Brain opioid receptor density relates to stereotypies in chronically stressed pigs

Opioid receptor densities were measured in the hippocampus of chronically stressed (tethered) pigs to study the involvement of endogenous opioid systems in stereotypy performance. Three groups of animals were housed tethered for 2 (n = 12), 5.5 (n = 12) and 8-9 months (n = 8), respectively, and the intensity of stereotypy performance was determined. Opioid receptor densities were measured post mortem, using membrane binding assays with [ 3H]naloxone as a ligand. A negative correlation was found between the density of opioid receptors and the intensity of stereotypy performance in the animals that had been housed tethered for 2 months. This correlation seemed to disappear with increasing duration of tethered housing. The data further suggest that, associated with the duration of tethered housing, there was a gradual decrease in the density of opioid receptors in the left hippocampal lobe of the low-stereotyping animals, but not in the right lobe, nor in the left and right lobes of the high-stereotypers. This suggests that chronic stress leads to a (asymmetrically expressed) progressive loss of opioid receptors in the hippocampus, and that stereotypies exert a mitigating effect on stress-induced changes in opioid receptor densities, supporting the hypothesis that stereotypies help the animals cope with the adverse effects of chronic stress.