Responses of rat medial prefrontal cortical neurons to Pavlovian conditioned stimuli and to delivery of appetitive reward.

In the present experiments, medial prefrontal cortical (mPFC) neurons were extracellularly recorded by means of tetrode electrodes to examine their possible role in the prediction of appetitive reward. Two different cue tones (CS) and sucrose solution or water reward (US) were associated in a Pavlovian conditioning paradigm. In order to test behavioral correlate of the CS-US association, the head acceleration before the first lick of licking cluster was measured. Neuronal activity changes in the mPFC were analyzed (i) during the CS presentations; (ii) before the first lick of licking clusters; (iii) during consummation; and (iv) we also examined whether consummation was represented in neurons responding to the CSs. There was a difference between the head accelerations to the different USs during early or late occurring first approaches, but there was no such a difference during intercluster approaches. A significant proportion of neurons changed their firing rate during the CS presentation, before the first lick of licking cluster or during licking of the reward. Both, excitatory and inhibitory responses were observed. A subpopulation of neurons responding to the CSs also responded during reward consumption. Differential population activities of excitatory neurons were recorded in response to the different CSs, CS evoked approach behaviors and consumption of different rewards. Neuronal responses also discriminated among the CSs and trials with or without consummation. These results provided evidence for the involvement of mPFC neurons in the prediction, representation and organization of conditioned behavioral actions, such as approaches to rewards and consummation.