Oral administration of kratom leaf extract alleviates anxiety-like behavior, urinary bladder pain, voiding dysfunction, and bladder hypercontractility via attenuating muscarinic receptor response in male mice exposed to chronic water avoidance stress

Psychological stress causes and deteriorates interstitial cystitis/painful bladder syndrome with urinary frequency, incontinence, bladder pain and urgency. The major alkaloid of kratom (Mitragyna speciosa), mitragynine, shows analgesic, anxiolytic, and smooth muscle relaxant effects. However, the effects of kratom leaf extract on stress-induced anxiety-like behavior, urinary bladder pain and urinary bladder dysfunction remain unknown. Therefore, this study aims to examine the effect of kratom leaf extract administration on anxiety-like behaviors, bladder pain, bladder contractile properties, and mast cell number in mice exposed to water avoidance stress. Male C57BL/6 mice were exposed to water avoidance stress (WAS) protocol for 10 consecutive days and compared with the stress-exposed mice receiving oral administration of kratom leaf extract (2.5 and 5 mg/kg of mitragynine) or solifenacin (10 mg/kg). Anxiety-like behaviors were assessed using open field test. Bladder pain sensitivity was evaluated with von Frey test, while voiding behavior was analyzed using voiding pattern analysis. Bladder contractility was examined using an in vitro organ bath technique, and urinary bladder mast cell infiltration was assessed by toluidine blue staining. Results show that mice receiving WAS had a reduction in the total duration and number of unsupported rearing behaviors, reduced voiding area, and increased bladder pain responses; however, these effects were reversed by treatment with kratom leaf extract (2.5 and 5 mg/kg of mitragynine). Interestingly, the WAS group also exhibited markedly increased tonic contractions in response to carbachol, a muscarinic agonist; these responses were attenuated in mice treated with kratom leaf extract (2.5 and 5 mg/kg) The enhanced tonic contractile response to carbachol was abolished by pre-incubation with ondansetron (a 5-HT₃ antagonist). The WAS group showed an increased total number of mast cells in the urinary bladder, which was reduced by treatment with kratom leaf extract at both 2.5 and 5 mg/kg. Our results indicate that treatment with kratom leaf extract attenuated chronic stress–induced bladder pain responses, voiding abnormalities, and mast cell numbers, and was associated with reduced contractile response to muscarinic stimulation, suggesting a potential modulatory effect on stress-induced bladder dysfunction.