Major depression is associated with both dysregulation of the hypothalamic pituitary adrenal axis and serotonergic deficiency, not the least of the 5-HT2A receptor. However, how these phenomena are linked to each other, and whether a low 5-HT2A receptor level is a state or a trait marker of depression is unknown. In mice with altered glucocorticoid receptor (GR) expression we investigated 5-HT2A receptor levels by Western blot and 3H-MDL100907 receptor binding. Serotonin fibre density was analyzed by stereological quantification of serotonin transporter immunopositive fibers. To establish an effect of GR activation on 5-HT2A levels, mature organotypic hippocampal cultures were exposed to corticosterone with or without GR antagonist mifepristone and mineralocorticoid receptor (MR) antagonist spironolactone. In GR under-expressing mice, hippocampal 5-HT2A receptor protein levels were decreased (26.3 +/- 1.6%, p < 0.05) and frontal 5-HT2A receptor binding was decreased (20 +/- 15%, p < 0.01) as compared to wild-type mice. Conversely, in over-expressing GR mice hippocampal 5-HT2A receptor protein levels were increased (60.8 +/- 4.0%, p = 0.0001) and 5-HT2A receptor binding was increased in dorsal hippocampus (77 +/- 35%, p < 0.05) as compared to wild-type mice. No difference in serotonin fibre density was observed in the GR over-expressing mice, while the GR under-expressing mice showed lower serotonergic innervation in the frontal cortex area. An effect of GR activation on 5-HT2A receptor levels was further corroborated by the culture studies as long-term exposure of 3 microM corticosterone to organotypic hippocampal cultures increased 5-HT2A receptor levels (p < 0.05). The corticosterone-induced 5-HT2A receptor up-regulation was blocked by addition of either spironolactone or mifepristone.