Expression of muscarinic cholinergic receptors during T cell maturation in the thymus.

Thymocytes at various stages of their ontogeny have been studied in relation to their ability to bind [3H]quinuclidinyl benzilate [( 3H]QNB), a specific radioligand of the muscarinic cholinergic receptors. [3H]QNB-specific binding to thymocytes from 15-19-day fetal, newborn and adult thymuses of mice and rats was compared and correlated. Our experiments showed that the kinetics of [3H]QNB binding to thymocytes at 37 degrees C was similar to that of the lymph node lymphocytes (LNL) with maximum after 5 min of incubation and subsequent decrease to 10% of the maximum after 90 min of incubation. Maximal binding for the entire thymocyte population was twice lower than for the cortisone-resistant thymocytes (CRT) or for LNL. Binding of [3H]QNB carried out at 4 degrees C resulted in disappearance of the maximum, but did not alter the difference between CRT and entire thymocyte population. Depletion of CRT detectable [3H]QNB-specific binding to thymocytes until 18th day of gestation but the maximal binding increased up to 20% at the day 19 and reached 90% of adult level on the third day after birth. Moreover, carbamylcholine (a muscarinic agonist) treatment in vivo induced a significant decrease in [3H]QNB binding to the thymocytes. We thus suggest that a subpopulation of thymocytes bearing muscarinic receptors in the periphery acquired these receptors in the thymus as one of the last steps of their maturation. We cannot exclude the possibility that cholinergic stimulation might trigger these lymphocytes to leave the thymus.