Comparative study of the influence of the acute administration of drugs of abuse on 50-kHz

Increasing evidence indicates that the emisison of ultrasonic vocalizations (USVs) may index the emotional state of rats, with the 50- kHz component of these vocalizations reflecting a "positive" state. A thorough characterization of the pharmacological stimuli capable of triggering the emission of 50-kHz USVs is however lacking. On these bases, this study examined the influence of some psychoactive drugs bearing to different pharmacological classes on the emission of 50-kHz USVs by male rats. Rats received one of the following drugs, in acute administration: methylenedyoxymethamphetamine (MDMA 5-15 mg/kg i.p), methylphenidate (2,5-10 mg/kg i.p.), morphine (1-7 mg/kg s.c.), nicotine (0.1-0.4 mg/kg s.c.). Additional group of animals received the acute administration of D-amphetamine (2 mg/kg i.p.), which is known to robustly stimulate the emission of 50-kHz USVs, by term of comparison. The results obtained show that methylphenidate stimulated the emission of 50-kHz by rats in a fashion comparable to that of D-amphetamine, whereas MDMA, morphine and nicotine influenced the emission of 50-kHz USVs only marginally. This study shows that important differences exist in the ability of psychoactive drugs to stimulate the emission of 50-kHz USVs by rats. The present findings may further elucidate the neurobiology of 50-kHz USVs and may help understanding the relevance of 50-kHz USVs to drug-induced reward.