Sensitivity to stress-induced reproductive dysfunction linked to activity of the serotonin system

To use a nonhuman primate model and determine whether individuals sensitive to stress-induced reproductive dysfunction have lower activity of central serotonergic neurons under nonstressed conditions. The activity of the central serotonergic system was assessed by measuring responsiveness to a fenfluramine challenge (5 mg/kg, IV) in sedated monkeys previously categorized as highly stress resistant (HSR; n = 4; normal menstrual cyclicity through two stress cycles), medium stress resistant (MSR; n = 5; ovulatory in the first stress cycle but anovulatory in the second stress cycle), or low stress resistant (i.e., stress sensitive, SS; n = 4; anovulatory as soon as stress is initiated). To control for differences in pituitary stores of prolactin or ACTH, the animals were subsequently administered a bolus of thyrotropin-releasing hormone (3 μg/kg) plus corticotropin releasing factor (CRF), (3 μg/kg). Oregon National Primate Research Center, Animal Services Building. Female cynomolgus macaques exhibiting normal menstrual cycles. Administration of fenfluramine, a serotonin-releasing drug. Serum concentrations of prolactin (PRL) and cortisol (F). Prolactin release in response to fenfluramine was significantly greater in the HSR group compared with the MSR or SS groups. In contrast, cortisol was higher in the SS group compared with the other two groups. Similar responses were not evident after thyrotropin-releasing hormone + CRF stimulation. The lower PRL response to fenfluramine in the stress-sensitive animals suggests that stress-sensitive individuals have decreased activity in central serotonergic neurons. However, the F data suggest that the hypothalamic-pituitary-adrenal axis in stress-sensitive individuals is highly responsive to even small increases in serotonin.