Brain water and proton magnetic resonance relaxation in preterm and term rabbit pups: their relation to tissue hyaluronan.

The present study was performed to investigate simultaneously total brain water, T1 and T2 relaxation times, and hyaluronan (HA) in fetal and neonatal rabbits. Attempts were also made to establish the relationship of HA to total brain water and to T2-derived motionally distinct water fractions, since HA is known to bind water and to limit tissue water mobility. Experiments were carried out in fetal Pannon white rabbit pups at gestational ages of 25, 27, 29, and 31 days and at a postnatal age of 4 days. The brain tissue water content (desiccation method), T1 and T2 relaxation times (H1-NMR method), and HA concentration (radioassay HA 50) were measured, and free and bound water fractions were calculated by using multicomponent fits of the T2 relaxation curves. Compared with values in newborn pups, water and HA contents were found to be highly elevated in the preterm brain and decreased markedly during early postnatal life. The trends and time courses of T1 and T2 relaxation times proved to be similar, but the postnatal decrease in T2 was preceded by a significant decline in late gestation. Maturity-related changes occurred in the T2 relaxation derived bound water fraction which amounted to 4-19% of brain water. The bound water fraction appeared to be independent of total brain water and HA concentration, and HA is, therefore, unlikely to be the only factor controlling brain water mobility. The clear dissociation of bound water fraction from total water suggests restructuring of brain water during the perinatal period.