Figure 1.

Basal and forskolin-stimulated cyclic AMP production in the different experimental groups. Cyclic AMP was determined in brain cortex slices incubated for 10 min in the absence (IBMX) or in the presence of 10 μM forskolin. Control animals (n=24) were treated with saline (1 μl h⁻¹) for 7 days. Tolerant animals (n=19) received sufentanil (2 μg h⁻¹) for 7 days. Supersensitive animals (n=8) were concurrently treated with sufentanil (2 μg h⁻¹) and nimodipine (1 μg h⁻¹) for 7 days. In the nimodipine withdrawal group (n=5), nimodipine minipump was removed on the 6th day of infusion, while sufentanil minipump remained in place for 7 days. Data are expressed as mean±s.e.mean. F=13.43, P<0.001 (ANOVA); ^**P<0.01, ^*P<0.05 vs control forskolin (Bonferroni test).