Exogenous CXCL1 decreases morphine analgesic efficacy and accelerates development of morphine tolerance in daily intraperitoneal morphine paradigm. CXCL1 was intrathecally administered via an osmotic pump at an infusion rate of 1.2 ng/h. Morphine was intraperitoneally injected at 10 mg kg−1 day−1. The time-course of analgesic action of acute morphine treatment was evaluated by assessment of the latency of the tail-flick response and the calculated % maximum possible effects on day 1 through day 3 (A–C). The area under the curve was summarized in (D). Note that intrathecal administration of CXCL1 accelerated tolerance induced by intraperitoneal injection of morphine in rats. *P < 0.05 as compared with morphine + saline control group at different time points tested by Bonferroni correction.
