Effect of interleukin-1beta (IL-1β on neonatal rat pain sensitivity in the tail-flick test (P5–P8). P5 rats were intracerebrally injected with saline, lipopolysaccharide (LPS) (1 mg/kg), or IL-1β (1 µg/kg). Similar to LPS exposure in the neonatal rat, IL-1β injection produced enhanced responses to painful stimuli in the neonatal rats, i.e., decreased latency to tail removal from a radiant heat source. The results are expressed as the mean ± SEM of 12 animals in each group, and were analyzed by two-way repeated measures ANOVA for data from tests conducted continuously at different postnatal days. * P < 0.05 represents a significant difference in the LPS or IL-1β group compared with the saline group on the same postnatal day. # P < 0.05 represents a significant difference in the IL-1β group compared with the LPS group on the same postnatal day.