AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 3 504-R511, Copyright © 1993 by American Physiological Society

ARTICLES

Differential timing of amino acid and 5-HIAA rhythms in suprachiasmatic hypothalamus

J. D. Glass, U. E. Hauser, J. L. Blank, M. Selim and M. A. Rea
Department of Biological Sciences, Kent State University, Ohio 44242.

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 hamsters. Diurnal variations in the extracellular concentrations of EAA and 5-HIAA observed under 14:10-h light-dark (LD) photoperiod were confirmed by cosinor analysis. Peak levels occurred during the night, with the 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 Ca(2+)-dependent manner and was not affected by tetrodotoxin. In the absence of light cues (constant dim red light, < 0.4 lx; DD) there was a significant time-of-day effect in peak glutamate concentration (occurring during subjective night), but the rhythmic pattern of 5-HIAA output was lost. There was no apparent direct temporal linkage between peaks 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 pattern of extracellular glutamate, but not 5-HIAA, is circadian in nature; and 2) this rhythm in glutamate is not based on Na+ channel-dependent action potentials. Finally, a discordance in the timing of peak nocturnal extracellular 5-HIAA and EAA concentrations was evident, possibly reflecting an interplay between serotonergic and EAA pathways in the SCN.

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