Effects of pregabalin and duloxetine on neurotransmitters in the dorsal horn of the spinal cord in a rat model of fibromyalgia.

Dysfunction of the monoamine systems in the nervous system is associated with the clinical symptoms of fibromyalgia. Reserpine-induced myalgia (RIM) rats are a putative model of fibromyalgia in which muscle pressure thresholds and monoamine content is reduced in the brain and spinal cord. We examined the effects of pregabalin and duloxetine, drugs approved for fibromyalgia treatment, on the levels of extracellular neurotransmitters in the dorsal horn of the spinal cord in RIM rats using microdialysis. Male SD rats were used for all experiments. To generate RIM rats, reserpine was injected at 1 mg/kg subcutaneously once daily for three consecutive days. The pressure threshold of the mid-gastrocnemius muscle was measured using a Randall-Selitto apparatus. Norepinephrine, dopamine, and serotonin were detected using high-performance liquid chromatography with electrochemical detection, and glutamate and γ-aminobutyric acid (GABA) were detected using liquid chromatography-mass spectrometry. The muscle pressure threshold in RIM rats was significantly lower than that in normal rats. While the levels of monoamines and glutamate were lower in the spinal cord of RIM rats than in normal rats, levels of GABA did not markedly differ. Duloxetine increased the levels of all three monoamines in normal and RIM rats in a dose-dependent manner. In contrast, pregabalin only increased norepinephrine levels in RIM rats. These results indicate that while both pregabalin and duloxetine ameliorate muscle pressure thresholds in RIM rats, their effects on the levels of extracellular neurotransmitters in the spinal cord differ considerably.

An automated method by which effects of compounds on locomotor activity and spontaneous neuropathic pain-specific movements can be simultaneously evaluated in rats with chronic-constriction nerve injury.

Neuropathic pain patients are characterized by evoked pain (hyperalgesia and allodynia) and spontaneous pain, the latter of which is the predominant symptom. In animal models of neuropathic pain, effects of test compounds on spontaneous pain-related behaviors have been evaluated by direct visual observation. In addition, by performing another locomotor activity experiment in normal animals, it is also indispensable to examine whether test compounds cause motor impairment to avoid overestimation of their analgesic activities. In the present study, we developed spontaneous pain-specific and automated evaluation method by improving a previous method for measuring movements of the injured hind limb in unilateral chronic constriction nerve injury (CCI) rats. Rats with unilateral CCI were implanted with strong and weak magnets in each hind limb, respectively. Limb movements were automatically detected as spiked waveforms in electromagnetic field analyzing system. Movements in each limb were analyzed separately according to differences in their respective wave amplitudes, and aberrant movements of injured limb were specifically detected on basis of the asymmetry between injured and uninjured limb movements. Consequently, the incidence ratio of spontaneous pain in injured limb, which was not affected by individual locomotive activities, was able to be obtained as a new evaluation index. The incidence ratio of spontaneous pain revealed substantial difference between CCI and sham rats with only a small variation in the value. Further, according to the frequency of movement of the uninjured limb, it could be determined simultaneously whether a test compound causes sedative effect or not, resulting in eliminating the need for another experiment in normal animals.

Effects of 2-[N-(4-chlorophenyl)-N-methylamino]-4H-pyrido[3.2-e]-1,3-thiazin-4-one (YM928), an orally active alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist, in models of generalized epileptic seizure in mice and rats.

The anticonvulsant activity of 2-[N-(4-chlorophenyl)-N-methylamino]-4H-pyrido[3.2-e]-1,3-thiazin-4-one (YM928), a novel alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, was studied in animal models of generalized seizure. YM928 exerted significant anticonvulsant effects in the maximal electroshock (MES) seizure test (ED50 = 7.4 mg/kg p.o.), pentylenetetrazol (PTZ)-induced seizure test (ED50 = 9.6 mg/kg p.o.), AMPA-induced seizure test (ED50 = 5.5 mg/kg p.o.), and strychnine-induced seizure test (ED50 = 14.0 mg/kg p.o.) in mice. Effects in rats were detected in the MES seizure test (ED50 = 4.0 mg/kg p.o.) and PTZ-induced seizure test (ED50 = 6.2 mg/kg p.o.). The profile of YM928 was compared with that of established antiepileptics. Valproate showed beneficial effects in all tests used. In contrast, carbamazepine, phenytoin, lamotrigine, phenobarbital, diazepam, ethosuximide, and gabapentin were not active against seizures induced by at least one stimulant. In the rotarod test, YM928 impaired motor coordination (TD50 = 22.5 mg/kg p.o.). The protective index (TD50 value of the rotarod test/ED50 value of MES seizure) was 3.0, suggesting that YM928 can exert antiepileptic effects with only minor motor disturbances. YM928 at doses of 2, 4, and 8 mg/kg p.o. did not significantly affect the threshold of electroshock seizure in rats after 16 days of repeated administration. These data indicate that YM928 does not induce tolerance after subchronic administration. These results indicate that YM928 is a broad-spectrum anticonvulsant that would prove useful for the treatment of generalized seizure in human epileptic patients.