Endogenous opioids may modulate the activity of the hypothalamus-pituitary-adrenocortical axis in domestic fowl.

The activity of the hypothalamus-pituitary-adrenocortical (HPA) axis is under complex neuronal, hormonal and peptidergic control. In order to determine the role of the endogenous opioids in the modulation of the HPA axis in hens we have examined the changes of the plasma levels of Met-enkephalin, alpha-neo-endorphin, catecholamines and corticosterone during rest, stress (30 min of overcrowding) and after naltrexone pretreatment. Short overcrowding induced an increase of all parameters, but the time of response and duration of elevation was different. Plasma Met-enkephalin showed a biphasic response: decrease followed by increase at the end of stress. Catecholamines and corticosterone peaked at 30 min of overcrowding and the highest level of alpha-neo-endorphin has been noticed at 20 min after the experiment was started. Naltrexone (2 mg/kg b.w., i.v) diminished the responses to stress of all parameters and did not change their basal levels, except for corticosterone. These results indicate that the endogenous opioids may take part in the mediation of HPA activity in hens during resting and stressful situations.