Intense noxious stimuli impair GABAergic inhibition in spinal dorsal horn, which has been proposed as a critical contributor to pathological
pain. However, how the reduced inhibition exacerbates the transfer of nociceptive information at excitatory glutamatergic synapses is still poorly understood. The present study demonstrated that one of the striking consequences of GABAergic disinhibition was to enhance the function of
N-methyl-D-aspartate subtype
glutamate receptors (NMDARs), a well-characterized player in central sensitization. We found that intrathecal application of
bicuculline, a
GABA(A) receptor antagonist, to remove the inhibition readily elicited
mechanical allodynia in naive mice, which could be dose-dependently attenuated by NMDARs antagonist
D-APV. Biochemical analysis demonstrated that
bicuculline did not affect the total expression levels of the obligatory NMDARs subunit NR1 and the regulatory subunit NR2A and NR2B. However,
bicuculline promoted NR1 phosphorylation at
Serine 897 (NR1-S897) by
cAMP-dependent protein kinase (PKA). This PKA-mediated phosphorylation incorporated NR1 along with NR2B into synapses. When
PKA inhibitor H-89 was intrathecally applied, it totally eliminated
bicuculline-induced NMDARs phosphorylation, synaptic redistribution as well as
pain sensitization. Importantly, the reduced inhibition also operated to enhance NMDARs functions after peripheral
inflammation, because
spinal injection of
diazepam to rescue the inhibition in inflamed mice greatly depressed PKA phosphorylation of NR1-S897, reduced the synaptic concentration of NR1/NR2B and meanwhile, alleviated the inflammatory
pain. These data suggested that removal of GABAergic inhibition allowed for PKA-mediated NMDARs phosphorylation and synaptic accumulation, thus exaggerating NMDARs-dependent nociceptive transmission and behavioral sensitization.