, 2002 and Fitzgerald et al., 1994). Roscovitine datasheet In adult rats, Aδ and C but not Aβ primary afferent fibers transmit painful stimuli. In contrast, in P7 rats Aβ primary afferents can also transmit such stimuli (Fitzgerald and Jennings, 1999). It has been hypothesized that increased activity in Aβ-fibers early in development may be modulated by sub-threshold

C-fiber depolarization that primes the spinal cord for Aβ-fiber input (Dickenson and Rahman, 1999). Functionally, in adult rodents opioid agonists selectively inhibit Aδ- and C-fiber nociceptors but not Aβ-fibers (Dickenson et al., 1987 and Rahman and Dickenson, 1999). In contrast, in young rats morphine can inhibit Aβ- and C-fiber-mediated activity in the lumbar spinal cord (Rahman et al., 1998), which parallels expression of μORs in both small (Aδ and C) and large (Aβ) diameter cell bodies in the dorsal root ganglion. Based on the results of our study, we suggest that at P16 the animals do not exhibit increased nociceptive behavior in the formalin test because repeated exposure to a μOR agonist has influenced the development of C-fibers during maturation. However, we did observe that following the formalin test, the treated animals presented an inflammation-like

UK-371804 concentration edema in the formalin-injected hindpaw, which was measured and compared to the volume of the non-injected hindpaw by plethysmometry. It is interesting to note that there were no differences between the volume of formalin-injected Tryptophan synthase hindpaws in the morphine and control groups (data not shown). Taking into account the importance of a deeper understanding of the effects throughout

life of opioid analgesia at birth, and that previous results from our group showed that morphine exposure in early life lead to changes in the analgesic response in adult life (Rozisky et al., 2008), we hypothesized that the use of opioids in early life can induce persistent changes in nociceptive and opioid analgesic responses. We conclude from the present results that the altered nociceptive response induced by repeated morphine exposure can change in an age-dependent manner. In addition, the altered nociceptive response was expressed until adulthood, and this effect was partially reversed by indomethacin and completely reversed by an NMDA receptor antagonist. However, it should be noted that the response is complex and unlikely to be predominantly caused by any single mediator. Taken together, our data indicate that opioids elicit glutamatergic adaptations at the system level. Finally, the behavioral changes seen in response to repeated exposure to morphine during early life illustrate the need to examine nociceptive processing in neonatal patients who have been exposed to therapeutic morphine; moreover, this indicates the importance of evaluating the clinical consequences of long-term opioid administration.