The Effects of Progesterone on Extracellular Serotonin Levels in Rat Hypothalamus

Progesterone effects on extracellular serotonin were monitored in the ventromedial hypothalamus (VMN), midbrain central grey (MCG) and medical preoptic area (POA) using microdialysis. Rats primed with estradiol benzoate (5 μg) were anesthetized with choral hydrate (400 mg/kg, i.p.). Samples were collected at 20 minute intervals. Once a stable serotonin baseline was obtained, rats were injected (s.c) with 0.5 mg progesterone or vehicle. In the POA, extracellular serotonin levels were not affected by progesterone. Significant decreases in extracellular serotonin levels were present in the MCG and VMN 20 and 60 minutes after progesterone, respectively. In the VMN, extracellular serotonin decreased to 57 ± 5.9 % of pretreatment values 100 minutes after progesterone. Serotonin levels in the MCG decreased 56 ± 5.1% of pretreatment values 80 minutes after progesterone. The decreases in extracellular serotonin in the VMN and MCG persisted during the remainder of the sampling period. Serotonin baseline levels were stable in the respective vehicle-treated groups. These results demonstrate that progesterone-influenced decreases in serotonin overflow in the MCG and VMN occur in vivo and support previous studies which suggest that decreases in serotonin activity in the VMN and MCG contribute to the facilitation of female receptivity