Effects of gestational methylmercury exposure on immunoreactivity of specific isoforms of PKC and enzyme activity during post-natal development of the rat brain

Abstract

Protein kinase C (PKC)-mediated phosphorylation has been implicated in neuronal growth and differentiation [R.S. Turner, R.L. Mazzei, G.J. Raynor, P.R. Girard, J.F. Kuo, Proc. Natl. Acad. Sci. U.S.A., 81 (1984) 3143–3147.]. We examined effects of gestational exposure to the neurotoxicant, methylmercury (CH 3Hg), on the developmental profile of immunoreactivity (IR) for α, β, γ and ϵ PKC isoforms and cytosolic PKC activity. Long–Evans dams were dosed on gestational days (GD)6–15 (p.o.) with 0, 1, or 2 mg kg −1 day −1 CH 3Hg dissolved in saline. Pups were sacrificed and perfused with buffered paraformaldehyde on post-natal days (PND) 1, 4, 10, 21, 45 and 85. The brains were sectioned sagittally, stained immunohistochemically, and examined throughout the medial to lateral extent. IR in neuronal cell bodies for PKC isoforms α, β, γ, and ϵ was densest in the olfactory bulb, hippocampus, shell of the inferior colliculus, pons, cerebral, piriform, and cerebellar cortex, whereas axonal staining was prominent in the brainstem, internal capsule, corpus callosum, anterior commissure, fornix and olfactory tract. In controls, the PKC α and ϵ IR was highest on PND1–4, decreased dramatically by PND10, and decreased further by PND21. In the neonate, the regional and cellular distributions of α and ϵ IR were similar. The PKC γ IR was greater at post-weaning ages (PND21–85) with the greatest regional density apparent in the hippocampus, cortex, and cerebellum. Only the highest dose of CH 3Hg (2 mg kg −1 day −1; GD6–15) produced a persistent decrease in regional α and ϵ, but not β or γ IR during the post-natal period. These regional and time-dependent changes in PKC isoforms were complemented by the examination of PKC activity in cortex, olfactory bulb, cerebellum and brainstem. Cytosolic PKC activity increased from PND1 to 10 in cortex, olfactory bulb, and cerebellum. On PND21, PKC activity decreased in the cortex and olfactory bulb, but remained high in the cerebellum. By contrast, PKC activity in the brainstem was highest on PND1 and 4 and decreased dramatically by PND21. CH 3Hg (2 mg kg −1 day −1) significantly decreased PKC activity on PND1 and 4 in the cortex. The present results characterize the cellular and regional ontogeny of PKC isoenzymes α, β, γ and ϵ, and indicate that developmental exposure to CH 3Hg can alter the ontogeny of specific isoforms and regional PKC activity.