Water permeability of capillaries in the subfornical organ of rats determined by Gd-DTPA enhanced H magnetic resonance imaging

The circumventricular organs (CVOs) are structures bordering the third and fourth ventricles and are considered to be important sites for communicating between neural cells, cerebrospinal fluid and blood. CVOs have been characterized as regions of the brain with permeable capillaries, which can pass marker dyes from blood (Davson et al. 1987). This high tracer transportability of vessels in the CVOs was explained by the histological findings of the capillary endothelium, which has cytoplasmic vesicles, caveolae, pits, fenestrations and a few tight junctions (Bouchaud & Bosler, 1986; Gross, 1992). Several substances such as horseradish peroxidase and aaminoisobutyric acid (AIB) have been used as markers, and it has been revealed that the various parts of the CVOs have different permeability values. The order of permeability of AIB from highest to lowest was pineal gland > the posterior lobe of the pituitary gland > the subfornical organ (SFO), area postrema (AO), and median eminence (ME) > subcommissural organ (SCO) (Brodie et al. 1960a,b; Broadwell & Brightman, 1976; Bouchaud & Bosler, 1986; Gross et al. 1987; Gross, 1992). The SFO might be a part of the sensory system involved in body fluid homeostasis, and it might transfer information on the osmolarity of serum to neurons. However, no direct observations on water permeability in the SFO have been reported.