Magnesium-maintained self-administration responding in cocaine-trained rats

Abstract

Magnesium chloride (MgCl₂) produces behavioral effects similar to those of psychomotor stimulants in a variety of behavioral situations. Because MgCl₂ appears to have stimulant properties, the ability of MgCl₂ to maintain responding in a rat self-administration paradigm was examined in seven experiments under different access and schedule conditions in cocaine-trained rats. These varied from the availability of MgCl₂ for a single day's test session subsequent to 1 h availability of cocaine, to the availability of MgCl₂ for 10 or 20 days after cocaine availability was totally discontinued. Fixed ratio 1, fixed ratio 5, and progressive ratio 1, 2 and 3 schedules of drug delivery were used. The results demonstrate that MgCl₂ may substitute for self-administered cocaine because it maintained responding; it did so dose dependently to maintain a constant level of MgCl₂ intake; and it did so over a 10-day period of time both with and without access to cocaine on test days. Responding maintained by MgCl₂ when cocaine was no longer available was similar under fixed ratio 1 and 5 schedule conditions. The progressive ratio breakpoints for MgCl₂ were significantly higher than those for saline, but significantly lower than those for cocaine. These data indicate that MgCl₂ has some reinforcing efficacy in cocaine-trained rats, particularly under fixed ratio 1 and 5 schedules, but has a low abuse potential compared to cocaine.