Aim: The selective serotonin (5-HT) and norepinephrine (NE) reuptake inhibitors (SNRIs) are commonly used for the treatment of
neuropathic pain and fibromyalgia. Ammoxetine ((±)-3-(benzo[d] [1,3]dioxol-4-yloxy)-N-methyl-3-(thiophen-2-yl)propan-1-amine) has
been identified as a novel potent SNRI. In this study, we evaluated the acute analgesic properties of ammoxetine in different animal
models of pain, and examined the involvement of monoamines in its analgesic actions.
Methods: The analgesic effects of ammoxetine were assayed using models of acetic acid- and formalin-induced pain in mice,
neuropathic pain induced by sciatic nerve injury (SNI), chronic constriction injury (CCI) and reserpine-induced fibromyalgia pain in rats.
The contents of 5-HT and NE in brain regions of fibromyalgia rats were measured using HPLC-ECD. In all the experiments, duloxetine
was used as a positive control drug.
Results: Oral administration of ammoxetine (0.625–10 mg/kg) or duloxetine (2.5–40 mg/kg) dose-dependently decreased the number
of acetic acid-induced writhing and formalin-induced first phase and second phase paw licking time in mice. Oral administration of
ammoxetine (2.5–10 mg/kg) or duloxetine (10 mg/kg) alleviated mechanical allodynia in SNI and CCI rats and thermal hyperalgesia
in CCI rats. The antiallodynic effect of ammoxetine in CCI rats was abolished by pretreatment with para-chlorophenylalanine methyl
ester hydrochloride (PCPA, a 5-HT synthesis inhibitor) or α-methyl-para-tyrosine methylester (AMPT, a catecholamine synthesis
inhibitor). Oral administration of ammoxetine (30 mg/kg) or duloxetine (50 mg/kg) significantly attenuated tactile allodynia in rats with
reserpine-induced fibromyalgia. In the fibromyalgia rats, administration of ammoxetine (10, 30 mg/kg) or duloxetine (30, 50 mg/kg)
dose-dependently increased the levels of 5-HT and NE, and decreased the metabolite ratio of 5-HT (5-HIAA/5-HT) in the spinal cord,
hypothalamus, thalamus and prefrontal cortex.
Conclusion: Ammoxetine effectively alleviates inflammatory, continuous, neuropathic and fibromyalgia-related pain in animal models,
which can be attributed to enhanced neurotransmission of 5-HT and NE in the descending inhibitory systems.