Inverse correlation of acetylcholinesterase (AChE) activity with the presence of neurofilament inclusions in dorsal root ganglion neurons cultured in the presence of a reversible inhibitor of AChE

Microtubule-associated targets in chlorpyrifos oxon hippocampal neurotoxicity

Prolonged exposure to organophosphate (OP) pesticides may produce cognitive deficits reflective of hippocampal injury in both humans and rodents. Recent work has indicated that microtubule trafficking is also adversely affected by exposure to the OP pesticide chlorpyrifos, suggesting a novel mode of OP-induced neurotoxicity. The present studies examined effects of prolonged exposure to chlorpyrifos oxon (CPO) on acetylcholinesterase (AChE) activity, immunoreactivity (IR) of microtubule-associated proteins, neuronal injury, and tubulin polymerization using in vitro organotypic slice cultures of rat hippocampus and bovine tubulin. Cultures were exposed to CPO (0.1-10 microM) in cell culture medium for 1-7 days, a regimen producing progressive reductions in AChE activity of 15-60%. Cytotoxicity (somatic uptake of the non-vital marker propidium iodide), as well as IR of alpha-tubulin and microtubule-associated protein-2 (a/b) [MAP-2], was assessed 1, 3, and 7 days after the start of CPO exposure. As early as 24 h after the start of exposure, CPO-induced deficits in MAP-2 IR were evident and progressive in each region of slice cultures at concentrations as low as 0.1 microM. CPO exposure did not alter alpha-tubulin IR at any time point. Concentration-dependent injury in the cornu ammonis (CA)1 pyramidal cell layer and to a lesser extent, CA3 and dentate cells, was evident 3 days after the start of CPO exposure (>or=0.1 microM) and was greatest after 7 days. Tubulin polymerization assays indicated that CPO (>or=0.1 microM) markedly inhibited the polymerization of purified tubulin and MAP-rich tubulin, though effects on MAP-rich tubulin were more pronounced. These data suggest that exposure to CPO produces a progressive decrease in neuronal viability that may be associated with impaired microtubule synthesis and/or function.

Comparative neurochemical effects of repeated methyl parathion or chlorpyrifos exposures in neonatal and adult rats

Several studies have reported higher sensitivity based on lethality in young animals compared to adults following acute exposure to organophosphorus insecticides (OPs). We propose that age-related differences in sensitivity to OPs may differ qualitatively and quantitatively with different OPs and varying exposure conditions (e. g., high vs. low dose, acute vs. repeated). To test this hypothesis, we treated neonatal (7 days of age) and adult (90 days of age) rats with either methyl parathion (MPS) or chlorpyrifos (CPF) daily for 14 days and measured neurochemical endpoints {cholinesterase (ChE) inhibition, total muscarinic receptor ([(3)H]quinuclidinyl benzilate, QNB) and muscarinic M2 subtype-preferential ([(3)H]AF-DX 384) binding} in frontal cortex and striatum at timepoints both during (1 day after the 7(th) and 14(th) dose) and after (8 days after the 14(th) dose) exposures. Repeated CPF exposures were associated with relatively similar degrees of ChE inhibition between the age groups during dosing but more extensive inhibition was noted in adults after termination of exposures. Relatively similar changes in muscarinic receptor binding were also noted between age groups following CPF exposures. Moreover, the degree of muscarinic receptor binding reduction relative to ChE inhibition appeared similar in both age groups following CPF exposures. In contrast, ChE activity and muscarinic receptor binding were generally more reduced in neonatal relative to adult brain regions following repeated MPS exposures. Furthermore, the relationship between the degree of ChE inhibition and the reduction in cortical muscarinic receptor binding appeared different between the age groups, i.e., more extensive reduction was noted in neonates compared to adults with a given level of ChE inhibition. We conclude that OP-selective differences in in vivo ChE sensitivity, differential rates of enzyme recovery following inhibition, and age-dependent differences in muscarinic receptor adaptations can all influence the nature of age-related susceptibility to OPs.

Cholinesterase inhibitors induce growth cone collapse and inhibit neurite extension in primary cultured chick neurons

Cholinesterase inhibitors are commonly prescribed medicines in neurological and anesthesiological clinical practices at relatively high doses. Recent studies report that cholinesterases, originally described as neurotransmitter degradation enzymes, are expressed in the developing central nervous systems. They are hypothesized to play an important role during the establishment of neuronal cytoarchitecture. In this study, the effects of several clinically utilized cholinesterase inhibitors on one potential aspect of neuronal development were examined using a primary culture system of chick central and peripheral neurons. Three cholinesterase inhibitors, physostigmine, neostigmine, and edrophonium, suppressed the activity of the leading edges of the extending axons (the nerve growth cones) dose dependently. Filopoda and lamellipodia of nerve growth cones were collapsed by the exposure to the cholinesterase inhibitors. This action was quick, mostly within 5 min, and partly irreversible. The lowest concentration that could induce statistically significant growth cone collapse was 10(-5) M for physostigmine, 10(-4) M for neostigmine, and 10(-2) for edrophonium, respectively. The ED50 values for the growth cone collapsing activity were approximately, 10(-3) M for physostigmine. 10(-2.5) M for neostigmine, and 10(-2) M for edrophonium both in dorsal root ganglion culture and in retinal culture. Even though the result of this in vitro study cannot be applied directly to the in vivo situation, physicians should consider the potential detrimental effects of cholinesterase inhibitors to growing and regenerating nervous tissues.

Acetylcholinesterase inhibitor treatment delays recovery from axotomy in cultured dorsal root ganglion neurons

We have previously reported that dorsal root ganglion neurons cultured in the presence of the highly specific, reversible acetylcholinesterase inhibitor 1,5-bis-(4-allyldimethylammoniumphenyl) pentan-3-one dibromide (BW284c51), showed significantly reduced neurite outgrowth and contained massive perikaryal inclusions of neurofilaments. In the present report we have more closely examined these changes in a time course study over a 21-day culture period using a combined morphological, immunocytochemical and enzymatic approach and additionally, describe, the effects of acetylcholinesterase inhibitor treatment on the state of neurofilament phosphorylation. Finally, we have examined the effects of co-administration of N6,2'-0-dibutyryladenosine 3':5'-cyclic monophosphate (dbcAMP) with BW284c51. At 1 day in culture, both control and treated cells displayed eccentrically located nuclei, numerous polysomes and perikaryal accumulations of neurofilaments which were immunoreactive with both phosphorylation- and nonphosphorylation-dependent neurofilament antibodies. These cytological changes, which are common features of the chromatolytic reaction following axotomy in vivo, rapidly resolved in the control neurons, where by 7 days in culture, the neurofilament accumulations had completely disappeared and neurite outgrowth was robust. In contrast, inhibitor-treated neurons retained the post-axotomy features up to 21 days and had significantly reduced neurite outgrowth. In addition, we have investigated a possible role of cyclic adenosine monophosphate (cAMP) in the recovery process since it has been shown to enhance neuritic outgrowth in cultured neurons. Our results demonstrate that the addition of dbcAMP, a membrane permeable analog of cAMP, significantly enhanced neuritic outgrowth and accelerated the recovery of BW284c51-treated dorsal root ganglion cells, as gauged by the disappearance of the axotomy-related cytological changes. Treatment with dbcAMP also increased acetylcholinesterase activity which has been positively correlated with neurite outgrowth both in vivo and in vitro. Together, these observations suggest that acetylcholinesterase has a non-cholinolytic, neurotrophic role in neuronal regeneration and development.