Negligible role of catecholaminergic receptors in the anteroventral third ventricular region in mediating vasopressin-releasing and cardiovascular actions of prostaglandin E2.
The aim of this study was to pursue the roles of the catecholamine receptors in the anteroventral third ventricular region (AV3V), a cerebral site engaged in various stress responses, in prostaglandin (PG) E2-evoked vasopressin (AVP) release and cardiovascular action. Experiments were conducted in conscious rats in which cerebral and vascular cannulae had been implanted chronically. Local infusion (0.5 microliter, 1 min) of dopamine (150 nmol), a D1-dopaminergic agonist SKF 38393 (17 nmol) and an alpha-adrenergic agonist phenylephrine (150 nmol), as well as PGE2 (7 nmol), into the AV3V enhanced plasma AVP 5 min later, without affecting plasma osmolality and electrolytes. In contrast to the increases in both arterial pressure and heart rate observed when PGE2 was applied, dopamine and SKF 38393 did not affect these variables, and phenylephrine elevated only arterial pressure. The AV3V infusion of a beta-agonist isoproterenol (100 nmol) did not change plasma AVP, although it decreased arterial pressure and increased heart rate. The increase in plasma AVP by dopamine was not blocked by the preinfusion of the D2-antagonist sulpiride (13 nmol) into the AV3V 10 min before, but was abolished by that of the D1-antagonist SCH-23390 (8 nmol). The effects of phenylephrine on both plasma AVP and the blood pressure were prevented by the preadministration of the alpha-antagonist phenoxybenzamine (13 nmol). However, the pretreatments with phenoxybenzamine, sulpiride or SCH 23390 did not inhibit the responses of AVP, arterial pressure and heart rate caused by PGE2. These antagonists were without significant effect on AVP and other variables when given alone. The infusion sites of PGE2 and the other drugs identified histologically included the AV3V structures such as the organum vasculosum laminae terminalis or its vicinity, median preoptic nucleus, medial preoptic nucleus and periventricular hypothalamic nucleus. Dopamine or phenylephrine administered into the cerebral ventricle at the same dose as used in the AV3V application did not exert a significant effect on plasma AVP, arterial pressure and heart rate. These results suggest that catecholamine receptors in the AV3V may not be involved in the AVP-secreting, tachycardiac and pressor responses evoked by topical action of PGE2 on this area, despite their ability to influence hormone release and cardiovascular function.