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

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.