VASOPRESSIN ACTS IN THE SUBFORNICAL ORGAN TO DECREASE BLOOD-PRESSURE

In addition to its traditional role as a circulating vasoactive peptid e, vasopressin (VP) has been shown to play significant roles in centra l cardiovascular processing. The recent description of VP receptors wi thin the subfornical organ (SFO) has suggested this circumventricular organ (CVO) as a potential locus for feedback actions of circulating V P on the brain. The well-established anatomical connections between SF O and hypothalamic autonomic control centers provide further arguments in support of such a view. This study was undertaken to determine the physiological consequences of activation of VP receptors within the S FO of urethane anesthetized rats. Microinjection (0.5 mu l) of 5 pmol VP into SFO resulted in significant decreases in blood pressure (BP, m ean AUC -638.3 +/- 110.3 mm Hg.s, p < 0.01, n = 13) without a change i n heart rate (HR, mean AUC 7.9 +/- 14.0 beats, p > 0.05, n = 12), effe cts which were repeatable. These depressor effects were specific to mi croinjection locations within this CVO as similar VP microinjections i nto non-SFO tissue were without effect on BP (mean AUC 245.4 +/- 111.5 mm Hg.s, p > 0.05, n = 10), or HR (mean AUC 1.8 +/- 3.1 beats, p > 0. 05, n = 9). In contrast to the former depressor effects, VP microinjec tion (5 pmol in 0.5 mu l) into the third ventricle produced large incr eases in BP (mean AUC 1,461.8 +/- 368.97 mm Hg.s, p < 0.05, n = 6) aga in with no change in HR (mean AUC 1.4 +/- 5.96 beats, p > 0.05, n = 6) . The hypotensive effects observed in response to VP microinjection in to SFO were abolished by systemic treatment with a V-1 receptor antago nist (mean AUC 89.5 +/- 67.7 mm Hg.s, p > 0.05) compared to BP respons e before V-1 receptor blockade (mean AUC -605.9 +/- 119.8 mm Hg.s, n = 4). These results suggest that the SFO may be an essential structure in the feedback control loop through which circulating VP influences d escending autonomic pathways involved in cardiovascular control.