Incentive motivation and ultrasonic vocalizations in rats
After experiencing a reward, the positive affective reactions it induces can become associated with its sensory properties and related cues. However, the manner in which such affective reward representations are expressed in animals remains unclear. Juvenile and adult rats communicate through ultras...
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|Summary:||After experiencing a reward, the positive affective reactions it induces can become associated with its sensory properties and related cues. However, the manner in which such affective reward representations are expressed in animals remains unclear. Juvenile and adult rats communicate through ultrasonic vocalizations (USVs), which also serve as situation-dependent affective signals. Since rats emit high frequency (i.e., 50-kHz) USVs in socially and non-socially rewarding situations, 50-kHz calls might prove to be a way incentive motivational state is signaled when training rats to anticipate food rewards under some predictable cues. In general, the results show that reward-cues become effective to elicit 50-kHz calls. Under certain conditions, however, the utterance of 50-kHz calls can be either suppressed during a highly motivational state, or more strikingly, can be elicited when food rewards were devalued by satiation. For rats, both a state of hunger and waiting for access to a daily meal can be negatively perceived if the food reward offered turns out to be less satisfying than expected. Learning to anticipate such a negative state seemed to suppress the otherwise positive affective reactions evoked by having access to a highly expected food. Such a frustration-like effect occurred only at the USVs level without being indicated behaviourally through changes in rats’ learning and motivation to approach and consume the reward. In contrast, providing continued access to the reward prevented the suppression of USVs. Surprisingly, in spite of being sated and no longer interested in seeking and consuming the reward, rats nevertheless continued to emit appetitive USVs in the presence of cues predicting a previously desired food. Rats as a whole, just as with humans, seem to represent rewards affectively beyond basal appetite requirements. However, the ability to attribute incentive salience to reward cues has been shown to strongly differ among individuals. The second study, therefore, focused on the analysis of individual differences in conditioned anticipatory activity elicited by reward-related cues as indicative of the predisposition of animals to attribute incentive salience to otherwise neutral stimuli. Across several experiments, individual rats prone to attribute incentive salience to reward cues –as indicated by high levels of either rearing activity, or sign-tracking behavior– showed heightened reward-induced affective responses, namely in the form of 50-kHz calls. When re-exposing rats to reward cues after a non-testing period, USVs were elicited even at higher rates than previously, especially in subjects prone to attributing incentive salience to reward cues. USVs appeared reliably expressed over time and persisted despite physiological needs have already been fulfilled. Interestingly, USVs were still elicited by reward cues even though reward-oriented behaviors and exploratory activity were drastically weakened by reward devaluation. Additionally, prone subjects seemed to undergo particular adaptations in their dopaminergic system related to incentive learning, as indicated by the attenuated response to the catecholamine agonist amphetamine and to the dopamine receptor antagonist flupenthixol. The investigation of the psychological and neurobiological factors underlying affective states as related to incentive motivation is of remarkable relevance in preclinical- and clinical-oriented research. The current findings may have translational potential, since for some individuals, excessive attribution of incentive salience to reward cues may lead to compulsive behavior disorders, such as overeating, pathological gambling, and drug addiction. Certain aspects of these disabling human conditions can be further investigated with the same animal models as implemented in the present studies.|