Are nuclear receptors involved in pituitary responsiveness to melatonin?

Evidence suggests that the pineal hormone melatonin modulates prolactin (PRL) secretion in part through direct effects on the anterior pituitary. However, high-affinity membrane receptors for melatonin are only found in the pars tuberalis (PT) of the anterior pituitary, whereas lactotrophs are confined to the pars distalis (PD). This study therefore sought to determine whether melatonin might have direct effects on the PD, through alternate pathways. Such a possibility had been suggested by recent reports of melatonin binding to members of the retinoid-related orphan nuclear receptor family (ROR/RZR). Expression of ROR(alpha)/RZR(alpha) isoforms was observed by reverse transcription PCR in the ovine PT and PD. Correspondingly unidentified nuclear proteins from these tissues showed binding to consensus DNA response elements for members of the ROR/RZR family. In contrast nuclear 2-[125I]iodomelatonin (IMEL) binding was not detectable in PD or PT extracts even at high ligand and tissue concentrations. Nevertheless, the conditions used allowed membrane and nuclear IMEL binding to be observed in PT and liver extracts, respectively. Overall these findings do not support the possibility of direct effects of melatonin on the adult PD, and by implication they reinforce the view that the melatonin-responsive PT is an intermediary in the control of PRL secretion.