Vasopressin produces inhibition on phrenic nerve activity and apnea through V _1A receptors in the area postrema in rats

The area postrema (AP) is the most caudal circumventricular organ in the central nervous system and contains arginine vasopressin (AVP) receptors. To investigate that AVP receptors in the AP might participate in the modulation of respiration, the adult rat was anesthetized with urethane (1.2 g/kg, i.p.), paralyzed, ventilated artificially, and maintained at normocapnia in hyperoxia. The phrenic nerve was separated at C ₄ level. Phrenic burst was amplified, filtered, integrated, and then stored in the hard disc via the PowerLab system. Three doses of AVP and an AVP V _1A receptor antagonist, [β-mercapto-β,β-cyclopentamethylenepropionyl ¹ ,-O-Me-Tyr ² ,Arg ⁸ ]-vasopressin, were microinjected into the AP through a pair of microelectrodes. The moderate and high doses of AVP reduced the PNA to 72 % and 45% of the control (P < 0.05), extended the mean T _E from 1.4 s before AVP to 4.0 s and 7.6 s, (P < 0.05), and decrease in BP by 26 and 37 mmHg (P < 0.05), respectively. These significant reductions in PNA and BP and elongation of TE were totally abolished by the pre-treatment of the AVP V _1A receptor antagonist and by application of lidocaine or CoCl ₂ at the nucleus tractus solitarius (NTS). Moreover, pulmonary inhibition caused by AVP was significantly attenuated by hypercapnia. These results strongly suggest that AVP V _1A receptors in the AP may participate in the modulation of cardiopulmonary functions through the activation of V _1A receptors and the pathway connected to the NTS. They may also indicate that a putative vasopressinergic pathway has a projection to the AP to alter the excitability of neurons having AVP V _1A receptors and results in an inhibition of cardiopulmonary functions via the connection between the AP and NTS.