Self-administration of cocaine and nicotine mixtures by rhesus monkeys

The concurrent use of cocaine and nicotine is associated with increases in their relative rates of intake. While this increase could be due to a high reinforcing effect of the drug combination, higher rates of intake could also be explained by a decrease in the drugs' relative reinforcing effects.

To determine if nicotine could modulate cocaine's reinforcing effects, the current study compared the reinforcing potency and strength of cocaine to cocaine mixed with various concentrations of nicotine.

Five rhesus monkeys were allowed to self-administer cocaine (25-400 microg/kg/inj), nicotine (12-50 microg/kg/inj), or combinations of the two under a progressive ratio schedule of reinforcement.

Nicotine alone did not function as a reinforcer. Cocaine injections increased in a dose-dependent manner when taken alone and when taken as a mixture with nicotine. Furthermore, adding nicotine to cocaine shifted the cocaine dose-response function to the left in four of the five monkeys. Analysis of the ED(50) values for cocaine and the mixtures indicated that some mixtures of cocaine and nicotine were more potent than cocaine alone. There were no differences in maximum injections between cocaine or any of the mixtures of cocaine and nicotine.

These results suggest that nicotine, under certain conditions, can increase cocaine's potency as a reinforcer without affecting its maximum reinforcing strength.