| Summary: | 博士 === 國立臺灣師範大學 === 生命科學研究所 === 96 === The area postrema (AP) is located at the dorsal surface of the medulla, lacks of blood-brain barrier, and have abundant AVP receptors. AVP-induced activation of neurons in the AP has been demonstrated to attenuate the exercise-evoked pressor effect. Microinjection of AVP into the ventrolateral medulla can produce inhibition on respiration and modulation on cardiovascular functions. The present study was aimed to evaluate whether activation of AVP receptors in the AP could modulate cardiopulmonary functions and to examine the mechanism. Male Wistar rats were anesthetized with urethane (1.2 g/kg, i.p.). The femoral artery and vein were catheterized for monitoring blood pressure (BP) and drug administration. Tracheostomy, bilateral cervical vagotomy, paralyzation, and artificial ventilation were performed. The rat was then placed in a stereotaxic instrument with a prone position. The phrenic nerve was separated and its activity (PNA) was monitored at normocapnia in hyperoxia and hypercapnia if needed. Microinjection of various doses of AVP (1.5×10-5, 3.0×10-5 and 4.5×10-5 IU) into the AP was performed. There were four projects completed in the present dissertation to determine that AVP microinjection into the AP could produce inhibition on the PNA and BP via the V1A receptor and that a putative signaling pathway from PLC-DAG-PKC to inactivation of potassium channels and in turn to activate the voltage-gated calcium channels might have been activated. In the first project, the microinjection of AVP into the AP produced a dose-dependent inhibition on the PNA reflecting a decrease of phrenic amplitude and an elongation of expiratory period (TE) immediately and also a decrease in BP. The cardiopulmonary inhibition caused by AVP was totally abolished by the pretreatment of AVP V1A receptor antagonist. Results obtained from the second project showed that this cardiopulmonary modulation induced by AVP could be completely reversed by a pretreatment of lidocaine and/or CoCl2 at the both sides of the nucleus tractus solitarius (NTS), suggesting that AVP-producing cardiopulmonary modulation might be mediated through a neural pathway projecting from the AP to the NTS. Moreover, respiratory inhibition evoked by AVP was significantly attenuated by hypercapnia. These results strongly suggest that AVP V1A receptors in the AP may participate in the modulation of cardiopulmonary functions through the activation of V1A receptors and the pathway connected to the NTS.
The third project was designed to search the putative signal transduction pathway for AVP-induced decrease in blood pressure. To elucidate this putative signaling pathway, response of the BP to calcium influx into the AP neurons caused by AVP was examined in adult Wistar rats. In the third project, we firstly demonstrated that hypotension induced by AVP was totally abolished by V1A antagonist, U73122 (phospholipase C blocker), and by BAPTA-AM (Ca++ chelator), suggesting that an increasing intracellular Ca++ is essential for AVP-induced hypotension. We then confirmed that this hypotension induced by AVP was completely abolished by EGTA (extracellular Ca++ chelator) and various Ca++ blockers such as nifedipine, nimodipine (L-types Ca++ blockers), and omega-conotoxin MVIIC (P/Q type Ca++ blocker), but not by omega-conotoxin GVIA (N-type Ca++ blocker). Finally, we verified that AVP-induced hypotension was blocked by calphostin C (protein kinase C inhibitor) and mimicked by an increase in intracellular K+ ions that was also reversed by EGTA, suggesting that Ca++ influx through voltage-gated calcium channels is essential for AVP-producing hypotension. The fourth project was aimed to confirm that Ca++ influx was specific for AVP-induced hypotension. This conclusion was based on the results that glutamate-induced hypotension was reversed by BAPTA-AM but not by EGTA or V1A antagonist. These results suggest that AVP-induced hypotension depends on Ca++ influx through a PLC-DAG-PKC signal pathway to inactivate K+ channels that may depolarize AP neurons to activate L- and P/Q-type Ca++ channels. It may also provide new insights into establishing a relationship between the signal pathway and physiological functions.
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