Does My Pet’s Skin Have Its Own HPA Axis?

September 12, 2022

Would you be surprised to know that the skin has developed its own local HPA axis to regulate its exposure to stressors?  Recent mapping has found that all regulatory elements found in the central HPA axis are expressed in skin!  Could this explain why skin issues tend to worsen when us humans are stressed?  More importantly, when skin issues are the top issue facing the majority of dog owners we see here at My Pet Nutritionist, is this something we need to consider?

The short answer is yes.  So, let us take a deeper dive at what the HPA axis is and how this can influence our pet’s skin issues.  

The HPA Axis

Most of us have heard at one time or another the mention of the HPA axis when we talk about the stress response.  It stands for hypothalamic-pituitary-adrenal axis. 

In short, the hypothalamus in the brain receives a message that we are being exposed to a stressor, it then sends a message to the pituitary gland which then sends another message to the adrenal glands.  

It is the adrenal glands that release the hormones we typically associate with stress, like cortisol and adrenaline/noradrenaline.

These stress hormones flood the body, and help it deal with whatever the stressor may be – and then when it has passed, the hormones stop being produced; the body then returns to homeostasis.

This exact response also occurs in our pets when faced with a stressful situation, and so our pets too have a HPA axis.  

Now we know what the HPA axis does, let’s see how the skin has developed its own local axis.  

The Skin’s HPA Axis 

The skin is the largest organ of your dog’s body.  It consists of three major layers:

  • The Epidermis – (Epi – upon or above) this is the outer layer of skin, the protective layer. 
  • The Dermis – the dermis supports and nourishes the outer layer.  It provides strength and elasticity.  Here you will find collagen fibres, sweat glands, sebaceous glands, and hair follicles.  You will also find cells that release histamine and other inflammatory mediators when faced with an allergy or injury. 
  • The Subcutis – (sub meaning under or below) this in the innermost layer of the skin, where you will find fat and muscles.  Subcutaneous fat provides insulation, padding and storage for reserve energy.

Not only does the structure of the skin prevent water and electrolyte loss to help maintain body homeostasis, but it forms a protective barrier which helps protect against infections, parasites, and the environment.

This is the often-forgotten role of the skin – that it forms part of the immune system.  When we think of the skin as the first layer of defence in the immune system, and therefore a type of radar, it makes absolute sense that it should have its own response to stress. 

It seems that skin cells, known as keratinocytes, can produce the same hormones and neurotransmitters that the “internal” stress response does.  

These include:

  • corticotropin-releasing-hormone (CRH), 
  • cortisol, 
  • adrenaline, 
  • noradrenaline, 
  • dopamine, 
  • histamine and 
  • acetylcholine.

All of these compounds help the body deal with a stressor, they include increasing heart rate, upregulating energy conversion and redirecting resources to those functions that are necessary. 

So, the skin, we can think of like a radar.  It constantly senses the environment and reacts to various stressors like humidity, temperature, changes in the skin microbiome and of course injuries.  

CRH is possibly the most studied hormone and in the skin, it is involved in the proinflammatory response.  When exposed to a stressor, CRH brings an inflammatory response to help manage the threat.  If your dog scratched himself, the inflammatory response would be a normal part of healing.  However, a natural by-product of this process is that it increases skin permeability which isn’t great long term.  The skin serves as a protective barrier and if it becomes permeable, harmful compounds can sneak in, which can cause further problems around the body.    

Acute Vs. Chronic Stress

Like everything, moderation is key – acute stress is manageable, the issue is when it becomes chronic. 

Where this becomes even more problematic is that animal studies have suggested that skin stress can initiate a central HPA axis response.

When researchers exposed rodents to UVB radiation, they found increased stress responses in the adrenal glands. 

So, not only can the skin respond directly to a stressor, but it can bring the central response to the party too!  

What this means is that skin stress can cause body wide stress, and again this becomes an issue in chronic skin issues – leading to chronic activation of the “internal” stress response.      

To add insult to injury, psychological stress increases the production of “internal” stress hormones which then affects skin cell growth and function, compromising structure and altering skin permeability.  

So not only can skin stress cause psychological stress, but psychological stress can cause skin stress.  You can see how this can become a challenging cycle to break.   

But what we must remember is that most functions in the body serve a necessary function and this mechanism (although challenging) is no different.  

HPA Axis and Skin Barrier

The HPA axis in the skin responds to various stressors to regulate homeostasis – it serves to counteract any threat so normal function can resume.  The compounds produced and released all serve important roles in maintaining a healthy skin barrier. 

ACTH (adrenocorticotropic hormone) is involved in melanin production which protects the skin from ultraviolet radiation.  In dogs, it is melanin that contributes to skin and hair colour.  In autoimmune conditions, melanocytes can be destroyed which results in depigmentation.  This is also why if you have a dog who has repeated skin trauma (scratching etc) they develop visibly darker skin in those areas.  Keratinocytes with more melanin display superior barrier function – which is why the body attempts to achieve it in repeated trauma. 

Endorphins are also released during skin stress.  This enhances epidermal turnover rate, again to protect skin function.  This could be why it feels so good to scratch an itch! 

Cortisol, the stress hormone we are all familiar with, doesn’t in fact deserve the bad rap it gets.  Cortisol doesn’t cause stress, it’s the guy who comes along to try to sort out the problem.  In acute stress, cortisol acts as an anti-inflammatory, but cortisol also affects the growth and development of skin cells, which long term, affects the skin structure and ultimately, its barrier formation.  This is why glucocorticoids are used in skin issues – they do function as anti-inflammatory, but long term, can compromise skin function so should be used with caution. 

So, when these compounds are released in moderation and in response to acute stress, they work well and manage the threat.  

The issue, as always, is when the stress becomes chronic. 

What Does This All Mean?

If you are tackling skin issues in your dog, it is important to consider the brain-skin cross talk.  Psychological stress can exacerbate skin issues, and when a skin stress is induced, this can be signalled to the brain, leading to a vulnerability and additional stress-perception.

Causes of Skin Stress:

-    Infection,

-    Heat/cold,

-    UV light,

-    Toxic products,

-    Mechanical damage (scratching),

-    Free radicals,

-    Allergens,

-    Microbial disruption.

Causes of Psychological Stress:

-    Separation,

-    Change in routine,

-    Anxiety/fear,

-    Multi-pet home,

-    Owner stress,

-    Lack of routine,

-    Poor sleep,

-    Inappropriate nutrition,

-    Lack of enrichment.

Top Tips:

-    Address psychological stress triggers where possible,

-    Ensure adequate rest,

-    Offer opportunities to sniff and chew as these activate the parasympathetic nervous system, the calming side of the stress response,

-    Opt for non-toxic grooming products and consider the use of probiotic shampoo if absolutely needed,

-    Limit mechanical damage from scratching,

-    Limit exposure to toxic home cleaning products which are known to disrupt the microbiome found on the skin.

It goes without saying that your dog’s diet can also support skin health.  

Nutrients For Skin Health 

Things To Think About: Skin Health in Dogs 

Skin issues in dogs are one of the most common concerns we deal with here at My Pet Nutritionist.   The process to establish the cause can be long and frustrating, but it’s clear, stress does play a part.  There is a bidirectional communication between the brain and the skin – skin stress can exacerbate psychological stress, and psychological stress can exacerbate skin stress, so in addressing any chronic skin issues, it's a consideration to make.

If you would like any support with your pet, then check out our services to see how we can help. 

Thanks for reading, 

MPN Team 

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