Figure 4. Effect of melatonin receptor antagonists on IOP reduction mediated by melatonin and 5-MCA-NAT in C57BL/6J (dark bars) versus DBA/2J (light bars) mice at 9 months old. Two intraocular pressure (IOP) measurements were taken before a single dose of either vehicle or 100 μM melatonin (A) or 5-MCA-NAT (B) was applied, and 3 h later, the IOP was measured again. When indicated, melatonin antagonists, all tested at 100 μM, were instilled 30 min before the agonist. Luzindole is a non-selective melatonin antagonist; 4-phenyl-2- propionamidotetralin (4PPDOT) is a MT2 melatonin antagonist, prazosin is a MT3 melatonin antagonist/alpha blocker, and corynanthine is an alpha blocker. Data are the mean ± standard error of the mean (SEM) of four mice (*p<0.05, **p<0.01, ***p<0.001 versus agonist compound of each strain; one-way analysis of variance (ANOVA) with Dunnett’s multiple comparisons test). One hundred percent were calculated as the values of the control C57BL/6J mice.