Endogenous monoamine oxidase inhibitory activity and HPA activation in thepig

Previously we have shown that an increase in endogenous monoamine oxidase A inhibitory activity (MAO-AI), measured in human saliva, both precedes and predicts psychological stress-induced activation of the hypothalamic pituit ary adrenal (HPA) axis, as determined by the cortisol response. We now repo rt the relationship between endogenous MAO-AI and the cortisol response in the plasma of prepubertal pigs (n=5 or 6) under two experimental paradigms of HPA activation. In the first condition, pigs were physically restrained (snaring) for 15 minutes. Blood samples were taken from indwelling catheter s at intervals before and after snaring (a sampling period of about 1 hour) , and at the same time intervals on a separate day to provide baseline meas ures. Both cortisol concentration and percentage MAO-AI were determined in each plasma sample. There was a pronounced cortisol response on the snaring day (cortisol peaked 30 minutes after the start of the snaring). There was also a significant MAO-AI response to snaring which peaked 15 minutes afte r the start of the stress challenge. In the second experimental paradigm, b acterial endotoxin (LPS: 20 mu g/pig) was used to induce HPA activation and plasma cortisol and MAO-AI were determined. This time, however, the cortis ol response was not preceded by any change in MAO-AI. We conclude that gene ration of MAO-AI, which is associated with HPA activation induced by psycho logical stress, is not a component of the pathways involved in immunologica l stimulation of the HPA axis.