Mechanical allodynia caused by intraplantar injection of P2X receptor agonist in rats: Involvement of heteromeric P2X(2/3) receptor signaling in capsaicin-insensitive primary afferent neurons
M. Tsuda et al., Mechanical allodynia caused by intraplantar injection of P2X receptor agonist in rats: Involvement of heteromeric P2X(2/3) receptor signaling in capsaicin-insensitive primary afferent neurons, J NEUROSC, 20(15), 2000, pp. NIL_16-NIL_20
Extracellular ATP has been known to activate sensory neurons via the ATP-ga
ted ion channels P2X receptors, indicating that the P2X receptors may play
a role in signal transduction of pain from the periphery to the spinal cord
in vivo. Here, we found a novel nociceptive response induced by ATP, mecha
nical allodynia (hypersensitivity to innocuous mechanical stimulus). Inject
ion of alpha, beta-methylene ATP (alpha beta meATP), an agonist to P2X rece
ptor, into plantar surface in rats produced the mechanical allodynia along
with previously described nocifensive behavior and thermal hyperalgesia. Th
is allodynic response was blocked by pretreatment with the P2 receptor anta
gonist pyridoxal-phosphate- 6-azophenyl-2',4'-disulfonate. Interestingly, o
nly the mechanical allodynia evoked by abmeATP selectively remained in neon
atal capsaicin-treated adult rats that had selectively lost the capsaicin-s
ensitive neurons. ATP has been shown to produce two distinguishable electro
physiological responses (inward currents with rapid and slow desensitizatio
n) in dorsal root ganglion (DRG) neurons. In the present electrophysiologic
al experiment, the percentage of DRG neurons that responded to alpha beta m
eATP with slow desensitizing inward current remained constant in capsaicin-
treated rats, whereas the percentage that responded with rapid desensitizin
g current dramatically decreased. Taken together with our previous finding
that the alpha beta meATP-activated slow desensitizing current in DRG neuro
ns is mediated by heteromeric P2X(2/3) (P2X(2) and P2X(3)) receptors, it is
hypothesized that activation of heteromeric P2X(2/3) receptors in peripher
al terminals of capsaicin-insensitive primary afferent fibers leads to the
induction of mechanical allodynia.