The effects of chronic cadmium exposure on schedule controlled responding and conditioned suppression in the adult rat.

Adult rats fed daily rations of laboratory chow with added CdCl2 were tested for schedule controlled responding and conditioned suppression (conditioned emotional response-CER). Determinations of tissue Cd, levels of metallothionein, and the ratio of Cd in the tissue to the Cd-binding capacity of metallothionein also were made. Animals were exposed daily to either 5 mg/kg Cd (Group Cd-5), 1 mg/kg Cd (Group Cd-1), or no Cd (Group Control). An initial phase of training on a variable interval 2 min (VI-2) food reinforcement schedule lasting 40 sessions (1 session/day) was followed by CER training and then an interpolated period (100 days) of continued exposure but with no training. Schedule retraining was introduced for 21 sessions (1 session/day). The results showed no group differences during original schedule training. On the CER test, Group Cd-1 was found to exhibit less suppression (defined in terms of lever press rates) than controls to preaversive stimuli. Group Cd-5 showed significantly greater suppression to the primary aversive stimulus (shock) than the other two groups. Both treatment groups showed significantly lower lever press rates than controls on the schedule retraining task. These behavioral disturbances occurred commensurate with increases in non-metallothionein bound Cd in the liver, kidney, and small intestine.