piperidines and Lead-Poisoning--Nervous-System

Recent interests are beginning to be directed towards toxic neurobiological dysfunctions caused by lead (Pb) in aquatic vertebrates. In the present work, treatment with a maximum acceptable toxic concentration of this heavy metal was responsible for highly significant (p<0.01) abnormal motor behaviors such as hyperactive movements in the teleost Thalassoma pavo and the same treatment accounted for significantly (p<0.05) enhanced hyperventilating states. On the other hand, greater abnormal motor behaviors were detected in the presence of the histamine (HA) receptor subtype 2 (H(2)R) antagonist cimetidine (Cim), as shown by the very robust (p<0.001) increases of the two behavioral states. Interestingly, elevated expression levels of stress-related factors, i.e. heat shock protein70/90 (HSP90/70) orthologs were reported for the first time in hypothalamic and mesencephalic areas of Pb-treated teleosts. In particular, an up-regulation of HSP70 was readily detected when this heavy metal was given concomitantly with Cim, while the histamine subtype 3 antagonist (H(3)R) thioperamide (Thio), instead, blocked Pb-dependent up-regulatory trends of both chaperones in mostly hypothalamic areas. Moreover, intense neuronal damages of the above brain regions coincided with altered expressions of HSP70 and HSP90 when treated only with Cim. Overall these first results show that distinct H(n)R are able to exert a net neuroprotective role arising from their interaction with chaperones in fish exposed to Pb-dependent stressful conditions making this a potentially key interaction especially for T. pavo, aquatic species which plays an important ecological role towards the survival of other commercially vital fishes.

Topics: Amino Acid Sequence; Animals; Behavior, Animal; Brain; Brain Chemistry; Cimetidine; Fishes; Fluoresceins; Fluorescent Dyes; Heat-Shock Proteins; Histamine H2 Antagonists; Histamine H3 Antagonists; HSP70 Heat-Shock Proteins; HSP90 Heat-Shock Proteins; In Situ Hybridization; Injections, Intraperitoneal; Lead Poisoning, Nervous System; Molecular Sequence Data; Organic Chemicals; Piperidines; Receptor Cross-Talk; Receptors, Histamine; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger

In the present study we show that chronic exposure to low levels of lead (Pb(2+)) during development alters the type of N-methyl-D-aspartate receptor (NMDAR) expressed in the developing and young adult rat brain. Ifenprodil inhibition of [3H]MK-801 binding to the NMDAR channel in cortical and hippocampal neuronal membranes expressed high and low affinity components. Previous studies have shown that the high affinity component is associated with NR1/NR2B receptor complexes while the low affinity component is associated with the appearance and insertion of the NR2A subunit to NMDAR complexes. Pb(2+)-exposed rats express a greater number of [3H]MK-801 binding sites associated with the high affinity and low affinity components of ifenprodil inhibition. Further, [3H]ifenprodil saturation isotherms and Scatchard analysis in cortical and hippocampal membranes showed a higher number of binding sites (B(max)) with no change in binding affinity (K(d)) in Pb(2+)-exposed animals relative to controls. Quantitative [3H]MK-801 autoradiography in response to glutamate and glycine provided evidence that NMDAR complexes in Pb(2+)-exposed rat brain were maximally activated in situ. Higher levels of ifenprodil-sensitive binding sites and increased sensitivity to agonists are properties characteristic of NR1/NR2B recombinant receptors. Thus, our results strongly suggest that a greater proportion of the total number of NMDAR are NR1/NR2B receptors in the Pb(2+)-exposed rat brain. This Pb(2+)-induced change in NMDAR subtypes in the rat brain was associated with reduced CREB phosphorylation in cortical and hippocampal nuclear extracts. These findings demonstrate that chronic exposure to environmentally relevant levels of Pb(2+) altered the subunit composition of NMDAR complexes with subsequent effects on calcium-sensitive signaling pathways involved in CREB phosphorylation.

Topics: Animals; Animals, Newborn; Binding Sites; Brain; Calcium Signaling; Cell Differentiation; Cyclic AMP Response Element-Binding Protein; Dizocilpine Maleate; Environmental Exposure; Female; Lead; Lead Poisoning, Nervous System; Male; Neurons; Phosphorylation; Piperidines; Pregnancy; Prenatal Exposure Delayed Effects; Protein Subunits; Rats; Rats, Long-Evans; Receptors, N-Methyl-D-Aspartate

NMDA channels are key targets for lead (Pb2+) neurotoxicity and Pb2+-induced inhibition of NMDA current is age- and subunit-dependent. In rat cerebellar granule cells maintained in high KCl, glycine affinity as well as sensitivity to ifenprodil change significantly with the days in vitro, indicating a reduction of NR2B subunit expression. Pb2+ blocked NMDA current with IC50 approximately 4 microM and this effect decreased significantly during the second week in vitro. In Xenopus laevis oocytes expressing recombinant NR1-NR2A, NR1-NR2B or NR1-NR2C receptors, Pb2+ inhibited glutamate-activated currents with IC50 of 3.3, 2.5 and 4.7 microM respectively. These data indicate that Pb2+ action is dependent on subunit composition and suggest that down-regulation of the NR2B subunit is correlated to a diminished sensitivity to Pb2+ inhibition.

Topics: Animals; Cells, Cultured; Cerebellar Cortex; Dose-Response Relationship, Drug; Down-Regulation; Drug Interactions; Excitatory Amino Acid Antagonists; Gene Expression Regulation; Glycine; Lead; Lead Poisoning, Nervous System; Membrane Potentials; Neurons; Oocytes; Piperidines; Potassium Chloride; Rats; Receptors, N-Methyl-D-Aspartate; Recombinant Proteins; Up-Regulation