Activation of adenosine A2A receptors suppresses the emission of pro-social and drug-stimulated 50-kHz ultrasonic vocalizations in rats: possible relevance to reward and motivation

RATIONALE: Rats emit 50-kHz ultrasonic vocalizations (USVs) in response to pleasurable stimuli, and these USVs are considered a tool for investigating reward and motivation. OBJECTIVES: This study aimed to clarify how activity of adenosine A2A receptors, which modulate reward and motivation, influences 50-kHz USV emission in rats. METHODS: Rats received one of the following treatments in a test cage: (1) acute administration of the A2A receptor agonist CGS 21680 (0.05-0.2 mg/kg, i.p.) during social interactions; (2) long-term amphetamine (1 or 2 mg/kg, i.p.) or morphine (7.5 mg/kg, s.c.) administration on alternate days, alone or with CGS 21680, followed after 7 days of discontinuation by test cage re-exposure, to assess drug-conditioning effects, and thereafter drug challenge; (3) acute administration of the D1/D2 receptor agonist apomorphine (4 mg/kg, i.p.), alone or with CGS 21680; and (4) long-term administration of the non-selective A1/A2A receptor antagonist caffeine (15 mg/kg, i.p.), on alternate days. USVs and locomotor activity were evaluated throughout the treatments. RESULTS: CGS 21680 attenuated 50-kHz USV emission stimulated by social interactions, amphetamine, apomorphine, and morphine, and rats administered CGS 21680 with amphetamine or morphine emitted fewer conditioned 50-kHz USVs upon test cage re-exposure, compared with rats administered amphetamine or morphine alone. Moreover, CGS 21680 administration prevented long-term changes in locomotor activity in amphetamine- and morphine-treated rats. Finally, caffeine had no effect on 50-kHz USVs. CONCLUSIONS: These results indicate that activation of A2A receptors attenuates 50-kHz USV emission in rats and further elucidate how these receptors modulate the motivational properties of natural and pharmacological stimuli.