Individual differences in the effects of environmental stimuli on cocaine choice in socially housed male cynomolgus monkeys
Studies in laboratory animals have demonstrated an influence of environmentally derived stress and enrichment on the reinforcing effects of stimulants.
To characterize the effects of acute exposure to ethologically valid environmental stimuli on the reinforcing strength of cocaine relative to food in socially housed monkeys.
Materials and methods
Choice between cocaine and food was assessed in subsets of 16 socially housed (4/pen) male cynomolgus monkeys immediately after the following manipulations: (1) treats placed in home cage, (2) a 10-min exposure to a rubber snake, or (3) 3 to 7 days of living in a larger environment without cage mates.
Placing treats in the home cage shifted the cocaine dose–response curve to the left in five monkeys tested and to the right in 4 of 12 animals. The rubber snake significantly shifted the cocaine choice curve to the left in dominant monkeys. Exposure to an enlarged environment decreased cocaine choice in 9 of 15 monkeys; this effect was transient and not related to social rank. Repeated testing did not affect cocaine choice.
Brief exposure to environmental events hypothesized to be stressors or enrichment altered cocaine choice, although not all individuals were affected and the effects were transient. Importantly, the data suggest that implementing positive changes in the environment produced effects that are clinically desirable. Understanding the behavioral and neurobiological mechanisms mediating sensitivity to environmental events in socially housed animals will lead to better treatment strategies for drug addiction.