A study was undertaken of the effects of iontophoretically applied amino acids, acetylcholine and monoamines on suprachiasmatic nucleus (SCN) neurons of rat pups, at the 11th postnatal day, using a hypothalamic slice preparation. GABA and taurine inhibited 97 and 42% of SCN neurons, respectively, while glycine had no effect. Glutamate excited 68% of them. These effects were similar to those in adult rats. Serotonin and noradrenaline inhibited 6 and 10% of SCN neurons, respectively, in rat pups, whereas 32 and 26% of them were inhibited in adult rats. Acetylcholine excited 7% but inhibited 26% of SCN neurons in rat pups and these values were almost the same as those of adult rats. These results reveal that acetylcholine and amino acids regulate SCN neurons even in rat pups, whereas monoamines play an important regulatory role in SCN activity in adult rats only.
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