A, Thermal latencies (in s) to noxious heat were recorded every 15 min following intrathecal administration of deltorphin II (2.5 μg) to control littermates (Litt.;
), pro-enkephalin-KO (pENK-KO; ○), β-endorphin-KO (β-END-KO; □), and pro-dynorphin-KO (pDYN-KO; △) mice. B, Comparative latencies to paw withdrawal (in s) in control littermates, pENK-, β-END-, and pDYN-KO mice before (0 min) and 15 min after intrathecal injection of deltorphin II (2.5 μg). Number in parenthesis indicates the number of animals in each group. #, p < 0.0001 when ipsilateral 0 min are compared with their respective ipsilateral preCFA; two-tailed unpaired t-test.