Justin E. Aldridge

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.

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.

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 nicotines 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.

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.

Nicotine therapy in adulthood reverses the synaptic and behavioral deficits elicited by prenatal exposure to phenobarbital.

A major objective in identifying the mechanisms underlying neurobehavioral teratogenicity is the possibility of designing therapies that reverse or offset drug- or toxicant-induced neural damage. In our previous studies, we identified deficits in hippocampal muscarinic cholinergic receptor-induced membrane translocation of protein kinase C (PKC)γ as the likely mechanism responsible for adverse behavioral effects of prenatal phenobarbital exposure. We therefore explored whether behavioral and synaptic defects could be reversed in adulthood by nicotine administration. Pregnant mice were given milled food containing phenobarbital to achieve a daily dose of 0.5–0.6 g/kg from gestational days 9–18. In adulthood, offspring showed deficits in the Morris maze, a behavior dependent on the integrity of septohippocampal cholinergic synaptic function, along with the loss of the PKCγ response. Phenobarbital-exposed and control mice then received nicotine (10 mg/kg/day) for 14 days via osmotic minipumps. Nicotine reversed the behavioral deficits and restored the normal response of hippocampal PKCγ to cholinergic receptor stimulation. The effects were regionally specific, as PKCγ in the cerebellum was unaffected by either phenobarbital or nicotine; furthermore, in the hippocampus, PKC isoforms unrelated to the behavioral deficits showed no changes. Nicotine administration thus offers a potential therapy for reversing neurobehavioral deficits originating in septohippocampal cholinergic defects elicited by prenatal drug or toxicant exposures.