Neonatal persistent inflammation alters pain response and NMDA receptor expression in adult mice

Infant pain is of critical interest, especially with respect to premature infants and other high-risk neonates that experience many invasive and traumatic procedures early in development. The early neonatal period is characterized by great plasticity and reorganization. Sustained activation of central nervous circuits, caused by protracted and recurrent pain, may cause long-lasting changes in central neural function thus affecting developmental outcome and behavioural responsiveness to pain or stress later in life. However little is known about the neurobiological substrates underlying this ``memory'' process. The aim of our study was twofold:to study whether timing of postnatal exposure to a persistent inflammatory insult alters the responsiveness to thermal pain in the adult animal;given the role of the NMDA receptor in pain processing as well as in learning and memory, to examine if NMDA receptor subtype gene expression in specific areas of the cns is influenced by neonatal inflammation.Methods: Newborn mice received a single injection of Complete Freund's Adjuvant (CFA) or saline on either postnatal day 1, 3 or 14 (P1, P3 and P14) into the left hind paw. At twelve weeks of age paw withdrawal latency (PWL) of each animal was tested both in basal condition and 24h after an unilateral injection of 100 μL of CFA in the left hind paw. Mice were then killed by cervical dislocation and cerebral areas were removed. Using a sensitive RNAse protection assay, NMDA receptor subunit (NR1, NR2A, NR2B, NR2C) gene expression was evaluated in different brain areas; all data were processed by one-way ANOVA (p < 0.05).Results: Baseline paw withdrawal latency was significantly decreased in animals exposed to CFA at day 1 and 14 as compared to their saline exposed counterparts. Animals exposed to CFA at postnatal day 3 showed a significant increase in paw withdrawal latency with respect to saline injected animals. Twenty-four hours later a unilateral injection of CFA into the left hind paw, a significant decrease in paw withdrawal latency was observed in all experimental groups with respect to baseline values. PWL of P1 saline treated animals after CFA exposure was significantly higher than P3 and P14 saline treated mice. Adult mice exposed to an injection with CFA on postnatal day 1 exhibited reduced expression of the NMDA receptor subtype NR1 and NR2C in the hippocampus while mRNA levels for NR2A and NR2B did not differ between CFA treated and untreated mice. Exposure to CFA on postnatal day 3 and 14 did not affect adult expression levels of NMDA receptor subunits in the hippocampus. NMDA receptor subunit expression displayed a different profile in the thalamus. Exposure to CFA at P1 and P3 did not alter NMDA receptor subunit expression while exposure to CFA at P14 resulted in enhanced expression of the NR2A and NR2B subunits.Conclusions: These findings indicate that changes in NMDA receptor subtype gene expression in adult mice exposed to persistent neonatal peripheral inflammation are brain region specific and that NMDA gene expression and pain reactivity differ according to the day of neonatal exposure to CFA.