Could Targeting a “Hunger Hormone” Receptor Help Constipation in Parkinson’s?

Constipation may not sound like the most serious symptom of Parkinson’s disease, but for up to 90% of people living with the condition it is one of the most persistent, uncomfortable, and quality-of-life–reducing problems. Despite its impact, current treatments for Parkinson’s do little to manage it effectively. Now, scientists in Australia believe they may have uncovered an important clue to why constipation develops—and how it might one day be treated. Researchers at the University of Queensland, led by Dr Sebastian Furness, have been looking deep into the spinal cord, specifically an area called the spinal defecation centre, which helps coordinate bowel movements. Normally, this system relies heavily on dopamine, the same chemical messenger that is progressively lost in Parkinson’s. Dopamine binds to receptors on nerve cells called D2 receptors, which then send signals to the muscles that trigger defecation. But dopamine alone cannot get the job done. The team discovered that another receptor, one that responds to ghrelin—the so-called “hunger hormone”—plays a vital supporting role. Even when ghrelin itself isn’t around, its receptor (called GHSR) has a kind of background activity that seems to “prime” the system. Without that background signal, dopamine cannot properly activate the neurons in the spinal cord, making bowel movements difficult to control. In their experiments, the scientists compared nerve cells that had both the dopamine D2 receptor and the ghrelin receptor to cells engineered to have only the dopamine receptor. When dopamine was introduced, only the cells with both receptors became activated. This suggested that the ghrelin receptor isn’t physically interacting with dopamine, but rather setting the stage through its constant low-level activity. To test this idea further, they created a mutated version of the ghrelin receptor that looked normal but could not generate background activity. In those cells, dopamine stimulation failed again. This confirmed their suspicion: the “silent” activity of the ghrelin receptor is essential for dopamine’s action in bowel control. This finding is important because in Parkinson’s, the loss of dopamine may disrupt this delicate balance in the spinal defecation centre. Without the priming effect of the ghrelin receptor, dopamine cannot work efficiently, leading to the chronic constipation so many people experience. The researchers believe medicines that mimic ghrelin or enhance the activity of its receptor could restore this missing link. There is already some precedent for this idea. A previous clinical trial in people with chronic constipation—not specific to Parkinson’s—showed that a ghrelin receptor drug called relamorelin helped speed up bowel movements. The Queensland team now hopes to investigate whether a similar approach could work for Parkinson’s patients. Their work has already attracted attention and funding. The U.S. Department of Defense has awarded $3 million to support further studies aimed at translating these findings into potential therapies. If successful, such treatments could offer a completely new way to address constipation in Parkinson’s, tackling the problem at its root in the nervous system rather than just easing symptoms. Dr Furness summarised the impact clearly: “Chronic constipation is debilitating and one of the biggest factors for reduced quality of life in people with Parkinson’s because it is so poorly managed. Our research gives us a new explanation for why it happens—and a new target for treatment.” While much more research is needed before any new therapy reaches the clinic, this discovery sheds light on a problem that has long been overlooked and gives real hope of improvement for people living with Parkinson’s disease.