Chronic morphine exposure increases the phosphorylation of MAP kinases andthe transcription factor CREB in dorsal root ganglion neurons: an in vitroand in vivo study

Tolerance to opiates reduces their effectiveness in the treatment of severe pain. Although the mechanisms are unclear, overactivity of pro-nociceptive systems has been proposed to contribute to this phenomenon. We have report ed that the development of morphine tolerance significantly increased calci tonin-gene-related-peptide-like immunoreactivity (CGRP-IR) in primary senso ry afferents of the spinal dorsal horn, suggesting that changes in pain-rel ated neuropeptides in the dorsal root ganglion (DRG) neurons may be involve d (Menard et al, 1996, J. Neurosci., 16, 2342-2351). Recently, we have show n that repeated morphine treatments induced increases in CGRP- and substanc e P (SP)-IR in cultured DRG, mimicking the in vivo effects (Ma et al., 2000 , Neuroscience, 99, 529-539). In this study, we investigated the intracellu lar signal transduction pathways possibly involved in morphine-induced incr eases in CGRP- and SP-IR in DRG neurons. Repeated morphine exposure (10-20 muM) for 6 days increased the number of neurons expressing phosphorylated ( p) mitogen-activated protein (MAP) kinases, Including the extracellular sig nal-regulated kinase (pERK), c-jun N-terminal kinase (pJNK) and P38 (pP38 M APK). The number of neurons expressing phosphorylated cAMP responsive eleme nt binding protein (pCREB) was also markedly increased in morphine-exposed cultured DRG neurons. pERK-, pP38-, pJNK- and pCREB-IR were colocalized wit h CGRP-IR in cultured DRG neurons. Naloxone effectively blocked these actio ns of morphine, whereas a selective MEK1 inhibitor, PD98059, inhibited the morphine-induced increase in the phosphorylation of ERK and CREB, and the e xpression of CGRP and SID. Moreover, in morphine-tolerant rats, the number of pCREB-, CGRP- and SP-IR neurons in the lumbar DRG was also significantly increased. These in vitro and in vivo data suggest that the phosphorylatio n of MAP kinases and CREB plays a role in the morphine-induced increase in spinal CGRP and SP levels in primary sensory afferents, contributing to the development of tolerance to opioid-induced analgesia.