Circulating relaxin acts on subfornical organ neurons to stimulate water drinking in the rat

Abstract

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...

Loss of Vitamin D Receptor Produces Polyuria by Increasing Thirst

“…Indeed, immunostaining with AngII-specific antibody revealed markedly increased AngII staining in the MHb (Figure 6Dd) and hypothalamic PVN (Figure 6Df) regions, as well as in the SFO ( Figure 6Db) of VDR(Ϫ/Ϫ) mice. Both PVN and SFO are known to be involved in regulation of water drinking 14 ; therefore, in addition to the systemic AngII, the brain-derived AngII most likely plays a critical role in the induction of water intake in VDR(Ϫ/Ϫ) mice.…”

“…Previous studies demonstrated an association of increased c-Fos expression in the SFO and hypothalamic PVN neurons with stimulation of water drinking. 14 Moreover, the SFO is long known to be an important region in the brain to control water drinking in response to AngII. 15,16 To link AngII overproduction to overdrinking in VDR(Ϫ/Ϫ) mice, we examined c-Fos expression in the brain.…”

“…Interestingly, in VDR(Ϫ/Ϫ) mice, c-Fos expression, which indicates activation of neurons, is also increased in the PVN, a region known to be involved in regulation of water drinking. 14,26 Because PVN is located inside the blood-brain barrier, the PVN neurons are likely activated by AngII locally generated inside the brain. One interesting finding of this study is the upregulation of renin in the brain of VDR(Ϫ/Ϫ) mice, indicating that vitamin D regulation of renin expression is not limited to the kidney.…”

Relaxin increases sympathetic nerve activity and activates spinally projecting neurons in the paraventricular nucleus of nonpregnant, but not pregnant, rats

“…Previous studies found that acute intracerebroventricular (icv) (33) or intravenous (iv) administration of RLX to rats (59) increased Fos-immunoreactivity (IR) in neurons in the SFO, organum vasculosum of the lamina terminalis, supraoptic nucleus (SON), and paraventricular nucleus of the hypothalamus (PVN), consistent with RLX-induced activation of forebrain volume regulation pathways. Some of the activated (Fos-IR) magnocellular neurons in the SON and PVN were also immunopositive for AVP or oxytocin, although the extent of activation was not quantified (33).…”

“…RLX has been shown to activate SFO neurons that project to the PVN and SON (59,60). Importantly, the pressor response, which is often seen following acute administration of relatively high doses of RLX, is attenuated by lesion of the SFO (37) or iv administration of a V1 vasopressin receptor antagonist (40).…”