Expression of functional mu-opioid receptors during T cell development

We have examined the chemotactic responsiveness of thymocytes to selective mu-, kappa-, and delta-opioid agonists. Our results show that developing T cells migrate in response to mu-, but not kappa- or delta-opioids. The mu-opioid response appears to be dependent on the classical mu-opioid receptor (MOR-1) since the chemotactic response is blocked by a selective mu-opioid antagonist, and is absent in thymocytes from MOR-1-deficient mice. Flow cytometric analysis of the mu-opioid responsive cells shows that these cells consist predominantly of highly immature CD4- CD8- T cells. These results represent the first demonstration of the functional expression of mu-opioid receptors by developing T cells.