The syndrome of inappropriate antidiuretic hormone (SIADH) is a disease characterized by hyponatremia and hyperosmolarity of urine where vasopressin and angiotensin II are implicated in the alteration of salt water balance and cardiovascular and blood pressure regulation. The aim of this study is to analyse the expression of substances related with cardiovascular and salt water regulation in the subfornical organ in a case of SIADH. Two brains, one taken from a 66-year-old man with SIADH and the other from a 63-year-old man without SIADH, were used. Immunohistochemical study was performed using anti-angiotensin II, anti-vasopressin, and anti-collagen-VI as primary antibodies. Angiotensin and vasopressin immunoreaction were found in neurons, in perivascular spaces, and in the ependymal layer in the subfornical organ in both cases. However, in the SIADH case, the angiotensin II and collagen-IV expression in the SFO were different suggesting this organ’s possible participation in the physiopathology of SIADH. 1. Introduction Diabetes insipidus (DI), syndrome of inappropriate antidiuretic hormone (SIADH), and syndrome cerebral salt-wasting (CSW) are three pathologies with hyponatremia and hyperosmolarity and differentiation between them is important to prescribe the most appropriate treatments [1, 2]. SIADH is a disease which is characterized by the hyponatremia and hyperosmolarity of urine [3–5]. There are well known causes for this syndrome, such as neoplasmatic processes, disorders of the central nervous system, lung diseases, and the side effects of drugs. A study [6] of a large group of patients has revealed that SIADH occurs in 3% of patients with head and neck cancer, in 0.7% of patients with non-small-cell lung cancer, and in 15% of cases of small-cell lung cancer [6]. The standard therapy for SIADH is to treat the underlying malignant disease. If this is not possible or if the disease has become refractory, other treatment methods are available such as water restriction, demeclocycline therapy, or, in severe cases, infusion of hypertonic saline together with furosemide during treatment [6]. Total body water and tonicity are strictly regulated by the renal action of the antidiuretic hormone (ADH), renin-angiotensin-aldosterone system, and norepinephrine and by the thirst mechanism. Abnormalities in water balance are manifested in SIADH as sodium disturbances-hyponatremia and hypernatremia [6]. On the other hand, the presence of VAS, AGII, and TH and their implication in cardiovascular, salt water balance and blood pressure regulation have long been
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