CIRCULATING VASOPRESSIN INFLUENCES AREA POSTREMA NEURONS

Extracellular single-unit recordings were obtained from 107 area postr ema and 74 nucleus tractus solitarius neurons in sodium pentobarbitol anaesthetized rats. Systemic administration of vasopressin (1-10 ng) d ecreased the firing frequency of 45.8% of area postrema neurons and 58 .1% of nucleus tractus solitarius neurons tested while the firing freq uency of 38.3% of area postrema neurons and 21.6% of nucleus tractus s olitarius neurons was increased by this peptide. To determine whether these neurons were specifically influenced by vasopressin or the accom panying presser response, the effects of alpha-adrenergic agonists on neuronal activity were also determined. Cells that responded similarly to vasopressin and the change in blood pressure elicited by alpha-adr energic agonists were classified as ''blood pressure-sensitive'', wher eas those neurons that responded differently to both agents were class ified as ''vasopressin-sensitive'' neurons. The majority (85.2%) of ar ea postrema cells that decreased firing frequency in response to vasop ressin were determined to be ''vasopressin-sensitive'', while 68.8% of area postrema neurons responding to vasopressin with increases in fir ing frequency were classified as ''blood pressure-sensitive''. In cont rast, 78.6% of nucleus tractus solitarius neurons that decreased firin g frequency in response to vasopressin and 55.5% of those that increas ed firing frequency were classified as ''blood pressure-sensitive'' ne urons. To determine whether the actions of vasopressin in the area pos trema were mediated by V-1 receptors the effect of vasopressin after V -1 receptor blockade was examined in seven ''vasopressin-sensitive'' a rea postrema neurons. All seven neurons tested showed no response to v asopressin after such V-1 receptor blockade.These data suggest that th ere exists a population of area postrema neurons specifically responsi ve to circulating vasopressin as a result of actions of this peptide a t V-1 receptors. They also implicate these neurons in the physiologica l mechanisms through which circulating vasopressin acts in the area po strema to influence baroreceptor reflex sensitivity.