Glucocorticoids, but not dopamine, negatively regulate the melanotrophic activity of the rabbit pituitary intermediate lobe.

The leporidae (rabbit and hare) pituitary intermediate lobe (IL) differs from that of other mammals by its neuroendocrine regulation. In particular, it is not submitted to the classic dopaminergic inhibitory control, which has been considered as a repressive factor for the expression 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 with (3H)-dexamethasone, we localized GR in the nuclei of IL cells and showed the presence of saturable and high-affinity type II receptor sites, with Kd approximately 3.9 nM. Also, exposure of cultured IL cells 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 which clearly demonstrates in a mammalian IL, namely the rabbit, the presence of functional GR, involved in the negative regulation of the melanotrophic activity of this gland.