2093 Environmental Toxicology and Chemistry, Vol. 20, No. 9, pp. 2093–2098, 2001  2001 SETAC Printed in the USA 0730-7268/01 $9.00 + .00 CHARACTERIZATION OF SALINITY-ENHANCED TOXICITY OF ALDICARB TO JAPANESE MEDAKA: SEXUAL AND DEVELOPMENTAL DIFFERENCES ABIR T. EL-ALFY,* SONJA GRISLE, and DAN SCHLENK Environmental Toxicology Research Program, Environmental and Community Health Research, Research Institute of Pharmaceutical Sciences, Department of Pharmacology, School of Pharmacy, University of Mississippi, University, Mississippi 38677, USA ( Received 21 June 2000; Accepted 20 February 2001) Abstract—The effects of salinity, gender, and development on the acute toxicity of aldicarb were examined in the euryhaline fish, Japanese medaka (Oryzias latipes). The 96-h median lethal concentrations (LC50s) at 1.5 parts per thousand (‰) salinity were not significantly different between adults and juveniles but larvae were significantly more sensitive to aldicarb. A two-week exposure to increased salinity significantly enhanced the toxicity of 0.5 ppm aldicarb to both sexually mature male and female medaka. After 48 h of aldicarb exposure, mortality significantly increased (p  0.05) in males from 13  5.7% at 1.5‰ salinity to 56  5.7% at 20‰; in females mortality significantly increased (p  0.01) from 17  5.7% to 76  5.6%. A time-course study was conducted in which muscle acetylcholinesterase (AChE) inhibition was monitored after exposure to aldicarb. In general, AChE in females was inhibited to a greater degree at 12.0 and 20.0‰ salinity regimens than AChE in males. Muscle AChE in females residing at 20.0‰ was inhibited 93  3.3% by 8 h of exposure to 0.95 ppm aldicarb, whereas in males the maximum inhibition was 80  7.4% after 8 h of exposure to 0.86 ppm aldicarb at 20‰ salinity. These results indicate that environmental factors, such as salinity, in addition to gender and development, have significant impacts on the acute toxicity of aldicarb to Japanese medaka. Keywords—Aldicarb Medaka Salinity Acetylcholinesterase INTRODUCTION Several abiotic characteristics of water, such as temperature, pH, dissolved oxygen content, hardness, and salinity, may af- fect the toxicity of chemicals to aquatic organisms. Salinity is a critical factor in influencing the distribution and maintenance of aquatic life in estuaries [1]. Thus, examining the effect of salinity on the toxicity of various water-borne contaminants is essential in assessing the risk of these compounds to estuarine organisms. The carbamate pesticide aldicarb is one of the most widely used insecticides in the United States. It ranks seventh in terms of total acres and fifth in pounds applied [2]. Aldicarb is ap- plied on many crops, including cotton, sugarcane, soybean, and potatoes, as a nematocide. These crops are frequently grown adjacent to estuary habitats in the southeastern United States [3]. Because of its high water solubility (4.9 g/L) and moderate half-life of 5 to 10 d in pond water [4], aquatic species are at risk from contamination of waterways via ag- ricultural runoff. We previously have reported that salinity enhanced the toxicity of aldicarb to Japanese medaka (Oryzias latipes), a euryhaline fish that tolerates a wide salinity range from 1.5 to 34 parts per thousand (‰) [5]. The mechanism of the enhanced toxicity seems to be via the upregulation of fla- vin-containing monooxygenases (FMOs) that bioactivate al- dicarb to the more potent acetylcholinesterase (AChE) inhib- itor, aldicarb sulfoxide [6]. Salinity also affected the sensitivity of AChE inhibition by aldicarb in these fish. As with other carbamates, aldicarb exerts its toxic action by the reversible binding and subsequent inhibition of AChE, producing increased and sustained cholinergic signaling lead- * To whom correspondence may be addressed (aelalfy@olemiss.edu). The current address of A.T. El-Alfy is 10 Crestwood Drive, Randolph, NJ 07869, USA. ing to convulsions and death [7]. Although aldicarb inhibits AChE without bioactivation, oxidation of the thioether moiety to the corresponding sulfoxide was shown to enhance AChE inhibition in rats [8] as well as in Japanese medaka [6]. Most studies examining AChE inhibition in fish have focused on brain AChE [9–12]. However, several studies in fish have shown that brain AChE inhibition does not correlate well with acute toxicity [13–15]. Aldicarb is rapidly distributed to mus- cle tissue, which constitutes the majority of fish weight [16]. A recent study in our laboratory also has shown that inhibition of muscle AChE showed a better correlation to lethality than to brain or serum AChE in channel catfish (Ictalurus punc- tatus) exposed to aldicarb, suggesting that muscle AChE in- hibition might be an important factor in the mortality of fish due to aldicarb [13]. Consequently, monitoring the degree of muscle AChE inhibition in fish exposed to aldicarb might pro- vide insight into the degree of acute toxicity of this compound to aquatic organisms. The primary goals of this study were to measure the acute toxicity of aldicarb to the euryhaline fish, Japanese medaka, as an estuarine model species; to examine the effect of salinity on the toxicity of aldicarb using fish at different developmental stages as well as gender; and to characterize the effect of salinity on aldicarb toxicity by monitoring the inhibition of muscle AChE in fish exposed to aldicarb after acclimation to various salinity regimens. MATERIALS AND METHODS Chemicals Analytical standard-grade aldicarb (98% purity) was pur- chased from ChemService (West Chester, PA, USA). All other reagents were obtained from Sigma Chemical (St. Louis, MO, USA).