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The Endocrine System (Cushings/PPID)


Principal Body System: Endocrine
Definition: All glands that produce hormones.
Function: Regulates body activities through hormones transported by the cardiovascular system.


Living longer can bring about health issues that than we didn’t need to consider in the past, and although age isn't always a factor with Cushings/PPID, the endocrine system, and specifically the pituitary gland, can be adversely affected.

Our own experience with Cushings came out of the blue. Many years ago on his daily walk, our then young daughter’s elderly second pony, Dinky, suddenly presented seriously lame with his breathing worryingly laboured – all the signs of laminitis. This literally happened overnight - the previous day he'd been fine; the next, he wasn't.

Although we instantly alleviated his symptoms, there were other signs that had niggled us - we hadn't had him long and were still getting to know him, but we were approaching summer and his winter coat wasn't shifting. He was also noticeably drinking and urinating more. We suspected Cushings and got him tested. It came back positive.

Then in 2014, our daughter's third pony, Cookie, who is still with us albeit now retired, also started showing the classic signs of non-shedding curly coat which seemed to appear from nowhere, alongside noticeable lethargy and sadness. She was 14 at the time.

The endocrine system

The body's endocrine system is an interconnected group of glands working together to secrete hormones that are essential for the body's processes, with hormones being a regulatory substance/messenger which stimulates specific cells or tissues into action. Hormones travel around the body in the blood.

Cushing's Disease, aka PPID (Pituitary Pars Intermedia Dysfunction), is the name given to a specific dysfunction of the pituitary gland which causes hormonal disturbances. The pituitary gland is made up of two parts, the anterior and posterior lobes, with the Pars intermedia being the boundary between the two lobes; it's here that the functioning of the secretory cells is controlled. It relies on dopamine as its neurotransmitter - a chemical messenger that helps transmit brain signals - to regulate the hormone secretions.

As horses age, as with us humans, the decrease in dopamine occurs naturally. As the older horse becomes susceptible to the loss of dopamine, the Pars intermedia produces an excess of hormones, including the hormone ACTH (Adrenocorticotropic) which stimulates the production of cortisol, a stress hormone produced by the adrenal gland.

And so begins the metabolic effect

As if managing the other symptoms of PPID isn't a juggling act enough, cortisol increases sugars in the bloodstream and enhances the brain’s use of glucose.

When the horse is anxious, the system sends signals to the adrenal glands to release adrenaline and cortisol. While the fight-or-flight reaction in the body stays active, cortisol continues to release sugars into the bloodstream.

Here come the side effects of high cortisol levels - we're looking at weight gain, typically cresty necks and rear/belly fat pads, insulin resistance (IR) and abnormal glucose metabolism, all the precursors to metabolic laminitis. It’s a vicious circle - as cortisol levels increase, insulin levels must then increase to try and keep glucose within what the body thinks are ‘normal’ levels. In humans, we call chronic, unregulated levels of insulin ‘diabetes’.

ACTH levels are also renowned for increasing around autumn when the days become shorter and the body’s natural hormone levels change in response to the natural Circadium Rhythm, the 24-hr cycle in the physiological process of all living beings, determining sleeping/feeding patterns, brainwave activity, hormone production, cell regeneration and other biological activities.

Getting the PPID/IR/EMS controlled and managed is essential in order to minimise the associated laminitis risk. Controlling the cortisol levels restores the insulin response in the hooves. There is no cure for PPID, but the good news is that once it's been diagnosed, management is fairly straightforward and can help the horse to return to a comfortable lifestyle.

Top Tips

  • Lower cortisol and increase dopamine levels naturally. Stress and certain health conditions can raise cortisol, lower dopamine levels and cause weight gain. Keep levels balanced by maintaining a happy equilibrium; exercising and allowing quality rest, alongside feeding a natural, species-appropriate diet and adding a vitamin/mineral supplement that support healthy cortisol and dopamine levels.
  • Exercise is really important, not just to rid the body of fat cells, but it's also increases dopamine levels. Regular exercise also helps to burn the extra blood sugar made available through elevated cortisol levels.
  • In addition to the countless physical benefits, exercise can also have psychological benefits. Studies show that exercise can increase the amounts of both dopamine and serotonin neurotransmitters in the brain, as well as helping your horse feel more energised overall. In a May 2007 article in the Journal of Neuroscience, it was noted that exercise could cause the brain cells that use dopamine to work more efficiently, and an August 2007 article in the journal Neuroscience Letters noted that continuous exercise also reduced damage to brain cells that release dopamine.
  • Quality rest time helps lower cortisol levels because otherwise the body’s nervous system stays in a state of alertness that requires cortisol. Getting proper rest also increases serotonin and dopamine, which help control feed cravings.
  • Feed omega-3 fats as they help trigger the production of serotonin, as well as being a good food source of the trace mineral selenium. A low intake of this mineral has been linked with depression.
  • Brewers Yeast is extremely beneficial for the metabolic horse, as it provides the full compliment of the B-Complex vitamins which can help lower cortisol levels.

Steps you can take to lower cortisol and stimulate dopamine levels naturally

  • Follow a daily regime of 20-30 minutes exercise, even if it’s just a brisk walk. The body's reaction to exercise creates brain activity that regulates hormone and brain chemicals.
  • Add a vitamin/mineral supplement that supports good overall health and balances your forage/grazing. Avoid sugar, grains, bad fats (PUFAs) and processed feeds which can cause dopamine levels to drop.
  • In addition to supporting overall health, Vitamin C is helpful in supporting dopamine - rosehips are jam-packed full of Vitamin C.
  • A calm, happy horse with horse buddies creates positive brain activity, increasing the amount of feel-good substances such as dopamine and seratonin.

Update - January 2019

Our Cookie recently had a worrying seizure episode which I suspected was related to her Cushings. After doing the usual googling, I found the following related notes courtesy of The Chronicle of the Horse forum: www.chronofhorse.com/forum

I should add that the forum post dates back to 2006 so in theory it’s outdated; however, here in 2019 I still found the following info useful. Credit due to poster Melyni (PhD) PAS, Dipl. ACAN, answering a question from poster ‘equineelders’, of Travellers Rest Equine Elders Sanctuary, whose horse had presented with similar symptoms to Cookie:

“At AAEP in early Dec I attended a couple of talks on Cushings. The current thinking was that it isn't tumor per se of the anterior pituatary but a loss of the dopaminergic neurons that descend from the hypothalmus into the pars intermediar.

If I understand it correctly these neurons have an inhibitory effect on the cells of the par intermedia, and thus the loss of the neurons means a loss of the inhibition, thus the pars intermediar cells begin to put out more and more hormones esp ACTH plus others, which leads to a secondary hypocorticoidism.

The disease is thus more similar to Parkinson's than to a tumor.

Patients with Parkinsons suffer from neurolgical symptoms, such as muscle rigidity, spasms and loss of balance. Loss of control of fine motor neurons occurs as well.

So if you think about Parkinsons as the underlying cause (loss of dopaminergic neurons) rather than a cancerous growth, the symptoms make more sense. Not that this helps anyone dealing with a seizuring horse.

There is a theoretical way to modify the intake on amino-acids to enhance the dopaminergic pathways as opposed to the seratonergic, eg don't feed tryptophan; supplements that contain the amino-acids threonine and tyrosine are effective, but this is not scientific only anecdotal.

Seratonin is made from different precursors than dopamine. They are both made from amino-acid precursors but not the same ones, and there is some indication that increasing the seratonin in Parkinsons’ patients exacerbates the symptoms, as the pathway may be competitive.

Dopamine comes from Tyrosine and Threonine, while Seratonin comes from Tryptophan via the 5HT pathway. Arginine may also be involved in the dopamine pathway.

How do chromium and magnesium fit into this? We have a few on supplements per vet's recommendation, but I don't know specifically what they are doing. From what I have read in such literature as exists, the Mg helps with the peripheral circulation, as in it keeps the small arterioles open and maintains the circulation to the extremities (all data from human studies) thus it (may) minimise the loss of blood flow to the feet, and the subsequent laminitis. It also seems (in humans) to have an anti-inflammatory effect, as in, it reduces C reactive proteins from the adipose tissue.

Chromium is part of the enzyme system that picks up the glucose from the blood stream and enters it into the cell. Thus the extra chromium helps to clear the bloodstream of the glucose, and the extra magnesium helps to maintain peripheral circulation.

All this information came from human studies, and thus you cannot be too literal in saying that this is what happens in horses, as these things don't always transfer from species to species, but as there is next to no research in horses, that is all we have.

I do know from what work I have done, that giving the Cr and Mg to pre-Cushings and Cushings horses you get better laminar bloodflow and a reduction in the symptoms, such that the pergolide dosage can be reduced.

We have measured the reduction in neck thickness and in the abnormal fat pockets, and we can show an improved circulation to the foot. But these effects are treating the symptoms not the underlying cause.

Giving essential amino-acids to these horses reduces the loss of muscle tissue and the muscle wasting. Again this is treating the symptoms not the cause, but overall the horses look and feel better, and are more active, which in itself improves their metabolism.”

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