Like primary reinforcers, the anticipation of reward ought to affect neurochemical release in brain regions, such as the medial prefrontal cortex (mPFC), which are associated with appraisal processes. To assess the neurochemical changes associated with anticipation, rats were exposed to the pairing of auditory (60-dB white noise), visual, and olfactory cues with the daily presentation of a palatable snack (Cue Relevant group). Rats of a second group were similarly trained, but for a 2-week period, the snack was no longer provided following cue presentation (Extinction group). In the third condition, the presentation of the snack and cues was uncorrelated (Cue Irrelevant group). Analyses of dialysates collected in vivo from the mPFC revealed that release of corticotropin-releasing hormone (CRH), gastrinreleasing peptide (GRP), and the 5-HT catabolite, 5-hydroxyindole acetic acid (5-HIAA), had increased bilaterally in response to the anticipatory cues, whereas DA release increased only within the right mPFC. In the case of CRH and GRP, these increases were also apparent in the extinction condition, despite the fact that behavioral arousal to the anticipatory cues (increased exploration, rearing, grooming, and vigilance) was only evident in the Cue Relevant condition. In contrast, the elevated DA and 5-HIAA were apparent exclusively in the Cue Relevant condition. Thus, CRH and GRP systems may serve to allocate salience and/or incentive reward value to biologically significant stimuli or reflect the emotional response to the anticipatory stimulus. The activity of DA and 5-HT neurons, in contrast, is more closely aligned with the cognitive appraisal of predictor stimuli.
- Incentive motivation