Upregulated expression of neuropeptide Y in hypothalamic–pituitary system of rats by chronic dexamethasone administration

To study the effect of adrenal steroids on neuropeptide Y (NPY) synthesis in the hypothalamic–pituitary system, we examined NPY expression in rats treated with dexamethasone (a synthetic glucocorticoid) by in situ hybridization and immunohistochemistry. Rats were injected daily with dexamethasone (0.2 mg/100 g/day for 10 days, sc) or sesame oil (vehicle control), or non-injected (intact control). Relative staining area for corticotropin-releasing hormone or neurophysin II, a vasopressin carrier protein, was increased in the external zone of the median eminence in vehicle control, but was equivalent to that of intact control in the dexamethasone-injected group. Density of NPY-stained fiber varicosities was drastically increased in the external, but not the internal, zone of dexamethasone-injected group, coinciding with the increased NPY hybridization signal level in the arcuate nucleus. Dual-labeling experiments revealed no colocalization of NPY with hypophysiotropic or other peptides examined in single fibers of the median eminence. In the dexamethasone-injected group, expressions of NPY mRNA and peptide were detectable in a few pituitary cells, with some being corticotropes. These results suggest that NPY plays hormonal roles in the hypothalamic–pituitary–adrenal axis.