VASOPRESSIN ACTIONS ON AREA POSTREMA NEURONS IN-VITRO

The area postrema (AP) is a circumventricular organ located on the dor sal surface of the medulla. Substantial evidence suggests that the AP is an important site involved in cardiovascular regulation. Arginine v asopressin (AVP) is thought to act at the AP to increase the sensitivi ty of the baroreceptor reflex. We have therefore examined the effects of AVP on AP neurons with the use of extracellular single unit recordi ngs in vitro. Coronal medullary brain slices (thickness = 400 mu m) we re obtained from male Sprague-Dawley rats and maintained in oxygenated artificial cerebrospinal fluid (aCSF). The slices were perfused with AVP (10(-8) to 10(-6) M), and the effect on single AP neurons was reco rded. A total of 79 AP neurons was tested of which 50 (63.39%) were ex cited by AVP and 5 (6.3%) were inhibited, whereas the remaining 24 (30 .3%) cells were unaffected. The excitatory effects of AVP were dose de pendent: firing rate increased 92.6 +/- 25.8% at 10(-8) M, 289.4 +/- 5 3.9% at 10(-7) M, and 456.8 +/- 113.1% at 10(-6) M, respectively. We a lso examined whether these effects of AVP resulted from direct actions of this peptide on AP cells by testing if responses were retained dur ing blockade of synaptic transmission (achieved by perfusion with a lo w Ca2+-high Mg2+ aCSF) in 11 cells excited by AVP. Nine of these cells were excited by AVP during such synaptic blockade. Finally, we demons trated that the excitatory responses of five AP cells to AVP were all totally abolished by perfusion of slices with aCSF containing the V-1 antagonist {[1-beta-mercapto-beta,beta-cyclopentamethylene propionic a cid,2-(O-methyl)tyrosine]-Arg(8)-vasopressin; Peninsula Laboratories, 10(-6) M}. In contrast to previous in vivo studies reporting both exci tatory and inhibitory effects of AVP on AP cells, these data demonstra te that the predominant direct effect of AVP on AP neurons in vitro is a V-1 receptor-mediated excitatory action.