D-ASPARTATE UPTAKE INTO CULTURED RAT PINEALOCYTES AND THE CONCOMITANTEFFECT ON L-ASPARTATE LEVELS AND MELATONIN SECRETION

Significant amounts of D-aspartate (Asp) are found in mammalian tissue s and D-Asp is presumed to play some significant, but as yet undefined physiological role. However, it is not known whether D-Asp is synthes ized in mammals. In this study, we addressed this issue in cultured ra t pinealocytes, parenchymal cells of the pineal gland, which contain s ignificant amounts of D-Asp. Biosynthesis of D-Asp was found to be min imal to non-existent in cultured rat pinealocytes. We then investigate d the mechanism of uptake of D-Asp into these cells and its consequent effect on cell function. D-Asp was efficiently taken up into cells, i n a time- and dose-dependent manner. Interestingly, the L-Asp levels i n the cells and media decreased concomitantly with the uptake of D-Asp . This decrease was not due to D-Asp cytotoxicity, since the cellular levels of othernted.D-Serine and D-alanine were not taken up efficient ly into the cells and the cellular levels of L-serine and L-alanine we re unchanged. Also, immunocytochemical staining with anti-D-Asp antibo dy showed that D-Asp, which had been taken up into the cells, was disp ersed throughout the cytoplasm. In response to norepinephrine stimulat ion, pinealocytes, which had been pretreated with D-Asp released D-Asp as well as L-Asp. In these cells, norepinephrine-induced secretion of melatonin, a pineal hormone, was suppressed. The mechanism of this su ppression is discussed here. (C) 1998 Academic Press.