Glutamic acid decarboxylase activity of the preoptic area and hypothalamus is influenced by the serotonergic system.

The effect of the serotonergic system on glutamic acid decarboxylase (GAD) activity of the preoptic area and the hypothalamus was studied in female rats on the day of proestrus. A circadian rhythm of GAD activity was observed with higher values in rats killed at 1130 h than in rats killed at 1500 h. In rats bearing lesions of the median raphe nucleus (MRn), a nucleus that sends 5-hydroxytryptamine nerve terminals to the areas under study decreased GAD activity. On the contrary, electrochemical stimulation of the MRn enhanced GAD activity in intact rats killed at 1500 h, but not in those killed at 1130 h, an effect that was prevented by the injection of the 5-hydroxytryptamine antagonist, methysergide. Furthermore, the injection of 5-hydroxytryptamine into the third ventricle, either in intact rats in the afternoon or in MRn-lesioned rats in the morning, also increased GAD activity. The results of the present study suggest that activation of the serotonergic system increases GAD activity in the preoptic area and hypothalamus.