However ERK activation is induced by noxious… , world congress neurology
• volume 2 no 12 • december 1999
1115
Fig. 1. Induction of ERK phosphorylation in the ipsilateral dorsal horn
by intraplantar capsaicin injection. (a) Transverse section of the L5 lum-
bar spinal cord showing increased pERK immunoreactivity (IR) in the
ipsilateral medial superficial dorsal horn (arrow) two min after capsaicin
injection into the hindpaw. Scale bar, 200 µm. (b) High-magnification
image from (a) showing pERK-IR in many small postsynaptic neurons in
the superficial dorsal horn and a solitary deeper neuron. Scale bar, 50
µm. (c) Confocal image showing induced pERK-IR in cytoplasm (arrow-
head) and nucleus (arrows). Scale bar, 5 µm. (d) Western blot obtained
from ipsilateral (I) and contralateral (C) spinal dorsal horn, indicating
that ERK2 (p42 MAPK) and ERK1 (p44 MAPK) are phosphorylated after
capsaicin stimulation. Bottom bands are controls for non-phosphory-
lated ERK2. 1, ERK 1; 2, ERK 2.
ulus were applied to the hindpaw. All three induced pERK in
superficial dorsal horn neurons two minutes later (Fig. 2b). How-
ever, repeated light touch to either the dorsal or plantar surface of
the hindpaw did not induce pERK in the spinal cord (Fig. 2b).
Activation of ERK in response to graded heat stimuli was inten-
sity dependent (Fig. 2c). There was no ERK activation at 42oC,
an innocuous warm stimulus. At the threshold for activation of
heat-sensitive nociceptors (45oC), a few pERK immunostained
cells appeared, and the numbers increased with increasing tem-
perature from 48 to 55oC (Fig. 2c).
Calcium entry into neurons via ionotropic glutamate recep-
tors may initiate the ERK signaling cascade24,25. We therefore
examined the effect of the NMDA receptor channel blocker MK-
801 on capsaicin-induced ERK activation. MK-801 was injected
into the subarachnoid space of the spinal cord 20 minutes before
capsaicin administration. MK-801 at 1.5 nmol reduced the num-
ber of pERK-positive neurons in the superficial dorsal horn by
44% (p 0.01), and at 15 nmol by 50% (p 0.01; Fig. 3).
Electrical stimulation was used to selectively recruit different
types of primary afferents in an adult rat spinal cord slice prepa-
ration with an attached dorsal root26. Cells positive for pERK
were not induced in the dorsal horn after Aβ-fiber stimulation
b
c
Fig. 2. Activation of pERK in the dorsal horn. (a) Rapid onset and
decline of ERK phosphorylation after capsaicin administration measured
by the number of pERK-positive neurons in the superficial laminae (I–II)
of the ipsilateral dorsal horn. (b) ERK activation is nociceptive specific.
Innocuous tactile stimulation (light touch for 2 min) does not induce
pERK. However ERK activation is induced by noxious stimuli: prick
(100 g for 2 min), heat (50oC for 1 min) or cold (4oC for 1 min). Scale
bar, 50 µm. (c) ERK activation measured by the number of pERK-posi-
tive neurons in the superficial laminae (I–II) increases with increasing
stimulating temperature.
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© 1999 . •
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