Early ontogeny of iodocompound-processing neural systems in rat brain

The distribution and localization of iodocompounds reaching the brain during early development were measured in rat pups nurtured on [125I]-containing milk from dams receiving daily [125I]-iodide injections. The regimen produced no measurable changes in growth and development of the offspring during the nursing period. Pup brains accumulated labeled iodocompounds at a faster rate than they grew and accumulated protein. The ratio of [125I]-iodocompounds in cerebrum relative to skeletal muscle increased progressively from day 11 through day 19. Significant differences in distribution of radioactivity in different brain regions were evident on day 1; developmental progress was associated with significantly different rates of regional accumulation of the isotope. On day 1 only 10% of the radioactivity in the postnuclear supernatant phase of brain homogenates was particle-bound; at the time of weaning, radioactivity in brain particles accounted for more than 50%. Growing nerve cell processes and myelin, known to be major targets of early thyroid hormone deficiency or excess, were also the major subcellular sites of [125I]-iodocompound localization in the developing rat brain. Overall, the ontogeny reflected progressive elaboration of iodocompound-processing neural systems resembling those recently recognized in adult brain.