Comparative developmental neurotoxicity of organophosphates in vivo: transcriptional responses of pathways for brain cell development, cell signaling, cytotoxicity and neurotransmitter systems

The sea urchin embryo, an invertebrate model for mammalian developmental neurotoxicity, reveals multiple neurotransmitter mechanisms for effects of chlorpyrifos: therapeutic interventions and a comparison with the monoamine depleter, reserpine

Lower organisms show promise for the screening of neurotoxicants that might target mammalian brain development. Sea urchins use neurotransmitters as embryonic growth regulatory signals, so that adverse effects on neural substrates for mammalian brain development can be studied in this simple organism. We compared the effects of the organophosphate insecticide, chlorpyrifos in sea urchin embryos with those of the monoamine depleter, reserpine, so as to investigate multiple neurotransmitter mechanisms involved in developmental toxicity and to evaluate different therapeutic interventions corresponding to each neurotransmitter system. Whereas reserpine interfered with all stages of embryonic development, the effects of chlorpyrifos did not emerge until the mid-blastula stage. After that point, the effects of the two agents were similar. Treatment with membrane permeable analogs of the monoamine neurotransmitters, serotonin and dopamine, prevented the adverse effects of either chlorpyrifos or reserpine, despite the fact that chlorpyrifos works simultaneously through actions on acetylcholine, monoamines and other neurotransmitter pathways. This suggests that different neurotransmitters, converging on the same downstream signaling events, could work together or in parallel to offset the developmental disruption caused by exposure to disparate agents. We tested this hypothesis by evaluating membrane permeable analogs of acetylcholine and cannabinoids, both of which proved effective against chlorpyrifos- or reserpine-induced teratogenesis. Invertebrate test systems can provide both a screening procedure for mammalian neuroteratogenesis and may uncover novel mechanisms underlying developmental vulnerability as well as possible therapeutic approaches to prevent teratogenesis.

Developmental diazinon neurotoxicity in rats: later effects on emotional response

Developmental exposure to the organophosphorus pesticides chlorpyrifos and diazinon (DZN) alters serotonergic synaptic function at doses below the threshold for cholinesterase inhibition, however there are some indications that the two agents may differ in several important attributes. Previously, we found that low-dose chlorpyrifos exposure in neonatal rats causes lasting changes in emotional response and in the current study we did a comparable evaluation for DZN. Male and female Sprague-Dawley rat pups (N=10-12 of each sex per treatment group) were given 0, 0.5 or 2 mg/(kg day) of DZN s.c. daily on postnatal days (PND) 1-4. These doses bracket the threshold for barely-detectable cholinesterase inhibition. Starting on PND 52, these rats began a battery of tests to assess emotional reactivity. In the elevated plus maze, there was a slight decrease in the time spent in the open arms for DZN-exposed males, while DZN-exposed females were not different from control females. In the novelty-suppressed feeding test, DZN-exposed males had significantly shorter latencies to begin eating than did control males, reducing the values to those normally seen in females. DZN-exposed rats of either sex showed reduced preference for chocolate milk in the anhedonia test that compared the consumption of chocolate milk to water. These findings show that neonatal exposures to DZN at a dose range below the threshold for cholinesterase inhibition nevertheless evokes specific, later alterations in emotional behaviors, particularly in males. The effects show not only some similarities to those of chlorpyrifos but also some differences, in keeping with neurochemical findings comparing the two agents.

Ameliorating the developmental neurotoxicity of chlorpyrifos: a mechanisms-based approach in PC12 cells

BACKGROUND

Organophosphate developmental neurotoxicity involves multiple mechanisms converging on neural cell replication and differentiation.

OBJECTIVES

We evaluated mechanisms contributing to the adverse effects of chlorpyrifos (CPF) on DNA synthesis, cell number and size, and cell signaling mediated by adenylyl cyclase (AC) in PC12 cells, a neuronotypic cell line that recapitulates the essential features of developing mammalian neurons.

RESULTS

In undifferentiated cells, cholinergic receptor antagonists had little or no protective effect against the antimitotic actions of CPF; however, when nerve growth factor was used to evoke differentiation, the antagonists showed partial protection against deficits in cell loss and alteration in cell size elicited by CPF, but were ineffective in preventing the deterioration of AC signaling. Nicotine, which stimulates nicotinic acetylcholine receptors but also possesses a mixture of prooxidant/antioxidant activity, had adverse effects by itself but also protected undifferentiated cells from the actions of CPF and had mixed additive/protective effects on cell number in differentiating cells. The antioxidant vitamin E also protected both undifferentiated and differentiating cells from many of the adverse effects of CPF but worsened the impact on AC signaling. Theophylline, which prevents the breakdown of cyclic AMP, was the only agent that restored AC signaling to normal or supranormal levels but did so at further cost to cell replication.

CONCLUSIONS

Our results show definitive contributions of cholinergic hyperstimulation, oxidative stress, and interference with AC signaling in the developmental neurotoxicity of CPF and point to the potential use of this information to design treatments to ameliorate these adverse effects.