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 a gainst reinforcement rates at each schedule value, are argued to provi de features useful in the evaluation of drug effects on feeding behavi or. In the present study the effects of s.c. insulin (5.0 IU/kg), i.c. v. neuropeptide Y (NPY) (5.0 mu g/5.0 mu l), i.c.v. insulin (0.5 mU/5. 0 mu l), i.c.v. 2-deoxyglucose (2-DG) (10.0 mu g/5.0 mu l), i.c.v. nal oxone (NLX) (50.0 mu g/5.0 yl) in conjuction with i.c.v. NPY (5.0 mu g /5.0 mu l), and i.c.v. NLX alone (50.0 mu g/5.0 mu l) were assessed. i .c.v. NPY, insulin and 2-DG caused an elevation of the response functi on obtained 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 reinforce r (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 grai n pellets). S.c. insulin produced no shift in the function from baseli ne, 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 reduc ing response rates at the higher schedule values, a shift in the funct ion similar to that observed following an increase in body weight. Sin ce the literature on NPY and insulin would suggest that their effects are mediated through mechanisms associated with internal regulation, t hese findings were not predicted.