Id1, Id2 and Id3 are induced in rat melanotrophs of the pituitary gland by dopamine suppression under continuous stress.

In rats under continuous stress (CS) there is decreased hypothalamic dopaminergic innervation to the intermediate lobe (IL) of the pituitary gland, which causes hyperactivation and subsequent degeneration of melanotrophs in the IL. In this study, we investigated the molecular basis for the changes that occur in melanotrophs during CS. Using microarray analysis, we identified several genes differentially expressed in the IL under CS conditions. Among the genes up-regulated under CS conditions, we focused on the inhibitor of DNA binding/differentiation (Id) family of dominant negative basic helix-loop-helix (bHLH) transcription factors. RT-PCR, Western blotting and in situ hybridization confirmed the significant inductions of Id1, Id2 and Id3 in the IL of CS rats. Administration of the dopamine D2 receptor agonist bromocriptine prevented the inductions of Id1–3 in the IL of CS rats, whereas application of the dopamine D2 antagonist sulpiride induced significant expressions of Id1–3 in the IL of normal rats. Moreover, an in vitro study using primary cultured melanotrophs demonstrated a direct effect on Id1–3 inductions by dopamine suppression. These results suggest that the decreased dopamine levels in the IL during CS induce Id1–3 expressions in melanotrophs. Because Id family members inhibit various bHLH transcription factors, it is conceivable that the induced Id1–3 would cooperatively modulate gene expressions in melanotrophs under CS conditions to induce hormone secretion.