Effect of extinction training on CREB phosphorylation. A, Representative blots and mean ± SE percentage of pCREB immunoreactivity from unpaired control rats (lane 1), conditioned rats (lane 2), and conditioned rats given light-alone training (lane 3). Fear conditioning resulted in an increase in pCREB, whereas extinction training caused a decrease. Bilateral infusion of FK-506 (10 μg dissolved in 1.6 μl of DMSO; 0.8 μl per side) before extinction trials blocked extinction-induced decrease in pCREB (lane 4). *p < 0.01 versus conditioned. B, Time course of calcineurin expression induced by light-alone trials in nuclear fraction. Figure shows the representative blots and mean ± SE of calcineurin immunoreactivities from rats decapitated at various time points (n = 6 rats in each time point) after extinction training (histone used as internal control). *p < 0.001 versus conditioned.