Docking, characterization and investigation of β-cyclodextrin complexed with citronellal, a monoterpene present in the essential oil of Cymbopogon species, as an anti-hyperalgesic agent in chronic muscle pain model

Citronellal (CT) is a monoterpene with antinociceptive acute effect. β-Cyclodextrin (βCD) has enhanced the analgesic effect of various substances.To evaluate the effect of CT both complexed in β-cyclodextrin (CT-βCD) and non-complexed, in a chronic muscle pain model (CMP) in mice.The complex containing CT in βCD was obtained and characterized in the laboratory. The anti-hyperalgesic effect of CT and CT-βCD was evaluated in a pre-clinical in vivo study in a murine CMP.The complex was characterized through differential scanning calorimetry, derivative thermogravimetry, moisture determination, infrared spectroscopy and scanning electron microscopy. Male Swiss mice were pre-treated with CT (50mg/kg, po), CT-βCD (50mg/kg, po), vehicle (isotonic saline, po) or standard drug (tramadol4 mg/kg, ip). 60 min after the treatment and then each 1h, the mechanic hyperalgesia was evaluated to obtain the time effect. In addition, the muscle strength using grip strength meter and hyperalgesia were also performed daily, for 7 days. We assessed by immunofluorescence for Fos protein on brains and spinal cords of mice. The involvement of the CT with the glutamatergic system was studied with molecular docking.All characterization methods showed the CT-βCD complexation. CT-induced anti-hyperalgesic effect lasted until 6h (p0.001) while CT-βCD lasted until 8h (p0.001vs vehicle and p0.001vs CT from the 6th h). CT-βCD reduced mechanical hyperalgesia on all days of treatment (p0.05), without changing muscle strength. Periaqueductal gray (p0.01) and rostroventromedular area (p0.05) showed significant increase in the Fos protein expression while in the spinal cord, there was a reduction (p0.001). CT showed favorable energy binding (-5.6 and -6.1) to GluR2-S1S2J protein based in the docking score function.We can suggest that βCD improved the anti-hyperalgesic effect of CT, and that effect seems to involve the descending pain-inhibitory mechanisms, with a possible interaction of the glutamate receptors, which are considered as promising molecules for the management of chronic pain such as CMP.