Water permeability of capillaries in the subfornical organ of rats determined by Gd-DTPA^<2-> enhanced ^1H magnetic resonance imaging

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The water permeability of capillaries in the subfomical organ (SFO) of rat was measured by a ^1H nuclear magnetic resonance method in combination with a venous injection of a relaxation reagent: gadolinium-diethylene triamine-N,N,N',N",N"-pentaacetic acid (Gd-DTPA^2), which could not pass through the blood-brain barrier (BBB). Judging from results of Gd-DTPA^2 dose dependency in the intact brain and the BBB permeabilized brain, Gd-DTPA^2 could not have leaked out from the capillaries in the cortex,thalamus or SFO, but it could have been extravasated in the posterior lobe of the pituitary gland. The longitudinal (T_1) relaxation time of water in the SFO region was measured by inversion-recovery magnetic resonance imaging at 4.7 T. The T_1 relaxation rates (1/T_1) before and after Gd-DTPA^2 infusion were 0.70 ^^+__-0.02s^<-1> (mean ^^+__- SEM, n=9) and 1.53 ^^+__- 0.11s^<-1> (n=9), respecitively. The rate constant for water influx to the capillaries was estimated to be 0.84 ^^+__- 0.11s^<-1> (n=9) which corresponds with a diffusive membrane permeability (Pd) of 3.7 x 10^<-3> cm s^<-1>. Compared with values found in the literature available on this subject, this Pd value of the capillaries in SFO was the same order as that of transmembrane permeability of water of the vasa recta, and it may be 10 - 100 times larger than that of the blood-brain barrier in the cortex. Areas of the cortex and thalamus showed minimal changes in T_1 relaxation rate (ca. 0.09s^<-1>), but these were not statistically significant, which corresponds to Pd values much smaller than that found in the SFO. From these results, we conclude that the capillaries in SFO have one of the highest water permeability values among all of the capillaries in SFO have one of the highest water permeability values among all of the capillaries in the brain. It is also suggested that this magnetic resonance imaging, based on T_1 relaxation rate, is a useful method to detect local water permeability in situ