Research reportUltrasonic vocalizations in rats anticipating circadian feeding schedules

- Activity and ultrasonic vocalizations (USVs) were recorded for 2 h or 24 h in rats fed ad libitum or 2 h per day in the light period.
- Food restricted rats exhibited a daily rhythm of food anticipatory activity.
- Food anticipation was accompanied by FM and flat ∼50 kHz USVs.
- Flat ∼50 kHz USVs were more dependent on the presence of food.
- An audible tone prior to mealtime modulated anticipatory locomotor activity but did not evoke ∼50 kHz USVs.

Rats readily learn to anticipate a reward signaled by an external stimulus. Anticipatory behaviors evoked by conditioned stimuli include 50 kHz ultrasonic vocalizations (USVs), a proposed behavioral correlate of positive affect and activation of midbrain dopamine pathways. Rats can also anticipate a reward, such as food, provided once daily, without external cueing. Anticipation of a daily reward exhibits formal properties of a circadian rhythm. The neural circuits that regulate the timing and amplitude of these rhythms remain an open question, but evidence suggests a role for dopamine. To gain further insight into the neural and affective correlates of circadian food anticipatory rhythms, we made 2 h and 24 h USV recordings in rats fed 2 h/day in the light period, a procedure that induces robust anticipation 2-3 h before mealtime. Potential interactions between internal and external time cues in USV production were evaluated by inclusion of a 3 kHz tone 15 min before mealtime. Prior to scheduled feeding, spontaneous 50 kHz USVs were rare during the light period. During scheduled feeding, flat and frequency modulated (FM) 50 kHz USVs occurred prior to and during mealtime. FM USVs were more closely related to anticipation, while flat USVs were more dependent on food access. USVs also occurred during spontaneous waking at other times of day. The tone did not evoke USVs but did modulate activity. Behavioral anticipation of a daily meal is accompanied by USVs consistent with a positive affective state and elevated dopamine transmission.