Interesting studies. I have a few questions:
Melatonin is a lipid soluble molecule. It should diffuse down the concentration gradient. Your studies indicate that melatonin is localized in the cytoplasm. If simple diffusion of melatonin is taking place, the nucleus should also have a concentration similar to the cytoplasm or, at the most, only slightly lower than that of the cytoplasm. In your study, the cytoplasm but not the nucleus is densely stained. Is it possible to conclude that melatonin in the cytoplasm is packaged in some way so that simple diffusion to the nucleus does not take place and nucleus cannot be stained by the antibody? In a few earlier reports, pulsatility of circulating of melatonin has been demonstrated suggesting that melatonin may be released in pulses just like packaged protein hormone in the pituitary.