Abstract
In animal models of neuropathic pain, changes in afferent and spinal cord neurones
after pe1ipheral nerve injury lead to hyperexcitability within the spinal dorsal horn,
termed "central sensitisation". This causes a persistent pain state with enhanced
responses to noxious stimuli (hyperalgesia) and pain-like responses to previously
innocuous stimuli (allodynia).
The role of the VPAC₂ receptor in CCI was investigated. In VPAC₂R(⁻/⁻) mice, the
enhanced reflex responses to noxious heat and innocuous mechanical stimulation
seen in wild type (WT) mke were attenuated. No morihological differences were
seen between peripheral nerves of WT and VPAC2R c--) mice. Furthermore,
intrathecal administration of a VPAC₂R inhibitor attenuated the enhanced reflex
withdrawal responses to noxious heat and innocuous mechanical stimuli in WT mice
following CCI, with no effect in VPAC₂R (⁻/⁻) mice.
In normal rats, intrathecal administration of PKA inhibitors attenuated the enhanced
reflex withdrawal responses due to CCI. In situ hybridisation for isofo1ms of PKA
regulatory (R) and catalytic (C) subunits showed a spinal increase in C-subunit , but
not R-subunit mRNA ipsilaterally at the peak of CCI sensitisation. Immunoblots
confiimed an ipsilateral increase in C-subunits and showed a bilateral decrease in the
RI~ subunit.
The role of the proteasome in neuropathic sensitisation was studied.
Electrophysiological recordings made from dorsal horn neurones in anaesthetised
rats showed that proteasome inhibitors applied by ionophoresis inhibi ted activity
evoked by innocuous brush and cold in CCI rats, while nociceptive responses were
inhibited in CCI and normal animals. Intrathecal administration of proteasome
inhibitors attenuated the enhanced paw withdrawal behaviours ipsilateral to CCI.
The mRNA and protein levels for UCH-Ll, (a key enzyme in proteasomal function)
were increased ipsilaterally. PKA enzymatic activity was increased in spinal cord
ipsilateral to nerve injury and this increment was prevented by topical application of
proteasome inhibitors.
This investigation demonstrates the involvement of the VPAC₂ receptor, the
corresponding cAMP/PKA signal transduction cascade and the proteasome (a
regulator of PKA activity) in the spinal sensitisation caused by CCI.
