the anti-allodynic and anti-hyperalgesic effects of zerumbone. b 1 -adrenoceptor antagonist only reversed the anti-allodynic effect of zerumbone. The anti-allodynic and anti-hyperalgesic effects of zerumbone were both absent when TRPV1 and NMDA receptors were antagonized in both nociceptive assays. Zerumbone treatment markedly decreased the expression of a 2Aadrenoceptor, while an up-regulation was observed of NMDA NR2B receptors. Expression of TRPV1 receptors however did not significantly change. The in vitro study, representing a peripheral model, demonstrated the reduction of both NMDA NR2B and TRPV1 receptors while significantly increasing a 2A -adrenoceptor expression in contrast to the brain samples. Our current findings suggest that the a 1 -, a 2 -, b 1 -and b 2 -adrenoceptors, TRPV1 and NMDA NR2B are essential for the anti-allodynic and antihyperalgesic effects of zerumbone. Alternatively, we demonstrated the plasticity of these receptors through their response to zerumbone's administration. Keywords: zerumbone, neuropathic pain, a 2A -adrenoceptor, TRPV1, NMDA NR2B, allodynia and hyperalgesia Chia et al. Zerumbone; a 2A -adrenergic, TRPV1 and NMDA NR2B Frontiers in Pharmacology | www.frontiersin.org March 2020 | Volume 11 | Article 92 Chia et al. Zerumbone; a 2A -adrenergic, TRPV1 and NMDA NR2B Frontiers in Pharmacology | www.frontiersin.org March 2020 | Volume 11 | Article 92 3

Cardamonin, a naturally occurring chalcone isolated from Alpinia species has shown to possess strong anti-inflammatory and anti-nociceptive activities. Previous studies have demonstrated that cardamonin exerts antihyperalgesic and antiallodynic properties in chronic constriction injury (CCI)-induced neuropathic pain animal model. However, the mechanisms underlying cardamonin’s effect have yet to be fully understood. The present study aims to investigate the involvement of the serotonergic system in cardamonin induced antihyperalgesic and antiallodynic effects in CCI-induced neuropathic pain mice model. The neuropathic pain symptoms in the CCI mice model were assessed using Hargreaves Plantar test and von-Frey filament test on day 14 post-surgery. Central depletion of serotonin along the descending serotonergic pathway was done using ρ-chlorophenylalanine (PCPA, 100 mg/kg, i.p.), an inhibitor of serotonin synthesis for four consecutive days before cardamonin treatment, and was found to reverse the antihyperalgesic and antiallodynic effect produced by cardamonin. Pretreatment of the mice with several 5-HT receptor subtypes antagonists: methiothepin (5-HT1/6/77 receptor antagonist, 0.1 mg/kg), WAY 100635 (5-HT1A receptor antagonist, 1 mg/kg), isamoltane (5-HT1B receptor antagonist, 2.5 mg/kg), ketanserin (5-HT2A receptor antagonist, 0.3 mg/kg), and ondansetron (5-HT3 receptor antagonist, 0.5 mg/kg) were shown to abolish the effect of cardamonin induced antihyperalgesic and antiallodynic effects. Further evaluation of the 5-HT1A receptor subtype protein expressions reveals that cardamonin significantly upregulated its expression in the brainstem and spinal cord. Our results suggest that the serotonergic pathway is essential for cardamonin to exert its antineuropathic effect in CCI mice through the involvement of the 5-HT1A receptor subtype in the central nervous system.

Background: Cardamonin is a naturally occurring chalcone from the Alpinia species. It is known to possess antioxidant and anti-inflammatory properties. Our previous studies have shown that cardamonin has antihyperalgesic and antiallodynic effects on CCI-induced neuropathic pain in mice. Although the evidence of the association between cardamonin and neuropathic pain has been reported in animal studies, specific targets using in vitro models are still lacking. Objectives/Methods: This study aims to investigate the effect of cardamonin on nitric oxide production using the LPS-induced neuropathic pain-like SH-SY5Y in vitro model through NMDA receptor expression. Results: Cardamonin administration in differentiated SH-SY5Y cells significantly reduced nitric oxide production assessed using Griess reagent. Western blot analysis demonstrated a significant reduction in GluN2B receptor expression in the cardamonin treated SH-SY5Y cells compared to the vehicle treated group. Conclusions: These data suggest that cardamonin reduces nitric oxide production modulated through NMDA GluN2B receptor subunit. Our results provides preliminary data to support the in vivo studies using cardamonin and may contribute to further understanding the mechanisms of action of cardamonin.

The human neuroblastoma cell line, SH-SY5Y cells, derived from the parental SK-N-SH cell line, is commonly used as an in vitro model for neuroscience and neurobiology research. Since SH-SY5Y cells are widely cultured for research, several different culture media have been used to optimize the growth of the cells, including Eagle's Minimum Essential Medium (EMEM), Dulbecco's modified Eagle's medium (DMEM) and other recently developed culture media. SH-SY5Y cells has the ability to reach confluency in culture flasks ranges from 5 days to 15 days, depending on the culture media used. Hence, the optimization of the culture media is crucial to achieve the fastest growth rate for the cells. The objective of the study is to evaluate the culture media for the proliferation of SH-SY5Y cells. We compared the growth rate of SH-SY5Y cells cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 15% heat-inactivated fetal bovine serum (hiFBS), Dulbecco's modified Eagle's medium: Nutrient mixture F-12 (DMEM:F12) + supplemented with 15% hiFBS and DMEM:F12 supplemented with 10% hiFBS. In DMEM:F12 supplemented with 15% hiFBS, cells grew up to 6.67E+05 cells. In DMEM:F12 supplemented with 10% hiFBS, cells grew up to 5.28E+05 cells. In DMEM supplemented with 15% hiFBS, the cells grew up to 4.76E+05 cells. There was a significant difference between culture media DMEM:F12 supplemented with 15% hiFBS as compared to DMEM:F12 supplemented with 10%hiFBS and DMEM supplemented with 15% hiFBS (p<0.05) following 24h incubation. However, there is no significant difference between DMEM:F12 supplemented with 10%hiFBS as compared to DMEM supplemented with 15% hiFBS (p>0.05). We found that DMEM:F12 supplemented with 15% hiFBS could serve as an optimized culture media for high proliferation rate of SH-SY5Y cells.

Establishing experimental models to study neuropathic pain has been challenging due to the complex mechanism underlying the condition. Although in vivo models have been useful in the observation of behavioural pain responses, it should be acknowledged that species-to-species variance can lead to differences in terms of molecular mechanism and genetic expression. The study of molecular and signal transduction of neuropathic pain using in vivo models faces limitations due to ethical considerations involving pain induction in animals and the intricacy of molecular interactions in the pathophysiology of the condition. Hence, developing relevant in vitro models to study neuropathic pain is important, as it considers the physiological microenvironment and reduces the use of experimental animals. Several considerations should be taken into account in developing an in vitro model of neuropathic pain, including the use of either primary culture of cell lines with considerations to their origins; human or animal, the method of neuropathic pain-like induction and the relevant assays to assess pain. This review recapitulates previous research employing in vitro models in investigating the molecular mechanism of neuropathic pain, intending to provide an alternative to the growing concerns on in vivo neuropathic pain models.