The rat pineal gland comprises an endocannabinoid system

In the mammalian pineal gland, the rhythm in melatonin biosynthesis depends on the norepinephrine (NE)-driven regulation of arylalkylamine N-acetyltransferase (AANAT), the penultimate enzyme of melatonin biosynthesis. A recent study showed that phytocannabinoids like tetrahydrocannabinol reduce AANAT activity and attenuate NE-induced melatonin biosynthesis in rat pineal glands, raising the possibility that an endocannabinoid system is present in the pineal gland. To test this hypothesis, we analyzed cannabinoid (CB) receptors and specific enzymes for endocannabinoid biosynthesis or catabolism in rat pineal glands and cultured pinealocytes. Immunohistochemical and immunoblot analyses revealed the presence of CB1 and CB2 receptor proteins, of N-acyl phosphatidyl ethanolamine hydrolyzing phospholipase D (NAPE-PLD), an enzyme catalyzing endocannabinoid biosynthesis and of fatty acid amide hydrolase (FAAH), an endocannabinoid catabolizing enzyme, in pinealocytes, and in pineal sympathetic nerve fibers identified by double immunofluorescence with an antibody against tyrosine hydroxylase. The immunosignals for the CB2 receptor, NAPE-PLD, and FAAH found in pinealocytes did not vary under a 12 hr light:12 hr dark cycle. The CB1 receptor immunoreaction in pinealocytes was significantly reduced at the end of the light phase [zeitgeber time (ZT) 12]. The immunosignal for NAPE-PLD found in pineal sympathetic nerve fibers was reduced in the middle of the dark phase (ZT 18). Stimulation of cultured pinealocytes with NE affected neither the subcellular distribution nor the intensity of the immunosignals for the investigated CB receptors and enzymes. In summary, the pineal gland comprises indispensable compounds of the endocannabinoid system indicating that endocannabinoids may be involved in the control of pineal physiology.