Involvement of AMPA receptors for Mesobuthus tamulus Pocock venom-induced depression of monosynaptic reflex in neonatal rat spinal cord in vitro.

Glutamate is a putative neurotransmitter at Ia-α motoneuron synapse in the spinal cord and mediate the action via N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. Since NMDA receptors are not involved in M. tamulus Pocock (MBT) venom-induced depression of spinal monosynaptic reflex (MSR), the present study was undertaken to evaluate the role of AMPA receptors in mediating the depression of MSR by MBT venom. The experiments were performed on isolated hemisected spinal cord from 4-6 day old rats. Stimulation of a dorsal root with supramaximal voltage evoked MSR and polysynaptic reflex (PSR) potentials in the corresponding segmental ventral root. Superfusion of MBT venom (0.3 µg/ml) depressed the spinal reflexes in a time-dependent manner. The maximum depression of MSR(~ 66%) was seen at 10 min  and it was 25 min for PSR (~ 75%). The time to produce 50% depression of MSR and PSR was 6.7 ± 1.5 and 10.8 ± 2.6 min, respectively. Pretreatment of the cords with 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, 0.1 μM), an AMPA receptor antagonist, blocked the venom-induced depression of MSR but not PSR. The results indicate that venom-induced depression of MSR is mediated via AMPA receptors.

Influence of NMDA and non-NMDA antagonists on acute and inflammatory pain in the trigeminal territory: a placebo control study / Influência dos antagonistas dos receptores NMDA e não-NMDA sobre a dor aguda e inflamatória ao nível do território do trigêmeo: um estudo placebo controlado

NMDA and non-NMDA receptors are involved in spinal transmission of nociceptive information in physiological and pathological conditions. Our objective was to study the influence of NMDA and non-NMDA receptor antagonists on pain control in the trigeminal system using a formalin-induced orofacial pain model. Motor performance was also evaluated. Male Rattus norvegicus were pre-treated with topiramate (T) (n=8), memantine (M) (n=8), divalproex (D) (n=8) or isotonic saline solution (ISS) (n=10) intraperitoneally 30 minutes before the formalin test. Formalin 2.5 percent was injected into the right upper lip (V2 branch) and induced two phases: phase I (early or neurogenic) (0-3 min) and phase II (late or inflammatory) (12-30 min). For motor behavior performance we used the open-field test and measured latency to movement onset, locomotion and rearing frequencies, and immobility time. Pre-treatment of animals with M and D only attenuated nociceptive formalin behavior for phase II. T increased locomotion and rearing frequencies and reduced immobility time. Treatment with M increased immobility time and with D reduced locomotion frequency. Our results showed that the NMDA antagonist (M) is more potent than the non-NMDA antagonists (D and T) in the control of pain in the inflammatory phase. The non-NMDA topiramate improved motor performance more than did D and M, probably because T has more anxiolytic properties.

Receptores NMDA e não-NMDA estão envolvidos na transmissão das informações nociceptivas em condições fisiológicas e patológicas. Com o objetivo de estudar a influência dos antagonistas dos receptores NMDA e não-NMDA sobre o controle de dor no sistema trigeminal utilizamos modelo de dor orofacial induzida pela formalina. Testes de desempenho motor foram também avaliados. Ratos machos da espécie Rattus norvegicus foram tratados com topiramato (T) (n=8), memantina (M) (n=8), divalproato de sódio (D) (n=8) ou solução salina isotônica (SSI) (n=10), por via intraperitoneal, 30 minutos antes dos testes com a formalina. Formalina 2.5 por cento foram injetadas na região do lábio superior dos animais (segundo ramo do trigêmeo) induzindo comportamento em duas fases distintas: fase I (precoce ou neurogênica) (0-3 min ) e fase II (tardia ou inflamatória) (12-30 min). Para avaliação da atividade motora utilizou-se o teste do campo aberto mensurando-se a latência para o início dos movimentos, número de casas andadas, freqüência de levantamentos e tempo de imobilidade. Animais pré-tratados com M e D atenuaram a fase inflamatória do teste da formalina. O T aumenta o número de casas andadas, freqüência de levantamentos e reduz o tempo de imobilidade. Nossos resultados mostram que o antagonista NMDA é mais potente do que os antagonistas não-NMDA para o controle da fase inflamatória da dor. O topiramato entretanto aumenta a atividade motora provavelmente porque apresente propriedades ansiolíticas.

N-methyl-D-aspartate (NMDA) and Non-NMDA Receptors are Involved in the Production and Maintenance of Nociceptive Responses by Intraplantar Injection of Bee Venom and Melittin in the Rat

Whole bee venom (WBV) and its major component, melittin, have been reported to induce long-lasting spontaneous flinchings and hyperalgesia. The current study was designed to elucidate the peripheral and spinal mechanisms of N-methyl-D-aspartate (NMDA) and non-NMDA receptors by which intraplantar (i.pl.) injection of WBV and melittin induced nociceptive responses. Changes in mechanical threshold and flinching behaviors were measured after the injection of WBV (0.04 mg or 0.1 mg/paw) and melittin (0.02 mg or 0.05 mg/paw) into the mid-plantar area of a rat hindpaw. MK-801 and CNQX (6-cyano-7-nitroquinoxaline-2, 3-dione disodium) were administered intrathecally (i.t. 10microgram) or i.pl. (15microgram) 15 min before or i.t. 60 min after i.pl. WBV and melittin injection. Intrathecal pre- and post- administration of MK-801 and CNQX significantly attenuated the ability of high dose WBV and melittin to reduce paw withdrawal threshold (PWT). In the rat injected with low dose, but not high dose, of WBV and melittin, i.pl. injection of MK-801 effectively suppressed the decrease of PWTs only at the later time-points, but the inhibitory effect of CNQX (i.pl.) was significant at all time-point after the injection of low dose melittin. High dose WBV- and melittin-induced spontaneous flinchings were significantly suppressed by i.t. administration of MK-801 and CNQX, and low dose WBV- and melittin-induced flinchings were significantly reduced only by intraplantarly administered CNQX, but not by MK-801. These experimental flinchings suggest that spinal, and partial peripheral mechanisms of NMDA and non-NMDA receptors are involved in the development and maintenance of WBV- and melittin-induced nociceptive responses.

The Role of NMDA and non-NMDA Receptors for cFos Protein Expression on the Spinal Cord in Formalin Pain Model / 대한정형외과연구학회지

The purpose of this study was to evaluate effect of NMDA and non-NMDA receptor antagonists on pain-related behavioral change and cFos-like protein expression on the spinal dorsal horn neurons following subcutaneous injection of diluted formalin into one paw. Depending on the drug pretreatment paradigm, animals were divided into four groups; the saline treated, the MK801 treated, CNQX treated, and both MK801 and CNQX treated. Each group was consisted of fifteen rats. After single injection of formalin, all animals exhibited signs of phasic pain(first pain) within 15 minutes, which was followed by a long lasting tonic pain(second pain), thereafter. MK801 pretreatment significantly reduced only the tonic pain. On the other hand, the intrathecal administration of CNQX significantly inhibited only the first phasic pain behavior. An injection of formalin into one hindpaw elicited cFos-like protein expression on the ipsilateral dorsal horn neurons from T12 to S1 spinal cord. The cFos-like protein expression was especially strong in the L3-5 segments and sustained more than 8 hours after the formalin injection. The cFos-like protein expression was peaked two after the formalin injection in superficial layer (laminar III-VII). The MK801 pretreatment reduced the number of cFos-like protein positive neurons significantly one hour after the formalin injection in the superficial dorsal horn. The reduction was observed in the superficial as well as deep layers four hours after the injection. The pretreatment of MK801+CNQX reduced significantly cFos-like protein expression on the both superficial and deep layers of the ipsilateral dorsal horn throughout the experimental session. However, CNQX application did not produce any significant reduction of cFos-like protein expression on the dorsal horn neurons. These results suggest that NMDA receptor may play an more important role in cFos-like protein expression of ipsilateral dorsal horn neurons on the formalin evoked pain model of the rat.