GLUCOCORTICOIDS, BUT NOT DOPAMINE, NEGATIVELY REGULATE THE MELANOTROPHIC ACTIVITY OF THE RABBIT PITUITARY INTERMEDIATE LOBE

The leporidae (rabbit and hare) pituitary intermediate lobe (IL) diffe rs from that of other mammals by its neuroendocrine regulation. In par ticular, it is not submitted to the classic dopaminergic inhibitory co ntrol, which has been considered as a repressive factor for the expres sion of glucocorticoid receptors (GR) in the mammalian IL. Hence, the present experiments aimed at examining the rabbit IL for the possible existence of GR. Using both immunocytochemistry and binding studies wi th (H-3)-dexamethasone, we localized GR in the nuclei of IL cells and showed the presence of saturable and high-affinity type II receptor si tes, with Kd approximate to 3.9 nM. Also, exposure of cultured IL cell s to 10 nM dexamethasone (DEX) resulted in the blockade of melanocyte- stimulating hormone (alpha MSH) secretion stimulated by oxytocin (OT). Importantly, the inhibitory effect was reversed by a 100-fold excess of the glucocorticoid antagonist RU 38486. This is the first study whi ch clearly demonstrates in a mammalian IL, namely the rabbit, the pres ence of functional GR, involved in the negative regulation of the mela notrophic activity of this gland.