Toosendanin facilitates [3H]noradrenaline release from rat hippocampal slices

The anti-botulism triterpenoid toosendanin elicits calcium increase and exocytosis in rat sensory neurons

Toosendanin, a triterpenoid from Melia toosendan Sieb et Zucc, has been found before to be an effective anti-botulism agent, with a bi-phasic effect at both motor nerve endings and central synapse: an initial facilitation followed by prolonged depression. Initial facilitation may be due to activation of voltage-dependent calcium channels plus inhibition of potassium channels, but the depression is not fully understood. Toosendanin has no effect on intracellular calcium or secretion in the non-excitable pancreatic acinar cells, ruling out general toosendanin inhibition of exocytosis. In this study, toosendanin effects on sensory neurons isolated from rat nodose ganglia were investigated. It was found that toosendanin stimulated increases in cytosolic calcium and neuronal exocytosis dose dependently. Experiments with membrane potential indicator bis-(1,3-dibutylbarbituric acid)trimethine oxonol found that toosendanin hyperpolarized capsaicin-insensitive but depolarized capsaicin-sensitive neurons; high potassium-induced calcium increase was much smaller in hyperpolarizing neurons than in depolarizing neurons, whereas no difference was found for potassium-induced depolarization in these two types of neurons. In neurons showing spontaneous calcium oscillations, toosendanin increased the oscillatory amplitude but not frequency. Toosendanin-induced calcium increase was decreased in calcium-free buffer, by nifedipine, and by transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine. Simultaneous measurements of cytosolic and endoplasmic reticulum (ER) calcium showed an increase in cytosolic but a decrease in ER calcium, indicating that toosendanin triggered ER calcium release. These data together indicate that toosendanin modulates sensory neurons, but had opposite effects on membrane potential depending on the presence or absence of capsaicin receptor/TRPV 1 channel.

Biological effects of toosendanin, a triterpenoid extracted from Chinese traditional medicine

Toosendanin (TSN) is a triterpenoid extracted from Melia toosendan Sieb et Zucc, which was used as a digestive tract-parasiticide and agricultural insecticide in ancient China. TSN was demonstrated to be a selective presynaptic blocker and an effective antibotulismic agent. By interfering with neurotransmitter release through an initial facilitation followed by a subsequent depression, TSN eventually blocks synaptic transmission at both the neuro-muscular junction and central synapses. Despite sharing some similar actions with botulinum neurotoxin (BoNT), TSN has a marked antibotulismic effect in vivo and in vitro. Studies suggest that the antibotulismic effect of TSN is achieved by preventing BoNT from approaching its enzymatic substrate, the SNARE protein. It is also found that TSN can induce differentiation and apoptosis in several cell lines, and suppress proliferation of various human cancer cells. TSN inhibits various K(+)-channels, selectively facilitates Ca(2+)-influx via L-type Ca(2+) channels and increases intracellular Ca(2+) concentration ([Ca(2+)](i)). The TSN-induced [Ca(2+)](i) increase and overload could be responsible for the TSN-induced biphasic effect on transmitter release, cell differentiation, apoptosis as well as the cytoxicity of TSN.

The pre-synaptic blocker toosendanin does not inhibit secretion in exocrine cells

AIM

Toosendanin is a pre-synaptic blocker at the neuromuscular junction and its inhibitory effect is divided into an initial facilitative/stimulatory phase followed by a prolonged inhibitory phase. The present study investigated whether the subsequent inhibitory phase was due to exhaustion of the secretory machinery as a result of extensive stimulation during the initial facilitative phase. More specifically, this paper examined whether toosendanin could directly inhibit the secretory machinery in exocrine cells.

METHODS

Rat pancreatic acinar cells were isolated by collagenase digestion. Secretion was assessed by measuring the amount of amylase released into the extracellular medium as a percentage of the total present in the cells before stimulation. Cholecystokinin (CCK)-induced increases in intracellular calcium in single cells were measured with fura-2 microfluorometry.

RESULTS

Effects of toosendanin on CCK-induced amylase secretion and calcium oscillations were investigated. Toosendanin of 87-870 microM had no effect on 10 pM-100 nM CCK-stimulated amylase secretion, nor did 8.7-870 microM toosendanin inhibit 5 pM CCK-induced calcium oscillations. In contrast, 10 nM CCK(1) receptor antagonist FK 480 completely blocked 5 pM CCK-induced calcium oscillations.

CONCLUSION

The pre-synaptic "blocker" toosendanin is a selective activator of the voltage-dependent calcium channels, but does not interfere with the secretory machinery itself.

Effect of Toosendanin on acetylcholine level of rat brain, a microdialysis study

The effect of prejunctional blocker Toosendanin (TSN) on acetylcholine (ACh) level in striatum and parietal cortex was investigated by means of method of brain dialysis coupled with microbore high-performance liquid chromatograph (HPLC)-electrochemical detection (ECD) with enzyme immobilized post-column at freely moving rats. The results are as follows: (1) TSN inhibits high K+ (100 mM)-induced ACh release from the mentioned two regions of brain, completely and irreversibly. (2) Before the inhibition, there appears a transient rise of ACh level. (3) This rise of ACh level can be blocked by TTX. (4) Calcium is necessary for the onset of this rise, but not for that of the sustained decrease of ACh level. These results indicate that the effect of TSN on central cholinergic synapses is similar to that of neuromuscular junction, namely, its blocking action is always preceded by a Ca(2+)-dependent facilitatory phase with result of a TSN-induced transient rise of ACh, but the sustained decline and final blockade of ACh release are Ca(2+)-independent.

Inhibition of Toosendanin on the delayed rectifier potassium current in neuroblastoma x glioma NG108-15 cells

The effect of Toosendanin (TSN), a presynaptic transmission blocker, on the outward delayed rectifier potassium current (IKD) of NG108-15 cells was studied by using the whole-cell voltage-clamp technique. It was observed that externally applying TSN not only reduced IKD amplitude in a dose-dependent and partial reversible manner but also accelerated its inactivation. The effect of internally applying TSN was also examined by including TSN in the electrode, and it was the same as that of externally applying TSN. Further, comparison observations with TEA, 4-AP, verapamil, nifedipine, and (+/-)-Bay K 8644 were also made, and the results were as follows. The time courses of TSN's inhibition effect as well as its recovery after washing were much slower than those of TEA and 4-AP. Externally applying TEA or 4-AP reduced IKD amplitude but did not accelerate its inactivation. Externally applying verapamil, nifedipine, or (+/-)-Bay K 8644, however, similarly to the effect of TSN, not only reduced IKD amplitude but also accelerated its inactivation. Thus, from the obtained results it is suggested that TSN might diffuse into the cell interior and act intracellularly, and the underlying mechanism might be different from that of TEA and 4-AP but similar to that of verapamil, nifedipine, and (+/-)-Bay K 8644 to some extent.