Reinforcement schedule effects in rats trained to discriminate 3,4-methylenedioxymethamphetamine (MDMA) or cocaine.
BACKGROUND
Relatively few studies have compared the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine, and findings from different laboratories are somewhat inconsistent. One possible reason for discrepant results may be the use of different reinforcement schedules during discrimination training.


OBJECTIVE
The present study compared fixed ratio (FR) 20 and variable interval (VI) 15-s reinforcement schedules to determine their influence on discrimination acquisition, response rates, frequency of reinforcements, and stimulus generalization in rats trained to discriminate cocaine or MDMA.


METHODS
Thirty-two male Sprague-Dawley rats were trained to discriminate cocaine (10 mg/kg; n=16) or MDMA (1.5 mg/kg; n=16) from saline under either a FR 20 or a VI 15-s schedule of food reinforcement. Stimulus generalization tests were conducted with a range of doses of cocaine, MDMA, d-amphetamine, and lysergic acid diethylamide in all four training groups.


RESULTS
The FR 20 schedule facilitated more rapid discrimination acquisition compared to the VI 15-s schedule and established differential response rates and frequency of reinforcement under drug and vehicle conditions. However, reinforcement schedule had little influence on stimulus generalization between MDMA and cocaine. Cocaine produced partial substitution for MDMA in both training groups (FR 20, 51%; VI 15-s, 58%). Likewise, MDMA produced only partial substitution for cocaine in both training groups (FR 20, 40%; VI 15-s, 72%).


CONCLUSIONS
The present findings suggest that the number of sessions required to establish discriminative stimulus control varies with different reinforcement schedules. Nevertheless, training schedules alone do not appear to have significant effects on stimulus generalization between MDMA and cocaine.
Kueh
Daniel
D
Department of Biological Sciences, Western Michigan University, Kalamazoo, MI 49008, USA.
Baker
Lisa E
LE
D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D000697:Central Nervous System Stimulants / Q000008:administration & dosage*; D003042:Cocaine / Q000008:administration & dosage*; D003913:Dextroamphetamine / Q000008:administration & dosage; D004193:Discrimination Learning / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D005795:Generalization, Stimulus / Q000187:drug effects; D006213:Hallucinogens / Q000008:administration & dosage*; D008238:Lysergic Acid Diethylamide / Q000008:administration & dosage; D008297:Male; D018817:N-Methyl-3,4-methylenedioxyamphetamine / Q000008:administration & dosage*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012055:Reinforcement Schedule; D013997:Time Factors
D000697:Central Nervous System Stimulants; D006213:Hallucinogens; D008238:Lysergic Acid Diethylamide; D003042:Cocaine; D018817:N-Methyl-3,4-methylenedioxyamphetamine; D003913:Dextroamphetamine