The inhibitory effect of melatonin on evoked dopamine release from whole hypothalamus as well as from its three regions (preoptic area, median and posterior hypothalamus) was studied in male rats and in female rats during various stages of the estrous cycle. Inhibition of dopamine release by 1 μM melatonin was significantly higher at the estrous and lower at the diestrous than at the metestrous and proestrous stages. The variations in inhibition of dopamine release during the estrous cycle occurred predominantly in the preoptic area, and were nonsignificant in the other hypothalamic regions. In male rats, a constant level of inhibition was observed. In addition, the concentration of melatonin needed to produce half maximal effect in the female shifted from approximately 0.6 nM at the estrous and metestrous stages to more than 1 μM at the proestrous and diestrous stages. The corresponding concentration in male rats was of the order of 1 μM, and did not change.
- Estrous cycle
- Rat brain