High Membrane Permeability for Melatonin
The pineal gland is the endocrine organ in the brain that synthesizes and secretes the circulating night hormone melatonin throughout the night. How is this hormone secreted? Is it by calcium dependent vesicular exocytosis or by simple diffusion across the pinealocyte plasma membrane? Experiments were designed to compare the cell membrane permeability to three indoleamines: melatonin and its precursors N acetyl serotonin (NAS) and serotonin. The three experimental approaches were (i) to measure the concentration of indoleamine amperometrically in the bath while cells were being loaded by a whole cell pipette, (ii) to measure the rise of intracellular indoleamine fluorescence as the compound was perfused in the bath, and (iii) to measure the rate of quenching of intracellular fura 2 dye fluorescence as indoleamines were perfused extracellularly in the bath. These measures showed that permeabilities of melatonin and NAS are high (both are uncharged) whereas that for serotonin (mostly charged) is much lower: melatonin NAS serotonin. Comparisons are made with predictions of solubility diffusion theory and compounds of known permeability, and a diffusion model is made to simulate all the measurements. The bottom line is that melatonin equilibrates across the plasma membrane in 3.5 s and has a membrane permeability of ~1.7 mm/s. Thus it and NAS will be "secreted" from pineal cells by membrane diffusion within seconds of synthesis. Circumstances are suggested when serotonin and possibly catecholamines (mostly charged) may also appear in the extracellular space passively by membrane diffusion from processes with high surface to volume ratio.Supported by National Institutes of Health grants R01GM 83913, R37NS 008174 and R01DK 080840.