Effects of 2,4-dichlorophenoxyacetic acid exposure on dopamine D2-like receptors in rat brain
Introduction
2,4-Dichlorophenoxyacetic acid (2,4-D), an aryloxoalkanoic acid, works as a systemic herbicide being used to control many types of broadleaf weeds. Several derivatives (esters, amines and salts) of 2,4-D are known, but chlorophenoxy herbicides are more extensively used than all other groups of herbicides in terms of tonnage employed annually [2], [32], [34]. Human exposure through agricultural use, food products, or through lawn and garden use has been demonstrated in several studies, but the risk of 2,4-D to human health has not been completely assessed [22], [23], [25], [36]. Early research indicates that the central nervous system (CNS) is a target organ for the effects of this herbicide in different animal species [11], [13], [14], [27], [28], [38]. It was previously reported that 2,4-D exposure causes a variety of behavioral deficits in rodents, including behavioral developmental pattern alterations and motor abnormalities [17], [29], [30], [33]. In addition, we described that rats exposed to 2,4-D during specific developmental periods elicited stereotypic behaviors, signs associated to serotonin syndrome, right-preference turning and catalepsy [6], [18]. More recently, we demonstrated that 2,4-D induces ipsilateral rotation after unilateral 2,4-D injection into striatum (CPu) and medial forebrain bundle in adult rats [8]. Functional disturbances in various neurotransmitter systems may contribute to these behavioral impairments. Interestingly, alterations of serotonin (5-hydroxytryptamine, 5-HT), dopamine (DA) and their metabolite levels in midbrain, cerebellum (Cer), nigrostriatal and mesocorticolimbic systems were observed in our laboratory [5], [8], [16], [19]. In view of these results, it seems most probable that the dopaminergic system might be involved in the neurotoxicity of 2,4-D. Recently, the effect of 2,4-D on lateralization activity in the monoamine systems of the basal ganglia of adult rats was described [10]. A reduction in DA, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovallinic acid (HVA) content in midbrain, ventral tegmental area (VTA) and prefrontal cortex (PFc) revealed an alteration in the mesocorticolimbic system. Moreover, an increase in DA and DOPAC levels in sustantia nigra (SN) and Cer—in both sexes—with no changes in CPu and nucleus accumbens (NAc) were detected. 5-HT contents also were significantly enhanced in PFc, CPu, midbrain, SN and Cer of adult rats. No reversion was observed when the herbicide was withdrawn with female rats being more affected than males. DA levels were found to be decreased in the right side with respect to the left side in striata and accumbens nuclei supporting the behavioral rotation previously reported by us in these rats [10].
In spite of the evidences showing the involvement of the dopaminergic system in 2,4-D deleterous effects, no attempts have been made to study the participation of the dopaminergic receptors. Moreover, several evidences indicate the existence of critical periods during the rat perinatal development that appears to be particularly sensitive to diverse environmental and pharmacological treatments, which may lead to permanent disturbances in adulthood [15], [39]. In the present work, we examined the impact of 2,4-D exposure on the postnatal development of dopaminergic D2-like receptors in specific brain areas of adult rats, including PFc, CPu, hippocampus (H) and Cer. We employed two models of 2,4-D exposure reported previously [6] to provide more precise information about the relationship between 2,4-D-induced changes and D2-like receptors. We evaluated the effect of the exposure of 2,4-D on the ontogeny of D2 receptors in offspring that were chronically exposed to the herbicide through their mother's diet until PND 23, with or without withdrawal from weaning to PND 90.
Section snippets
Chemicals and drugs
R,S-(±)-[N-methyl-3H]nemonapride (YM-09151-2; 85 Ci/mmol) was purchased from New England Nuclear (Boston, MA). 1,3-Ditolylguanidine (DTG), pindolol, flupenthixol and S(−)-sulpiride were obtained from Research Biochemicals International (RBI; Natick, MA). 2,4-D and all other chemicals were purchased from Sigma (St. Louis, MO).
Animals
Nulliparous female rats, Wistar origin, approximately 3 months of age, were separately placed with fertile males on the proestrus night, and the presence of spermatozoa was
Postnatal development of brain D2-like receptors
D2-like receptors postnatal ontogeny in different brain regions of female and male rats is shown in Fig. 1. A similar course of development between female and male rats was observed in CPu, H and Cer (Fig. 1A, C, and D). In contrast, we observed sexual differences in PFc (Fig. 1B). In CPu, D2-like receptor levels increased from PD 6 to PD 30 (154% and 130% in female and male rats, respectively; P<0.001), then slightly declined at PD 45 (by 26% and 10% in female and male rats, respectively), to
Discussion
The aim of this study was to examine the hypothesis that DA system alterations would contribute to understand the 2,4-D-associated behavior changes in rats previously reported by us [6], [7], [17], [18]. The ontogeny of DA D2-like receptors was evaluated in adult offspring whose mothers were exposed to subchronic doses of 2,4-D during gestation and lactation. The effect of a dietary exposure of the herbicide to the pups from weaning to PD 90 was also studied.
The development of D2-like receptors
Acknowledgments
This research was supported by a grant from the Consejo Nacional de Investigaciones Cientı́ficas y Técnicas (CONICET), Argentina to Ana Marı́a Evangelista de Duffard and by a grant from Agencia Nacional de Promoción Cientı́fica y Tecnológica (PICT 0287) and CONICET (PIP 6266) to Marta Antonelli. Analı́a Bortolozzi was supported by a fellowship from FOMEC (UNA).
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Present address: Department of Neurochemistry, Institut d'Investigacions Biomediques de Barcelona (CSIC-IDIBAPS), Rossello 161, 6th floor, room 32 08036 Barcelona, Spain.