The cholinergic septo-hippocampal pathway is critically involved in learning and memory functions of the hippocampus and a likely substrate for the neurotoxic effects of low-level lead (Pb) exposure in children. We observed that exposure of neonatal rats to low levels of lead causes selective reduction in ontogenic expression of cholinergic marker enzyme cholineacetyltransferase (ChAT) and muscarinic receptors in the septum without inducing similar changes in the hippocampus. This implicates the cholinergic septo-hippocampal neurons as a particularly vulnerable target for the developmental insult by inorganic lead. In this proposal we will investigate in detail, the mechanism and functional consequences of lead's effects on the septa) cholinergic cells. (1) Receptor radioligand binding to septal homogenates from control and lead-exposed neonatal rats will be used to determine the developmental time course of the Pb-induced reduction in muscarinic receptors and its persistence into adulthood; (2) Receptor subtype-selective radioligand binding and a combination of receptor autoradiography and ChAT immunocytochemistry in brain sections and/or dissociated septal cultures will be used to determine which muscarinic receptor subtype(s) are reduced and establish their localization to the cholinergic neurons; (3) a hypothesis that Pb interferes with the regulation of receptor expression by nerve growth factor (NGF) will be tested by examining the interaction between NGF and Pb on the expression of muscarinic receptors in dissociated septal cultures: (a) 125I-NGF binding to dissociated cells will be used to determine if lead interferes with binding and uptake of NGF by the target neurons and (b) receptor hybridization will be used to determine if NGF induces and Pb inhibits the induction of receptor subtypes-specific mRNA; (4) Whole cell patch-clamp recording from dissociated septal neurons in culture will be used to determine the effect of Pb exposure on the development of chemosensitivity to ACh in the septal cholinergic neurons and test Pb's effect on the electrophysiological characteristics of these neurons; specifically, we will test a hypothesis that Pb modifies the characteristic rhythmic, pacer-like firing properties of cholinergic cells by (a) attenuating muscarinic abbreviation of postspike afterhyperpolarization (AHP) , (b) altering Ca conductances, and (c) modifying the outward Ca-dependent potassium currents in the Pb-exposed cells. The phenotype of the cells will be identified after recordings by immunocytochemical staining with anti-ChAT antibodies. These experiments should provide new insights into the neurobiology of the cholinergic septo-hippocampal neurons and the extent to which alterations in their chemosensitive and electrical membrane properties might be involved in Pb-induced behavioral toxicity.

Agency
National Institute of Health (NIH)
Institute
National Institute of Environmental Health Sciences (NIEHS)
Type
Research Project (R01)
Project #
5R01ES006365-03
Application #
2155230
Study Section
Toxicology Subcommittee 2 (TOX)
Project Start
1993-04-01
Project End
1996-07-31
Budget Start
1995-04-01
Budget End
1996-07-31
Support Year
3
Fiscal Year
1995
Total Cost
Indirect Cost
Name
University of Cincinnati
Department
Physiology
Type
Schools of Medicine
DUNS #
City
Cincinnati
State
OH
Country
United States
Zip Code
45221
Zhou, M; Tian, X; Suszkiw, J B (2000) Developmental stage-dependent protective effect of NGF against lead cholinotoxicity in the rat septum. Brain Res 866:268-73
Tian, X; Sun, X; Suszkiw, J B (2000) Upregulation of tyrosine hydroxylase and downregulation of choline acetyltransferase in lead-exposed PC12 cells: the role of PKC activation. Toxicol Appl Pharmacol 167:246-52
Sun, X; Tian, X; Suszkiw, J B (1997) Reduction of vesicular acetylcholine transporter mRNA in the rat septum following lead exposure. Neuroreport 8:891-4
Sun, X; Tian, X; Suszkiw, J B (1997) Reduction of choline acetyltransferase mRNA in the septum of developing rat exposed to inorganic lead. Neurotoxicology 18:201-7
Bourjeily, N; Suszkiw, J B (1997) Developmental cholinotoxicity of lead: loss of septal cholinergic neurons and long-term changes in cholinergic innervation of the hippocampus in perinatally lead-exposed rats. Brain Res 771:319-28
Tian, X; Sun, X; Suszkiw, J B (1996) Developmental age-dependent upregulation of choline acetyltransferase and vesicular acetylcholine transporter mRNA expression in neonatal rat septum by nerve growth factor. Neurosci Lett 209:134-6
Bielarczyk, H; Tian, X; Suszkiw, J B (1996) Cholinergic denervation-like changes in rat hippocampus following developmental lead exposure. Brain Res 708:108-15
Sun, L R; Suszkiw, J B (1995) Extracellular inhibition and intracellular enhancement of Ca2+ currents by Pb2+ in bovine adrenal chromaffin cells. J Neurophysiol 74:574-81
Tian, X; Bourjeily, N; Bielarczyk, H et al. (1995) Reduced densities of sodium-dependent [3H] hemicholinium-3 binding sites in hippocampus of developmental rats following perinatal low-level lead exposure. Brain Res Dev Brain Res 86:268-74
Sun, L R; Suszkiw, J B (1994) Pb2+ activates potassium currents in bovine adrenal chromaffin cells. Neurosci Lett 182:41-3

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