SEROTONERGIC INHIBITION OF LIGHT-INDUCED FOS PROTEIN EXPRESSION AND EXTRACELLULAR GLUTAMATE IN THE SUPRACHIASMATIC NUCLEI

The present experiments were undertaken to explore a role for serotoni n (5-HT) in modulating photic signal transduction and extracellular gl utamate (Glu) concentration in the suprachiasmatic nuclei (SCN) of the Syrian hamster. Pretreatment with an i.p. injection of the serotonerg ic, quipazine, caused a marked decrease in the number of SCN cells exp ressing Fos protein-like immunoreactivity (Fos-LI) induced by a light pulse delivered during the latter part of the dark phase (7 h after li ghts-off; 55.6 +/- 7.5% of vehicle controls, P < 0.004). This effect o f quipazine was dose-dependent and was limited principally to the vent rolateral region of the SCN. In a likewise manner, intra-SCN microinje ction of quipazine inhibited light-induced Fos-LI in the ventrolateral SCN, indicating that the suppressive action of quipazine is centered in the SCN. In a separate experiment, localized perfusion of the SCN r egion with 5-HT using the microdialysis technique caused a significant reduction in the extracellular concentration of Glu. The effect was g reater during the dark phase, compared to the light phase of the day-n ight cycle (60.7 +/- 6.8% vs. 39.3 +/- 6.8% maximal suppression, respe ctively; P < 0.05). Similar localized application of quipazine also de creased extracellular Glu (48.0 +/- 6.1% maximal suppression; P < 0.05 ). Collectively, these results are evidence for a serotonergic modulat ion of retinohypothalamic input in the SCN, which could involve a pres ynaptic inhibition of Glu release.