Dissociations in dopamine release in medial prefrontal cortex and ventral striatum during the acquisition and extinction of classical aversive conditioning in the rat

Dual perfusion in vivo brain microdialysis was used to monitor extracellular levels of dopamine in the medial prefrontal cortex and ventral striatum during the acquisition and extinction of a classical aversive conditioning paradigm in rats. The main finding was a dissociation in the pattern of release in the two brain areas. The first stimulus-footshock pairing elicited large increases in cortical dopamine over baseline levels that were much greater than the increases elicited by different stimuli of equivalent salience that were unpaired with footshock. In contrast, dopamine levels in ventral striatum were unchanged under these conditions. Over the next two pairings, there was a decline in the cortical response and an increase in the response in ventral striatum. The first presentation of the aversive conditioned stimulus in a separate context elicited the largest response in ventral striatum. Post-conditioning, the cortical response to the conditioned stimulus was smaller than that elicited by the initial stimulus-footshock pairing and was equivalent in magnitude to that elicited by stimuli unpaired with footshock. Over the final two conditioned stimuli presentations, in the absence of the footshock reinforcer (extinction), responses declined in both brain areas. Simultaneous monitoring of behaviour indicated that the neurochemical events were accompanied by effective aversive learning, as indexed by conditioned freezing responses. The data are discussed in terms of the hypothesis that medial prefrontal cortex is especially engaged during novel circumstances which may, potentially, require new learning, whilst ventral striatal dopamine more closely follows the expression of conditioned responding during learning and extinction.