Citrulline extracellular level in the nucleus accumbens during acquisition and extinction of classical conditioned fear response with pain reinforcement

By means of in vivo microdialysis combined with HPLC analysis, we have shown that extracellular levels of citrulline (NO co-product) and arginine (NO precursor) increase in the rat n. accumbens during acquisition and expression of a classical fear response. The conditioned rise of citrulline and arginine levels gradually decreased in the course of extinction. The renewal of the response produced an increase in extracellular citrulline and arginine levels. These data suggest that the acquisition of conditioned fear response causes an increase in NO production in the n. accumbens that weakens during the extinction and is restored during the reinstatement of the response.