Diurnal variation in 5-hydroxyindole-acetic acid output in the suprachiasmatic region of the Siberian hamster assessed by in vivo microdialysis: evidence for nocturnal activation of serotonin release.

In vivo brain microdialysis was used to characterize the daily pattern of 5-hydroxyindole-acetic acid (5-HIAA) release in the region of the suprachiasmatic nuclei (SCN) in freely behaving male Siberian hamsters housed under 16L:8D. A marked diurnal variation in the concentration of extracellular 5-HIAA was apparent, with peak levels (147 +/- 5% of the daily mean; p < 0.05) occurring 2-3 h after lights-off. Smaller nocturnal rises in extracellular 5-HIAA were observed in the posterior hypothalamus and preoptic area (128 +/- 4 and 123 +/- 8% of the daily mean, respectively; both p < 0.05 vs. average daytime levels). Tryptophan loading increased 5-HIAA in SCN microdialysates by 44 +/- 6%, and this response was enhanced by localized perfusion with tetrodotoxin (TTX; 5 microM). Localized applications of KCl (150 mM) or veratridine (100 microM) decreased 5-HIAA by 62 +/- 5 or 49 +/- 11%, respectively. The effect of KCl was not significantly affected by specific calcium channel blockers. Perfusion with TTX markedly decreased SCN 5-HIAA during the dark phase, but had little effect during the light phase (42 +/- 8 vs. 12 +/- 5% suppression, respectively; p < 0.01). Addition of serotonin (3 microM) to the perfusate significantly stimulated 5-HIAA output. This treatment increased the release of 5-HIAA more during the dark than during the light phase (61 +/- 8 vs. 25 +/- 5%, respectively; p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)