Increased hippocampal 5-lipoxygenase mRNA content in melatonin-deficient, pinealectomized rats

Melatonin is neuroprotective because of its antioxidative action, but it also can modify neuronal vulnerability by altering gene expression. 5-Lipoxygenase (5-LO) gene expression is suppressed by the binding of melatonin to its high-affinity nuclear receptors. Recently, we reported that in rats the melatonin deficiency elicited by pinealectomy increases hippocampal susceptibility to excitotoxic injury. Here we have hypothesized that pinealectomy may increase hippocampal vulnerability by eliminating the tonic inhibitory action of melatonin on 5-LO gene expression. Sham-pinealectomized controls and pinealectomized rats were killed 15 days after surgery. Their hippocampi were dissected, and total RNA was extracted and processed for quantitative reverse transcription-polymerase chain reaction assay of 5-LO and cyclophilin mRNAs. Mutated primers were used as internal standards to assay attomole quantities of these two specific mRNAs per microgram of total RNA; the ratio 5-LO/cycophilin was used to compare samples from control and pinealectomized rats. Pinealectomy increased hippocampal 5-LO mRNA content by about threefold. These results support our hypothesis that melatonin deficiency may abate the tonic inhibition of 5-LO mRNA expression and thereby up-regulate 5-LO gene expression, which in turn would increase the brain's synthesis rate of potentially harmful eicosanoids, leukotrienes.