Diurnal and Daily Variations in Growth Hormone and Growth Hormone Stimulation Test in Male Cynomolgus Monkeys

Background/Aim: Growth hormone (GH) secretion patterns differ across species. Humans exhibit a nocturnal surge, while rodents exhibit ultradian pulses. In cynomolgus monkeys, diurnal and daily variations and responsiveness to exogenous GH-releasing hormone (GHRH) remain insufficiently defined in non-clinical studies. This study aimed to characterize GH secretion patterns and evaluate responsiveness to exogenous GHRH in adult male cynomolgus monkeys for pituitary toxicity studies. Materials and Methods: Serum from 10 animals was collected between 10:00 and 22:30 and again at 10:30 on the following day to evaluate diurnal variation. Serum from 10 additional animals was collected once daily between 9:00 and 10:00 across five days to evaluate daily variation. In the GH stimulation test, four animals received intravenous pralmorelin hydrochloride (as GHRH) and four received physiological saline between 11:00 and 11:30. Serum was collected before and at 0.5, 1, and 2 h after administration. GH concentration was measured with enzyme-linked immunosorbent assay. Results: Diurnal variation was observed, with concentrations increasing from 10:00 to 11:30, transiently dropping to their lowest at 12:30 and peaking at 22:30, similar to the pattern in humans. Daily variation was also observed inter- and intra-individually across five days. In the stimulation test, compared to the control group, the GHRH group showed higher GH concentrations at 0.5 h (p