Releasing hormones e.g. cortisol releasing hormone (CRH) produced in the hypothalamus under nervous and hormonal control, travel in the portal blood to the pituitary where hormones e.g. ACTH are produced and released into the general blood system where they travel to the adrenal gland. Under the influence of ACTH, the adrenal cortex produces and releases corticosteroids into the blood. The corticosteroids then affect, amongst other things, inflammation and metabolism. They also have a feed back (inhibitory) action in the hypothalamus, helping control its own production.
Activity of this system is most easily monitored using measurements of corticosteroid hormone values in plasma, serum, saliva, extracellular fluid (Cook 2002) or faeces (Mostle et al 2002; Schatz & Palme 2001). The corticosteroid of choice varies with species e.g. corticosterone is used in rats and hens, cortisol in dogs, cats, horses, pigs, sheep and cattle. Base line values of plasma cortisol can vary within and between species e.g. for horses values range from 80 - 180 nmol/l (Pritchett et al 2003) but are less than 20 nmol/l in sheep (Kent et al 1993) and cattle (Robertson et al 1994). Base line values also tend to be higher in animals less than a week of age. A series of measurements, taken at appropriate intervals, should be made before and after the painful treatment. Changes in plasma cortisol values closely follow behavioural changes after castration and tail docking procedures (Kent et al 1993).
However, plasma cortisol values appear to show evidence of a 'ceiling effect' suggesting that assessment of the relative severity of intense pains cannot be achieved using this parameter. However, the parameter can be useful in assessing pain of more moderate severity. Measurements of corticosteroids are mainly of use after acute episodes of pain. Changes in plasma cortisol values during chronic pain may not be consistent (Ley et al 1991,1994). Other factors that may limit its use for assessment of pain include - individual variation, circadian changes, age and breed effects and the wide variety of alternative stressors both pleasurable and stressful (transportation), which activate the HPA system.