Early chronic exposure to low-level lead alters total hippocampal microglia in pre-adolescent mice.

Developmental lead (Pb) exposure alters brain function through mechanisms that are not yet understood. A previous study showed that early lead exposure reduced microglia number in the dentate gyrus region of the hippocampus. Given the critical role of microglia in brain development, it is important to determine whether these differences are unique to the dentate gyrus, or occur throughout the hippocampus. Unbiased stereology was used to quantify microglia mean cell body number in total hippocampus, and compare the proportion of microglia in the ventral vs. dorsal regions. Total hippocampal volume was also measured and compared. The study included brain tissue from 30 pre-adolescent C57BL/6 J mice, exposed to 30 ppm Pb acetate (n = 10, mean BLL 3.4 µg/dL at sacrifice), 330 ppm Pb acetate (n = 10, mean BLL 14.1 µg/dL at sacrifice), or 0 ppm Pb acetate (n = 10, negative controls). In lead exposed animals, microglia mean cell body number was reduced in total hippocampus; total hippocampal volume was reduced. Importantly, effects in low- and high-dose exposure groups did not differ. Contrary to hypotheses, the distribution of hippocampal microglia in the ventral vs. dorsal hippocampal regions did not differ. Thus, lowest and higher levels of lead exposure during development appear to have strikingly similar disruptive effects in the neuroimmune system. Studies are needed to determine the immune and other mechanisms responsible for these effects. Future studies would benefit from larger samples to determine whether in fact there is a group by sex interaction driving the effects of early lead exposure on microglia.