Toxicokinetic and toxicodynamic modeling explains carry-over toxicity from exposure to diazinon by slow organism recovery

Carry-over toxicity occurs when organisms exposed to an environmental toxicant survive but carry some damage resulting in reduced fitness. Upon subsequently encountering another exposure event stronger effects are possible if the organisms have not yet fully recovered. Carry-over toxicity was observed after exposure of the freshwater amphipod Gammarus pulex to repeated pulses of diazinon with varying intervals. Uptake, biotransformation and depuration kinetics were determined. Metabolites were identified and quantified (diazoxon, 2-isopropyl-6-methyl-4-pyrimidinol, one nonidentified metabolite). Parameters of a process-based toxicokinetic-toxicodynamic model were determined by least-squares fitting followed by Markov Chain Monte Carlo parameter estimation. Model parametrization was based on the time-course of measured internal concentrations of diazinon and its metabolite diazoxon in combination with the pulsed toxicity experiment. Prediction intervals, which take the covariation between parameters into account, were calculated for bioaccumulation factors, organism recovery time and simulations of internal concentrations as well as the time-course of survival under variable exposure. Organism recovery time was 28 days (95% prediction interval 25-31 days), indicating the possibility for carry-over toxicity from exposure events several weeks apart. The slow organism recovery and carry-over toxicity was caused by slow toxicodynamic recovery; toxicokinetic processes alone would have resulted in a recovery time of only 1-2 days.

Alarm substance recognition and predator avoidance by chinook salmon (Oncorhynchus tschawytscha) following exposure to an organophosphate pesticide

The sense of smell in fish is thought to be important for behaviors associated with alarm scent recognition and predator avoidance. Olfactory function of juvenile chinook salmon was evaluated in two separate experiments following 2-h immersion exposures to 0, 1, 10, or 100 microg diazinon/L. In the first test system, groups of 10 salmon were transferred to the prechoice zone of a Y-maze fluviarium with parallel streams of water. A dilute alarm scent composed of either chinook skin extract or L-serine was pumped into the water supply of one arm. Fish were allowed to choose either upstream arm by raising a containment screen for 10 min, then their positions were photographed and the screen lowered. In the second test system the relationship of diazinon exposure to predation susceptibility was directly examined by transferring groups of 20 salmon (10 diazinon-exposed, 10 control) to troughs containing a protective cover. These salmon were given two olfactory warnings by delivering conspecific skin extract to both their transfer container and the predation tank they entered. After a 2-min prey acclimation period, larger predator rainbow trout were allowed access to the downstream section of the tank containing the test salmon. The combined fish were observed until approximately 50% of the prey fish had been consumed. Statistical comparisons indicated that although the salmon did avoid the water streams containing alarm scents, none of the diazinon exposure levels resulted in significantly different outcomes from the controls (alpha=0.05) in either the two-choice maze study or the predation study. Possible environmental and population implications are discussed in light of related olfaction and diazinon studies.

Degradation of diazinon in apple juice by ultrasonic treatment

The degradation of diazinon spiked in apple juice treated by ultrasonic treatment was investigated in this paper. Results showed that the ultrasonic power and initial concentration of diazinon significantly influenced the degradation percentage of diazinon (p<0.05) and the degradation of diazinon followed the first-order kinetics model well. Based on the gas chromatography-mass spectrometry (GC-MS) analysis, seven degradation products of diazinon have been identified. A degradation pathway involving hydrolysis of the ester moiety, oxidation, hydroxylation, dehydration, and decarboxylation was proposed. Simultaneously, the toxicity of apple juice was mitigated by the ultrasonic treatment based on the photobacterium bioassay.

Role of food and clay particles in toxicity of copper and diazinon using Daphnia magna

Toxicity changes in copper and diazinon were investigated in the presence of food, clay, and their mixture by using Daphnia magna. In sorption equilibrium experiments, copper was significantly attracted (>34% sorbed) to food, clay, and food-clay mixture due to their negative zeta potential, while diazinon was less sorbed (<11%). In the exposure test with food and clay particles, it was revealed that copper was remarkably reduced in the presence of clay particles indicating the change in bioavailability of copper by sorption to clay. This was considered as the primary mechanism for toxicity reduction whereas diazinon toxicity was food dependent in the analysis of toxicity using toxicity change index (TCI). It was also shown that certain foods could not only act as a sorbent to copper and diazinon, but also as a material of energy source to alleviate the toxic damage. Meanwhile, clay can be considered as a prominent sorbent to copper but not to diazinon and can inhibit the sorption interaction between foodstuffs and toxicants through the aggregation and sedimentation processes. Furthermore, clay particles, as shown in TCI analysis, may be a potentially risky material as a physiological stressor or a toxicant carrier in contaminated environments.

Variation in plasma cholinesterase activity in the clay-colored robin (Turdus grayi) in relation to time of day, season, and diazinon exposure

Cholinesterase (ChE) activity in birds is subject to interspecific and intraspecific species variations. Factors that influence enzyme activity have to be taken into account in order to obtain an accurate estimation of cholinesterase inhibition due to pesticide exposure in wild birds. This study evaluates variation of plasma cholinesterase activity in clay-colored robin (Turdus grayi) in relation to time of day, season, and exposure to diazinon. Other variables that can affect cholinesterase activity such as weight are also taken into account. The birds were marked, weighed and sexed using the cloacal technique. One dose of commercial diazinon mixed with papaya was fed to each bird at concentrations of 0.0, 0.5, 1.5 and 3.0 mg/kg ai. The results showed differences in ChE activity between seasons (t = -3.07, P < 0.05). Also, diurnal plasma cholinesterase variations were observed (20% in 2 h). The highest inhibition values were 73% for birds dosed with 1.5 mg/kg ai. Our study provides field and laboratory data on variation of ChE activity in a tropical bird species. Knowledge of the variation of ChE in the clay-colored robin will enable us to use this species as an indicator of exposure to ChE inhibiting pesticides in tropical agroecosystems.

Disparate developmental neurotoxicants converge on the cyclic AMP signaling cascade, revealed by transcriptional profiles in vitro and in vivo

Cell-signaling cascades are convergent targets for developmental neurotoxicity of otherwise unrelated agents. We compared organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)) for their effects on neuronotypic PC12 cells, assessing gene transcription involved in the cyclic AMP pathway. Each agent was introduced during neurodifferentiation at a concentration of 30 microM for 24 or 72 h and we assessed 69 genes encoding adenylyl cyclase isoforms and regulators, G-protein alpha-and beta,gamma-subunits, protein kinase A subtypes and the phosphodiesterase family. We found strong concordance among the four agents across all the gene families, with the strongest relationships for the G-proteins, followed by adenylyl cyclase, and lesser concordance for protein kinase A and phosphodiesterase. Superimposed on this pattern, chlorpyrifos and diazinon were surprisingly the least alike, whereas there was strong concordance of dieldrin and Ni(2+) with each other and with each individual organophosphate. Further, the effects of chlorpyrifos differed substantially depending on whether cells were undifferentiated or differentiating. To resolve the disparities between chlorpyrifos and diazinon, we performed analyses in rat brain regions after in vivo neonatal exposures; unlike the in vitro results, there was strong concordance. Our results show that unrelated developmental neurotoxicants can nevertheless produce similar outcomes by targeting cell signaling pathways involved in neurodifferentiation during a critical developmental period of vulnerability. Nevertheless, a full evaluation of concordance between different toxicants requires evaluations of in vitro systems that detect direct effects, as well as in vivo systems that allow for more complex interactions that converge on the same pathway.

Rhinitis associated with pesticide exposure among commercial pesticide applicators in the Agricultural Health Study

OBJECTIVES

Rhinitis is common, but the risk factors are not well described. To investigate the association between current rhinitis and pesticide use, we used data from 2245 Iowa commercial pesticide applicators in the Agricultural Health Study.

METHODS

Using logistic regression models adjusted for age, education and growing up on a farm, we evaluated the association between current rhinitis and 34 pesticides used in the past year.

RESULTS

74% of commercial pesticide applicators reported at least one episode of rhinitis in the past year (current rhinitis). Five pesticides used in the past year were significantly positively associated with current rhinitis: the herbicides 2,4-D, glyphosate and petroleum oil, the insecticide diazinon and the fungicide benomyl. The association for 2,4-D and glyphosate was limited to individuals who used both in the past year (OR 1.42, 95% CI 1.14 to 1.77). Both petroleum oil and diazinon showed consistent evidence of an association with rhinitis, based on both current use and exposure-response models. We saw no evidence of confounding by common agricultural rhinitis triggers such as handling grain or hay.

CONCLUSIONS

Exposure to pesticides may increase the risk of rhinitis.

Comparative toxicity of pesticides to stingless bees (Hymenoptera: Apidae: Meliponini)

Stingless bees are potential pollinators of commercial tropical crops and their use may increase in the short term. However, studies comparing the toxicity of pesticides to different individuals and species are lacking, making it difficult to evaluate their short- and long-term effects on colonies and populations of these insects. In this work, we tested the lethality of compounds from the main pesticide groups on stingless bees of the species Melipona beecheii Bennett, Trigona nigra Provancher, and Nannotrigona perilampoides Cresson. The LDo (in micrograms per bee) for each pesticide was calculated for callow workers and foragers of the three species as well as for gynes and drones of M. beecheii. The results showed that all species were highly susceptible to the evaluated compounds. Nicotinoid pesticides were the most toxic, followed in descending order by permethrin, diazinon, and methomyl. We found evidence of a relationship between the body weight of the species and their LD50 for permethrin and methomyl (r = 0.91 and 0.90, respectively) but not for diazinon (r = -0.089). An analysis of contingency tables showed that within each species, callow workers had higher mortalities than foragers (P < 0.01). In M. beecheii at similar pesticide dose more males died compared with females [chi2((0.0),1) = 10.16]. However, gynes were less resistant than workers [chi2((0.01),1)) = 8.11]. The potential negative consequences of pesticides to native stingless bees are discussed considering the reproductive biology of these insects. It is important to take actions to prevent damage to these key species for the ecology and agriculture of Mexico and Latin America

Comparing the inhibitory effects of five protoxicant organophosphates (azinphos-methyl, parathion-methyl, chlorpyriphos-methyl, methamidophos and diazinon) on the spontaneously beating auricle of Sparus aurata: an in vitro study

Organophosphates (OPs) can provoke toxicity by inhibiting acetylcholinesterase (AChE) in non-target organisms, like fish. In a previous pilot study, the anticholinesterase effects of paraoxon on the heart of Sparus aurata were examined [Tryfonos, M., Antonopoulou, E., Papaefthimiou, C., Chaleplis, G., Theophilidis, G., 2009. An in vitro assay for the assessment of the effects of an organophosphate, paraoxon, and a triazine, atrazine, on the heart of the gilthead sea bream (Sparus aurata). Pest. Biochem. Physiol. 93, 40-46]. The objective of the present study was to investigate the effects of the five protoxicant OPs, azinphos-methyl (MeAZP), parathion-methyl (MePS), chlorpyriphos-methyl (MeCCP), methamidophos (MET) and diazinon (DZ), on the spontaneously beating auricle of S. aurata. The results showed that: (1) MeAZP and MET induced exclusively cholinergic effects on auricle contractility. These effects were expressed as a significant decrease in the force and frequency of contractions and were fully reversible (140%) after the application of the muscarinic cholinergic receptor antagonist, atropine (15 microM). MeAZP was found to be the most effective anticholinesterase compound, with an IC(50) of 2.19+/-1.05 microM (n=6), while MET was less effective, with an IC(50) of 72.3+/-1.2 microM (n=6). (2) DZ and MePS, although classified as OPs, induced non-cholinergic effects. These effects were observed as an irreversible decrease in force and frequency of the auricle in all the concentrations examined; the depression is retained even after application of 15 microM atropine. (3) MeCCP was halfway between a typical OP and an OP lacking anticholinesterase properties, since there was a partial recovery in the force, but no recovery in the frequency of the auricle contractions. (4) The toxicity order, based on the IC(50), was as follows: MeAZP, 2.19+/-1.05 microM>paraoxon, 3.2+/-1.5 microM"MET, 72.3+/-1.2 microM>MePS, 80.3+/-1.03 microM>MeCPP, 93.7+/-1.01 microM>DZ, 164+/-1.01 microM. (5) There was a good correlation (r=0.779, p=0.04, n=5) between IC(50) and the previously determined logP (octanol:water partition coefficient) values for MeAZP, paraoxon, MeCCP, MePS and DZ. The results indicated that the increase in lipophilicity of MePS, MeCCP and DZ is accompanied by a decrease in their acute cardiotoxic properties in vitro. The non-cholinergic effects of these relatively high lipophilic OPs, might be caused by their tendency to distribute preferentially in the lipid bilayer of cardiac cells, affecting the proper functioning of the ionic channels which regulate the force (Ca(2+) channels) and the frequency (K(+) channels) of the spontaneous auricle contractions.

Immunologic parameters evaluations in Nile tilapia (Oreochromis niloticus) exposed to sublethal concentrations of diazinon

Fish resistance to microorganisms depends basically on the immune response. Although there are several studies on the diazinon mammalian immunotoxicity, in the case of fish there are only few. The aim of present study was to evaluate the effect of diazinon on immunological parameters (relative spleen weight, splenocytes count, lysozyme activity, respiratory burst and IgM concentration) in Nile tilapia. Diazinon at sublethal concentrations (0.39 and 0.78 mg/L) did not alter RSW, splenocytes count or lysozyme activity. However, at the highest concentration tested (1.96 mg/L) diazinon significantly increased respiratory burst and IgM concentration. In summary, diazinon (and perhaps other pesticides) could alter immunological response and induce oxidative stress.

Diazinon resistance in different selected strains of Chilo suppressalis (Lepidoptera: Crambidae) in northern Iran

Asiatic rice borer, Chilo suppressalis (Walker) (Lepidoptera: Crambidae), is a cosmopolitan and destructive pest in rice fields of the world. This pest was reported in 1973 in Iran, and it has since spread widely in rice, Oryza sativa L., fields throughout the country. In this study, we tried to evaluate comparative toxicity of diazinon in five colonies of C. suppressalis, collected from Babol (Ba), Amol (Am) of Mazandaran Province and Rasht (Ra), Sheikhmahale (Sh), and Gourabzarmikh (Go) of Guilan Province, northern Iran. The LD50 values were compared. We also evaluated the general esterases, alkaline phosphatase (ALP), glutathione transferase (GST), and acetylcholinesterase (AChE) activities from the five populations. The LD50 values of Ra, Ba, Am, and Sh (12.64, 11.4, 7.17, and 3.71 microg/mg larva(-1)) were 13.67-, 12.33-, 7.75-, and 4.02-fold higher than Go population (0.924 microg/mg larva(-1)). Using alpha-naphthyl acetate as substrate, the general esterase activities in Ra, Ba, Am, and Sh colonies were, respectively, 1.81-, 1.68-, 1.75-, and 1.35-fold more than those in Go population. When beta-naphthyl acetate was used as the substrate, activity ratio was measured 1.98-, 2.58-, 1.25-, and 1.24-fold compared with the Go population. Glutathione transferase activities in Ra, Ba, Am, and Sh populations were 1.27-, 1.68-, 0.98-, and 1.7-fold more than those in Go, when 1-chloro-2,4-dinitrobenzene was used as the substrate. When 1,2-dichloro-4-nitro-benzene was used as the substrate, activity ratio was measured 1.14-, 1.42-, 0.56-, and 0.95-fold compared with Go population. The ALP activity demonstrated a significant difference among these populations and in Ra, Ba, Am, and Sh larvae were 3.54-, 4.62-, 3.84-, and 2.18-fold more than Go. The AChE inhibition or I50 value was 0.19, 0.22, 0.31, 0.19, and 0.26 mM in Ra, Ba, Am, Sh and Go populations, respectively. However, the results showed no significant differences in studied colonies. These biochemical characterizations of general esterases ALP, GST, and AChE were consistent with diazinon bioassay in the five populations. It is inferred from increased esterase, alkaline phosphatase and glutathione transferase, activities that might play an important role in the increasing resistance in C. suppressalis to diazinon among these five populations.

Efficiency of experimental rice (Oryza sativa L.) fields in mitigating diazinon runoff toxicity to Hyalella azteca

This study assessed the viability of using planted, mature rice fields in mitigating diazinon (an organophosphate insecticide) runoff toxicity using aqueous 48 h Hyalella azteca whole effluent toxicity bioassays. Rice fields decreased diazinon concentrations 80.1%-99.9% compared with 10.8% in the unvegetated field control. H. azteca survival responses coincided with observed diazinon concentrations. Estimated LC50 effects dilutions (%) ranged from 1.15 to 1.47 for inflow samples and 1.66 (unvegetated), 6.44 (rice field A), and >100 (rice field B) outflow samples. Decreases in inflow versus outflow aqueous toxicity were 77.1%-100% in rice fields compared with 18.7% in the unvegetated field.

Oxidative and excitatory mechanisms of developmental neurotoxicity: transcriptional profiles for chlorpyrifos, diazinon, dieldrin, and divalent nickel in PC12 cells

BACKGROUND

Oxidative stress and excitotoxicity underlie the developmental neurotoxicity of numerous chemicals.

OBJECTIVES

We compared the effects of organophosphates (chlorpyrifos and diazinon), an organo-chlorine (dieldrin), and a metal [divalent nickel (Ni2+)] to determine how these mechanisms contribute to similar or dissimilar neurotoxic outcomes.

METHODS

We used PC12 cells as a model of developing neurons and evaluated transcriptional profiles for genes for oxidative stress responses and glutamate receptors.

RESULTS

Chlorpyrifos had a greater effect on oxidative-stress-related genes in differentiating cells compared with the undifferentiated state. Chlorpyrifos and diazinon showed significant concordance in their effects on glutathione-related genes, but they were negatively correlated for effects on catalase and superoxide dismutase isoforms and had no concordance for effects on ionotropic glutamate receptors. Surprisingly, the correlations were stronger between diazinon and dieldrin than between the two organophosphates. The effects of Ni2+ were the least similar for genes related to oxidative stress but had significant concordance with dieldrin for effects on glutamate receptors.

CONCLUSIONS

Our results point to underlying mechanisms by which different organophosphates produce disparate neurotoxic outcomes despite their shared property as cholinesterase inhibitors. Further, apparently unrelated neurotoxicants may produce similar outcomes because of convergence on oxidative stress and excitotoxicity. The combined use of cell cultures and microarrays points to specific end points that can distinguish similarities and disparities in the effects of diverse developmental neurotoxicants.

Effects of 3,4-dichloroaniline and diazinon on different biological organisation levels of zebrafish (Danio rerio) embryos and larvae

In this study the effects of 3,4-dichloroaniline (3,4-DCA), a decomposition product of the herbicides propanil and diuron (and other pesticides), and diazinon, a neurotoxic insecticide, on early life stages of zebrafish Danio rerio were assessed. The toxicity of these substances with different modes of action (acetylcholine esterase inhibitor vs. polar narcosis) was tested for single substances as well as in binary mixtures. To study effects on different biological organisation levels (from the molecular up to the whole organism level) the molecular stress response regarding Hsp70, the embryonic and larval development and the locomotor activity were investigated as integrative biomarkers. In single substance tests 3,4-dichloroaniline elicited deformations at > or = 0.25 mg/l during the 11 days subchronic test, whereas locomotor activity and mortality were impaired at > or = 0.5 mg/l. Diazinon effects on those parameters were obvious at > or = 2 mg/l, except for the deformation rate (11 days:1 mg/l). In equitoxic mixtures of both substances concentration additivity was observed for deformation rate and mortality (11 days). An increase in the Hsp70 content occurred in zebrafish exposed to 0.25 mg 3,4-DCA/l as well as to 0.05 mg diazinon/l; in mixtures concentration additivity could be shown. The investigated endpoints varied in respect to their sensitivity, with deformations and Hsp70 levels as most sensitive parameters concerning 3,4-DCA and Hsp70 as most sensitive parameter concerning diazinon. Accordingly, for an integrated understanding of the effects of chemicals and their mixtures on fish, a battery of different test methods should be applied.

Effects of repeated exposure of diazinon on cholinesterase activity and growth in snakehead fish (Channa striata)

The organophosphate insecticide diazinon is widely used in the Mekong river delta and often applied several times per rice crop. In the present study, juvenile snakehead fish Channa striata, which is a commercially important inhabitant of rice fields, were exposed twice to 4-day pulses of 0.016, 0.079 or 0.35mg/L of diazinon, separated by a 2 week interval to imitate the exposure conditions in the field. After the 4-day exposures to these environmentally realistic concentrations, the fish were moved to clean water for recovery. During this experiment, which lasted a total of 2 months, the individual growth rates and brain cholinesterase levels were measured. We show not only that diazinon caused long term inhibition of brain ChE activity, which was still significantly depressed at the termination of the experiment, but also that the highest of these realistic concentrations caused a significant 30% growth inhibition.

Melatonin prevents damage elicited by the organophosphorous pesticide diazinon on the mouse testis

Organophosphates like O,O-diethyl O-2-isopropyl-6-methyl pyrimidinyl-4-g-1-phosphorothioate (diazinon) are pesticides used worldwide, which can affect both animals and man even after a single exposure. Whereas their toxicity is due to acetylcholinesterase inhibition, their secondary toxic effects have been related to free oxygen radicals. This study evaluates the effects of a single dose of diazinon and melatonin-a powerful antioxidant-on plasmatic acetylcholinesterase activity and testis histopathology in adult mice 1 and 32 days post-treatment. Diazinon diminished the plasma acetylcholinesterase activity on day 1 post-treatment, although testosterone levels remained unaffected. Morphometrical analysis showed a decrease in seminiferous epithelium height (days 1 and 32), whereas an increase in testicular superoxide dismutase (SOD) activity was detected (day 32). Melatonin pretreatment prevented every alteration induced by diazinon, except the diminution of acetylcholinesterase plasmatic activity. Testicular damage might be due to elevated concentrations of free oxygen radicals released upon diazinon exposure, inducing alterations in the DNA and promoting local apoptosis; however, antioxidant pretreatment with melatonin prevents or diminishes this damage.

Melatonin prevents damage elicited by the organophosphorous pesticide diazinon on mouse sperm DNA

Toxic effects of pesticides are commonly associated with DNA damage. To evaluate the effect of the organophosphate diazinon on sperm DNA and to test whether melatonin could prevent this damage, male mice were intraperitoneally treated with melatonin, diazinon (1/3 or 2/3 LD50) or both; cauda epididymal spermatozoa were obtained on days 1 and 32 postinjection and tested for DNA alterations. On day 1, sperm from diazinon-treated mice showed augmented DNA breakages and reduced chromatin packaging, whilst DNA damage increased only in the diazinon 2/3 LD50 group. Micronucleus test of bone marrow cells demonstrated somatic cell chromosomal damage in both diazinon-treated groups. Pretreatment with melatonin before diazinon acute administration improved all parameters studied on day 1 pi. The organophosphorous pesticide diazinon is a dose-dependent testicular toxicant that alters the sperm DNA structure; melatonin is able to prevent this damage.

Biochemical and behavioral effects of diazinon exposure in hybrid striped bass

The effects of environmental stimuli on biochemical processes may influence behavior. Environmental contaminants that alter behavior can have major impacts on populations as well as community structures by changing species' interactions. One important behavior is the ability to capture prey. We hypothesized that sublethal exposure to diazinon, an organophosphate pesticide, may lead to feeding behavior abnormalities in hybrid striped bass (Morone saxatilis x M. chrysops) through inhibition of brain acetylcholinesterase (AChE) activity. This can potentially reduce the organism's survival by affecting its ability to find and capture food. To test this hypothesis, bass were exposed to diazinon for 6 d, followed by a 6-d recovery period in clean water. Brain AChE activity and the ability of bass to capture prey fathead minnows (Pimephales promelas) were measured every third day. Exposed fish exhibited a concentration- and duration-dependent decrease in ability to capture prey. While bass in all diazinon treatment groups had significantly inhibited brain AChE activity, only the medium and high treatment groups showed a dose- and time-dependent increase in time to capture prey. Acetylcholinesterase activity also decreased in an exposure duration- and concentration-dependent manner. The AChE levels in exposed fish did not recover to control levels during the 6-d recovery period. These results suggest that sublethal exposure to AChE-inhibiting substances may decrease the ecological fitness of hybrid striped bass by reducing their ability to capture prey.

Effects of diazinon and diazoxon on the lymphoproliferation rate of splenocytes from Nile tilapia (Oreochromis niloticus): the immunosuppresive effect could involve an increase in acetylcholine levels

The lymphoproliferation rate of spleen cells from Nile tilapia (Oreochromis niloticus) exposed to the organophosphorus pesticide diazinon, to its metabolite diazoxon and to the neurotransmitter acetylcholine, was evaluated in order to explore the immunotoxic mechanism of action of this widely used insecticide. The lymphoproliferative response of spleen cells to mitogenic stimulus was not affected by either diazinon or diazoxon, indicating that these xenobiotic substances do not have direct immunotoxic properties. Conversely, ex vivo assays showed that spleen from fish exposed to diazinon presented a lower acetylcholinesterase activity and a higher acetylcholine concentration than non-exposed controls. Lymphoproliferation assays also indicated that pre-exposure to acetylcholine depleted the proliferative function of spleen cells. Thus the combined information from in vitro and ex vivo experiments suggest that the immunotoxic properties of diazinon in Nile tilapia are indirect and could involve the cholinergic system of lymphocytes.

Use of toxicity identification evaluations to determine the pesticide mitigation effectiveness of on-farm vegetated treatment systems

Evidence of ecological impacts from pesticide runoff has prompted installation of vegetated treatment systems (VTS) along the central coast of California, USA. During five surveys of two on-farm VTS ponds, 88% of inlet and outlet water samples were toxic to Ceriodaphnia dubia. Toxicity identification evaluations (TIEs) indicated water toxicity was caused by diazinon at VTS-1, and chlorpyrifos at VTS-2. Diazinon levels in VTS-1 were variable, but high pulse inflow concentrations were reduced through dilution. At VTS-2, chlorpyrifos concentrations averaged 52% lower at the VTS outlet than at the inlet. Water concentrations of most other pesticides averaged 20-90% lower at VTS outlets. All VTS sediment samples were toxic to amphipods (Hyalella azteca). Sediment TIEs indicated toxicity was caused by cypermethrin and lambda-cyhalothrin at VTS-1, and chlorpyrifos and permethrin at VTS-2. As with water, sediment concentrations were lower at VTS outlets, indicating substantial reductions in farm runoff pesticide concentrations.

Effects of diazinon on larvae of the Asian common toad (Bufo melanostictus, Schneider 1799)

Continuous subchronic exposure experiments were conducted to assess the effects of diazinon, an organophosphate pesticide, on the survival, growth and activity of larvae of the Asian common toad Bufo melanostictus. Two larval stages, the gill stage (Gosner stages 21 and 22) and gill-atrophy stage (Gosner stages 24 and 25), were continuously exposed to 4 microg/L, 400 mirog/L, and 10 mg/L of commercial-grade diazinon for 30 d. Treatments and untreated controls were maintained in triplicate with water changed and pesticide concentrations renewed every 3 d. Observations showed that subchronic exposure to 400 microg/L and 10 mg/L diazinon caused a significant dose-dependent increase in mortality compared to the control, regardless of the age at which larvae were exposed. One hundred percent mortality was observed in larvae exposed to 10 mg/L. No clear age-related sensitivity was evident in this study. The lethal concentrations at which 50% of the tadpoles (LC50) died during 30 d of continuous exposure were 6 and 7.5 mg/L for gill stage and gill-atrophy stage larvae, respectively. Diazinon impaired larval growth and activity. Tail abnormalities were apparent in larvae exposed to 400 microg/L and 10 mg/L of diazinon. This investigation provides the first empirical evidence of the negative effects of diazinon on the survival, growth and activity of B. melanostictus. The high degree of diazinon toxicity in this study highlights the need to consider important nontarget groups when recommending safe levels of pesticide application.

Sublethal Effects of Organophosphate Diazinon on the Brain ofCyprinus Carpio

Effects of diazinon, at different concentrations and exposure times, were investigated in freshwater fish, Cyprinus carpio, to elucidate the possible mode of action on lipid peroxidation together with the inhibitory effect of diazinon on acetylcholinesterase activity and changes in tissue protein levels. Cholinesterase inhibition is considered to be a specific biomarker of exposure to organophosphorus pesticides. Fish were exposed to 0.0036 microg/L, 0.018 microg/L, and 0.036 microg/L (sublethal) concentrations of diazinon for 5, 15, and 30 days, and biochemical measurements were carried out spectrophotometrically. Brain was chosen as an indicator tissue because it is a target system for the organophosphorus action. More than 20% decline in acetylcholinesterase activity relative to mean activity of the controls was observed in the diazinon-exposed groups. Protein content decreased significantly after 15 days of exposure to 0.018 microg/L and 0.036 microg/L diazinon and after 30 days of exposure to 0.036 microg/L. Malondialdehyde level declined markedly compared with the control levels. This study showed that prolonged exposures of C. carpio to diazinon had significant effects on brain acetylcholinesterase activity and that environmentally relevant concentrations of diazinon can significantly inhibit brain acetylcholinesterase activity. Altered protein content was probably due to the high energy demand under pesticide stress or inhibition of de novo enzyme synthesis. The decreased malondialdehyde content may reflect the possibility of better protection against oxidative stress.

Brain cholinesterase response in the snakehead fish (Channa striata) after field exposure to diazinon

The snakehead Channa striata is an economically important air-breathing fish species in the Mekong delta of Vietnam. Rice paddies, which are disturbed by the frequent application of agro-chemicals, are among the preferred habitats for this species during the rainy season. Diazinon is one of most commonly used chemicals in rice paddies. In the present study, exposure of adult snakehead fish to a single diazinon application in cages within a rice field resulted in long-term brain cholinesterase inhibition, while the water concentration of this insecticide fell below the detection limit within 3 days. In addition, incubation of brain homogenates with 2-PAM caused reactivation of the cholinesterase diazinon complex to within 80% of the control level. These experiments also showed that chemical ageing of the diazinon cholinesterase binding occurred, which may explain the long-term effects of this pesticide.

Effects of the insecticides malathion and diazinon on the early oogenesis in mice in vitro

Malathion and diazinon are two of the most commonly used organophosphorous (OP) agrochemicals. Several studies show that these pesticides exert several effects on mammalian spermatogenesis. Nevertheless, there are no studies concerning their effects on oogenesis. The objective of this study was to evaluate the effects of these insecticides on the viability of in vitro cultured mouse oocytes during the early oogenesis and to get a further understanding of the molecular mechanisms by which OP insecticides act and affect germinal cells. Oocytes were cultured from fetal ovaries for 10 days, when most oocytes had reached the diplotene stage (germinal vesicle stage). Cultures were exposed to different concentrations of malathion or diazinon for 24 h, and the effect on oocyte viability was assessed. Gene expression in oocytes exposed to the insecticides was analyzed by generating cDNA libraries and performing differential screenings. Results show a significant decrease in oocytes survival after 24-h exposure to 250 microM malathion or 900 nM diazinon, and the effect of these insecticides on the regulation of genes encoding proteins involved in transcription (BP75), translation (ribosomal protein S5), and mitochondrial function (cytochrome oxidase subunits I and III), providing evidence for OP insecticides as toxicants for mammals oocytes during the early oogenesis.

Developmental neurotoxicity of low dose diazinon exposure of neonatal rats: effects on serotonin systems in adolescence and adulthood

The developmental neurotoxicity of organophosphate pesticides targets serotonin (5HT) systems, which are involved in emotional and appetitive behaviors. We exposed neonatal rats to daily doses of diazinon on postnatal days 1-4, using doses (0.5 or 2mg/kg) spanning the threshold for barely-detectable cholinesterase inhibition. We then evaluated the effects on 5HT(1A) and 5HT(2) receptors, and on the 5HT transporter in cerebral cortical regions and the brainstem in adolescence through adulthood. Diazinon evoked a lasting deficit in 5HT(1A) receptors in males only, whereas it caused a small but significant increase in 5HT transporters in females; neither effect showed a significant regional selectivity. This pattern differed substantially from that seen in earlier work with another organophosphate, chlorpyrifos, which at pharmacodynamically similar doses spanning the threshold for cholinesterase inhibition, evoked a much more substantial, global upregulation of 5HT receptor expression; with chlorpyrifos, effects on receptors were seen in females, albeit to a lesser extent than in males, and were also regionally distinct. The effects of diazinon were nonmonotonic, showing larger alterations at the lower dose, likely reflecting positive trophic effects of cholinergic stimulation once the threshold for cholinesterase inhibition is exceeded. Our results reinforce the idea that different organophosphates have fundamentally distinct effects on the developmental trajectories of specific neurotransmitter systems, unrelated to their shared action as cholinesterase inhibitors. The effects on 5HT circuits expand the scope of behavioral endpoints that need to be considered in evaluating the developmental neurotoxicity of organophosphates.