Fos expression in mesocorticolimbic areas during heroin place conditioning.

We studied Fos immunoreactivity within targets of the mesocorticolimbic dopamine systems at different stages of acquisition of heroin place conditioning. Fos was quantified in groups of rats after one or four exposures to 0.3-mg/kg heroin only, or to the conditioning compartment only, or to both heroin and the conditioning compartment, or to neither stimuli. Increases in Fos were found primarily after the first exposure. Further, the medial prefrontal cortex was the only region displaying higher Fos expression in the group exposed to both heroin and conditioning compartment. It is concluded that the medial prefrontal cortex may be the neural site whereby neural representations of novel stimuli with motivational value are associated during associative learning.