Abstract:The daily rhythm in circulating melatonin is driven by a circadian rhythm in the expression of the arylalkylamine N-acetyltransferase gene in the rat pineal gland. Turning off expression of this gene at the end of night is believed to involve inhibitory transcription factors, among which Fos-related antigen 2 (Fra-2) appears as a good candidate. Circadian rhythms in the expression of three proteins of activating protein-1 (AP-1) complexes, namely, Fra-2, c-Jun, and Jun-D, are shown here to account for circadian variations in AP-1 binding activity. Quantitative variations in the Fra-2 component over the circadian cycle were associated with qualitative variations in protein isoforms. Destruction of the suprachiasmatic nucleus resulted in decreased nocturnal AP-1 activity, showing that AP-1 circadian rhythm is driven by this nucleus. Exposure to light during subjective night and administration of a serotonin 5-HT 1A /5-HT 7 receptor agonist during subjective day, respectively, induced a 50% decrease and a 50% increase in both AP-1 and Fra-2 expression. These effects were impaired by suprachiasmatic nucleus lesions. These data show that pineal AP-1 binding activity, which results from Fra-2 expression, can be modulated by light and serotonin through the suprachiasmatic nucleus according to a "phase dependence" that is characteristic of the rhythm of clock sensitivity to both zeitgebers.

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Glutamic acid and glycine were quantified in cells and medium of cultured rostral rhombencephalic neurons derived from fetal rats. In the presence of 1 mM Mg2+, NMDA (50 microM) significantly stimulated (by 69%) release of newly synthesized 5-[3H]hydroxytryptamine ([3H]5-HT). D-2-Amino-5-phosphonopentanoate (AP-5; 50 microM) blocked the stimulatory effect of NMDA. AP-5 by itself inhibited [3H]5-HT release (by 25%), suggesting a tonic control of 5-HT by glutamate. In the absence of Mg2+, basal [3H]5-HT release was 60% higher as compared with release with Mg2+. AP-5 blocked the increased [3H]5-HT release observed without Mg2+, suggesting that this effect was due to the stimulation of NMDA receptors by endogenous glutamate. Glycine (100 microM) inhibited [3H]5-HT release in the absence of Mg2+. Strychnine (50 microM) blocked the inhibitory effect of glycine, indicating an action through strychnine-sensitive inhibitory glycine receptors. The [3H]5-HT release stimulated by NMDA was unaffected by glycine. In contrast, when tested in the presence of strychnine, glycine increased NMDA-evoked [3H]5-HT release (by 22%), and this effect was prevented by a selective antagonist of the NMDA-associated glycine receptor, 7-chlorokynurenate (100 microM). 7-Chlorokynurenate by itself induced a drastic decrease in [3H]5-HT release, indicating that under basal conditions these sites were stimulated by endogenous glycine. These results indicate that NMDA stimulated [3H]5-HT release in both the presence or absence of Mg2+. Use of selective antagonists allowed differentiation of a strychnine-sensitive glycine response (inhibition of [3H]5-HT release) from a 7-chlorokynurenate-sensitive response (potentiation of NMDA-evoked [3H]5-HT release).

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