Vasopressin Acts in the Subfornical Organ to Decrease Blood Pressure

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Abstract
In addition to its traditional role as a circulating vasoactive peptide, vasopressin (VP) has been shown to play significant roles in central cardiovascular processing. The recent description of VP receptors within the subfomical organ (SFO) has suggested this circumventricular organ (CVO) as a potential locus for feedback actions of circulating VP on the brain. The well-established anatomical connections between SFO 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 SFO of urethane anesthetized rats. Micro injection (0.5 µl) of 5 pmol VP into SFO resulted in significant decreases in blood pressure (BP, mean AUC-638.3 ± 110.3 mm Hg·s, p 0.05, n = 12), effects which were repeatable. These depressor effects were specific to microinjection locations within this CVO as similar VP microinjections into 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 microinjection (5 pmol in 0.5 µl) into the third ventricle produced large increases in BP (mean AUC 1,461.8 ± 368.97 mm Hg·s, p 0.05, n = 6). The hypotensive effects observed in response to VP microinjection into SFO were abolished by systemic treatment with a V1 receptor antagonist (mean AUC 89.5 ± 67.7 mm Hg·s, p > 0.05) compared to BP response before V1 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 descending autonomic pathways involved in cardiovascular control.

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