Dopamine D1 and μ-opioid receptor antagonism blocks anticipatory 50 kHz ultrasonic vocalizations induced by palatable food cues in Wistar rats.

RATIONALE: Fifty kilohertz ultrasonic vocalizations (USVs) have been sometimes shown to reflect positive affective-like states in rats. Rewarding events, such as access to palatable food or drugs of abuse, increase the number of anticipatory 50-kHz USVs. However, little is known about the predictability of USVs, subtypes of USVs involved, and underlying neurobiological mechanisms.
OBJECTIVES: We examined whether cue-induced anticipatory 50-kHz USVs predict palatable food intake and tested the effects of dopamine D1 and μ-opioid receptor antagonism on anticipatory USVs. MATERIALS: Food-restricted rats received repeated sessions of a 2-min cue light immediately followed by a 5-min access to palatable food. Ultrasonic vocalizations were recorded during cue presentation. After 24 pairing sessions, the rats were pretreated with the D1 receptor antagonist SCH 23390 (5, 10, and 20 μg/kg) and μ-opioid receptor antagonist naltrexone (0.03, 0.06, 0.13, 0.25, 0.5, and 1 mg/kg) in a Latin-square design, and USVs were recorded during cue presentation.
RESULTS: Rats emitted 50-kHz USVs during cue presentation, and the number of USVs increased across sessions with robust and stable interindividual differences. Escalation in USVs was subtype-dependent, with nontrill calls significantly increasing over time. Palatable food intake was positively correlated with anticipatory 50-kHz USVs. Moreover, anticipatory USVs were dose-dependently prevented by antagonism of D1 and μ-opioid receptors.
CONCLUSIONS: These findings demonstrate that anticipatory 50-kHz USVs represent a stable phenotype of increased motivation for food, and dopamine and opioid systems appear to mediate anticipatory 50-kHz USVs.