Fyn kinase-regulated NMDA receptor- and AMPA receptor-dependent pain sensitization in spinal dorsal horn of mice.

Inhibition of Src-family protein tyrosine kinases (SFKs) in spinal dorsal horn has been established as an effective strategy for the alleviation of chronic pathological pain. As one of the important SFKs members, Fyn kinase is critical for synaptic plasticity and many pathophysiological processes. However, whether Fyn is involved in spinal sensitization is far from being elucidated. We manipulated Fyn activity by expressing a constitutively active Fyn mutant [Fyn(Y528F) ] or a catalytically null mutant [Fyn(K296M) ] in the spinal dorsal horn of mice, and performed behavioural and biochemical experiments to investigate the role of Fyn in regulating the nociceptive responses and the synaptic expression of ionotropic glutamate receptors. Spinal expression of Fyn(Y528F) alone in intact mice was sufficient to elicit persistent mechanical allodynia and thermal hyperalgesia, which lasted for at least 12 days. Fyn(Y528F) simultaneously enhanced the concentrations of N-methyl-D-aspartic acid (NMDA)-subtype and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-subtype glutamate receptors at synaptosomal membrane fraction. Pharmacological inhibition of NMDA receptors or AMPA receptors greatly alleviated Fyn(Y528F)-induced pain hypersensitivity. To evaluate the contribution of Fyn to inflammatory pain, we expressed Fyn(K296M) before intradermal injection of complete Freund's adjuvant (CFA), finding that Fyn(K296M) had no effect on the induction of inflammatory pain within 3 h post-CFA injection, which, however, repressed the synaptic accumulation of NMDA receptors and AMPA receptors to attenuate the maintenance of chronic pain states. Fyn played a key role in the sustained sensitization of nociceptive behaviours by up-regulating the functions of ionotropic glutamate receptors in spinal dorsal horn.