Circadian rhythm and response to light of extracellular glutamate and aspartate in rat suprachiasmatic nucleus

Extracellular concentrations of glutamate and aspartate were measured in the vicinity of rat suprachiasmatic nucleus (SCN) by means of in vivo microdialysis. The concentrations of both excitatory amino acids (EAAs) were higher during the dark phase than during the light under the light-dark cycle, showing pulsatile fluctuations throughout the day. When rats were released into the complete darkness, the 24-h pattern in the aspartate continued for at least one cycle, whereas that in the glutamate disappeared. The nocturnal increases in the EAA levels were not due to the increase of locomotor activity during the nighttime, because the 24-h rhythms were also detected in animals under urethan anesthesia. The patterns of extracellular EAA levels were changed when rats were released into the continuous light. Circadian rhythm was not detected in the glutamate, whereas the 24-h pattern was maintained in the aspartate with the levels increased to various extents. A 30-min light pulse given either at zeitgber time (ZT) 1 or ZT 13 elevated the EAA levels during the latter half of the light pulse, except glutamate by a pulse at ZT 1. The extracellular EAA levels in the vicinity of the rat SCN showed the circadian rhythm with a nocturnal peak and increased in response to the continuous light and a brief light pulse. The aspartate level is considered to be regulated by the endogenous circadian rhythm, but the glutamate level seems to be modified by the light-dark cycle.