Ecotoxicology and Environmental Safety 59 (2004) 217–222 Effects of monocrotophos and its analogs in acetylcholinesterase activity’s inhibition and its pattern of recovery on euryhaline fish, Oreochromis mossambicus J. Venkateswara Rao  Toxicology Unit, Biology Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 3 April 2003; received in revised form 14 August 2003; accepted 8 September 2003 Abstract Acute toxicity of monocrotophos (MCP) and its newly synthesized thiol analogs 2-butenoic acid-3-(diethoxyphosphinothionyl) methyl ester (RPR II) and 2-butenoic acid-3-(diethoxyphosphinothionyl) ethyl ester (RPR-V) was studied on euryhaline fish, Oreochromis mossambicus. The median lethal concentrations of MCP, RPR-II, and RPR-V are 11.506, 0.167, and 0.174 mg L 1 , respectively. Both the analogs were found to be 65-fold more toxic than MCP. Inhibitory and recovery patterns of brain, gill, and muscle acetylcholinesterase (AChE) was studied in vivo after exposure to LC 50 (96 h) concentrations. Recovery study was performed of regular intervals of day-1, -7, -14, -21, and -28 to establish the time course of 50% and 100% recovery in neurotransmitter enzyme of brain, gill, and muscle tissues. The sensitivity of the tissue AChE was in the order gill4brain4muscle. The relative toxicity of these compounds with regard to AChE was RPR-II4RPR-V4MCP. The MCP-treated fishes recovered at a faster rate than for RPR-V, followed by RPR-II. r 2003 Elsevier Inc. All rights reserved. Keywords: Fish; Monocrotophos; Analogs; Acetylcholinesterase 1. Introduction Chemical pesticides are well recognized as an eco- nomic approach to controlling pests; at the same time such chemicals are highly toxic to other species in the environment. Now, there is growing concern worldwide over the indiscriminate use of such chemicals, which result in environmental pollution and toxicity risk to nontarget organisms. Monocrotophos (MCP), com- monly known as Azodrin, is one of the organopho- sphorus (OP) insecticides extensively used in agriculture and animal husbandry. A number of studies were conducted on the toxicity of MCP on different organ- isms and indicated it as a potent neurotoxicant (Qadri et al., 1994; Rao et al., 1991; Rao et al., 1992a; Venkateswara Rao et al., 2001). Current critical situa- tion is that many of the previously used and economic- ally affordable insecticides are no longer effective and are also banned in many countries. Therefore, there is an urgent to develop newer, more effective, target- specific, and less persistent insecticides. In the search for developing newly insecticides, our Institute, the Indian Institute of Chemical Technology, developed the RPR series of pesticides based on the existing insecticide monocrotophos (Renu Rani et al., 1989). Among the newly developed analogs those named RPR-II [2-butenoic acid-3-(diethoxyphosphinothionyl) methyl ester] and RPR-V [2-butenoic acid-3-(diethoxyphosphi- nothionyl) ethyl ester] have enough potential to develop as new insecticides (Fig. 1). They were found to be quite effective against target species and had low mammalian toxicity when compared with MCP (Qadri et al., 1986). Further studies have indicated that these two analogs are less neurotoxic than MCP on rats (Siddiqui et al., 1991). RPR-V disrupts embryonic development in the chick and causes teratogenic effects but it cannot be reliably predicted that it will have similar effects in mammals (Rao et al., 1992b). Responses to organophosphorus (OP) insecticides by aquatic organisms are broad-ranged, depending on the compound, exposure time, water quality, and species (Eisler, 1970; Fisher, 1991; Richmonds and Dutta, 1992). Acetylcholinesterase (AChE, E.C: 3.1.1.7) enzyme is widely used for rapid detection to predict early warning of pesticide toxicity (Dutta and Arends, 2003; Pan and ARTICLE IN PRESS  Fax: +97-40-2717-3387. E-mail address: jv@iict.res.in. 0147-6513/$ - see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.ecoenv.2003.09.015