Could i just pick up on the physiological side of stress that was mentioned earlier?
There is a huge difference between the physiology of acute stress & that of chronic stress. Acute stress is when the catecholamine release is triggered ~ usually called the adrenaline rush, but it's not really because adrenaline, noradrenaline & dopamine are also released. It is these neurotransmitters that cause increase heart rate, increased respiration, increased supply of glucose the muscles, inhibition of less vital functions like digestion etc. This is the flight/fight response when the body is goes into survival mode. It happens in predatory behaviours as the predator is often at risk from injury or even death during predatory chase & very often the body is required to respond rapidly & strongly.
The acute stress response in these situations is quite normal & necessary & evolved especially to help animals survive. Acute stress is relatively short lived partly because once the animal is safe, or the hunt is over & mission accomplished, the body's homeostatic responses kick in to turn it back to normal, & partly because it is impossible for the body to keep this acute stress response going for long.
But chronic stress is a very different thing & is governed by very different physiology. In chronic stress it is the hypothalamus-pituitary-adrenal axis that is affected & it is this system that releases other hormones & neurotransmitters, ultimately ACTH (adrenocorticotrophic hormone for those that like big words!). ACTH is also known as the "coping" hormone & allows the body to try to adapt to its environment. If ACTH is present in large amounts it is a sign that the animal is not coping and indicates that it is under chronic stress. In chronic stress, activation of the HPA axis also produces cortisol. The immune system also produces cortisol when fighting infection. To keep the homeostatic balance, the immune system detects cortisol levels are too high and gives negative feedback to stop producing cortisol. If the animal gets an illness, the immune system is in negative feedback so no cortisol is produced to help fight the illness.
The interplay between the immune and endocrine systems also affects protein synthesis, enzyme activity and glucose. In chronic stress there can be a delayed anti-body production to fight infection and may cause allergic reactions, e.g. asthma.
ACTH and Cortisol can be measured through saliva, urine or blood but ACTH has a circadian rhythm, i.e. it peaks at midnight to 6.00 am. It is important, therefore, to take measurements at different times of the day and night.
So if you're still with me
you can see that there are very different responses to stress!!
Then on top of this, there is the way that the neural pathways in the brain are altered by exposure to acute & chronic stress, and there is the interplay of the physiology of the learning processes, both operant & classical conditioning.
So how does this relate to dog training you may well ask!!! We all know the the adages "practice makes perfect" and "use it or lose it" but these sayings do have a scientific foundation. And we all know that the rush of adrenalin can be addictive & internally rewarding. So if a dog finds that the adrenalin rush of e.g. chasing cats is addictive & internally rewarding, & if the ability to "practice" this behaviour is frequently available, then it can be no surprise to see that the dog is continually looking for opportunities to chase cats!
So how to stop it without causing frustration, unhappiness & more than likely chronic stress, because of the dog's inability to perform behaviours that it enjoys, be they hard-wired instinctive behaviours or learned behaviours?
First avoid the places, scenarios, circumstances etc under which the dog experiences the adrenalin rush. From the physiological perspective this withdraws the adrenalin rush, from the learning perspective, this stops the continual rehearsing or practising of chasing cats. From the brains point of view, this stops the frequent activation of the neural pathways associated with cat chasing.
But, as this is a behaviour the dog enjoys & is probably instinctive, innate & hard-wired (more so in some breeds thn others), then to stop the frustration, unhappiness & ultimately the chronic stress caused by the inability to act out normal behaviours, we have to replace it with something mmore acceptable, e.g. chasing a ball, or a frisbee, or only chasing when off lead in a particular location, or only chasing on the evening walk & not the morning walk, or only chasing seagulls & not anything else. In other words it becomes a management issue & a training issue. But IMO unless you understand all the other factors that you can be bringing into play, such as the physiological, neurological, learning theory & ethological aspects of this huge "jigsaw", then it can be very easy to get wrong. This in turn leads to frustrated owners saying it doesn't work, & frustrated dogs who aren't allowed acceptable outlets for normal behaviours.
Not easy problems to sort out & each individual dog may need this managed & redirected in very different ways ~ so no, there's no "one size fits all", but the basic underlying scientific principles are the same whatever method you use.