Genetic influences on tolerance development with chronic oxotremorine infusion

Mice of four inbred strains (BALB, C57BL, DBA and C3H) were administered either saline or oxotremorine, a muscarinic agonist, at a dose of 0.5 mg/kg/hr by constant infusion through cannulas implanted in the right jugular veins. Chronic treatment resulted in the development of tolerance to the effects of oxotremorine both on rotarod performance and on body temperature. For drug-treated BALB mice, the dose-response curves for both measures were parallel to those for saline-treated mice, while for DBA and C3H mice the slopes of the dose-response curves were significantly less for treated mice than they were for controls. The equi-effective doses for the drug-treated animals were at least 8-fold greater than those for saline-treated mice. Drug treatment resulted in a significant decrease in the total number of muscarinic receptors in cortex as measured by the binding of [3H]quinuclidinyl benzilate (QNB) without effect on the KD for this ligand. Similarly, drug treatment did not affect the affinity of carbamylcholine as an inhibitor of QNB binding, but did significantly decrease the levels of both the high- and low-affinity agonist binding sites in cortex. The number of M1 muscarinic receptors measured by high affinity [3H]pirenzepine (PZ) binding was also significantly decreased in cortex without effect on the KD. The experiments were extended to five other brain regions. Full saturation curves were not constructed, however. Oxotremorine treatment significantly reduced QNB binding in every brain region. While the binding to agonist affinity states measured by carbamylcholine inhibition of QNB binding and M1 receptor levels measured by high affinity PZ binding tended to decrease with oxotremorine treatment not all changes were statistically significant. The changes in muscarinic receptor subtype levels induced by oxotremorine infusion did not differ among the strains. The results demonstrate that chronic treatment with a muscarinic agonist results in substantial tolerance to the effects of the drug in all four mouse strains. Although some differences in tolerance development exist, these differences are not readily explained by differences in the number or affinity states of brain muscarinic receptors.