Plasma neurophysin levels in monkeys: emphasis on the hypothalamic response to estrogen and ovarian events

Specific radioimmunoassays for human neurophysins released in response to estrogen (estrogen-stimulated neurophysin, ESN) and nicotine (nicotine-stimulated neurophysin, NSN) have been used to measure two similar neurophysins in rhesus monkey plasma. As in the human, concentrations of rhesus monkey neurophysins in plasma were specifically produced a marked increase of plasma NSN concentrations in the monkey. Estradiol benzoate administered intramuscularly consistently produced an increase in plasma ESN concentrations in normal cycling and castrate monkeys. ESN response to estrogen was exclusively positive and occurred approximately 10 hours after an injection of estradiol benzoate intramuscularly. Plasma samples obtained throughout the mid-cycle were measured and a characteristic rise in estrogen and LH, and a more prolonged rise in ESN were found. Our data indicate that the ESN and LH responses to estrogen stimulation are temporally related events and that the assay of ESN in plasma may be of unique value as it directly reflects the hypothalamic response to changes in estrogen secretion.