Individual differences in food cue responsivity are associated with acute and repeated cocaine-induced vocalizations, but not cue-induced vocalizations.

RATIONALE: Individuals prone to attribute incentive salience to food-associated stimuli ("cues") are also more sensitive to cues during drug seeking and drug taking. This may be due in part to a difference in sensitivity to the affective or other stimulus properties of the drug. In rats, these properties are associated with 50-kHz ultrasonic vocalizations (USVs), in that they are elicited during putative positive affective and motivational states, including in response to drugs of abuse.

OBJECTIVES: We sought to determine whether individual differences in the tendency to attribute incentive salience to a food cue (as measured by approach) were associated with differences in cocaine-induced USVs. We also tested whether the food cue would elicit USVs and if this response was related to approach to the food cue.

METHODS: In experiment 1, rats underwent Pavlovian conditioned approach (PavCA) training where they learned to associate a cue (an illuminated lever) with the delivery of a food pellet into a food cup. Subjects were categorized based on their approach to the cue ("sign-trackers") or to the food cup ("goal-trackers"). Rats subsequently underwent nine testing days in which they were given saline or cocaine (10 mg/kg i.p) and placed into a locomotor chamber. In experiment 2, rats were first tested in the locomotor chambers for one saline-treated day followed by one cocaine-treated day and then trained in PavCA. USVs were recorded from a subset of individuals during the last day of PavCA to determine if the food cue would elicit USVs.

RESULTS: Sign-trackers produced 5-24 times more cocaine-induced 50 kHz USVs compared to goal-trackers for all days of experiment 1, and this response sensitized with repeated cocaine, only in sign-trackers. Similarly in experiment 2, individuals that produced the most cocaine-induced USVs on a single exposure also showed the greatest tendency to sign-track during PavCA. Lastly, while sign-trackers produced more USVs during PavCA generally, the cue itself did not elicit additional USVs in sign- or goal-trackers.

CONCLUSIONS: These results indicate a robust and consistent relationship between approach to a food cue and cocaine-induced USV production. Thus, these USVs may index the neurobiological differences underlying the behavioral distinctions of sign- and goal-trackers.