Ovine arylalkylamine N-acetyltransferase in the pineal and pituitary glands: Differences in function and regulation

The enzyme arylalkylamine N-acetyltransferase (AANAT; EC 2.3.1.87) has been conventionally linked with the biosynthesis of melatonin within the pineal gland and retina. This study establishes that AANAT messenger RNA (mRNA) a nd functional enzyme occurs within the pars tuberalis (PT) and to a lesser degree within the pars distalis (PD) of the sheep pituitary gland; expressi on in these tissues is approximately 1/15th (PT) and 1/300th (PD) of that i n the ovine pineal gland. AANAT mRNA in the PT appears to be expressed in t he same cells as the Mel1a receptor. No evidence was obtained to indicate t hat either PT or PD cells have the ability to synthesize melatonin, suggest ing that this enzyme plays a different functional role in the pituitary. We also found that cAMP regulation of the abundance of AANAT mRMA differs bet ween the PT and pineal gland. Forskolin(10 mu M) has no effect on pineal AA NAT mRNA levels, yet represses expression in the PT. This suppressive influ ence could be mediated by ICER (inducible cAMP response early repressor), w hich is induced by forskolin in both tissues. Although it appears that the specific function and regulation of AANAT in the pituitary gland differ fro m that in the pineal gland, it seems likely that AANAT may play a role in t he broader area of signal transduction through the biotransformation of ami nes.