Fipronil (Active Ingredient of Acaricide Frontline 1 ) Acting on the Mice Thyroid MATEUS FERREIRA, 1 PATRI ´ CIA ROSA DE OLIVEIRA, 1 SANDRA ELOISI DENARDI, 1 GERVA ´ SIO HENRIQUE BECHARA, 2 AND MARIA IZABEL CAMARGO MATHIAS 1 * 1 Department of Biology, Institute of Biosciences, Sa ˜o Paulo State University-UNESP, 13506-900 Rio Claro, SP, Brazil 2 Department of Animal Pathology, Faculty of Agronomic and Veterinary Sciences, Sa ˜o Paulo State University-UNESP, Via de Acesso Prof. Paulo Castellane, 14884-900 Jaboticabal, SP, Brazil KEY WORDS pesticides; phenyl-pyrazolic; nontarget organisms; histology; thyroidean tissue ABSTRACT Fipronil, active ingredient of Frontline 1 acaricide, is a phenyl-pyrazolic deriva- tive, and its efficacy in the elimination of several plagues, even in lower concentrations, has already been demonstrated; however, its effect on nontarget organisms has not been thoroughly explained. In this sense, the present study had the objective to evaluate the effects of different doses of fipronil in mice and determine its cytotoxic potential and possible compromises on nontar- get organisms in artificial conditions. The results showed that animals exposed to fipronil presented significant alterations in the thyroidean tissue with evident follicular disorganization, decreased in the size of most follicles culminating with the rupture of smaller follicles and still with posterior fusion in a big and irregular ‘‘follicle’’ filled with colloid. The present study also revealed that the action of fipronil not only causes disorganization in the thyroidean tissue but also alters the chemical composition of the colloid itself (prehormone), modifying its proteic basis, which was confirmed by histochemical test for protein detection, making it clear that, in thyroids exposed to fipronil, the colloid was weakly positive or even negative for the presence of proteins in its constitution. Microsc. Res. Tech. 75:265–270, 2012. V V C 2011 Wiley Periodicals, Inc. INTRODUCTION The chemical control used in agriculture and in pub- lic health is important due to its high efficacy against target organisms and supposed selectiveness among the chemical receptors from both insects and verte- brates. However, the indiscriminate practice has caused environmental impacts on the soil and water in a drastic way, which is sometimes difficult to measure. In this sense, some studies have demonstrated the toxic and carcinogenic effects of the plaguecides, espe- cially those used in agriculture, which would contain organochlorate and pyrethroids, today forbidden in most developed countries, as well as in those in devel- opment (Flores et al., 2004; Rubin et al., 2007; Shafer et al., 2005). Currently, the so-called second-generation insecticides have proved to be more efficient and less toxic (Tingle et al., 2003). However, their effects on nontarget organisms have still to be clarified as well as the toxic potential of several products. Fipronil is part of this new generation of synthetic products. It is a phe- nyl-pyrazolic derivate of broad insecticide and acari- cide spectrum (Tingle et al., 2003) and has been used to control several domestic and plantation plagues such as ants, cockroaches, termites, grasshoppers, and bugs, presenting excellent results even in low concentrations (Gunasekara et al., 2007; Tingle et al., 2003). In veterinary, medicine field presents excellent ther- apeutic action and persistent activity against fleas and ticks of domestic dogs and cats when topically used, popularly known under the commercial names Front- line 1 and Top Spot 1 (Oliveira et al., 2008; Postal et al., 1995). Furthermore, its use has helped to solve public health problem, controlling the main anthropozoonosis vectors such as the larvae of Aedes aegypti (Ali et al., 1998; Pridgeon et al., 2008). However, this spread utili- zation of fipronil also affects nontarget organisms such as algae, aquatic plants, reptiles, birds, and mammals as human (Tingle et al., 2000, 2003). The actuation of fipronil on the organisms is via in- terference in the synapses of the neurons, specifically in g-aminobutyric acid receptors, acting as a noncom- petitive blocker of the clorate channels (Tingle et al., 2003). The effect of this block is to eliminate the ability to inhibit nerve impulses, resulting in an excessive neuronal activity, which, depending on the dosage, leads to a state of severe paralysis and consequent death of the individual (Rho ˆne-Poulenc, 1995). The selectivity of the product in relation to target organ- isms is guaranteed due to its condition to attach more efficiently to invertebrate rather than to vertebrate receptors (Hainzl et al., 1998; Matsuda et al., 2001). In vertebrate, especially mammals, fipronil has toxic effect when ingested, with DL 50 for Wistar rats esti- mated at 97 mg kg 21 (USEPA, 1998). Opposite to this, the topic application is less toxic (with DL 50 superior to 2,000 mg kg 21 ) due to low dermal permeability (Gunasekara et al., 2007; Tingle et al., 2000; USEPA, *Correspondence to: Maria Izabel Camargo Mathias, Department of Biology, Institute of Biosciences, Sa ˜o Paulo State University-UNESP, Av. 24A, no. 1515, 13506-900 Rio Claro, SP, Brazil. E-mail: micm@rc.unesp.br Received 19 April 2011; accepted in revised form 8 June 2011 Contract grant sponsor: FAPESP; Contract grant numbers: 2006/52599-0, 2008/59020-0 DOI 10.1002/jemt.21053 Published online 5 August 2011 in Wiley Online Library (wileyonlinelibrary.com). V V C 2011 WILEY PERIODICALS, INC. MICROSCOPY RESEARCH AND TECHNIQUE 75:265–270 (2012)