hMOR-T and hMOR exhibit similar acute and chronic DAMGO-induced behavioral tolerance. (A) Acute tolerance. On day 1, 4 μg DAMGO produced 88.6% MPE in hMOR rats and, 8 μg DAMGO produced 87.1% MPE in hMOR-T rats. On day 2, similar reduction in DAMGO responses (hMOR: 39.4%, hMOR-T: 44.4%, n = 5-7) were observed. (B) Chronic tolerance was obtained by applying 4 μg/10 μl DAMGO to hMOR and 8 μl/10 μl to hMOR-T twice daily for 4 consecutive days. The rates and extents of DAMGO-induced tolerance developed similarly in hMOR and hMOR-T rat groups.