Central administration of the peptide ghrelin and its receptor agonists reduced morphine-induced pain relief in mice, and this anti-opioid effect occurred through a pathway independent of the known ghrelin receptor GHS-R1a.
Anti-opioid effect independent of the known ghrelin receptorGhrelin reduced morphine analgesia in mice, but blocking its established receptor GHS-R1a did not prevent this effect — pointing to an unknown signaling pathway.
What the researchers found
Intracerebroventricular injection of ghrelin at doses of 0.1 to 100 nmol/L inhibited the pain-relieving effect of systemic morphine (6 mg/kg) in the tail withdrawal test in mice. The GHS-R1a receptor agonists GHRP-6 and GHRP-2 similarly reduced morphine analgesia.
Critically, pretreatment with the selective GHS-R1a antagonist [d-Lys(3)]-GHRP-6 (100 nmol/L) did not block the anti-opioid effects, demonstrating that ghrelin's interference with morphine pain relief occurs through a receptor mechanism independent of GHS-R1a. This points to an unidentified receptor or signaling pathway mediating the ghrelin-opioid interaction in the brain.
Why it matters
Understanding how different peptide systems in the brain interact with opioid pain pathways could lead to better pain management strategies and help explain why opioids work differently in different people. The discovery that ghrelin reduces morphine effectiveness through an unknown receptor also opens a new area of pharmacological research that could eventually yield novel targets for pain treatment or opioid optimization.
How the study worked
Male mice received ghrelin, GHRP-6, or GHRP-2 via intracerebroventricular (brain) injection, followed by systemic (intraperitoneal) morphine at 6 mg/kg. Pain sensitivity was measured using the tail withdrawal test. To determine receptor involvement, some mice were pretreated with the GHS-R1a antagonist [d-Lys(3)]-GHRP-6 before receiving ghrelin and morphine.
What this study cannot tell us
This is a mouse study using direct brain injection, which is far from how either ghrelin or morphine is used clinically. The specific receptor or mechanism responsible for ghrelin's anti-opioid effect was not identified. Only one pain test (tail withdrawal) was used, so results may not generalize to other pain types. Dose-response relationships and potential differences between acute and chronic morphine exposure were not fully explored.
How to read the evidence
This is a preclinical mouse study using invasive brain injection techniques. While it provides clear mechanistic evidence of a ghrelin-opioid interaction, the methodology is far from clinical application. It sits in the basic research tier of the evidence hierarchy.
When this study was published
Published in 2013, this study provided early evidence for ghrelin-opioid interactions in the brain. Research on ghrelin's role in pain and addiction has continued to grow since, though the specific non-GHS-R1a mechanism identified here may still be under investigation.
The bigger picture
Ghrelin is best known as the 'hunger hormone,' but this study adds to growing evidence that it has wide-ranging effects in the brain beyond appetite — including modulating pain. The finding that ghrelin opposes opioid analgesia through a non-classical receptor pathway adds complexity to the ghrelin system and connects it to the opioid crisis-relevant question of what factors influence morphine effectiveness.
Questions still open
- What receptor or signaling pathway mediates ghrelin's anti-opioid effect if not GHS-R1a?
- Does circulating ghrelin (from the gut) also influence morphine analgesia, or is this effect specific to brain ghrelin?
- Could ghrelin levels explain some of the variability in how well opioids work for pain relief in different patients?
Common questions
Why would a hunger hormone affect how well painkillers work?
Does this mean ghrelin could make pain medications less effective in people?
Read the original research
Attenuation of systemic morphine-induced analgesia by central administration of ghrelin and related peptides in mice.
Peptides, 50, 42-9
Citation
Zeng, Ping; Chen, Jia-Xiang; Yang, Bei; Zhi, Xing; Guo, Fa-Xian; Sun, Meng-Li; Wang, Jing-Lei; Wei, Jie. (2013). Attenuation of systemic morphine-induced analgesia by central administration of ghrelin and related peptides in mice.. Peptides, 50, 42-9. https://doi.org/10.1016/j.peptides.2013.09.017