Our previous study showed that
cocaine- and
amphetamine-regulated transcript
peptide fragment 55-102 (CARTp) specifically potentiated spinal
N-methyl-D-aspartate (
NMDA)-mediated nociceptive transmission in vivo and in vitro. The cellular mechanisms underlying CARTp potentiation of
NMDA receptor function remains unclear. The present study was carried out to test the hypothesis that CARTp changes the phosphorylated state of
NMDA receptors by activating intracellular signals and subsequently increasing the function of
NMDA receptors. We found that the potentiating effect of CARTp on spinal
NMDA-induced
hyperalgesia in rats was reduced by intrathecal pretreatment with
KT5720 (a selective
PKA inhibitor) or GF109206X (a selective PKC inhibitor), but was increased by pretreatment with
calyculin A (a
protein phosphatase inhibitor). In the in vitro electrophysiological study, CARTp potentiation of
NMDA-induced depolarizations was blocked by superfusion of PKA or PKC inhibitor applied 10 min before the application of CARTp. The levels of
phosphoserine 897 on the NR1 subunit (pNR1-ser897) and
phosphoserine 896 on the NR1 subunit (pNR1-ser896) in the dorsal horn of spinal lumbar segments significantly increased following CARTp superfusion in vitro or
intrathecal injection in vivo. The increases in pNR1-ser897 and pNR1-ser896 in the in vivo and in vitro studies were inhibited by pretreatment with
KT5720 and GF109206X, respectively. The results provide the first evidence that CARTp increases the phosphorylation of
NMDA receptor NR1 subunit via activation of PKA and PKC signals, which may play a crucial role in CARTp regulation of spinal
NMDA receptor-mediated nociceptive responses.