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-alpha motoneuron synapse in the spinal cord and mediate the action via N-methyl-D-aspartate (NMDA) and a-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 microg/ml) depressed the spinal reflexes in a time-dependent manner. The maximum depression of MSR(approximately 66%) was seen at 10 min and it was 25 min for PSR (approximately 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 microM), 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.

Involvement of NO-guanylyl cyclase pathway for the depression of spinal monosynaptic reflex by Mesobuthus tamulus venom in neonatal rat in vitro.

AIMS: The present study was undertaken to evaluate the role of nitric oxide (NO) in Mesobuthus tamulus (MBT) venom-induced depression of spinal reflexes. MAIN METHODS: Experiments were performed on isolated hemisected spinal cords from 4 to 6day old rats. Stimulation of a dorsal root with supramaximal strength evoked monosynaptic (MSR) and polysynaptic reflex (PSR) potentials in the corresponding segmental ventral root. KEY FINDINGS: Superfusion of MBT venom (0.3µg/ml) depressed the spinal reflexes in a time-dependent manner and the maximum depression was seen at 10min (MSR by 63%; PSR by 79%). The time to produce 50% depression (T-50) of MSR and PSR was 7.7±1.3 and 5.7±0.5min, respectively. Pretreatment with bicuculline (1µM; GABA(A) receptor antagonist) or strychnine (1µM; glycine(A) receptor antagonist) did not block the venom-induced depression of spinal reflexes. However, Nω-nitro-L-arginine methyl ester (L-NAME, 100 or 300µM; NO synthase inhibitor) or hemoglobin (Hb, 100µM; NO scavenger) antagonized the venom-induced depression of MSR. Further, soluble guanylyl cylase inhibitors (1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one, ODQ; 1µM or methylene blue, 100µM) also antagonized the venom-induced depression of MSR but not PSR. Nitrite concentration (indicator of NO activity) of the cords exposed to venom (0.3µg/ml) was not different from the control group. SIGNIFICANCE: The results indicate that venom-induced depression of MSR is mediated via NO-guanylyl cyclase pathway without involving GABAergic or glycinergic system.

Involvement of 5-hydroxytryptaminergic transmission for the Mesobuthus tamulus venom-induced depression of spinal reflexes in neonatal rat in vitro.

Mesobuthus tamulus (MBT) venom is shown to depress the spinal reflexes through a mechanism unrelated to the NMDA receptors. 5-Hydroxytryptamine (5-HT) is another excitatory transmitter in the spinal cord therefore, the present study was undertaken to examine the involvement of 5-HT in the venom-induced depression of reflexes. The experiments were performed on isolated hemisected spinal cords from 4 to 6-day-old rats. Stimulation of a dorsal root with supramaximal strength evoked monosynaptic reflex (MSR) and polysynaptic reflex (PSR) potentials in the corresponding segmental ventral root. MBT venom (0.3 microg/ml) depressed the spinal reflexes in a time-dependent manner and the maximal depression was seen at 10 min. The time to produce 50% depression (T-50) of MSR and PSR was 8.1+/-1.41 and 6.8+/-0.5 min, respectively. Pretreatment with pindolol (1 microM; 5-HT(1A/1B) receptor antagonist) blocked the reflexes up to 15 min. On the other hand, ketanserin (10 microM; 5-HT(2A/2C) receptor antagonist) or ondansetron (0.1 microM; 5-HT(3) receptor antagonist) blocked the venom-induced depression of MSR and PSR during entire exposure time (30 min). The 5-HT concentration of the cords exposed to venom (1.6+/-0.04 microg/g tissue) was significantly greater than the control group (0.98+/-0.08 microg/g tissue). The results indicate that venom-induced depression of spinal reflexes is mediated via 5-HTergic transmission involving 5-HT(1A/1B), 5-HT(2A/2C) and 5-HT(3) receptors.

Indian red scorpion venom depresses spinal synaptic transmission without involving NMDA receptors.

Stings of Indian red scorpion (Mesobuthus tamulus, MBT) produce neurological abnormalities such as convulsions and paralysis. These parameters indicate the activity at alpha-motoneuron. The present study was therefore, undertaken to evaluate the effect of MBT-venom on spinal reflexes and the involvement of N-methyl-d-aspartate (NMDA) receptors. The experiments were performed on isolated hemisected spinal cords from 4 to 6 days old rats. Stimulation of a dorsal root with supramaximal strength at 0.1Hz evoked monosynaptic (MSR) and polysynaptic reflex (PSR) potentials in the corresponding segmental ventral root. Superfusion of MBT-venom depressed the spinal reflexes in a time- and a concentration-dependent (0.1-1microg/ml) manner. MBT-venom at 0.1, 0.3 and 1.0microg/ml produced maximal depression of 55, 75 and 90% at 30, 10 and 7min, respectively. The time required to produce 50% depression (T-50) of MSR was 19.0, 8.0, and 3.6min and for PSR was 15.0, 5.6, and 2.9min at 0.1, 0.3 and 1microg/ml of venom, respectively. Pre-treatment with DL-alpha-2-amino-5-phosphonovaleric acid (APV) decreased MSR by 26% and abolished PSR. In the presence of APV, the MBT-venom-induced depression of MSR was not different from the venom only group. The results indicate that venom-induced depression of spinal reflexes did not involve NMDA receptors.

5-HT-induced depression of the spinal monosynaptic reflex potential utilizes different types of 5-HT receptors depending on Mg2+ availability.

Receptor subtypes involved in the 5-hydroxytryptamine (5-HT)-induced depression of synaptic transmission in neonatal rat spinal cords in vitro were evaluated in the absence or presence of Mg(2+) in the medium. Stimulation of a dorsal root evoked monosynaptic reflex potential (MSP) and polysynaptic reflex potential (PSP) in the segmental ventral root in Mg(2+)-free medium where the voltage-dependent blockade of NMDA receptors is absent. The 5-HT (0.3-50 microM) in the Mg(2+)-free medium depressed the MSP and PSP in a concentration-dependent manner. At 30 microM of 5-HT, the depression was 57% and 95% for MSP and PSP, respectively, and no further depression was seen at 50 microM. The 5-HT-induced depression of the reflexes in the Mg(2+)-free medium was blocked by ondansetron (5-HT(3) receptor antagonist), but not by spiperone (5-HT(2A/2C) antagonist). In the Mg(2+)-free medium, phenylbiguanide (5-HT(3) agonist) also depressed the MSP and PSP in a concentration-dependent manner and was blocked by ondansetron. Addition of Mg(2+) (1.3 mM) to the medium abolished the PSP and decreased the MSP by 30%. In the presence of Mg(2+), 5-HT (1-50 microM) also depressed the MSP in a concentration-dependent manner. At 10 microM of 5-HT, there was approximately 20% depression and at 50 microM the depression was 100%. The 5-HT-induced depression of MSP in the Mg(2+)-containing medium was antagonized by spiperone (p < 0.05, two-way ANOVA), but not by ondansetron. The results indicate that the 5-HT-induced depression of MSP involves 5-HT(3) receptors in the Mg(2+)-free medium and 5-HT(2A/2C) in the presence of Mg(2+) when NMDA receptors are in the closed state.