Reinforcement magnitude modulation of rate-dependent effects of fluvoxamine and desipramine in the rat.

Reinforcement magnitude modulates the effects of the antidepressants fluvoxamine and desipramine in the pigeon. Increasing reinforcement magnitude diminishes the rate-dependent effects of these drugs. Whether this is also the case in other species is unknown. Rats were trained to respond under a multiple fixed-interval (FI 300 s) schedule of reinforcement. In one FI component, rats earned two food pellets, and in the other component they earned 10 food pellets when they completed the FI requirement. The effects of fluvoxamine (3, 5.6, 10, and 17.8 mg/kg) or desipramine (1, 3, 5.6, 10, 30 mg/kg) given 30 min presession (intraperitoneally) on overall response rate were examined. Local rates of responding (during each 10th of the component) increased throughout the FI as is typical, and were higher during the component reinforced with 10 pellets. Fluvoxamine and desipramine decreased overall response rates similarly in both components. Both drugs exerted limited rate-dependent effects, shown by a negative slope for the regression of log (drug rate/control rate) on log (control rate) using data from each 10th of the FI. The slope for the two-pellet condition was, however, significantly steeper than the slope for the 10-pellet condition after 3 and 10 mg/kg fluvoxamine and after 30 mg/kg desipramine. This result is consistent with those obtained in pigeons and shows that reinforcement magnitude can modulate rate-dependent effects of fluvoxamine and perhaps desipramine in rats.