CIRCULATING VASOPRESSIN INFLUENCES AREA POSTREMA NEURONS

Extracellular single-unit recordings were obtained from 107 area postrema and 74 nucleus tractus solitarius neurons in sodium pentobarbitol anaesthetized rats. Systemic administration of vasopressin (1-10 ng) decreased the firing frequency of 45.8% of area postrema neurons and 58.1% of nucleus tractus solitarius neurons tested while the firing frequency of 38.3% of area postrema neurons and 21.6% of nucleus tractus solitarius neurons was increased by this peptide. To determine whether these neurons were specifically influenced by vasopressin or the accompanying 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-adrenergic agonists were classified as ''blood pressure-sensitive'', whereas those neurons that responded differently to both agents were classified as ''vasopressin-sensitive'' neurons. The majority (85.2%) of area postrema cells that decreased firing frequency in response to vasopressin were determined to be ''vasopressin-sensitive'', while 68.8% of area postrema neurons responding to vasopressin with increases in firing frequency were classified as ''blood pressure-sensitive''. In contrast, 78.6% of nucleus tractus solitarius neurons that decreased firing frequency in response to vasopressin and 55.5% of those that increased firing frequency were classified as ''blood pressure-sensitive'' neurons. To determine whether the actions of vasopressin in the area postrema were mediated by V-1 receptors the effect of vasopressin after V-1 receptor blockade was examined in seven ''vasopressin-sensitive'' area postrema neurons. All seven neurons tested showed no response to vasopressin after such V-1 receptor blockade. These data suggest that there exists a population of area postrema neurons specifically responsive to circulating vasopressin as a result of actions of this peptide at V-1 receptors. They also implicate these neurons in the physiological mechanisms through which circulating vasopressin acts in the area postrema to influence baroreceptor reflex sensitivity.