Repeated amphetamine administration and long-term effects on 50-kHz ultrasonic vocalizations: possible relevance to the motivational and dopamine-stimulating properties of the drug

Ultrasonic vocalizations (USVs) of 50kHz are thought to indicate positive affective states in rats, and are increasingly being used to investigate the motivational properties of drugs of abuse. However, previous studies have observed that only dopaminergic psychostimulants of abuse, but not other addictive drugs, stimulate 50-kHz USVs immediately after their administration. This would suggest that 50-kHz USVs induced by addictive dopaminergic psychostimulants might reflect rewarding dopaminergic effect, rather than motivational effect. To elucidate this issue, our study compared the effects of the psychostimulant of abuse amphetamine and the dopamine receptor agonist apomorphine on 50-kHz USVs. Rats that received five drug administrations on alternate days in a novel test-cage, were first re-exposed to the test-cage 7 days after treatment discontinuation to assess drug-conditioning, and then received a drug challenge. USVs were recorded throughout the experiments together with locomotor activity. To further clarify how amphetamine and apomorphine influenced 50-kHz USVs, rats were subdivided into “low” and “high” vocalizers, and time-dependence of drug effects was assessed. Amphetamine and apomorphine stimulated both 50-kHz USVs and locomotor activity, though they elicited dissimilar changes in these behaviors, depending on drug dose, rats’ individual predisposition to vocalize, and time. Moreover, only amphetamine-treated rats displayed both sensitized 50-kHz USVs emission and conditioned vocalizations on test-cage re-exposure. These results indicate that the effects of amphetamine on 50-kHz USVs are not mimicked by a dopaminergic agonist with a low abuse potential, and may further support the usefulness of 50-kHz USVs in the study of the motivational properties of psychoactive drugs.