Expression of Fos-related antigens in the nucleus accumbens and associatedregions following exposure to a cocaine-paired environment

This study examined whether conditioned hyperactivity measured in a cocaine -paired environment was associated with increased expression of Fos-related antigens (FRA) within the nucleus accumbens (NAc) and associated forebrain regions of rats. Three groups of rats were given repeated injections of ei ther cocaine in the test environment and saline in the colony room (group P aired), saline in the test environment and cocaine in the colony room (grou p Unpaired), or saline in both environments (group Control). All rats were subsequently given a drug-free test for conditioned hyperactivity in the te st environment, and their brains were removed so that FRA immunohistochemis try could be conducted. Rats in the Paired group showed conditioned hyperac tivity during the conditioning test, and this behavioural response was asso ciated with increased FRA expression within the caudal NAc, the medial pref rontal cortex and the lateral septum relative to the Unpaired and Control g roups. Paired rats also showed increased FRA expression within the orbital prefrontal cortex, the claustrum, the caudal amygdala (basolateral and cent ral regions), the paraventricular thalamic nucleus, the subiculum of the hi ppocampus, and the lateral habenula relative to the Control group. However, the FRA levels in these latter sites were not significantly increased rela tive to those of Unpaired rats, indicating that genomic responses in these regions were not entirely context dependent. The correspondence between con ditioned hyperactivity and enhanced FRA expression within the caudal NAc, t he medial prefrontal cortex and lateral septum suggests that these regions may participate in the expression of conditioned responses to cocaine-relat ed stimuli.