Besides the pineal gland, melatonin is reported to be produced in a number
of extrapineal sites, where it could act as an intracellular mediator or pa
racrine signal in addition to its endocrine effects. In view of the suggest
ed immunoregulatory role of melatonin, we compared lymphoid organs and seve
ral other tissues of the rat for their potential to synthesize melatonin. U
sing the reverse transcription-polymerase chain reaction (RT-PCR) method. w
e determined the tissue-specific expression of mRNAs encoding two key enzym
es of the melatonin biosynthesis: serotonin-N-acetyltransferase (NAT) and h
ydroxyindole-O-methyltransferase (HIOMT). The minimal number of PCR cycles
required to obtain a positive signal served as a measure for the abundance
of a given mRNA. NAT and HIOMT mRNAs were detected in all tested tissues at
high numbers of PCR cycles (40 and 45, respectively). At 35 cycles, only g
ut, testis, spinal cord, raphe nuclei, stomach fundus and striatum yielded
positive signals for both enzymes. In conclusion, the presence of NAT and H
IOMT mRNAs in a wide range of tissues corroborates and extends the notion o
f extrapineal melatonin synthesis. Comparatively low levels of the HIOMT me
ssages in lymphoid organs, however, indicate a limited significance of mela
tonin synthesis within the immune system.