Aging unmasks adverse effects of gestational exposure to methylmercury in rats

The consequences of developmental exposure to methylmercury on behavior in aged animals were investigated. Methylmercury exposure was arranged by plac ing 0, 0.5 or 6.4 ppm Hg in the drinking water of female rats at least 4 we eks before mating and continuing until post natal (PN) day Ih. Brain Hg con centrations in cohorts of low- and high-dose offspring were 0.5 and 9.1 ppm at birth and 0.04 and 0.52 ppm at weaning (described in another report). L ever pressing of female offspring was maintained under a Multiple Different ial Reinforcement of High Rate 9:4 Extinction schedule of food reinforcemen t (Mult DRH 9:4 EXT). Under the DRH 9:4 schedule, a food reinforcer was del ivered when nine responses occurred within 4 s. Under the Extinction schedu le, responding had no programmed consequences. No exposure-related differen ces in reinforcement rate under the DRH schedule or discrimination between the DRH and extinction components were apparent initially. At 950 days of a ge, the overall response rates of controls had shown a gradual decline over the previous 500 days to about 80% of their beginning levels, but, otherwi se, most controls were healthy. A gradual decline in the reinforcement rate began to appear in low- and high-dose rats at about 500 and 800 days of ag e, respectively Microanalyses of the nine-response burst maintained by the DRH schedule revealed that the lever-press duration increased, the inter-re sponse time (IRT) was unaffected, and the time between response bursts incr eased. Overall, the nine-response burst remained intact as a coherent respo nse unit. The increased time between response bursts caused the decline in reinforcement rate. All rats displayed these effects as they aged, but the mercury-exposed rats did so sooner. (C) 2000 Elsevier Science Inc. All righ ts reserved.