Background & Aims

The concurrent opioid and pain epidemics have driven efforts for new, analgesic therapeutics lacking side effects of addiction, reward. Agmatine, decarboxylated L-arginine, preferentially antagonizes GluN2B-containing N-Methyl-D-Aspartate (NMDA) receptors in the spinal cord and inhibits nitric oxide synthase[1, 2]. Our previous work has shown that agmatine does not demonstrate side effects associated with high affinity, broad spectrum NMDA receptor antagonists. To improve distribution of agmatine, a highly polar molecule, to the central nervous system (CNS), we formulated compounds with strategic substitutions to agmatine (strategically-substituted agmatine, SSA) and assessed them for alleviation of neuropathic pain, inflammatory pain, post-operative pain, motor side effects, and reward typical of NMDA antagonists.

Methods

Spared nerve injury: The sciatic nerve was exposed and the tibial and peroneal branches were ligated and cut, while the sural branch was spared[3]. Inflammation: Chronic Freund’s adjuvant was injected into the hindpaw. Post-operative pain: A incision was made through the skin of the hindpaw of mice and rats. The muscle was retracted, incised, and the skin was sutured. In Vivo Pharmacological Responses: Hindpaw withdrawal to stimulation were assessed using an electronic von Frey anesthesiometer. Open field monitoring: Subjects were injected intravenously 15 minutes prior to placement in an open field chamber. Movement within the chamber was recorded for ambulatory time and rate of movement. Self Administration: Female and male rats were allowed to self-administer either oxycodone or SSA reward through an indwelling intravenous catheter. Dependence: Rats were subcutaneously injected twice daily with SSA compound.Behavioral responses were observed and recorded twice daily.

Results

: SSA compounds demonstrated dose-dependent reversal of tactile hypersensitivity with different time to onset of peak effect (with some earlier than agmatine) in neuropathic, inflammatory, and post-operative pain models. No SSA compound demonstrated increased motor output typical of NMDA receptor antagonists as compared to MK-801 positive control, which significantly increased total ambulatory distance. Rats did not lever press to receive SSA reward, in contrast to the positive control, oxycodone; these results support the premise that the SSAs are not reinforcing. None of the SSA compounds displayed alterations in behavior following twice daily injection, as compared to the locomotor sensitization displayed by ketamine- and morphine-treated subjects. Morphine-treated subjects displayed characteristic opioid withdrawal behaviors following naloxone precipitation, whereas the SSA-treated subjects did not.

Conclusions

The presented studies support that the SSA provide analgesia in models of neuropathic, inflammatory, and post-operative pain without the characteristic side effects associated with broad spectrum NMDA receptor antagonists. Importantly, the SSA compounds do not demonstrate a concerning side effect profile including lacking motor alterations, dependence, and reward that limit the use of currently available NMDA receptor analgesic therapeutics. These studies support continued development of the SSA compounds for addressing ongoing neuropathic pain.

References

1.Waataja, J.J., et al., Agmatine preferentially antagonizes GluN2B-containing N-methyl-d-aspartate receptors in spinal cord. J Neurophysiol, 2019. 121(2): p. 662-671.
2.Peterson, C.D., et al., Agmatine requires GluN2B-containing NMDA receptors to inhibit the development of neuropathic pain. Mol Pain, 2021. 17: p. 17448069211029171.
3.Decosterd, I. and C.J. Woolf, Spared nerve injury: an animal model of persistent peripheral neuropathic pain. Pain, 2000. 87(2): p. 149-58.

Presenting Author

Cristina D Peterson

Poster Authors

Cristina Peterson

PhD

University of Minnesota

Lead Author

E-mail

Topics

  • Mechanisms: Biological-Molecular and Cell Biology