Effects of Chronic Lead Exposure on Cocaine-induced Disturbance of Fixed-interval Behavior

Abstract

Chronic lead exposure has been shown to attenuate cocaine-induced increases in extracellular dopamine levels in the region of the nucleus accumbens, and antagonize the locomotor stimulating effects of the drug. The purpose of this study was to determine if similar lead-induced disturbances in the effects of cocaine include the impact of the drug on schedule-controlled responding. Adult male rats exposed ad libitum to water containing 500 ppm lead acetate (Group Lead), or a comparable concentration of sodium acetate (Group Control), were placed on a restricted diet (12–15 g food/day) prior to commencing fixed-interval (FI-5 min) schedule training on Day 33 of exposure. After 27 days of operant training, animals received a sequence of no injection, saline injection, and cocaine injection tests, repeating the sequence for 3, 10, 20 and 40 mg/kg cocaine HCl (ip). Local rates were determined for successive 30 s segments of the interval and the pattern of responding was compared under conditions of saline and cocaine injection. For both groups, cocaine increased responding, especially early in the interval. However, the rate enhancing effects of cocaine were less pronounced in lead-exposed animals than controls, at least at the 20 mg/kg dose. These data extend earlier findings and accent the need to examine further the interactive relations between the external chemical environment and drug sensitivity.