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.