Disinhibition Opens the Gate to Pathological Pain Signaling in SuperficialNeurokinin 1 Receptor-Expressing Neurons in Rat Spinal Cord

摘要

Blockade of local spinal cord inhibition mimics the behavioral hypersensitivity that manifests in chronic pain states. This suggests that there is a pathway capable of mediating allodynia/hyperalgesia that exists but is normally under strong inhibitory control. Lamina I and III neurokinin 1 (NK1) receptor expressing (NK1R+) dorsal horn neurons, many of which are projection neurons, are required for the development of this hypersensitivity and are therefore likely to be a component of this proposed pathway. To investigate, whole-cell patch-clamp recordings were made from lamina I and III NK1R+ neurons in the spinal cord slice preparation with attached dorsal root. Excitatory postsynaptic currents were recorded in response to electrical stimulation of the dorsal root. Lamina I NK1R+ neurons were shown to receive high-threshold (Aδ/C fiber) monosynaptic input, whereas lamina III NK1R+ neurons received low-threshold (Aβ fiber) monosynaptic input. In contrast, lamina I neurons lacking NK1 receptor (NK1R−) received polysynaptic A fiber input. Blockade of local GABAergic and glycinergic inhibition with bicuculline (10 μm) and strychnine (300 nm), respectively, revealed significant A fiber input to lamina I NK1R+ neurons that was predominantly Aβ fiber mediated. This novel A fiber input was polysynaptic in nature and required NMDA receptor activity to be functional. In lamina I NK1R−and lamina III NK1R+ neurons, disinhibition enhanced control-evoked responses, and thiswas also NMDA receptor dependent. These disinhibition-induced changes, in particular thenovel polysynaptic low-threshold input onto lamina I NK1R+ neurons, may be an underlyingcomponent of the hypersensitivity present in chronic pain states.