Colocalisation of c-Fos and glucocorticoid receptor as well as of 5-HT(1A) and glucocorticoid receptor immunoreactivity-expressing cells in the brain structures of low and high anxiety rats.

We sought to determine the colocalisation of c-Fos (a marker of neuronal activation) and glucocorticoid receptors (GRs) as well as of 5-HT(1A) and glucocorticoid receptor immunoreactivity-expressing cells (ir) in the dorsomedial prefrontal cortex (M2), dentate gyrus of the hippocampus (DG), and basolateral nucleus of the amygdala (BLA) in low and high anxiety rats (i.e., rats with duration of a freezing response in the conditioned fear test one standard error or more below or above the mean value: low responders (LR) and high responders (HR), respectively). It was found that 1.5 h after a testing session of the conditioned fear test, the LR animals had a higher activity of the cortical M2 area and DG (c-Fos), a higher expression of GRs-ir, as well as an increased number of cells co-expressing c-Fos and GRs-ir in the same brain areas. In the case of HR rats, they had similar expression of c-Fos in the BLA, but a significantly higher concentration of GRs-ir and c-Fos/GR colocalised neurons in the same amygdala nucleus. The pattern of distribution of 5-HT(1A) and GR receptor-ir in LR and HR animals was similar to the c-Fos and GRs-ir expression. LR animals showed a higher density of 5-HT(1A) and GRs-ir in the cortical M2 area and DG as well as an increased number of cells co-expressing 5-HT(1A) and GR-ir in the same brain areas. HR rats had a significantly higher concentration of 5-HT(1A) and GR-ir as well as a greater number of c-Fos/GR protein colocalised neurons in the BLA. The present data add to the arguments for the neurobiological background of differences in individual responses to aversive conditioned stimuli.