Ketamine and ARA 290 differ in effects on acute nociceptive pain and side effects.

<p><b>A.</b> Ketamine administration increases the latency of tail withdrawal to a thermal stimulus (treatment effect, p<0.001), whereas ARA 290 does not. <b>B.</b> Ketamine treatment had significant biphasic effects on stereotypic behavior: after a period of transient sedation, the animals showed signs of psychomimetic disturbances that lasted for about 20 minutes (treatment effect, p<0.001). ARA 290 did not display these side effects. <b>C.</b> Treatment with ketamine was associated with a biphasic activation of generalized activity (treatment effect, p<0.001) causing an increase in restlessness and explorative behavior after a period of transient sedation.</p

QRT-PCR primers and probes used in this study.

<p>Primers and probes used for the quantification of mRNA from NMDA receptor subtypes NR1, NR2A and NR2B (Grin); microglia marker Iba-1 (AIF-1), astrocyte (GFAP) and CCL2; f, Reporter dye1 (FAM:6-carboxyfluorescein); t, Reporter dye2 (TET:Tetrachloro-6-carboxyfluorescein); q, Quencher dye (TAMRA: 6-carboxytetramethyl1-rhodamine).</p

Ketamine and ARA 290 have similar effects on allodynia.

<p><b>A.</b> Treatment with both ketamine and ARA 290 prevented the full development of allodynia (treatment effect, p = 0.049 and p = 0.03, respectively). <b>B.</b> The effects of ketamine on acute nociceptive pain remained unchanged over time (treatment effect, p<0.001).</p

Relief of allodynia depends upon the ß-common receptor (ßcR).

<p><b>A.</b> Both ketamine and ARA 290 did not prevent the development of allodynia in ßcR^−/− mice. <b>B.</b> However, the effect of ketamine on nociceptive pain is unchanged in ßcR^−/− animals (treatment effect, p<0.001).</p

Neuropathic pain involves a pathway that utilizes the Innate Immune Receptor.

<p>Nerve injury results in microglial recruitment, increased expression of NMDAR, and proinflammatory cytokine production, ultimately resulting in allodynia. Activation of the innate immune receptor (IRR), e.g., by ARA 290, antagonizes this pathway. Ketamine also requires the IRR to reduce allodynia. This may be via a direct interaction with the IRR or alternatively, via modulation of intermediate processes that are upstream of the IRR. Additionally, ketamine interacts with NMDARs that mediate antinociception and psychomotor effects. ARA 290 does not interact with the NMDAR and therefore lacks these additional effects.</p

Ketamine and ARA 290 reduce inflammatory mediators in the spinal cord following sciatic nerve injury.

<p>One week post surgery, animals showed a marked elevation of CCL2 (panel <b>A</b>), Iba1 (panel <b>B</b>), and GFAP (panel <b>C</b>) compared to naïve controls. Both ketamine and ARA 290 significantly reduced the mRNA levels of these genes to a similar extent. *p<0.05 <i>versus</i> vehicle, #p<0.05 between ketamine and ARA 290 treatments, **p<0.05 <i>versus</i> naïve.</p

Ketamine and ARA 290 reduce mRNA for NDMA receptor subunits in established neuropathy.

<p>Real time PCR data show that NMDA receptor subunits 1 (panel <b>A</b>), 2A (panel <b>B</b>), and 2B (panel <b>C</b>) are all modestly elevated one week following sciatic nerve injury. Administration of ketamine significantly reduces mRNA to baseline levels. In contrast, ARA 290 reduced mRNA for these receptor subunits to substantially below baseline (naïve). *p<0.05 <i>versus</i> vehicle, #p<0.05 between ketamine and ARA 290 treatments, **p<0.05 <i>versus</i> naïve.</p