DIFFERENTIAL TIMING OF AMINO-ACID AND 5-HIAA RHYTHMS IN SUPRACHIASMATIC HYPOTHALAMUS

In vivo brain microdialysis was used to characterize the daily pattern of extracellular excitatory amino acids (EAA; glutamate and aspartate ), glutamine and 5-hydroxyindoleacetic acid (5-HIAA) in the region of the suprachiasmatic nucleus (SCN) in freely behaving male Syrian hamst ers. Diurnal variations in the extracellular concentrations of EAA and 5-HIAA observed under 14:10-h light-dark (LD) photoperiod were confir med by cosinor analysis. Peak levels occurred during the night, with t he mean acrophasis for 5-HI-AA concentration preceding that for EAA by 1-2 h. Release of EAA was stimulated by K+-induced depolarization in a Ca2+-dependent manner and was not affected by tetrodotoxin. In the a bsence of light cues (constant dim red light, <0.4 lx; DD) there was a significant time-of-day effect in peak glutamate concentration (occur ring during subjective night), but the rhythmic pattern of 5-HIAA outp ut was lost. There was no apparent direct temporal linkage between pea ks in extracellular EAA or 5-HIAA and bouts of wheel-running activity under LD or DD. These results are evidence that 1) the daily release p attern of extracellular glutamate, but not 5-HIAA, is circadian in nat ure; and 2) this rhythm in glutamate is not based on Na+ channel-depen dent action potentials. Finally, a discordance in the timing of peak n octurnal extracellular 5-HIAA and EAA concentrations was evident, poss ibly reflecting an interplay between serotonergic and EAA pathways in the SCN.