Cocaine sensitization in adult Long-Evans rats perinatally exposed to polychlorinated biphenyls

Chronic early-life lead exposure sensitizes adolescent rats to cocaine: Role of the dopaminergic system

Exposure to heavy metals has been associated with psychiatric disorders and recent studies suggest an association between childhood lead (Pb²⁺) intoxication and schizophrenia (SZ). In animal models, Pb²⁺ exposure recapitulates key neuropathological and dopaminergic system alterations present in SZ. Given the high comorbidity of mental disorders such as SZ and substance abuse, coupled with evidence showing that Pb²⁺ exposure affects addiction circuits, we hypothesized that early life Pb²⁺ exposure could sensitize neuronal systems relevant to SZ and substance abuse. To this goal, we examined the effects of chronic developmental Pb²⁺ exposure on the acute locomotor response to cocaine (0, 5, and 15 mg kg^–1) and behavioral sensitization. We also examined the role of the dopaminergic system in the psychostimulant effects of cocaine, and measured D1-dopamine receptor (D1R) levels in the rat brain using [³H]-SCH23390 quantitative receptor autoradiography, as well as the ability of the D1R antagonist SCH23390 to block the cocaine effects on locomotor activation. These studies were performed in male and female rats at different developmental ages consisting of juveniles (postnatal, PN14), early-adolescent (PN28), late adolescent (PN50), and adults (PN120). Our results show that chronic developmental Pb²⁺ exposure increases the acute locomotor response to the higher dose of cocaine in Pb²⁺-exposed male adolescent (PN28 and PN50) rats, and to the lower dose of cocaine in adolescent female rats. No changes in the locomotor activity were detected in adult rats. Behavioral sensitization experiments showed a sustained sensitization in early adolescent Pb²⁺-exposed male but not female rats. The cocaine-induced effects on locomotor activity were abrogated by injection of a D1R antagonist suggesting the involvement of this dopamine receptor subtype. Furthermore, Pb²⁺-induced increases D1R levels in several brain regions were prominent in juveniles and early adolescence but not in late adolescence or in adults. In summary, early chronic developmental Pb²⁺ exposure results in age and sex-dependent effect on the locomotor response to cocaine, suggesting differential susceptibilities to the neurotoxic effects of Pb²⁺ exposure. Our data provides further support to the notion that Pb²⁺ exposure is an environmental risk factor for psychiatric disorders and substance abuse.

Ahr and Cyp1a2 genotypes both affect susceptibility to motor deficits following gestational and lactational exposure to polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are persistent organic pollutants known to cause adverse health effects and linked to neurological deficits in both human and animal studies. Children born to exposed mothers are at highest risk of learning and memory and motor deficits. We developed a mouse model that mimics human variation in the aryl hydrocarbon receptor and cytochrome P450 1A2 (CYP1A2) to determine if genetic variation increases susceptibility to developmental PCB exposure. In our previous studies, we found that high-affinity Ahr^bCyp1a2(-/-) and poor-affinity Ahr^dCyp1a2(-/-) knockout mice were most susceptible to learning and memory deficits following developmental PCB exposure compared with Ahr^bCyp1a2(+/+) wild type mice (C57BL/6J strain). Our follow-up studies focused on motor deficits, because human studies have identified PCBs as a potential risk factor for Parkinson's disease. Dams were treated with an environmentally relevant PCB mixture at gestational day 10 and postnatal day 5. We used a motor battery that included tests of nigrostriatal function as well as cerebellar function, because PCBs deplete thyroid hormone, which is essential to normal cerebellar development. There was a significant effect of PCB treatment in the rotarod test with impaired performance in all three genotypes, but decreased motor learning as well in the two Cyp1a2(-/-) knockout lines. Interestingly, we found a main effect of genotype with corn oil-treated control Cyp1a2(-/-) mice performing significantly worse than Cyp1a2(+/+) wild type mice. In contrast, we found that PCB-treated high-affinity Ahr^b mice were most susceptible to disruption of nigrostriatal function with the greatest deficits in Ahr^bCyp1a2(-/-) mice. We conclude that differences in AHR affinity combined with the absence of CYP1A2 protein affect susceptibility to motor deficits following developmental PCB exposure.

Prenatal exposure to PCBs in Cyp1a2 knock-out mice interferes with F1 fertility, impairs long-term potentiation, reduces acoustic startle and impairs conditioned freezing contextual memory with minimal transgenerational effects

Polychlorinated biphenyls (PCBs) are toxic environmental pollutants. Humans are exposed to PCB mixtures via contaminated food or water. PCB exposure causes adverse effects in adults and after exposure in utero. PCB toxicity depends on the congener mixture and CYP1A2 gene activity. For coplanar PCBs, toxicity depends on ligand affinity for the aryl hydrocarbon receptor (AHR). Previously, we found that perinatal exposure of mice to a three-coplanar/five-noncoplanar PCB mixture induced deficits in novel object recognition and trial failures in the Morris water maze in Cyp1a2^-/- ::Ahr^b1 C57BL6/J mice compared with wild-type mice (Ahr^b1  = high AHR affinity). Here we exposed gravid Cyp1a2^-/- ::Ahr^b1 mice to a PCB mixture on embryonic day 10.5 by gavage and examined the F₁ and F₃ offspring (not F₂ ). PCB-exposed F₁ mice exhibited increased open-field central time, reduced acoustic startle, greater conditioned contextual freezing and reduced CA1 hippocampal long-term potentiation with no change in spatial learning or memory. F₁ mice also had inhibited growth, decreased heart rate and cardiac output, and impaired fertility. F₃ mice showed few effects. Gene expression changes were primarily in F₁ PCB males compared with wild-type males. There were minimal RNA and DNA methylation changes in the hippocampus from F₁ to F₃ with no clear relevance to the functional effects. F₀ PCB exposure during a period of rapid DNA de-/remethylation in a susceptible genotype produced clear F₁ effects with little evidence of transgenerational effects in the F₃ generation. While PCBs show clear developmental neurotoxicity, their effects do not persist across generations for effects assessed herein.