Using a high-performance liquid chromatography (HPLC) system coupled with an electrochemical detector, the concentrations of dopamine (DA) and 5-hydroxytryptamine (5-HT) and their major specific metabolites 3,4-dihydroxy-phenylacetic acid (DOPAC) and 5-hydroxyindole-3-acetic acid (5-HIAA), respectively, were measured in the median eminence (ME) throughout the rat estrous cycle. The ME DA content remained fairly constant throughout the estrous cycle except on estrus when Π.OOh values were significantly lower than 10.00 h values (40% decrease, p < 0.05). The ME 5-HT content determined at 10.00 h was higher on proestrus than on any other day of the cycle. The ME DOPAC concentrations did not differ between 10.00 and 17.00 h on diestrus I, diestrus II or estrus. On the contrary, there was an almost linear decline between 10.00 and 17.00 h on proestrus (36% decrease, p < 0.05). The ME 5-HIAA content did not differ between 10.00 and 17.00 h on any day of the estrous cycle. Significant changes were recorded for the DOPAC/DA and 5-HIAA/ 5-HT ratio in the ME on proestrus. There was a progressive decrease, starting from 10.00 in the DOPAC/DA ratio with minimal values (42% decrease, p < 0.05) at 16.00 h followed by an increase from 16.00 to 19.00 h. On the other hand, the 5-HIAA/5-HT ratio increased between 10.00 and 17.00 h (97% increase, p < 0.05) and subsequently declined until 19.00 h (67% decrease vs. 17.00 h, p < 0.05). These data show that a concomitant decrease in dopaminergic activity and increase in serotoninergic activity in the ME occur in association with the preovulatory prolactin and luteinizing hormone preovulatory surges.

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