Long-term effects of short-lasting early local inflammatory insult.

We examined the long-term effects of a short-lasting (approximately 24 h) inflammatory insult generated by injections of 0.25% carrageenan (1 microl/g) into the hindpaws of newborn (P0) rat pups. At P60 animals which experienced this early inflammatory insult showed significant alterations in the withdrawal responses to noxious stimulation of the affected paws. Furthermore, in the absence of ongoing inflammation, the withdrawal latencies to heat stimulation and withdrawal thresholds to mechanical stimulation were increased by such experience. In the presence of ongoing CFA-induced inflammation, however, the same early experience decreased these parameters of response to noxious stimulation. These data suggest that early inflammatory insult may differentially affect the aspects of nociceptive circuitry involved in transient pain sensitivity and in inflammation-induced hyperalgesia.