Chronic pain is thought to be partly caused by a loss of GABAergic inhibition and resultant neuronal hyperactivation in the central
pain-modulating system, but the underlying mechanisms for
pain-modulating neurons in the brain are unclear. In this study, we investigated the cellular mechanisms for activation of brainstem descending
pain facilitation in rats under persistent
pain conditions. In the nucleus raphe magnus (NRM), a critical relay in the brain's descending
pain-modulating system, persistent inflammatory
pain induced by complete
Freund's adjuvant decreased the
protein level of
K(+)-Cl(-) cotransporter (KCC2) in both total and synaptosomal preparations. Persistent
pain also shifted the equilibrium potential of GABAergic inhibitory postsynaptic current (EIPSC) to a more positive level and increased the firing of evoked action potentials selectively in μ-
opioid receptor (MOR)-expressing NRM neurons, but not in MOR-lacking NRM neurons. Microinjection of
brain-derived neurotrophic factor (
BDNF) into the NRM inhibited the KCC2
protein level in the NRM, and both
BDNF administration and KCC2 inhibition by
furosemide mimicked the
pain-induced effects on EIPSC and excitability in MOR-expressing neurons. Furthermore, inhibiting
BDNF signaling by NRM infusion of
tyrosine receptor kinase B-
IgG or blocking KCC2 with
furosemide prevented these
pain effects in MOR-expressing neurons. These findings demonstrate a cellular mechanism by which the hyperactivity of NRM MOR-expressing neurons, presumably responsible for descending
pain facilitation, contributes to
pain sensitization through the signaling cascade of BDNF-KCC2-GABA impairment in the development of
chronic pain.