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 μg/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 μM; 5-HT1A/1B receptor antagonist) blocked the reflexes up to 15 min. On the other hand, ketanserin (10 μM; 5-HT2A/2C receptor antagonist) or ondansetron (0.1 μM; 5-HT3 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 μg/g tissue) was significantly greater than the control group (0.98 ± 0.08 μg/g tissue). The results indicate that venom-induced depression of spinal reflexes is mediated via 5-HTergic transmission involving 5-HT1A/1B, 5-HT2A/2C and 5-HT3 receptors.