INTERACTION OF CIRCULATING HORMONES WITH THE BRAIN - THE ROLES OF THESUBFORNICAL ORGAN AND THE ORGANUM VASCULOSUM OF THE LAMINA TERMINALIS

1. Most circulating peptide hormones are excluded from much of the bra in by the blood-brain barrier, However, they do have access to the cir cumventricular organs (CVO), which lack the blood-brain barrier. Three of the CVO, the subfornical organ (SFO), organum vasculosum of the la mina terminalis (OVLT) and area postrema, contain neurons responsive t o peptides such as angiotensin II (AngII), atrial natriuretic peptide and relaxin. 2. We have studied the patterns of neuronal activation, a s shown by Fos expression, in the SFO and OVLT in response to systemic ally infused AngII, relaxin or hypertonic saline and have found subgro ups of neurons activated by the different stimuli. 3. Systemic infusio n of relaxin or hypertonic saline activated neurons almost exclusively in the outer regions of the SFO and in the dorsal cap of the OVLT, Ma ny of these neurons send axonal projections to regions of the brain su bserving vasopressin secretion and thirst, such as the median preoptic , supraoptic and hypothalamic paraventricular nuclei. 4. At moderate b lood concentrations, AngII only stimulates neurons in the inner core o f the SFO and lateral regions of the OVLT, Higher levels of AngII in t he bloodstream activate additional neurons in the outer parts of the S FO that connect to the supraoptic, paraventricular and median preoptic nuclei and these probably mediate water drinking and vasopressin secr etion induced by blood-borne AngII, The efferent connections and the f unctions mediated by angiotensin-sensitive neurons in the inner core o f the SFO and lateral part of the OVLT are unknown.