Direct actions of ACTH on ovarian function of pseudopregnant rabbits

The present study sought to assess whether the receptors for adrenocorticotropic hormone (ACTH), MC2R, and for glucocorticoid (GR) are expressed in corpora lutea (CL) of pseudopregnant rabbits and whether ACTH and cortisol exert any direct action on luteal function. By immunohistochemistry, positive reaction for MC2R and GR was detectable within luteal cells of CL. The MC2R mRNA levels were five-fold less abundant in day 9 than in day 4 CL (P < 0.01). At both stages, ACTH agonist (ACTH 1-24) increased progesterone and prostaglandin (PG) E2 (PGE2) (P < 0.01), but reduced PGF2α releases (P < 0.01) in vitro. ACTH 1-24 injection increased plasma cortisol levels within 4 h (P < 0.01), but decreased (P < 0.01) progesterone 24 h later and for the following two days. ACTH administration to estrous rabbits caused a transitory increase in blood progesterone concentrations (P < 0.01). Daily injections of ACTH did not modify progesterone profile following ovulation. In conclusion, ACTH directly up-regulates CL progesterone production in vitro via MC2R, but indirectly hampers luteal function via cortisol-GR associated mechanism.