Studies on the possible sites of chlordecone-induced tremor in rats

Public health and chronic low chlordecone exposure in Guadeloupe, Part 1: hazards, exposure-response functions, and exposures

BACKGROUND

Inhabitants of Guadeloupe are chronically exposed to low dose of chlordecone via local food. The corresponding health impacts have not been quantified. Nevertheless the public authority implemented an exposure reduction program in 2003. We develop methods for quantifying the health impacts of chlordecone and present the results in 2 articles: 1. hazard identification, exposure-response functions (ERF) and exposure in Guadeloupe, 2. Health impacts and benefits of exposure reduction. Here is the first article.

METHODS

Relevant data are extracted from publications searched in Medline and Toxline. Available knowledges on mode of action and key-event hazards of chlordecone are used to identify effects of chlordecone that could occur at low dose. Then a linear ERF is derived for each possible effect. From epidemiological data, ERF is the delta relative risk (RR-1) divided by the corresponding delta exposure. From animal studies, ERF is the benchmark response (10 %) divided by the best benchmark dose modeled with BMDS2.4.0. Our goal is to obtain central values for the ERF slopes, applicable to typical human populations, rather than lower or upper bounds in the most sensitive species or sex.

RESULTS

We derive ERFs for 3 possible effects at chronic low chlordecone dose: cancers, developmental impairment, and hepatotoxicity. Neurotoxicity in adults is also a possible effect at low dose but we lack quantitative data for the ERF derivation. A renal toxicity ERF is derived for comparison purpose. Two ERFs are based on epidemiological studies: prostate cancer in men aged >44y (0.0019 per μg/Lblood) and altered neurodevelopment in boys (-0.32 QIpoint per μg/Lcord-blood). Two are based on animal studies: liver cancer (2.69 per mg/kg/d), and renal dysfunction in women (0.0022 per mg/kg/d).

CONCLUSION

The methodological framework developed here yields ERFs for central risk estimates for non-genotoxic effects of chemicals; it is robust with regard to models used. This framework can be used generally to derive ERFs suitable for risk assessment and for cost-benefit analysis of public health decisions.

ATSDR evaluation of health effects of chemicals. II. Mirex and chlordecone: health effects, toxicokinetics, human exposure, and environmental fate

This document provides public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective of the toxicology of mirex and chlordecone. It contains descriptions and evaluations of toxicological studies and epidemiological investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health. Additional substances will be profiled in a series of manuscripts to follow.

Quantification of motor function in toxicology

Disturbances of movement and other motor functions can result from exposure to toxicants and drugs. Sometimes, as with acute exposure to ethanol or solvents, these effects disappear when exposure ends. Other times, as with manganese, haloperidol, or chronic ethanol, motor disturbances are irreversible and may even lie undetected until after exposure has ended. Motor disturbances can take on many guises, including tremor, difficulty in positioning, fatigue, or rigidity. Techniques for measuring these different endpoints in primates will be addressed. One preparation that enables the simultaneous monitoring of positioning, tremor, and operant behavior in nonhuman primates is described, and tactics for obtaining spectral estimates of tremor from a positioning task are outlined. The spectra obtained from this preparation are reliable and valid: they are stable over a period of a year, they correspond to spectra obtained from accelerometers, and are altered by acute administration of ethanol or oxotremorine. These two drugs had opposite effects on tremor but affected bar positioning in a similar manner.

Effects of chlordecone and chlordecone alcohol on isolated ovine erythrocytes

Chlordecone (CHLO, 14-30 microM) and chlordecone alcohol (CHLO ALC, 10-23 microM) altered the permeability of isolated ovine erythrocytes as evidenced by a concentration- and time-dependent induction of K+ efflux and hemolysis. Hemolysis, but not K+ efflux, was markedly delayed when the erythrocytes were suspended in isotonic sucrose. CHLO-induced and CHLO ALC-induced hemolysis and K+ efflux were dependent on the pH of the external media. Raising the pH from 6.5 to 9.4 inhibited CHLO-induced K+ efflux and hemolysis, whereas CHLO ALC-induced K+ efflux and hemolysis were increased. Low concentrations of both compounds (1-4 microM) protected erythrocytes against hypotonic hemolysis. Neither CHLO (30 microM) nor CHLO ALC (23 microM) induced the release of trapped K+ from KSCN-loaded unilamellar liposomes made from erythrocyte lipids. It is postulated that CHLO and CHLO ALC interact with membrane proteins and increase the permeability of the membrane to cations. This interaction leads to colloid-osmotic hemolysis.

Modulation of p,p'-DDT-induced tremor by catecholaminergic agents

p,p'-DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane; 75 mg/kg) or corn oil was administered po to male Fischer 344N rats. Tremor was quantified 8 hr later by spectral analysis of whole body movements. The effect of sc injection of pharmacological challenges on the spectral profile of body movements was determined. The alpha antagonist phenoxybenzamine decreased the intensity of movements over most of the power spectra in animals exposed to DDT, but decreased spectral power only at lower frequencies in control subjects. The alpha-1 antagonist prazosin had similar effects in animals given DDT. The alpha-2 antagonist yohimbine and the beta antagonist propranolol produced lethality and increased the intensity of movements in animals administered DDT, without significantly affecting control animals. The alpha-2 agonist clonidine decreased the spectral profile over a wide range of frequencies in animals exposed to DDT, while depressing the spectral power of control animals only at higher frequencies. The dopamine antagonist haloperidol increased the intensity of movements in DDT-treated animals, without altering the spectral profile of controls. The dopamine agonist apomorphine induced stereotypy in control animals, but failed to significantly alter the power spectra in subjects given DDT. These data suggest a facilitatory and inhibitory role, respectively, for alpha-1 and alpha-2 receptors in the modulation of DDT-induced tremor. Dopamine and beta receptors may be involved in the tonic inhibition of tremor produced by DDT.

Pharmacological modification of tremor and enhanced acoustic startle by chlordecone and p,p'-DDT

Pretreatment of rats with phenoxybenzamine (5 mg/kg; SC), an alpha adrenergic antagonist, decreased the peak tremor power and startle magnitude of rats subsequently given DDT (75 mg/kg; PO) or chlordecone (60 mg/kg; IP), without having a significant effect on control animals. Pretreatment with an intracerebroventricular injection of calcium (3.75 microM in 5 microliters NaCl) decreased the peak tremor power due to subsequently administered DDT, while increasing the tremor response in rats later dosed with chlordecone. The effects of phenoxybenzamine are postulated to be due to a blockade of an excitatory influence of the adrenergic system. Calcium may decrease DDT-induced tremor by acting as a neuronal stabilizer. Potentiation of the tremorigenic effect of chlordecone by calcium may be due to increased levels of intracellular calcium, resulting in augmented release of neurotransmitters in chlordecone-exposed animals.

5,5-Diphenylhydantoin antagonizes neurochemical and behavioral effects of p,p'-DDT but not of chlordecone

Rats were given 75 mg/kg of 5,5-diphenylhydantoin (phenytoin) or vehicle 30 min prior to 75 mg/kg of 1,1,1-trichloro-bis(p-chlorophenyl)ethane (p,p'-DDT) (p.o.) or chlordecone (i.p.) and tremor was measured 12 h later. Rats were then killed, and regional brain levels of biogenic amines and their acid metabolites and amino acids were determined. Pretreatment with phenytoin significantly attenuated the tremor produced by p,p'-DDT but enhanced that produced by chlordecone. p,p'-DDT had significant effects on the levels of aspartate, glutamate, 5-hydroxyindoleacetic acid (5-HIAA), and 3-methoxy-4-hydroxyphenylglycol (MHPG), whereas chlordecone increased glycine, 5-HIAA, and MHPG levels. Pretreatment with phenytoin blocked p,p'-DDT-induced increases of aspartate in the brainstem and spinal cord, 5-HIAA in the hippocampus, and MHPG in the brainstem and hypothalamus. Phenytoin significantly enhanced chlordecone-induced increases of MHPG in the brainstem. These data indicate that organochlorine-induced increases in noradrenergic activity in the brainstem and spinal cord may be directly related to the tremorigenic effects of these chemicals.

Neurochemical effects of DDT in rat brain in vivo

p,p'-DDT and related agents act to hold the sodium channel open once opened and this effect is believed to be responsible for neurological effects of tremor and hyperexcitability in vivo. There is a good correlation between DDT-induced tremor and an increase in the levels of the metabolites of norepinephrine (NE), serotonin (5HT) and, to a lesser extent, dopamine (DA) in the brain stem (BS), hypothalamus (HYP), striatum (STR), or hippocampus (HPC). DDT also increases levels of excitatory amino acids glutamate (GLU) and aspartate (ASP), but the effect occurs only in the brain stem. These effects are dose- and time-related. Pharmacological studies found that blockade of alpha 1-adrenergic receptors attenuate DDT-induced tremor, while blockade of serotonergic, cholinergic muscarinic, and dopaminergic receptors augment the toxicity of DDT. Tremor was almost completely blocked in rats pretreated with hydantoin, an anticonvulsant believed to block repetitive firing of nerves by interfering with the inactivation gate of the sodium channel. A similar antagonism was observed for permethrin, a Type I pyrethroid believed to have a mechanism of action very similar to that of DDT. However, hydantoin increased the tremorigenic effects of chlordecone, an organochlorine whose mechanism has not been linked to the sodium channel. These data are consistent with the hypothesis that the in vivo neurotoxicity of some organochlorine insecticides is related to their effects on the sodium channel.

Evaluation of long-term consequences in behavioral and/or neural function following neonatal chlordecone exposure

The effects of neonatal exposure of rats to chlordecone, during the major postnatal period of neuroendocrine differentiation were assessed after the animals matured to 90 days of age. On day 4 postpartum, pups received a s.c. injection of either DMSO vehicle or chlordecone (1 mg/pup) dissolved in DMSO. The neonatal exposure produced a significant sex-dependent alteration in adult body weight; chlordecone-exposed males were lighter than vehicle-exposed controls and chlordecone-exposed females were heavier than vehicle-exposed controls. Behavioral tests sensitive to neonatal chlordecone exposure during preweaning development--i.e., spectral analysis of movement, activity, and auditory startle responsiveness--gave no statistically significant evidence for residual effects of the early organochlorine treatment. When challenged with harmine, a known tremorogen with putative effects on olivocerebellar pathways, chlordecone-exposed males were less responsive than vehicle-exposed littermates in a spectral analysis of movement. The movement spectrum of chlordecone-exposed females was not differentially sensitive to the harmine challenge. However, subsequent evaluation of the auditory startle reflex indicated that harmine interacted with the neonatal treatment and sex of the animal; chlordecone-exposed males were less responsive and chlordecone-exposed females more responsive than same sex vehicle-exposed littermates. The responsiveness to a d-amphetamine challenge, expressed as a ratio of baseline activity in a pre- and post-test design, suggested the chlordecone-exposed males gave an exaggerated response to the drug challenge. Collectively, these findings suggest that the neonatal chlordecone exposure had a significant organizational effect on the development of behavioral and/or neural function. These findings also suggest the predictive utility of early behavioral tests; that is, long-term alterations were noted in each component of the toxicological syndrome previously identified during preweaning development.

Evaluation of the immunotoxic potential of chlordecone with comparison to cyclophosphamide

The immunotoxic potential of chlordecone was evaluated in male Fischer-344 rats following 10 d of acute dosing by oral gavage. These results were compared with those obtained following a comparable dosing regimen with the known immunosuppressive drug cyclophosphamide. Significant changes in ratios of spleen and thymus to body weight, blastogenic responsiveness of lymphocytes to concanavalin A, and natural killer (NK) cell activity against allogeneic W/Fu-G1 rat lymphoma target cells and xenogeneic YAC-1 mouse lymphoma target cells were observed only at the highest chlordecone dosage (10 mg/kg . d). A significant decrement in body weight also occurred at this dosage which suggests that the observed changes in the immune parameters measured were most likely due to the overt toxicity of chlordecone. In contrast, rats dosed over 10 d by oral gavage with cyclophosphamide showed significant decreases in spleen and thymus to body weight ratios at a dosage as low as 1.5 mg/kg . d. Body weight decrements were observed only at dosages of 12 mg/kg . d or greater. At dosages of 1.5 mg/kg . d or greater, cyclophosphamide caused a significant decrease in the total leukocyte and absolute lymphocyte counts in peripheral blood, and a decrease in the lymphoproliferative responses to both T- and B-lymphocyte mitogens. Significant suppression of NK cell activity was observed at dosages of 6 mg/kg . d or greater. These results suggest that T and B lymphocytes are more sensitive than are NK lymphocytes to the immunosuppressive effects of cyclophosphamide. The results of these two studies indicate that the failure to detect chlordecone-induced immunotoxic effects in the absence of overt toxicity is not due to the possible insensitivity of the parameters examined. Instead, since cyclophosphamide affected all of the parameters tested, these results indicate that the endpoints examined here are capable of detecting immunotoxicity with sufficient sensitivity to permit distinction between probable indirect immunosuppression due to overt toxicity and subtle, direct impairment of the immune system.

DDT-induced tremor in rats: effects of pharmacological agents

Rats given a tremorigenic dose of DDT (75 mg/kg, PO) were treated with pharmacological agents either 30 min prior to DDT or 1-2 h prior to testing at the time of peak effect (12 h postdosing). The administration of mephenesin (a centrally acting muscle relaxant) or Dilantin (an anticonvulsant) prior to DDT significantly attenuated tremor. Pretreatment with pizotifen (a serotonergic receptor antagonist) had no significant effect on tremor. Administration of the same agents 1-2 h prior to measurement had minimal effects. Trihexyphenidyl (a muscarinic cholinergic receptor antagonist) exacerbated the tremor produced by DDT. These data suggest that cholinergic neurotransmitter systems may be involved in DDT-induced tremor. That DDT-induced tremor was significantly attenuated by mephenesin and Dilantin is in accord with the conclusion that DDT-induced tremor is a manifestation of repetitive discharge due to interference with ionic conductance.