In the horse, the hormone dopamine exerts a number of effects through its actions on the pituitary gland. Both dopamine and ergovaline, a dopaminergic agonist (a drug that stimulates dopamine receptors), can significantly impact the mare’s reproductive cycle, said Valerie J. Linse, MS, DVM, of Hagyard Equine Medical Institute, during her presentation at the Hagyard Bluegrass Equine Reproduction Symposium 2006, which was held Oct. 18-21 in Lexington, Ky. The effects of dopamine deficiency can be observed in horses with pituitary pars intermedia dysfunction (PPID or Cushing’s disease), while the effects of a relative dopamine excess are evident in cases of fescue toxicosis.

Pituitary pars intermedia dysfunction is the most common equine endocrinopathy (disorder of the endocrine gland). While the disease isn’t fully understood, research suggests that it occurs as a result of a dopamine deficiency at the level of the pituitary. The resulting decrease in dopamine inhibition leads to a loss of circadian rhythm and chronically elevated cortisol levels. 

Pituitary pars intermedia dysfunction is most frequently diagnosed in horses older than 15 years of age. Typical signs of the condition include muscle wasting, hirsutism (characterized by a long, often curly hair coat that does not shed out well), and obesity. Horses with PPID are predisposed to chronic laminitis, parasitic and bacterial infections, and infertility.

While the dexamethasone suppression test has long been the gold standard for diagnosing the condition, its safety and repeatability have been questioned. According to Linse, “The reliability of the dexamethasone suppression test may be overestimated. In addition, it has the potential to exacerbate laminitis