Chlorpyrifos exposure during neurulation: cholinergic synaptic dysfunction and cellular alterations in brain regions at adolescence and adulthood

Disruption of rat forebrain development by glucocorticoids: critical perinatal periods for effects on neural cell acquisition and on cell signaling cascades mediating noradrenergic and cholinergic neurotransmitter/neurotrophic responses

Glucocorticoids are the consensus treatment for the prevention of respiratory distress in preterm infants, but there is evidence for increased incidence of neurodevelopmental disorders as a result of their administration. We administered dexamethasone (Dex) to developing rats at doses below or within the range of those used clinically, evaluating the effects on forebrain development with exposure in three different stages: gestational days 17-19, postnatal days 1-3, or postnatal days 7-9. At 24 h after the last dose, we evaluated biomarkers of neural cell acquisition and growth, synaptic development, neurotransmitter receptor expression, and synaptic signaling mediated by adenylyl cyclase (AC). Dex impaired the acquisition of neural cells, with a peak effect when given in the immediate postnatal period. In association with this defect, Dex also elicited biphasic effects on cholinergic presynaptic development, promoting synaptic maturation at a dose (0.05 mg/kg) well below those used therapeutically, whereas the effect was diminished or lost when doses were increased to 0.2 or 0.8 mg/kg. Dex given postnatally also disrupted the expression of adrenergic receptors known to participate in neurotrophic modeling of the developing brain and evoked massive induction of AC activity. As a consequence, disparate receptor inputs all produced cyclic AMP overproduction, a likely contributor to disrupted patterns of cell replication, differentiation, and apoptosis. Superimposed on the heterologous AC induction, Dex impaired specific receptor-mediated cholinergic and adrenergic signals. These results indicate that, during a critical developmental period, Dex administration leads to widespread interference with forebrain development, likely contributing to eventual, adverse neurobehavioral outcomes.

ELISA methods to measure cholinergic markers and nerve growth factor receptors in cortex, hippocampus, prefrontal cortex, and basal forebrain from rat brain

The central cholinergic system has a fundamental role in normal cognitive function, and in diseases that exhibit cognitive dysfunction. The purpose of this study was to design ELISA methods to measure proteins that have essential functions in the central cholinergic system. We were particularly interested in quantifying proteins that respond directly or indirectly to nerve growth factor (NGF). ELISAs offer advantages over Western blot analyses and other methods, such as increased sensitivity, decreased assay variability, increased efficiency, and decreased cost. We developed indirect ELISA methods for: choline acetyltransferase (ChAT); the vesicular acetylcholine transporter (VAChT); the high affinity choline transporter (HACT/CHT); TrkA, the high affinity NGF receptor; the p75 neurotrophin receptor (p75(NTR)). A sandwich ELISA was developed to measure tyrosine-phosphorylated TrkA in brain lysates. We used these ELISAs to compare levels of the above proteins in important memory-related brain regions--basal forebrain, hippocampus, cortex, and prefrontal cortex--from old and young rats. We identified age-related differences in the levels of the aforementioned proteins (e.g., VAChT and HACT/CHT in hippocampus). Thus, these ELISA methods should be particularly useful for comparing the effects of age, disease, drugs, and toxicants on brain levels of key cholinergic and growth factor-related proteins.

The alterations in CNS serotonergic mechanisms caused by neonatal chlorpyrifos exposure are permanent

Fetal or neonatal exposure to chlorpyrifos (CPF) or related organophosphate pesticides leads to abnormalities of brain cell development, synaptic function, and behavior. Recent studies in rats indicate profound effects on serotonin (5HT) systems that originate during CPF exposure and that are still present at 2 months posttreatment in the young adult. To determine if these changes are permanent, we administered 1 mg/kg of CPF daily to neonatal rats on postnatal days 1-4, a regimen devoid of systemic toxicity, and examined 5HT synaptic markers at 5 months of age: radioligand binding to 5HT1A and 5HT2 receptors and to the 5HT transporter. There were global elevations in all three synaptic proteins, with pronounced sex selectivity (effects on males>females) and a regional hierarchy of effects, viz. striatum>midbrain approximately brainstem>cerebral cortex. Because there is a normal sex disparity for 5HT synaptic proteins, with females having higher values than males, the increase caused by CPF exposure in males completely eliminated this difference. Our findings at 5 months of age replicate those seen in young adulthood and strongly suggest that the effects of neonatal CPF exposure on 5HT systems are permanent.

Alterations in central nervous system serotonergic and dopaminergic synaptic activity in adulthood after prenatal or neonatal chlorpyrifos exposure

Exposure to chlorpyrifos (CPF) alters neuronal development of serotonin (5HT) and dopamine systems, and we recently found long-term alterations in behaviors related to 5HT function. To characterize the synaptic mechanisms underlying these effects, we exposed developing rats to CPF regimens below the threshold for systemic toxicity, in three treatment windows: gestational days (GD) 17-20, postnatal days (PN) 1-4, or PN11-14. In early adulthood (PN60), we assessed basal neurotransmitter content and synaptic activity (turnover) in brain regions containing the major 5HT and dopamine projections. CPF exposure on GD17-20 or PN1-4 evoked long-term increases in 5HT turnover across multiple regions; the effects were not secondary to changes in neurotransmitter content, which was unaffected or even decreased. When the treatment window was shifted to PN11-14, there were no long-term effects. Dopamine turnover also showed significant increases after CPF exposure on GD17-20, but only when the dose was raised above the threshold for overt toxicity; however, hippocampal dopamine content was profoundly subnormal after exposures below or above the acute, toxic threshold, suggesting outright neurotoxicity. These results indicate that, in a critical developmental period, apparently nontoxic exposures to CPF produce lasting activation of 5HT systems in association with 5HT-associated behavioral anomalies.

Developmental exposure of rats to chlorpyrifos leads to behavioral alterations in adulthood, involving serotonergic mechanisms and resembling animal models of depression

Developmental exposure to chlorpyrifos (CPF) causes persistent changes in serotonergic (5HT) systems. We administered 1 mg/kg/day CPF to rats on postnatal days 1-4, a regimen below the threshold for systemic toxicity. When tested in adulthood, CPF-exposed animals showed abnormalities in behavioral tests that involve 5HT mechanisms. In the elevated plus maze, males treated with CPF spent more time in the open arms, an effect seen with 5HT deficiencies in animal models of depression. Similarly, in an anhedonia test, the CPF-exposed group showed a decreased preference for chocolate milk versus water. Developmental CPF exposure also has lasting effects on cognitive function. We replicated our earlier finding that developmental CPF exposure ablates the normal sex differences in 16-arm radial maze learning and memory: during acquisition training, control male rats typically perform more accurately than do control females, but CPF treatment eliminated this normal sex difference. Females exposed to CPF showed a reduction in working and reference memory errors down to the rate of control males. Conversely, CPF-exposed males exhibited an increase in working and reference memory errors. After radial-arm acquisition training, we assessed the role of 5HT by challenging the animals with the 5HT2 receptor antagonist ketanserin. Ketanserin did not affect performance in controls but elicited dose-dependent increases in working and reference memory errors in the CPF group, indicating an abnormal dependence on 5HT systems. Our results indicate that neonatal CPF exposures, classically thought to be subtoxic, produce lasting changes in 5HT-related behaviors that resemble animal models of depression.

Quantitative morphological assessment reveals neuronal and glial deficits in hippocampus after a brief subtoxic exposure to chlorpyrifos in neonatal rats

Neurochemical and behavioral studies indicate that the widely used organophosphorus insecticide, chlorpyrifos (CPF), evokes neurobehavioral teratogenicity with a wide window of vulnerability, ranging from embryonic life through postnatal development. Few studies have detailed morphological damage that corresponds to the operational deficits. We administered 5 mg/kg of CPF sc daily on postnatal days (PN) 11-14, a regimen that is devoid of systemic toxicity, but that elicits long-term cognitive impairment and disruption of cholinergic, catecholaminergic, and serotonergic synaptic function. On PN15 and 20, we conducted quantitative morphologic examinations of neurons and glia in CA1, CA3, and dentate gyrus regions of the hippocampus. Although hippocampal morphology after CPF exposure was normal on gross observation, morphometric analysis revealed a significant overall reduction in the total number of neurons and glia. Superimposed on this basic effect, CPF elicited a delayed-onset increase in the neuron/glia ratio that emerged by PN20, connoting selective gliotoxicity. The alterations in cell numbers were accompanied by significant perikaryal swelling and by enhanced development of astrocytic processes. Layer thickness also showed delayed-onset effects of CPF, with thinning of the CA1 and CA3 layers and enlargement of the dentate gyrus. Our results indicate that there are subtle morphological changes in the juvenile rat brain after neonatal CPF exposure that are detectable only with quantitative analysis and that correlate with regional and cell-specific targets identified earlier in neurochemical studies. The simultaneous targeting of neurons and glia by CPF is likely to play an important role in its developmental neurotoxicant effects.

Effects of prenatal nicotine exposure on primate brain development and attempted amelioration with supplemental choline or vitamin C: neurotransmitter receptors, cell signaling and cell development biomarkers in fetal brain regions of rhesus monkeys

Studies in developing rodents indicate that nicotine is a neuroteratogen that disrupts brain development by stimulating nicotinic acetylcholine receptors (nAChRs) that control neural cell replication and differentiation. We administered nicotine to pregnant Rhesus monkeys from gestational day 30 through 160 by continuous infusion, achieving maternal plasma levels comparable to those in smokers (30 ng/ml). Fetal brain regions and peripheral tissues were examined for nAChR subtypes, other neurotransmitter receptors, and indices of cell signaling and cell damage. Nicotine evoked nAChR upregulation, but with distinct regional disparities indicative of selective stimulatory responses. Similarly, indices of cell loss (reduced DNA), cell size and neuritic outgrowth (protein/DNA and membrane/total protein ratios) were distinct for each region and did not necessarily follow the rank order of nAChR upregulation, suggesting the involvement of additional mechanisms such as oxidative stress. We then attempted to offset the adverse effects of nicotine with standard dietary supplements known to interact with nicotine. By itself, choline elicited nicotine-like actions commensurate with its promotion of cholinergic neurotransmission. When given in combination with nicotine, choline protected some regions from damage but worsened nicotine's effects in other regions. Similarly, Vitamin C supplementation had mixed effects, increasing nAChR responses while providing protection from cell damage in the caudate, the brain region most susceptible to oxidative stress. Our results indicate that nicotine elicits neurodevelopmental damage that is highly selective for different brain regions, and that dietary supplements ordinarily thought to be neuroprotectant may actually worsen some of the adverse effects of nicotine on the fetal brain.