Central injections of growth hormone-releasing factor increase operant responding for food reward

1. Male Wistar rats were prepared with chronic ventricular cannulae and trained to bar press for food pellets in operant cages. 2. Some rats were given icv injections of 0 (vehicle), 0.4, 4.0 and 40.0 picomole GRF on separate days and tested on a continuous reinforcement (CRF) reward schedule. 3. Other rats were given icv injections of 0 (vehicle) and 4 picomole GRF on separate days and tested on a progressive ratio (PR) reward schedule. 4. In comparison with vehicle injections, GRF significantly increased the average response rate under the CRF schedule and the average breaking point under the PR schedule. 5. The increased operant responsiveness displayed by rats suggests that central GRF possesses motivational efficacy, increasing the relative reinforcing strength of food.