Gestational exposure to methylmercury alters the developmental pattern of trk-like immunoreactivity in the rat brain and results in cortical dysmorphology

• Published in: Brain research. Developmental brain research 1998-07, Vol.109 (1), p.13-31
• Main Authors: Barone, Stan, Haykal-Coates, Najwa, Parran, Damani K, Tilson, Hugh A
• Format: Article
• Language: English
• Subjects: Animals; Blotting, Western; Brain Chemistry - drug effects; Brain Chemistry - genetics; Brain Chemistry - physiology; Cell Size; Cerebral Cortex - abnormalities; Cerebral Cortex - drug effects; Cerebral Cortex - embryology; Cerebral Cortex - metabolism; Developmental neurotoxicology; Female; Glial Fibrillary Acidic Protein - biosynthesis; Glial Fibrillary Acidic Protein - genetics; Image Processing, Computer-Assisted; Immunohistochemistry; Methylmercury Compounds - toxicity; Nerve Growth Factor receptor; Neuroglia - drug effects; Neuroglia - physiology; Neuroglia - ultrastructure; NGF; PC12 Cells; Pregnancy; Protein phosphorylation; Rats; Rats, Long-Evans; Receptor Protein-Tyrosine Kinases - biosynthesis; Receptor Protein-Tyrosine Kinases - genetics; Receptors, Nerve Growth Factor - biosynthesis; Receptors, Nerve Growth Factor - genetics; trk; Tropomyosin receptor kinase
• ISSN: 0165-3806
• DOI: 10.1016/S0165-3806(98)00038-8

Nerve growth factor signal transduction mediated through the trk receptor has been implicated in neuronal growth, differentiation, and survival. In this study, we examined the effects of gestational exposure to the developmental neurotoxicant methylmercury (CH 3Hg) on the ontogeny of trk-immunoreactivity (IR). Long–Evans dams were dosed on gestational days 6–15 (p.o.) with 0, 1, or 2 mg/kg CH 3Hg dissolved in saline. Pups were sacrificed and perfused with buffered paraformaldehyde on postnatal days (PND) 1, 4, 10, 21 and 85. The brains were sectioned sagitally, Nissl-stained or stained immunohistochemically for trk receptors or glial fibrillary acidic protein (GFAP), and examined throughout the medial to lateral extent of the brain. The greatest density of IR in neural cell bodies was seen in the olfactory bulb, hippocampus, cerebral, and cerebellar cortex, striatum, septum, nucleus basalis, inferior colliculus, pons, and brain stem nuclei. trk IR was not limited to nerve cell bodies, with prominent axonal and dendritic staining in the brainstem, neocortex, hippocampus, cerebellum, and olfactory tract. The regional pattern of trk IR varied in an age-dependent manner. In controls, trk-like IR appeared to peak in most regions between PND4–10 and decreased dramatically after PND21. This age-related difference in trk IR was supported by western blot analysis of PND10 and adult neocortex. This reduced and more adult-like pattern of trk IR was apparent on PND21 with some persistent trk-like IR in the olfactory bulb, hippocampus, neocortex, cerebellum and basal forebrain. In contrast to the normal regional patterns of trk IR, CH 3Hg produced a dose-related decrease in trk-like IR in the absence of overt maternal toxicity or neonatal toxicity. CH 3Hg-induced decreases in trk-like IR were especially apparent during the early postnatal period when trk IR was the greatest. The effects of CH 3Hg exposure were restricted regionally, with the largest decrease in trk-like IR apparent in cortical regions, basal forebrain nuclei, and brain stem nuclei. Subsequent to the effects of CH 3Hg on cortical trk-like IR were alterations in the development of cortical laminae on PND10 and 21 of neocortex. These alterations were characterized by quantifiable decreases in cell density, cell size and the widths of the layers of posterior neocortex. Not all of the CH 3Hg-induced effects were characterized by decreased trk-like IR. Robust increases in trk IR in glial cells in the corpus