Regional differences in receptor reserve for analogs of oxotremorine in vivo: implications for development of selective muscarinic agonists.

The muscarinic actions of several analogs of oxotremorine were compared in the mouse. Compounds such as AKS 19 and BM 5 having low intrinsic efficacy (partial agonists) differentiated between centrally mediated muscarinic effects as they, like oxotremorine, were potent in producing analgesia and hypothermia but did not produce tremor. Instead they antagonized oxotremorine-induced tremor. They resembled oxotremorine in their ability to stimulate salivary secretion. Excellent correlations were found between the various muscarinic effects measured in vivo and spasmogenic activity on the guinea pig ileum and between tremorolytic potency and affinity for ileal muscarinic receptors. The results suggest that there are regional differences in the receptor reserve for muscarinic agonists that may affect in vivo responses to partial agonists in a qualitative manner. Responses to more efficacious agonists such as oxotremorine are affected only quantitatively by such differences. Efficacious agonists appeared to have a large receptor reserve with respect to salivation, analgesia and hypothermia, whereas their receptor reserve for the tremor response was lower. The possibility is raised of achieving selective muscarinic actions, even in the absence of distinct subtypes of muscarinic receptors, by exploiting regional differences in receptor density, in the efficiency of receptor-effector coupling and in endogenous levels of acetylcholine.