The percentile IRT schedule: High rate behavior as a tool for examining the toxic motor effects of methylmercury

Abstract

Methylmercury exposure has become a topic of interest in both the toxicology literature and among the general media within the last decade. Concern about methylmercury’s toxicity has grown with the discovery that fish-consuming individuals can be exposed to the toxicant at varying levels. One dietary constituent thought to prevent or counteract the toxicity of methylmercury is selenium, which is also available through fish consumption. The scientific community is trying to develop a balanced approach to fish consumption, by designating which fish are safest, and the amount of that fish which would be unlikely to cause any noticeable physical or behavioral deficits. Previously, it was found that prenatal methylmercury exposure resulted in deficits in high rate responding reinforced under a differential reinforcement of high-rates 9:4 (DRH 9:4) schedule. The differences may have been due to motor deficits or differential reinforcer rates. The current study, using a percentile schedule instead of a DRH, was designed to reduce the influence of reinforcer rate on behavior. The percentile IRT 10:0.5 schedule reinforces an interresponse time (IRT) if it is shorter than the median of the previous ten. The schedule produces high response rates, and minimizes direct effects of reinforcement rate. In experiment 1, behavior of 8 rats under the percentile IRT 10:5 schedule was characterized using a log survivor plot analysis. Bouts of responses were separated by periods of disengagement. Overall response rates ranged from 50 to 150 responses per minute. These overall rate differences were most closely correlated with within-bout response rates and bout lengths, with a smaller contribution from bout-initiation rates. Inter-correlations among these measures were low, indicating that they contribute independently to overall response rates. In experiment 2, forty eight rats were used in a 2x3 design to examine the interaction of methylmercury and selenium on behavior. Rats were chronically exposed to 0, 0.5 or 5 ppm of methylmercury through drinking water daily. Rats consumed diets which contained either high (0.05 PPM) or low (0.5 PPM) levels of selenium. At 56 weeks of age, (247 days on methylmercury and 267 days on selenium), animals were placed on a percentile IRT 10:5 schedule of reinforcement. Methylmercury exposure resulted in slower behavioral acquisition, lower steady state response rates, and longer post-reinforcement pause durations. Selenium deficiency resulted in slower behavioral acquisition and lower steady state response rates. High selenium levels were associated with decreasing bout initiation rates over time. There were no statistically significant interactions between methylmercury and selenium.