Effect of the 5-HT3 receptor antagonists, MDL72222 and ondansetron on morphine place conditioning.

The purpose of the present study was to reassess the original findings of Carboni et al. (1988) who suggested that 5-HT3 receptor antagonists may block morphine-induced place conditioning in rats. These workers used a biased protocol with treatments allocated to compartments based on initial preference. In the present study we have adopted an unbiased approach with treatments randomly assigned to conditioning compartment in a counter-balanced fashion. Thus treatments were equally paired between distinct environmental cues. Using this protocol, morphine produced a dose-related place preference (0.3-3 mg/kg SC). Thirty-minute pretreatment with the selective 5-HT3 antagonists, MDL72222 (1 mg/kg SC) and ondansetron (0.01 mg/kg SC) before morphine (1.5 mg/kg SC), significantly antagonized the place conditioning to this treatment. However, with higher doses of ondansetron (0.1-1 mg/kg SC), the antagonism of morphine-induced place preference became variable and dependent on the conditioning compartment. This was probably a reflection of the fact that ondansetron when administered alone also appeared to produce an environmentally dependent place conditioning at these doses. Therefore it is concluded that at certain doses, 5-HT3 receptor antagonists may antagonize morphine place conditioning in a manner consistent with a blockade of the appetitive effects of this drug. However, at higher doses, at least with ondansetron, this antagonism became non-specific and dependent on the training environment. It is suggested that other animal models of opioid reinforcement (e.g., self-administration) are now needed to validate the hypothesis that 5-HT3 receptor antagonists may modify opioid reward.