Role Of NMDA And Non-NMDA Receptors In The Nucleus Tractus Solitarius Of The Rabbit Following The Acute Hypoxia

Objective By microinjecting N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (AP-5) and non-NMDA receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (DNQX) into the nucleus tractus solitarius (nTS), we try to observe the effects of the two kinds of antagonists on the respiration excitation caused by acute hypoxia, and then to investigate the role of NMDA and non-NMDA receptors in peripheral chemosensitive afferent inputs. Meanwhile, we employed several immunohistochemical methods in our experiments in order to explore the function of tyrosine kinase pathway in the intracellular signaling transmission after NMDA and non-NMDA receptors on nTS neurons have been activated by acute hypoxia.Methods 1. Electrophysiological experiment: The experiment was conducted on 45 healthy adult rabbits and 30 results were available. Experimental rabbits were divided into five groups at random as follow, and rabbits in different experimental groups received microinjection of different substance into nTS: Group A(n=6): microinjecting physiological saline solution and DMSO; Group B(n=6): microinjecting AP-5; Group C(n=6): microinjecting DNQX; Group D(n=6): microinjecting both AP-5 and DNQX; Group E(n=6): no microinjection.Phrenic nerve were separated to recorded the respiratory frequency, integral amplitude of the discharge of phrenic nerve, and inspiratory time of the rabbits in acute hypoxic circumstance. All rabbits' phrenic nerve were recorded before microinjection as Group F(n=30). These records were used to investigate the effects of each experimental factor. Comparisons of these records were also made among different groups to find the difference of effects caused by different experimental factors.2. Immunohistochemical experiment: As the same grouping method in electrophysiological experiment, 30 healthy adult rabbits were divided into five groups, from Group A to Group E. Rabbits in each group were killed rapidly after exposed in the circumstance of acute hypoxia. The brain stem tissue were then fixed in 10% formalin PBS solution. In this part, the level of the expression of tyrosine phosphorylated Src tyrosine kinase in the brain stem tissue under acute hypoxic circumstance were selected as observational indexes. By using of the immunohistochemical SP method, we observed and compared the different level of tyrosine phosphorylated Src tyrosine kinase in the brain stem tissue in different group.Results 1. Results in electrophysiological experiment have shown that both AP-5 and DNQX can depress the effects of hyperpnea caused by acute hypoxia. The respiratory frequency, integral amplitude of the discharge of phrenic nerve and inspiratory time under hypoxic circumstance for 60 seconds were significantly depressed by microinjecting AP-5. Only respiratory frequency and inspiratory time were influenced by microinjecting DNQX, but the integral amplitude of the discharge of phrenic nerve remained. Also, we found that the two antagonists have interactive effects on respiratory frequency and inspiratory time (P<0.01), while the interactive effects were not found on integral amplitude of the discharge of phrenic nerve (P>0.05). 2. Results in immunohistochemical experiment have shown that there are different level of expression of tyrosine phosphorylated Src tyrosine kinasemasculine neurons in nTS in different group. The strongest expression were foundin Group A, followed by Group C, Group B, Group D, and Group E. Thecooperative effects between the two antagonists, AP-5 and DNQX, weresignificantly higher than their separate effects (P<0.01).Conclusion 1. NMDA and non-NMDA receptors located in the nTS could exertboth separate effects and a cooperative influence on the peripheralchemosensitive afferent inputs caused by acute hypoxia, and NMDA receptorplays a more significant role.2. The tyrosine kinase pathway may have an important effect in the nTSneurons' intracellular signaling pathways after NMDA receptor and non-NMDAreceptor have been activated by acute hypoxia. But we are not sure aboutwhether there still exist other pathways in the intracellular signalingtransmission of nTS under hypoxic circumstance.Significances The present study will contribute to further identifying theimportant role of different receptor in the signaling pathway from peripheralchemoreceptors to respiratory nervous centre in acute hypoxia. It will also helpto investigate the effects of tyrosine kinase pathway in intracellular signalingtransmission of nTS under hypoxic circumstance.