Researchers studied how opioid signaling affects growth hormone levels in nursing pigs by immunizing some animals against growth hormone-releasing factor (GRF). Blocking opioid receptors with naloxone reduced growth hormone in all animals, but the effect was mostly dependent on GRF, confirming that opioids regulate growth hormone during lactation primarily through the GRF pathway. The study also found a secondary, GRF-independent route through which opioids influence growth hormone secretion.
Armstrong, J D; Esbenshade, K L; Coffey, M T; Heimer, E; Campbell, R; Mowles, T; Felix, A