Endosulfan Exposure Disrupts Pheromonal Systems in the Red-Spotted Newt: A Mechanism for Subtle Effects of Environmental Chemicals

Environmental Health Perspectives, July, 2001 by Daesik Park, Steven Hempleman, C., Catherine R. Propper

Because chemicals introduced into the environment by humans can affect both long-term survivorship and reproduction of amphibians, discovering the specific mechanisms through which these chemicals act may facilitate the development of plans for amphibian conservation. We investigated the amphibian pheromonal system as a potential target of common environmental chemicals. By treating female red-spotted newts, Notophthalmus viridescens, to a commonly used insecticide, endosulfan, we found that the pheromonal system is highly susceptible to low-concentration exposure. The impairment of the pheromonal system directly led to disrupted mate choice and lowered mating success. There were no other notable physiologic or behavioral changes demonstrated by the animals at the insecticide concentrations administered. Our findings suggest that the amphibian pheromonal system is one of the systems subject to subtle negative effects of environmental chemicals. Key words: amphibian declines, electro-olfactogram, endosulfan, environmental chemicals, insecticides, pheromones, olfaction. Environ Health Perspect 109:669-673 (2001). [Online 22 June 2001] http://ehpnet1.niehs.nih.gov/docs/2001/109p669-673park/abstract.html

Chemicals released into the environment may have either overt toxic effects or more subtle effects that influence long-term survival and reproduction. In several amphibian populations, such toxic effects of environmental chemicals have been reported as one of the major pathways underlying population declines (1-3). Unlike the detection of overt toxic effects of chemicals, conventional toxicologic screening and testing methods are generally poor at exploring more subtle effects such as changes in behavioral or hormonal responses. For this reason, there are relatively few studies that have investigated such subtle effects of exposure to environmental chemicals (4). Discovering more subtle, but nevertheless damaging, mechanisms through which environmental chemicals act may help in the development of plans for amphibian conservation.

Endosulfan is a chlorinated hydrocarbon insecticide of the cyclodiene subgroup, which acts as a poison to a variety of insects. It is used on a wide variety of crops including tea, coffee, cotton, fruits, and vegetables, as well as on rice, cereals, maize, or other grains (5). Several studies reported the presence of endosulfan in cotton crop soils [0.1-0.4 mg/kg (6)] and estuaries [0.5-4.0 [micro]g/L (7)], and relatively high residual levels have been found in some marketed fruits such as Brazilian strawberries and tomatoes [4-510 ppb (8)]. These reports indicate that endosulfan exposure can occur in both human and wildlife populations. Endosulfan not only acts as a hormone disruptor, but it also affects neurotransmitter systems of many species such as rats, catfish, and bullfrog tadpoles (9-14). For example, endosulfan suppressed testosterone and 17[Beta]-estradiol concentrations in neonatal rats (9), increased thyroxine (T.sub.4) levels in catfish (10), and induced neurotoxic effects including increased excitability, trembling, and deficits in operant learning performance via the disruption of cholinergic, dopaminergic, and serotonergic neurotransmitter systems in freshwater fishes, pigeons, and rats (11-14).

Pheromonal systems play important roles during many stages of amphibian life history including searching for migration routes, advertising individual territories, identifying conspecific individuals, presenting aggressive or alarm signals, and alluring potential mates (15). The pheromonal systems of the red-spotted newts, Notophthalmus viridescens, are well studied (16,17). Females have cloacal glands that produce attractants for males, and males have hedonic and cloacal glands. Male hedonic glands open into epidermal pits that are arranged in a row on either side of the head, and the pheromone produced increases female receptivity (18). Male cloacal pheromones may function by both increasing female receptivity (18) and repelling conspecific males (19). The growth of alveoli in the glands and the production of pheromones are under the control of prolactin, dopamine, and sex steroid hormones (20). Cholinergic neurotransmitter systems are responsible for the secretion of pheromones (21).

The purpose of our study was to determine whether exposure of female red-spotted newts to endosulfan limits individual interactions by disrupting pheromonal communication.

Materials and Methods

Animal husbandry and endosulfan exposure. Experiments were conducted from 26 March to 11 June during the breeding season in 1998. A total of 99 red-spotted female and male newts were purchased from a commercial supplier (Charles D. Sullivan Co. Inc., Nashville, TN). The use of animals in this study was approved by the institutional animal care and use committee of Northern Arizona University (#98-584). Newts were kept as described by Verrell (22). Endosulfan (ps-81, 99% mixture of isomers, Chemical Service, West Chester, PA) dissolved in 10 [micro]L acetone was delivered into individual containers containing one female and 1 L of water, and then immediately and gently mixed throughout the water using a glass rod. Final exposure concentrations were 0, 5, and 10 ppb endosulfan. These concentrations were based on the results of a study that exposed frog tadpoles to endosulfan (12) and concentrations found in the field (7). Animals were treated for 4 days. We neither changed the water containing endosulfan nor reintroduced endosulfan. During exposure, control animals only received 10 [micro]L acetone.