Abstract
The overflow and metabolism of serotonin (5-HT) from transplants of embryonic medullary and mesencephalic raphé neurones in the previously 5-HT-denervated hippocampus have been analyzed in vivo using microdialysis. Output of 5-HT and its metabolite 5-hydroxyindolacetic acid (5-HIAA) from such grafts was as great (medullary) or greater (mesencephalic) than that measured in the normal intact hippocampus. Systemic administration of the putative 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) led to a 50–80% reduction in 5-HT levels in hippocampal dialysates from both intact and raphé grafted hippocampus. Since 8-OH-DPAT normally reduces 5-HT release from the nerve terminal via activation at the somatodendritic 5-HT1A autoreceptor and subsequent inhibition of impulse flow, the results indicate that 5-HT output from both medullary and mesencephalic raphé grafts can be regulated by autoreceptors and is dependent on serotoninergic neuronal activity.
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