Monitoring dopamine transmission in the rat nucleus accumbens shell and core during nose poking and lever pressing for sucrose

Several studies investigated the dopamine (DA) responsiveness in the nucleus accumbens (NAc) shell and core during food self-administration (SA), but some discrepancies between results, probably due to the different methodological procedures used in each study, are shown. In order to clarify this issue we investigated by microdialysis assays the responsiveness of DA transmission in the NAc shell and core in rats responding for sucrose, using two different operant schedules: nose poking versus lever pressing. In rats trained to respond for sucrose pellets by nose poking fixed ratio 1 (FR1) schedule, dialysate DA increased in the shell but not in the core during active responding, as well as during extinction in the presence of sucrose cues. Instead, rats responding for sucrose by lever pressing FR1 schedule shown a strengthening of DA transmission either in the NAc shell and core. Non-contingent sucrose presentation and feeding in nose poking and lever pressing FR1 trained animals increased dialysate DA to a similar extent in both shell and core. We can conclude that using the nose poking training, under FR1 schedule of responding, the non-contingent sucrose feeding activates DAergic transmission in the shell and core, while, the response-contingent feeding activates DA release selectively in the NAc shell, as a result of the action of sucrose conditioned cues. At the opposite, in lever pressing trained rats, during responding for sucrose we found an increase of DA in both areas as a result of the different instrumental task required to obtain the reward. These findings can explain most of the discrepancies existing in the literature on the responsiveness of shell and core DA during food self-administration behavior.