1. Most circulating peptide hormones are excluded from much of the brain by the blood-brain barrier. However, they do have access to the circumventdcular organs (CVO), which lack the blood-brain barrier. Three of the CVO, the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT) and area postrema, contain neurons responsive to peptides such as angiotensin I1 (AngII), atrial natriuretic peptide and relaxin.2. We have studied the patterns of neuronal activation, as shown by Fos expression, in the SFO and OVLT in response to systemically infused AngII, relaxin or hypertonic saline and have found subgroups of neurons activated by the different stimuli.3. Systemic infusion of relaxin or hypertonic saline activated neurons almost exclusively in the outer regions of the SFO and in the dorsal cap of the OVLT. Many of these neurons send axonal projections to regions of the brain subserving vasopressin secretion and thirst, such as the median preoptic, supraoptic and hypothalamic paraventricular nuclei. 4. At moderate blood concentrations, AngII only stimulates neurons in the inner core of the SFO and lateral regions of the OVLT. Higher levels of AngII in the bloodstream activate additional neurons in the outer parts of the SFO that connect to the supraoptic, paraventricular and median preoptic nuclei and these probably mediate water drinking and vasopressin secretion induced by blood-borne AngII. The efferent connections and the functions mediated by angiotensin-sensitive neurons in the inner core of the SFO and lateral part of the OVLT are unknown.

Relaxin, a peptide hormone secreted by the corpus luteum during pregnancy, exerts actions on reproductive tissues such as the pubic symphysis, uterus, and cervix. It may also influence body fluid balance by actions on the brain to stimulate thirst and vasopressin secretion. We mapped the sites in the brain that are activated by i.v. infusion of a dipsogenic dose of relaxin (25 g͞h) by immunohistochemically detecting Fos expression. Relaxin administration resulted in increased Fos expression in the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus, and magnocellular neurons in the supraoptic and paraventricular nuclei. Ablation of the SFO abolished relaxininduced water drinking, but did not prevent increased Fos expression in the OVLT, supraoptic or paraventricular nuclei. Although ablation of the OVLT did not inhibit relaxin-induced drinking, it did cause a large reduction in Fos expression in the supraoptic nucleus and posterior magnocellular subdivision of the paraventricular nucleus. In vitro single-unit recording of electrical activity of neurons in isolated slices of the SFO showed that relaxin (10 ؊7 M) added to the perfusion medium caused marked and prolonged increase in neuronal activity. Most of these neurons also responded to 10 ؊7 M angiotensin II. The data indicate that bloodborne relaxin can directly stimulate neurons in the SFO to initiate water drinking. It is likely that circulating relaxin also stimulates neurons in the OVLT that influence vasopressin secretion. These two circumventricular organs that lack a blood-brain barrier may have regulatory influences on fluid balance during pregnancy in rats.R elaxin is a peptide hormone secreted by the corpus luteum of the ovary during pregnancy. Relaxin acts on reproductive tissues such as the pubic symphysis, uterus, and cervix (1, 2), but it may also influence the brain (3). Evidence of this influence is presented in reports that the intracerebroventricular (i.c.v.) (1) injection of relaxin results in stimulation of oxytocin and vasopressin secretion, water drinking, and a pressor response (3-8). i.c.v. administration of relaxin also stimulates increased expression of c-fos in groups of neurons in the supraoptic nucleus (SON) and hypothalamic paraventricular nucleus (PVN), as well as in the subfornical organ (SFO), median preoptic nucleus (MnPO), and organum vasculosum of the lamina terminalis (OVLT) (9). Circulating relaxin probably has endocrine actions on the brain, because high-affinity-binding sites for relaxin are present in the SFO and OVLT of the rat (10), two brain regions that are accessible to circulating relaxin because they lack a blood-brain barrier (11). Also, i.v. infusion of relaxin causes vasopressin secretion and water drinking in the rat, effects that may be mediated through brain angiotensinergic mechanisms (12,13). Evidence that relaxin [together with other hormones such as angiotensin II (Ang II) and vasopressin] plays a physiological role in regulating body fluid homeostasis d...