Effects of bufalin and related cardiotonic steroids in the neuromuscular junction

Assessment of neuropharmacological potential of low molecular weight components extracted from Rhinella schneideri toad poison

BACKGROUND

Studies on toad poison are relevant since they are considered a good source of toxins that act on different biological systems. Among the molecules found in the toad poison, it can be highlighted the cardiotonic heterosides, which have a known mechanism that inhibit Na+/K+-ATPase enzyme. However, these poisons have many other molecules that may have important biological actions. Therefore, this work evaluated the action of the low molecular weight components from Rhinella schneideri toad poison on Na+/K+-ATPase and their anticonvulsive and / or neurotoxic effects, in order to detect molecules with actions of biotechnological interest.

METHODS

Rhinella schneideri toad (male and female) poison was collected by pressuring their parotoid glands and immediately dried and stored at -20 °C. The poison was dialysed and the water containing the low molecular mass molecules (< 8 kDa) that permeate the dialysis membrane was collected, frozen and lyophilized, resulting in the sample used in the assays, named low molecular weight fraction (LMWF). Na+/K+ ATPase was isolated from rabbit kidneys and enzyme activity assays performed by the quantification of phosphate released due to enzyme activity in the presence of LMWF (1.0; 10; 50 and 100 µg/mL) from Rhinella schneideri poison. Evaluation of the L-Glutamate (L-Glu) excitatory amino acid uptake in brain-cortical synaptosomes of Wistar rats was performed using [3H]L-glutamate and different concentration of LMWF (10-5 to 10 µg/µL). Anticonvulsant assays were performed using pentylenetetrazole (PTZ) and N-methyl-D-aspartate (NMDA) to induce seizures in Wistar rats (n= 6), which were cannulated in the lateral ventricle and treated with different concentration of LMWF (0.25; 0.5; 1.0; 2.0; 3.0 and 4.0 µg/µL) 15 min prior to the injection of the seizure agent.

RESULTS

LMWF induced a concentration-dependent inhibition of Na+/K+-ATPase (IC50% = 107.5 μg/mL). The poison induces an increased uptake of the amino acid L-glutamate in brain-cortical synaptosomes of Wistar rats. This increase in the L-glutamate uptake was observed mainly at the lowest concentrations tested (10-5 to 10-2 µg/µL). In addition, this fraction showed a very relevant central neuroprotection on seizures induced by PTZ and NMDA.

CONCLUSIONS

LMWF from Rhinella schneideri poison has low molecular weight compounds, which were able to inhibit Na+/K+-ATPase activity, increase the L-glutamate uptake and reduced seizures induced by PTZ and NMDA. These results showed that LMWF is a rich source of components with biological functions of high medical and scientific interest.

Entomotoxic activity of Rhinella icterica (Spix, 1824) toad skin secretion in Nauphoeta cinerea cockroaches: An octopamine-like modulation

Rhinella icterica is a poisonous toad whose toxic secretion has never been studied against entomotoxic potential. Sublethal doses of Rhinella icterica toxic secretion (RITS) were assayed in Nauphoeta cinerea cockroaches, in order to understand the physiological and behavioral parameters, over the insect central and peripheral nervous system. RITS (10 μg/g) injections, induced behavioral impairment as evidenced by a significant decrease (38 ± 14%) in the distance traveled (p < .05), followed by an increase (90 ± 6%) of immobile episodes (p < .001, n = 28, respectively). In cockroaches semi-isolated heart preparations, RITS (16 μg/200 μl) induced a significant irreversible dose-dependent negative chronotropism, reaching ~40% decrease in heart rate in 20 min incubation. In in vivo cockroach neuromuscular preparations, RITS (20, 50 and 100 μg/g of animal weight) induced a time-dependent inhibition of twitch tension that was complete for 20 μg/g, in 120 min recordings. RITS (10 μg/g) also induced a significant increase in the insect leg grooming activity (128 ± 10%, n = 29, p < .01), but not in the antennae counterparts. The RITS increase in leg grooming activity was prevented in 90% by the pretreatment of cockroaches with phentolamine (0.1 μg/g). The electrophysiological recordings of spontaneous neural compound action potentials showed that RITS (20 μg/g) induced a significant increase in the number of events, as well as in the rise time and duration of the potentials. In conclusion, RITS showed to be entomotoxic, being the neuromuscular failure and cardiotoxic activity considered the main deleterious effects. The disturbance of the cockroaches' behavior together with the electrophysiological alterations, may unveil the presence of some toxic components present in the poison with inherent biotechnological potentials.

The T-cell suppressive effect of bufadienolides: structural requirements for their immunoregulatory activity

Many studies indicate that substances similar to cardenolides and bufadienolides naturally occur in mammals. The majority of previous studies focused on their cardiovascular, renal, and central nervous action. We analyzed the immunoregulatory property of 52 bufadienolides. Human T-cells were stimulated "in vitro" with mitogens or alloantigens in the presence of bufadienolides. The most active compound totally inhibited T-cell activity at a concentration of 0.75 pmol/10(5) cells. This effect is 16,384 x stronger than that of cortisol and 256 x stronger than that of cyclosporin A or tacrolimus. Preactivated T cells were downregulated and, most importantly, suppressed viable T cells could not be restimulated. Lack of the 17 beta-lactone ring dramatically reduced the activity of bufadienolides. Substitution at C3 also affected their function: components with a 3-OH group were up to 1000 x stronger than those without. The replacement of 14 beta-OH with an epoxy-group slightly decreased the activity. Because there is evidence that the latter change abolishes the cardiac activity, this finding is relevant for therapeutic applications in which immunosuppression without the risk of cardiotoxicity is attempted. One of the substances analyzed in this study was Proscillaridin A. A similar bufadienolide occurs naturally in mammals. We speculate that bufadienolides represent an important bioregulatory link between the cardiovascular, nervous and immune systems.