Electrophysiology of the circumventricular organs.

Since the first anatomical description of the circumventricular organs (CVOs) as a structurally distinct group of regions in the central nervous system (CNS), considerable information has implicated these structures as physiologically significant autonomic control centers located at the blood-brain interface. Specialized features of these structures, such as their extensive vasculature, lack of the normal blood-brain barrier (BBB) (i.e., capillaries have a fenestrated endothelium), and dense aggregations of a variety of peptidergic receptors, support an involvement of the CVOs in communication between the circulation and the CNS. The two best understood examples of CVOs with the ability to sense circulating substances impermeable to the BBB are the subfornical organ (SFO) and the area postrema (AP). Specifically, the ability of numerous peptides to influence CNS function, as the result of actions on the neural substrate of these structures has been especially well documented. Considerable anatomical, biochemical, pharmacological, and physiological evidence has implicated these structures as CNS sites at which angiotensin (ANG), atrial natriuretic peptide (ANP), vasopressin (VP), and endothelin (ET) act to influence neuroendocrine and other more classical autonomic functions. In the following sections, we review neurophysiological studies which have provided new and exciting insights regarding the specific neural pathways and cellular mechanisms through which CVO neurons are able to exert their profound influences over central autonomic control.