Utilization of a novel model of food reinforced behavior involving neuropeptide Y, insulin, 2-deoxy-D-glucose and naloxone

The cyclic-ratio schedule methodology exposes animals to an ascending followed by a descending sequence of ratio values over six consecutive cycles. The response functions, obtained by plotting response rates against reinforcement rates at each schedule value, are argued to provide features useful in the evaluation of drug effects on feeding behavior. In the present study the effects of s.c. insulin (5.0 IU/kg), i.c.v. neuropeptide V (NPY) (5.0 μg/5.0 μl), i.c.v. insulin (0.5 mU/5.0 μl), i.c.v. 2-deoxyglucose (2-DG) (10.0 μg/5.0 μl), i.c.v. naloxone (NLX) (50.0 μg/5.0 μl) in conjunction with i.c.v. NPY (5.0μg/5.0 μl), and i.c.v. NLX alone (50.0 μg/5.0 μl) were assessed. i.c.v. NPY, insulin and 2-DG caused an elevation of the response function ohtained by plotting response rates against reinforcement rates but did not affect the slope of the function. This elevation was similar to that observed after increasing the incentive value of the reinforcer (i.e., similar to increasing a 5.0% sucrose concentration reinforcer to 10.0%, and to substituting 45 mg sweet food pellets for 45 mg grain pellets). S.c. insulin produced no shift in the function from baseline, and i.c.v. NLX blocked the effect of i.c.v. NPY. I.c.v. NLX given alone reduced the slope of the response function, by selectively reducing response rates at the higher schedule values, a shift in the function similar to that observed following an increase in body weight. Since the literature on NPY and insulin would suggest that their effects are mediated through mechanisms associated with internal regulation, these findings were not predicted.