Stress, endogenous opioids and stereotypies in tethered pigs

Tether housing of female pigs in narrow, individual boxes represents a chronic stressor for the animals. Pigs that are housed tethered often develop behavioural disturbances, such as stereotypies, and changes in physiological regulation. The results of the studies described in the present thesis confirm and extend previous suggestions that there is an association between stereotypies and brain opioid activity. We found a negative correlation between the intensity of stereotypy performance and opioid receptor densities in the hippocampus and the hypothalamus of pigs which had been housed tethered for two months. This correlation seemed to disappear with increasing duration of tether housing, likely as a consequence of the gradual decrease in receptor density that occurred in pigs with low levels of stereotypies. This receptor decrease might reflect glucocorticoid-induced neuronal cell loss, since we also found a negative correlation between he salivary cortisol concentration and the number of neurons in the hippocampus in long-term tether housed pigs. These results accord with the idea that stereotypies represent a strategy to reduce adverse effects of chronic stress.The chronic stress of tether housing not only leads to the development of stereotypies, it can also induce changes in physiological responsivity to further stressful stimulation. The mechanisms underlying these changes likely include alterations in endogenous opioid systems. This is indicated by our finding that antagonism of endogenous opioid activity increased the heart rate response of pigs to a stressful challenge after long-term tether housing but not after loose housing. These results provide evidence indicating that long-term tether housing leads to an increased impact of endogenous opioid systems that attenuate physiological responses to additional acute stress.Taken together, the present thesis highlights changes in endogenous opioid activity that are induced by chronic stress and appear to prevent or reduce potentially harmful stress effects.