Serotonin metabolism in various brain areas and dopamine metabolism in striatum were determined in rats injected intraperitoneally with metergoline or methiotepin. Metergoline (1 mg/kg) increased serotonin metabolism in various brain areas. The effect was particularly evident 4 hours after administration, whereas after 1 hour only a slight increase of 5-hydroxy-indoleacetic acid was observed. 5 mg/kg of methiotepin was found to increase serotonin metabolism in various brain areas 2 hours after injection. The data are compatible with the hypothesis that metergoline, like methiotepin, acts on serotonin receptors involved in the feedback control of serotonin synthesis and metabolism. Metergoline raised striatal concentrations of homovanillic acid 1 h but not 4 h after injection, indicating that at the latter time it has a more selective effect on central serotoninergic mechanisms in rats.
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