Thomas R. Ward

Organophosphorus compounds preferentially affect second messenger systems coupled to M2/M4 receptors in rat frontal cortex

Recent reports indicate that organophosphate insecticides, in addition to inhibiting acetylcholinesterase activity, can bind directly at a subset of muscarinic receptors, which also bind cis-methyldioxolane with high affinity. Muscarinic receptors are known to act through at least two second messenger systems, either the stimulation of phosphoinositide turnover (mediated through the M1 and M3 receptor subtypes) or the inhibition of cAMP formation (mediated through the M2 and M4 receptor subtypes). We have investigated the action of the active forms of parathion, malathion, and chlorpyrifos (paraoxon, malaoxon, and chlorpyrifos oxon, respectively) on these second messenger systems in cortical slices from adult male Long-Evans rats. Paraoxon, malaoxon, and chlorpyrifos oxon (10(-8) to 10(-4) M) inhibited forskolin-stimulated cAMP formation in a concentration-dependent manner. The effect on cAMP formation was blocked by the muscarinic antagonist atropine (10 microM). These results suggest that paraoxon, malaoxon, and chlorpyrifos oxon can act as agonists at the M2 and/or M4 subset of muscarinic receptors. In addition, chlorpyrifos may have another site of action. In contrast, none of the organophosphates had any effect on basal or carbachol-stimulated phosphoinositide hydrolysis. The differential activity on these two second messenger systems make it unlikely that the observed effects on cAMP formation are due to increases in endogenous acetylcholine resulting from inhibition of acetylcholinesterase.

Inhibition of microsomal and mitochondrial Ca2+-sequestration in rat cerebellum by polychlorinated biphenyl mixtures and congeners. Structure-activity relationships.

Recent studies from our laboratory indicate that polychlorinated biphenyl (PCB) congeners in vitro perturbed signal transduction mechanisms including cellular Ca2+-homeostasis and protein kinase C translocation. We have now investigated the structure-activity relationship (SAR) of three PCB mixtures, 24 PCB congeners and one dibenzofuran for their effects on microsomal and mitochondrial Ca2+-sequestration in rat cerebellum. Ca2+-sequestration by these intracellular organelles was determined using radioactive 45CaCl2. All three mixtures studied, Aroclor 1016, Aroclor 1254 and Aroclor 1260, were equally potent in inhibiting microsomal and mitochondrial Ca2+-sequestration with IC50 values of 6–8 μM. 1,2,3,7,8-Pentachlorodibenzofuran had no effect on Ca2+-sequestration by these organelles. The SAR among the congeners revealed: (1) congeners with ortho-/meta- or ortho-, para-chlorine substitutions were the most potent in inhibiting microsomal and mitochondrial Ca2+-sequestration (IC50=2.4–22.3 μM); (2) congeners with only para- but without ortho-substitutions were not effective in inhibiting Ca2+-sequestration by microsomes and mitochondria; (3) increased chlorination was not related to the effectiveness of these congeners. The present SAR studies indicate that the effects of most PCB congeners in vitro may be related to an interaction at specific sites having preference for low lateral substitution or lateral content (meta- or para) in the presence of ortho-substitution.

Developmental Exposure to Polychlorinated Biphenyls Interferes with Experience-Dependent Dendritic Plasticity and Ryanodine Receptor Expression in Weanling Rats

Background Neurodevelopmental disorders are associated with altered patterns of neuronal connectivity. A critical determinant of neuronal connectivity is the dendritic morphology of individual neurons, which is shaped by experience. The identification of environmental exposures that interfere with dendritic growth and plasticity may, therefore, provide insight into environmental risk factors for neurodevelopmental disorders. Objective We tested the hypothesis that polychlorinated biphenyls (PCBs) alter dendritic growth and/or plasticity by promoting the activity of ryanodine receptors (RyRs). Methods and Results The Morris water maze was used to induce experience-dependent neural plasticity in weanling rats exposed to either vehicle or Aroclor 1254 (A1254) in the maternal diet throughout gestation and lactation. Developmental A1254 exposure promoted dendritic growth in cerebellar Purkinje cells and neocortical pyramidal neurons among untrained animals but attenuated or reversed experience-dependent dendritic growth among maze-trained littermates. These structural changes coincided with subtle deficits in spatial learning and memory, increased [3H]-ryanodine binding sites and RyR expression in the cerebellum of untrained animals, and inhibition of training-induced RyR upregulation. A congener with potent RyR activity, PCB95, but not a congener with negligible RyR activity, PCB66, promoted dendritic growth in primary cortical neuron cultures and this effect was blocked by pharmacologic antagonism of RyR activity. Conclusions Developmental exposure to PCBs interferes with normal patterns of dendritic growth and plasticity, and these effects may be linked to changes in RyR expression and function. These findings identify PCBs as candidate environmental risk factors for neurodevelopmental disorders, especially in children with heritable deficits in calcium signaling.

Effects of the chlorotriazine herbicide, cyanazine, on GABAA receptors in cortical tissue from rat brain ☆

Chlorotriazine herbicides disrupt luteinizing hormone (LH) release in female rats following in vivo exposure. Although the mechanism of action is unknown, significant evidence suggests that inhibition of LH release by chlorotriazines may be mediated by effects in the central nervous system. GABA(A) receptors are important for neuronal regulation of gonadotropin releasing hormone and LH release. The ability of chlorotriazine herbicides to interact with GABA(A) receptors was examined by measuring their effects on [3H]muscimol, [3H]Ro15-4513 and [35S]tert-butylbicyclophosphorothionate (TBPS) binding to rat cortical membranes. Cyanazine (1-400 microM) inhibited [3H]Ro15-4513 binding with an IC50 of approximately 105 microM (n=4). Atrazine (1-400 microM) also inhibited [3H]Ro15-4513 binding, but was less potent than cyanazine (IC50 = 305 microM). However, the chlorotriazine metabolites diaminochlorotriazine, 2-amino-4-chloro-6-ethylamino-s-triazine and 2-amino-4-chloro-6-isopropylamino-s-triazine were without significant effect on [3H]Ro15-4513 binding. Cyanazine and the other chlorotriazines were without effect on [3H]muscimol or [35S]TBPS binding. To examine whether cyanazine altered GABA(A) receptor function, GABA-stimulated 36Cl- flux into synaptoneurosomes was examined. Cyanazine (50-100 microM) alone did not significantly decrease GABA-stimulated 36Cl- flux. Diazepam (10 microM) and pentobarbital (100 microM) potentiated GABA-stimulated 36Cl- flux to 126 and 166% of control, respectively. At concentrations of 50 and 100 microM, cyanazine decreased potentiation by diazepam to 112 and 97% of control, respectively, and decreased potentiation by pentobarbital to 158 and 137% of control (n = 6). Interestingly, at lower concentrations (5 microM), cyanazine shifted the EC50 for GABA-stimulated 36Cl- flux into synaptoneurosomes from 28.9 to 19.4 microM, respectively (n = 5). These results suggest that cyanazine modulates benzodiazepine, but not the muscimol (GABA receptor site) or TBPS (Cl- channel), binding sites on GABA(A) receptors. Furthermore, at low concentrations, cyanazine may slightly enhance function of GABA(A) receptors, but at higher concentrations, cyanazine antagonizes GABA(A) receptor function and in particular antagonizes the positive modulatory effects of diazepam and pentobarbital.

Effect of repeated organophosphate administration on carbachol-stimulated phosphoinositide hydrolysis in the rat brain.

The effects of repeated exposure to two organophosphates on the turnover of phosphoinositides, the second messenger system coupled to the M1 and M3 subtypes of muscarinic receptors, were examined in the rat hippocampus. Repeated diisopropylfluorophosphate (DFP) exposure (0.2-0.8 mg/kg, SC) decreased brain acetylcholinesterase activity and muscarinic receptor density. The incorporation of [3H]myoinositol into brain slices was also decreased. Phosphoinositide turnover was measured as the accumulation of [3H]inositol phosphates (IP) in the presence of lithium. DFP did not affect basal IP accumulation, but decreased carbachol-stimulated IP accumulation in the hippocampus after 0.4 and 0.8 mg/kg. The effects of repeated disulfoton administration (2.0 mg/kg, IP) were also examined in the hippocampus. Similar to DFP, repeated disulfoton exposure decreased acetylcholinesterase activity, receptor density, and carbachol-stimulated IP accumulation. The incorporation of myoinositol, however, was increased in disulfoton-treated rats. These data indicate that repeated organophosphate exposure results in a functional decrease in muscarinic receptor activity, as well as changes in myoinositol incorporation into phospholipids.

Measurement of the Nitric Oxide Synthase Activity Using the Citrulline Assay

The free-radical gas nitric oxide (NO) recently has been identified as an important biological messenger molecule in both the central and peripheral nervous system. NO is generated by the enzyme NO synthase (NOS) by the oxidation of the amino acid L: -arginine. As a dissolved gas, NO is an unusual neurotransmitter. It is not packaged in synaptic vesicles and released by exocytosis upon membrane depolarization, but rather diffuses from its site of production to surrounding neurons where it acts directly on specific intracellular targets. The activity of NO terminates when it chemically reacts with a target substrate. Although all of the targets of NO are not yet known, NO can bind to the iron associated with heme groups or result in nitrosylation of proteins, leading to conformational changes. One of the best-described targets of NO in the central nervous system is the heme-containing protein guanylyl cyclase. NO is a relatively long-lived free radical and does not react readily with most cellular components. This allows it to diffuse to several surrounding neurons and integrate neuronal activity on a local scale. NO is involved in a number of physiological processes including morphogenesis and synaptic plasticity. However, under conditions in which NOS is overstimulated, excessive formation of NO may mediate cell injury in a variety of disorders of the nervous system that result in neurodegeneration (1).