Until recently, equine researchers knew little about how dopamine regulates insulin secretion in horses. In two recent studies scientists examined this connection, with implications for managing conditions such as pituitary pars intermedia dysfunction (PPID, formerly called equine Cushing’s disease) and insulin dysregulation (ID).

“A significant number of horses and ponies with PPID produce very large amounts of insulin in response to dietary nonstructural carbohydrates (starches, sugars, and fructans), and this puts them at increased risk of laminitis,” said Simon Bailey, BVMS, PhD, FHEA, Dipl. ECVPT, FRCVS, professor of veterinary biosciences at The University of Melbourne’s Veterinary School, in Victoria, Australia.

Bailey and a research team funded by the Australian Research Council, the Waltham Petcare Science Institute, and Boehringer Ingelheim Vetmedica, including PhD candidate Nicolas Galinelli, have been working to identify the link between PPID and insulin production so they can find ways to better reduce the risk of laminitis in and manage these horses.

Dopamine’s Role in Equine Insulin Response

“Horses and ponies with PPID have reduced dopamine production in the brain (which causes the production of certain hormones from the pituitary gland to get out of control), and we have recently found that it is decreased in the bloodstream as well,” said Bailey.

To investigate the link between dopamine depletion and insulin metabolism in horses, the researchers administered alpha-methyl-para-tyrosine (AMPT), a modified amino acid that inhibits dopamine production, to six healthy Standardbred horses in a placebo-controlled crossover study. Bailey and his colleagues aimed to determine how short-term dopamine reduction affects insulin sensitivity and postprandial (post-meal) insulin responses.

In healthy horses, insulin—a hormone produced by the pancreas—regulates blood sugar by signaling cells to take up glucose and store it as glycogen. In insulin-resistant horses, reduced sensitivity to insulin limits glucose uptake, prompting the pancreas to produce more insulin.

The researchers found that reducing dopamine levels in the horses led to increased postprandial insulin responses. “By temporarily reducing dopamine levels in the brain as well as in the rest of the body, we found that dopamine is not linked to the sensitivity to insulin (in horses, unlike in some other species), but depleting dopamine increases the insulin response to a high-glucose meal,” said Bailey. “Therefore, normal dopamine levels must be helping to keep insulin in check.”

Implications for Treating PPID in Horses

Bailey and his team used the study findings to explore the effects of pergolide—a drug commonly used to treat PPID in horses—on dopamine in horses and ponies with and without PPID. “This drug stimulates the receptors for dopamine, telling the pituitary gland to reduce hormone production, but now we find that it may also affect the pancreas, reducing insulin production,” said Bailey.

However, Bailey added that the drug only partially reduced postprandial insulin response to a high-starch meal, and horses without PPID did not benefit from treatment. “Therefore, in PPID animals or those with equine metabolic syndrome, who are at risk of laminitis due to high insulin levels, they still need to be fed diets low in starch and sugars to avoid excessive insulin levels. Pergolide will not protect them from the effects of high-carbohydrate diets (including lush grass).”

Take-Home Message

Dopamine influences insulin secretion in other species, but its role in equine metabolism has been unclear. Many horses with PPID also experience ID, marked by heightened insulin responses and an increased risk of laminitis. Researchers are still exploring the link between dopamine and insulin production and how this information can be used to more effectively treat horses with metabolic problems. Pergolide had no effect on tissue insulin sensitivity; however, the study results suggest that postprandial hyperinsulinaemia might be limited by this dopamine receptor agonist in animals with PPID and ID. If your horse has metabolic issues, work with your veterinarian to develop an appropriate management and treatment plan.

“We are certainly doing more research in this area to try to better understand these conditions (PPID, EMS, and laminitis) and how they fit together,” said Bailey. “These are such common conditions with huge effects on the well-being of our horses and ponies, so we need to do everything we can in the effort to find new treatments and countermeasures.”

The first study, “Effect of short-term dopamine reduction on insulin sensitivity and post-prandial insulin and glucose responses in Standardbred horses,” was published in Domestic Animal Endocrinology in January 2025. The second study, “Effect of pergolide treatment on insulin dysregulation in horses and ponies with pituitary pars intermedia dysfunction,” was published in Equine Veterinary Journal in February 2025.