Pineal nitric oxide synthase, but not heme oxygenase, mRNA is suppressed by continuous exposure to light.

We have previously shown that exposure of rats to constant light (LL) induced a decrease in NO synthase (NOS) activity in the pineal gland. We report here that the use of the sensitive technique of RT-PCR has demonstrated that mRNA for neuronal NOS is present in the pineal, and that it is photoneurally regulated. There was a marked decrease in pineal neuronal NOS mRNA levels in continuous light conditions, similar to the changes seen in NOS enzyme activity. Inducible NOS was not present in the pineal, and there was evidence that the photoregulatable form was not endothelial NOS. The mRNA for two isoforms of heme oxygenase, the enzyme responsible for the generation of the putative neuromodulator carbon monoxide, was also present in the pineal, but neither isoform was photoregulated. Using immunodetection, it was not possible to identify the presence of NOS protein, other than to a minimal extent, even though NOS activity was clearly present. NADPH-diaphorase staining and in situ hybridization were carried out in an attempt to identify the precise location of neuronal NOS message. A strong NADPH-diaphorase reaction was present in sympathetic nerve fibers of the pineal, but pinealocytes showed no or only very weak labelling. In situ hybridization was also unable to identify neuronal NOS message in pinealocytes. These data thus also suggest the possible presence of a pineal-specific NOS isoenzyme.