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.1; Mehrle et al., 1988).Reduced feeding wasevident after 7 days of exposure to 789 pg L" 1; hypoactivity and diminishedresponsiveness to external stimuli were evident by day 10 of exposure; abnormalswimming postures were noted by day 12; and severe lethargy occurred by day 19.Theappearance of a particular abnormality varied with concentration.For example, reducedfeeding appeared after 7 days of exposure at 789 pg L" 1; by day 14 at 176 pg L" 1; and byday 17 at 38 pg L" 1.Thus, as with other toxicologic endpoints, the magnitude or severityof behavioral response varies with the duration and concentration of exposure.Although there are many ways that behavior can be used to determine effects of exposureto toxicants, the use of behavior as an index for toxicant identification is somewhat morelimited because there have been few systematic studies examining behavioral responsesthat are indicative of exposure to specific toxic agents.In an approach developed byDrummond et al.(1986), behavioral and morphologic responses were divided into tencategories containing a total of forty unique descriptors.Responses were monitored duringacute exposures of fathead minnows (Pimephales promelas) to 139 single compounds.Statistical pattern recognition, based on the types of behavioral changes induced byexposure, was used to identify three general responses (types I, II, and III) whichcorrelated with three classes of contaminants.Type I responses were indicative ofnarcosis-producing chemicals such as ethers, alcohols, ketones, and phthalates.Thesechemicals depress central and peripheral nervous system activities.Exposed fish exhibitdepressed locomotory activity, loss of startle responses, rapid shallow opercular rates,darkened coloration, and tetany.Type II responses were indicative of chemicals such asrotenone, benzene, and phenol, which disrupt metabolic activity.Exposed fish exhibitheightened locomotory activity, hyperactivity to stimulation, increased rate and amplitudeof opercular activity, slight darkening, and edema.Type III responses were indicative ofneurotoxic chemicals such as carbamates, organophosphates, caffeine, and strychnine.Exposed fish exhibit depressed locomotory activity with hyperactivity to stimulation, Figure 4.1 Days of exposure to the dioxin TCDD to induce behavioral changes in rainbow troutexposed as free-swimming juveniles for 28 days (redrawn from Merhle et al., 1988).EDWARD E.LITTLE AND SANDRA K.BREWER143 144 EDWARD E.LITTLE AND SANDRA K.BREWERconvulsions, spasms, tetany, scoliosis, lordosis, or vertebral hemorrhage.Thus,behavioral changes can be induced through direct effects to the nervous system as well asthrough indirect physiologic alterations.Recently, the approach by Drummond et al.(1986) of using behavioral alterations as anindex for toxicant identification was used by Rice et al.(1997), who exposed 30-day-oldJapanese medaka (Oryzias latipes) to five single compounds with different toxicologicmechanisms.Unique behavioral and morphologic abnormalities were observed for eachchemical except 2, 4-dinitrophenol (2, 4-DNP), an uncoupler of oxidativephosphorylation.2, 4-DNP displayed the fewest behavioral symptoms, with a loss ofequilibrium being the most common.These results were similar to the work ofDrummond et al.(1986) and Drummond and Russom (1990).Because behavioral changescan be induced through direct effects to the nervous system as well as through indirectphysiologic alterations, the development of behavioral toxicity syndromes is a promisingtool for assessing both mechanisms of toxicity and, possibly, identifying toxicants incomplex environmental samples.Neural basis of behavioral toxicityIn the simplest form, behavior results when a stimulus from the environment is encodedby the sensory cell as neural impulses which induce a muscle to respond (Figure 4.2).Thesynaptic connection between sensory cell and interconnecting neuron and betweenneuron and muscle fiber is the focus of communication between environmental stimuliand behavioral response.Contained within that neural chain is a network of chemicalreactions that include: the synthesis and transport of neurosecretory substances to thepresynaptic membrane (Figure 4
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