| Chlorpyrifos is a developmental neurotoxicant, causing impairment in the immature brain at doses below the threshold for systemic toxicity, so exposures relevant to adverse neurobehavioral endpoints may initially go undetected. We determined if exposure to chlorpyrifos at doses below the threshold for systemic toxicity affects the serotonergic system by administering chlorpyrifos (subcutaneous injection) to developing rats in four critical windows of development: gestational days (GD) 9-12, GD17-20, postnatal (PN) days 1-4, and PN11-14. Results demonstrated that the serotonin system represents a major target for chlorpyrifos in the developing brain. Immediately following chlorpyrifos exposure and in adulthood, we found critical windows for effects on serotonin receptors, the presynaptic serotonin transporter site, and cell signaling mediated by serotonin that range from the embryonic neural tube stage to terminal differentiation and synaptogenesis. We determined the functional consequences of the serotonin alterations by examining the effects of chlorpyrifos on behaviors with serotonin involvement in adulthood. Our results indicate that chlorpyrifos evoked a uniform set of behaviors consistent with decreased serotonin synaptic function: increased open-arm time in the elevated plus-maze, anhedonia in the chocolate consumption test and elimination of sex differences in cognition. Furthermore, chlorpyrifos-treated animals demonstrated an abnormal dependence on serotonin systems to complete cognitive tasks. To characterize the synaptic mechanisms underlying these effects, we examined serotonin content and synaptic activity (presynaptic turnover) in brain regions containing the major serotonin projections and found that chlorpyrifos exposure during the perinatal period evoked long-term increases in serotonin turnover across multiple regions, indicating presynaptic hyperactivity. The final objective of this dissertation was to determine if exposure to the beta-adrenoceptor agonist terbutaline preconditions the developing brain to enhance the subsequent vulnerability to chlorpyrifos. Both terbutaline and chlorpyrifos exposure alone resulted in upregulation of serotonin biomarkers and alterations in serotonin-mediated cell signaling. When animals were exposed sequentially to both agents, the outcomes were no more than additive, suggesting convergence of the two agents on a common set of developmental mechanisms. We conclude that chlorpyrifos, acting through non-cholinergic mechanisms, targets the serotonin system in the developing brain, resulting in disrupted architectural assembly of the serotonin system. |
