Short, medium, and long-term behavioral and molecular effects of the preweaning exposure to Chlorpyrifos in Wistar rats.

Abstract

Chlorpyrifos (CPF) is one of the most widely used Organophosphate pesticides (OP) in both developed and developing countries. Although its importance FOR human needs is unquestionable, its use has been systematically linked with several specific behavioral and physiological alterations, as well as with the incidence of numerous psychiatric, neurologic, neurodevelopmental, and neurodegenerative disorders. Even though the main mechanism of toxicology of CPF is the irreversible inhibition of Acetylcholinesterases (AChE) in the central nervous system (CNS), several authors have found alternative molecular targets (e.g., different components within the cholinergic system and from other neurotransmitter systems) without the implication of AChE by using doses wellbellow the range of systemic inhibition, defining a non-cholinesterase profile of neurotoxicology (NChEI). Interestingly, this profile has primarily been observed following developmental exposure protocols when the immature CNS appears to be more sensitive to OP intoxication. However, the different behavioral and physiological alterations induced by developmental exposure to NChEI doses of CPF can vary depending on the specific developmental stage, with different or opposing results for a specific variable according to whether the exposure occurs during gestation (early vs. late ages) or postnatally (neonatal vs. preweaning exposures). To date, preweaning ages are one of the least studied developmental periods. This is relevant, since the CNS of rodents at these ages is equivalent to the perinatal CNS in humans in developmental terms, an age known to be particularly sensitive to external stressors, including environmental chemicals. Based on this, we considered that the short, medium and long-term behavioral and molecular effects of exposure to NChEI doses of CPF must be analyzed in more depth, particularly at preweaning ages (Postnatal day 10 to 15) and using lower doses (1mg/kg/day) than those commonly reported in the current literature. In addition to this, we were also interested in studying the existence of other molecular targets that could explain the alternative mechanism of actions of CPF exposure. Following other authors, we also included both sexes, since the effects of CPF exposure have been shown to be sex-dependent for a multitude of behaviors. Our main results indicate that preweaning exposure to NChEI doses of CPF hypo and hypersensitized the cholinergic and the GABAergic systems, respectively when challenged with specific drugs. This was also accompanied by the up-regulation of both the muscarinic 2 receptor and the GABA-A- α2 subunit receptor genes in the dorsal striatum and the frontal cortex, respectively, during early adulthood. A significant ecrease in the mRNA expression of the brainderived neurotrophic factor was also found at the dorsal striatum during adulthood. CPF exposure altered motricity by both increasing (adolescence) and decreasing (adulthood and late-adulthood) various locomotor outcomes, and also hypersensitized females to stressors (following an i.p. injection of saline) during adolescence. CPF exposure had no effect on sociability (adolescence and adulthood) or dominance (adulthood), whilst it did not alter basal attention (adulthood), inhibitory control (adulthood), spatial learning/memory (late-adulthood) skills or anxiety state. However, exposed females showed enhanced learning during adulthood in sustained attention-based tasks, which was not observed later in aging rats in spatial-memory-based mazes, whilst the exposed animals showed increased impulsive action behaviors, which were revealed when the contextual contingencies were manipulated (increasing the inter-trial interval in the 5- choice serial reaction time task). In addition to these behavioral, pharmacological, and brain gene expression effects, CPF also induced both medium (adolescence) and longterm (adulthood) gut dysbiosis involving several bacteria at both genus and species levels. CPF exposure also altered the levels of numerous plasma metabolites by inducing a longterm (adulthood) hyperlipidemic, hypoglycemic, and an apparent decreased cell energy production exclusively in adult female rats. All of the empirical data derived from the present doctoral thesis is novel and relevant with regard to the exposure protocol used, completely new regarding the exposure to CPF at any developmental stage, or even the exposure to CPF at any moment of the life-span. However, future research concerning this exposure protocol should include a more in-depth analysis of components of the cholinergic and GABAergic systems, at the both pharmacological and molecular levels, and we emphasize the necessity to analyze the protein expression level along with mRNA outcomes. The physiological basis of the different motoric and impulsive-like behaviors should also be further explored.