Figure 1. Glycine inhibition in the dorsal horn is lost in
chronic pain states, but may be restored by GlyT2 inhibitors to
alleviate pain: Glycinergic inhibition to lamina II excitatory
interneurons is lost in chronic pain states, leading to enhancement of
nociceptive signals, and aberrant activation of the nociceptive pathways
in response to non-noxious stimuli such as touch (via Aβ fibres). Drugs
that rectify this glycinergic dysfunction, such as GlyT2-inhibitors, are
able to alleviate pain by inhibiting the abnormal signalling.
Figure 2. Analgesia (von Frey) testing in a mouse model of
chronic neuropathic (CCI) pain: A Ol-D-lys
(purple) produced analgesia in mice in a dose-dependent manner from 10
mg kg-1 to 30 mg kg-1 at which a
peak effect was seen. Higher doses of 50 mg kg-1 and
100 mg kg-1 were less effective than the lower dose of
30 mg kg-1. B ORG25543 produced statistically
significant analgesia from 60 min to 4 h at the 30 mg
kg-1 dose. At the highest tested dose of 50 mg
kg-1, responses to the von Frey stimuli were markedly
reduced, however this coincided with severe side effects and toxicity,
which precluded testing the full data set and this dose was excluded
from statistical analysis. Significant post-hoc multiple comparisons are
indicated by asterisks (* p < 0.05). n’s per group are
indicated in parentheses in legend.
Figure 3. Analgesia (von Frey) testing in a mouse model of
inflammatory (CFA) pain: No analgesia was observed for any dose of
ol-D-lys or for 30 mg kg-1 of
ORG25543. n’s per group are indicated in parentheses in legend.
Figure 4. Acute pain (hotplate) testing in mice: No
anti-nociception was observed for 30 mg kg-1 of
ol-D-lys or for 30 mg kg-1 of ORG25543
while non-parametric Kruskal-Wallis one-way test showed significant
anti-nociception at 30-min in the morphine (orange, 10 mg
kg-1) group (* p < 0.05). n’s per group are
indicated in parentheses in legend.
Figure 5. Side effects and convulsions caused by
GlyT2-inhibitors: A(i)-(ii) show side effects of ol-D-lys. B(i)-(ii)
show side effects of ORG25543. C shows convulsions caused by ol-D-lys. D
(i)-(ii) convulsions caused by ORG25543. Summary shows mean +/- SEM.
Panel (ii) of A, B and D show the scatter of the doses that caused
significant adverse behaviours compared to t=0, for which Wilcoxon
signed rank test was performed (* p < 0.05). The cross marked
after 1 hour in B(i) and D(i) indicates the time at which one animal
died. n’s per group are indicated in parentheses in legend.
Figure 6. Motor coordination Rotarod test: AOl-D-lys caused no dose-dependent motor incoordination.
However, an enhanced performance was observed at 60 and 90 minutes, at
the 16 mg kg-1 group performed (* p < 0.05).B ORG25543 caused profound and immediate incoordination in 2
out of 3 animals at 50 mg kg-1, however due to the low
numbers tested, this could not be statistically analysed. n’s per group
are indicated in parentheses in legend.
Figure 7. Respiratory side effects: Whole body plethysmography
of freely moving mice showed that a sub-maximal analgesic dose of
morphine (orange) caused respiratory depression, measured by respiratory
frequency and minute volume (MVb), but ol-D-lys did not.
Significant post-hoc multiple comparisons are indicated by asterisks (*p < 0.05). n’s per group are indicated in parentheses
in legend.