophagus through NMDA and non-NMDA glutamate receptors. These reports clue to us whether the effect of L-Glu microinjected into NA on gastric motility was also through NMDA and AMPA glutamate receptors in rats. L-Glu caused gastric contraction when it was injected into the rostral part of the DMV and relaxation when it was injected into the caudal part of the DMV by activating cholinergic preganglionic neurons in the DMV. The result suggested whether the effect of L-Glu microinjected into NA on gastric motility was also via activating NA cholinergic preganglionic neurons.Nitric oxide (NO) is a nonadrenergic-noncholinergic (NANC) neurotransmitter in the vagus nerve mediating relaxation of the gastrointestinal tract. NO is present in and released from NANC nerves and mediates gastrointestinal relaxation following vagal stimulation. NOS positive neurones and processes were seen in the NA. Endogenous NO may reinforce the output activity of the medullary respiratory network, modulate heart rate by vagus nerves, modulate activity of oesophagus and pharynx. According to these reports we assume whether NO within NA modulates gastrointestinal motility. Is the vagal pathway also involved in the modulation in rats? Our primary results showed that microinjection of sodium nitroprusside (SNP) and L-arginine (L-Arg) into the right NA respectively significantly inhibited gastric motility via vagally mediated pathways. In the previous study we found that microinjection of L-Glu into the right NA significantly inhibited gastric motility by activating the cholinergic preganglionic neurons in the NA. Whether was the effect of NO in the NA on gastric motility also via activating cholinergic preganglionic neurons within NA?We have reported that both L-Glu and NO participated in the regulation of gastric relaxation in the right NA. Whether is there some correlation between the L-Glu and NO? More and more evidences indicate the coupling of NMDA receptor activation with the synthesis of NO. Microinjection of L-Glu into dorsal motor nucleus of the vagus excites gallbladder motility through NMDA receptor-nitric oxide-cGMP pathway. There is a close relationship in humans between gallbladder motility and gastrointestinal motility during the fasting state, as well as in the postprandial period. These reports illume us whether microinjection of L-Glu into NA inhibits gastric motility through NMDA receptor-nitric oxide pathway. Therefore, we explored all the questions in this paper.A latex balloon connected with a pressure transducer was inserted into the pylorus through the forestomach for continuous recording of the gastric motility. The amplitude, duration, frequency, and motility index of gastric contraction waves within 5 minutes before microinjection and after microinjection were measured. In the intraperitoneal injection ACh group, frequency, amplitude, and average resting intragastric pressure of gastric contraction waves within 3 minutes before microinjection and after microinjection were measured.The results showed that L-Glu (5 nmol, 10 nmol and 20 nmol) microinjected into the right NA significantly inhibited gastric motility in a dose-dependent manner, however, microinjection of physiological saline (PS) at the same position and the same volume did not initiate any change of the gastric motility. Bilateral subdiaphragmatic vagotomy abolished the inhibitory effect of microinjection of L-Glu into NA on gastric motility. Microinjection of L-Glu into NA still significantly inhibited gastric motility by vagus nerves with intraperitoneal injection ACh in advance. The pretreatment of D-AP5, the specific NMDA receptor antagonist completely abolished the inhibitory effect of L-Glu on gastric motility. But the pretreatment of CNQX, the non-NMDA receptor antagonist, did not change the inhibitory effect of L-Glu on gastric motility. The pretreatment of intravenous injection hexamethonium bromide abolished the inhibitory effect of L-Glu on gastric motility too.Microinjection of SNP and L-Arg into the right NA significantly inhibited gastric motility by vagus nerves, respectively. However, microinjection of L-NAME, the inhibitor of nitric oxide synthase, into the right NA significantly enhanced gastric motility. The pretreatment of intravenous injection hexamethonium bromide abolished the inhibitory effect of SNP on gastric motility too.The pretreatment of L-NAME did not abolish the inhibitory effect of microinjection of L-Glu into the right NA on gastric motility completely, but the inhibitory effect was lower than that in the Glu group. We also found that the pretreatment of D-AP5 completely abolished the inhibitory effect of SNP on gastric motility.In conclusion, the data of these experiments suggested that microinjection of L-Glu into the right NA inhibited gastric motility through specific NMDA receptor activating the cholinergic preganglionic neurons in the NA and the efferent pathway was the vagal nerves. NO inhibited gastric motility by activating the cholinergic preganglionic neurons including NMDA receptor in the NA and the inhibitory effect was mediated by vagus nerves. Microinjection of L-Glu into NA inhibited gastric motility through NMDA receptor-nitric oxide pathway minorly.