ABSTRACT
The effects of the receptor antagonists MDL 72222 (MDL, 5-HT3) and naltrindole (delta-opioid) on ethanol reward and its discrimination were examined in ethanol-preferring C57BL/6 (C57) mice. MDL attenuated lever responding for 12% ethanol delivered on a fixed-ratio 8 reinforcement schedule at a dose that did not influence responding for water reward, thus confirming a previous report that ICS 205-930 reduced ethanol reward for Long-Evans rats. Our study in combination with the reduced ethanol consumption reported for C57 mice injected with odansetron indicates that 5-HT3 receptor systems are involved in mediating behavior directed toward obtaining ethanol as well as its consumption. By attenuating the rewarding effects of ethanol or of ethanol conditioned cues (e.g., the operant environment), 5-HT3 antagonists may be useful in the treatment of alcohol abuse. The 5-HT3 antagonist effects in this study are comparable with the effects of naltrexone on ethanol reward in C57 mice, although higher doses were required to reduce operant responding for ethanol reward. In contrast to the 5-HT3 antagonist and naltrexone effects, naltrindole, an antagonist with greater specificity for the delta-opioid receptor, was without effect on ethanol reward. This result and recent reports for rats and monkeys suggests that the general antagonists might be more efficacious in attenuating ethanol reward. Both MDL and naltrindole produced only slight reductions in the ethanol discriminative cue, suggesting that the rewarding and discriminative effects of ethanol are not likely mediated by identical neural mechanisms as previously suggested.
Subject(s)
Discrimination Learning/drug effects , Ethanol/pharmacology , Narcotic Antagonists/pharmacology , Receptors, Opioid, delta/antagonists & inhibitors , Receptors, Serotonin/drug effects , Reward , Serotonin Antagonists/pharmacology , Animals , Conditioning, Operant/drug effects , Dose-Response Relationship, Drug , Female , Male , Mice , Mice, Inbred C57BL , Naltrexone/analogs & derivatives , Naltrexone/pharmacology , Receptors, Serotonin, 5-HT3 , Reinforcement, Psychology , Tropanes/pharmacologyABSTRACT
The effects of the opioid antagonist, naltrexone, on operant responding for oral ethanol reward delivered on a fixed-ratio schedule, and on the discriminative stimulus properties of intraperitoneally injected ethanol, was examined in two separate experiments. The ages, food/water motivational conditions, and naltrexone doses for the two experiments were similar to allow a direct comparison of naltrexone effects on the two measures. Male food-deprived C57BL/6 mice responded for ethanol during either preprandial (low thirst, high hunger motivation) or postprandial (high thirst, low hunger motivation tests). The reinforcing value of ethanol relative to water was greater during the preprandial tests; however, the amounts of ethanol consumed was greater during the postprandial tests, with some mice becoming unconscious during the 15-min test session. Naltrexone produced dose-responsive reductions in responding for ethanol under either testing condition. During postprandial tests, naltrexone reduced responding for ethanol reward at a dose (1.25 mg/kg) that had little effect on responding for water reward, suggesting some selectivity for ethanol reward. In addition, doses of naltrexone that reduced responding for ethanol rewards did not alter the discrimination of ethanol (g/kg) in an operant discrimination task, but did reduce the total number of responses made during these tests. Thus, under similar motivational and dosing conditions, the opiate antagonist attenuated the reinforcing, but not the discriminative properties of ethanol, suggesting that the latter is mediated by either different or additional neural mechanisms in C57BL/6 mice.
