Hydroxyindole-O-methyltransferase is another target for L-glutamate-evoked inhibition of melatonin synthesis in rat pinealocytes.

Rat pinealocytes use L-glutamate as a modulator for melatonin synthesis. Upon binding of L-glutamate to the class II metabotropic glutamate receptor, norepinephrine (NE)-dependent formation of cAMP was inhibited, resulting in decreased serotonin-N-acetyltransferase (NAT) activity and melatonin output. Although L-glutamate at 1 mM caused 90% inhibition of melatonin synthesis, about 30% of the NAT activity remained, suggesting the presence of another target for L-glutamate. In this study, we found that L-glutamate also inhibits hydroxyindole-O-methyltransferase (HIOMT). The inhibition is reversible and dose-dependent: the maximal inhibition was obtained with more than 0.4 mM L-glutamate. Contrary to L-glutamate-evoked inhibition of NAT, agonists for class II metabotropic receptors such as (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) had no effect on HIOMT. Neither (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG), an specific antagonist for class II mGluRs, nor dibutyryl cAMP restored the L-glutamate-evoked inhibition of HIOMT. Northern blot analyses revealed that L-glutamate significantly inhibits the expression of mRNA of NAT, but not that of HIOMT. These results indicated that HIOMT is an another target for L-glutamate due to its inhibition of melatonin synthesis, and the signaling pathway toward the inhibition is distinct from that of NAT.