Atividade entomotóxica do veneno do sapo Rhinella schneideri em insetos da ordem Blattodea

Abstract

Rhinella schneideri is a common toad found in South America, whose insecticidal potential of its parotid toxic secretion has never been explored. In order to evaluate its insecticidal potential, cockroaches of the species Nauphoeta cinerea and Periplaneta americana were used as experimental organisms in biochemical, physiological, behavioral and electrophysiological experiments. Lethality tests with the toxic secretion of Rhinella schneideri (RSPS) determined the LD50 value after 24h (58.07µg/g) and after 48h (44.07µg/g) (R2=0.882 and 0.954, respectively). Acetylcholinesterase (AChE) activity after the highest dose of RSPS promoted enzymatic inhibition of 40%, a similar effect was observed with the administration of neostigmine (p < 0.001, n=5). Locomotor tests revealed that RSPS caused a decrease in the distance travelled by 37% with a concomitant increase of 85% in the immobility episodes (p < 0.001, n=36). The addition of RSPS in the semi-isolated heart in vivo preparation promoted an irreversible and dose-dependent heart rate decrease, leading to a complete stop after 30min of recording (p < 0.001, n ≥ 6). In addition, RSPS induced dose-dependent neuromuscular blockade in nerve-muscle preparation, reaching total blockade at 70min for the highest applied dose (p < 0.001, n ≥ 6). The effect of RSPS on spontaneous sensory action potentials was characterized by a 61% increase in the number of action potentials (p < 0.01). Meanwhile, a 42% decrease in the area of these potentials was observed (p < 0.05, n ≥ 6). Extracellular electrophysiological tests did not reveal presynaptic alteration in the presence of RSPS, however, an 86.07% postsynaptic decrease in nerve activity was observed. In intracellular experiments carried out with unpaired dorsal median neurons (DUM) it was observed that the RSPS caused a brief increase in the frequency of action potentials, which was soon reverted to a decrease that approached zero. Factors such as amplitude and membrane potential were not altered by RSPS in DUM-type neurons. Assays to assess intracellular calcium release in DUM-type neurons demonstrate that RSPS caused an increase of about 300% in intracellular calcium levels. The results obtained highlight the relevance of RSPS as a potential insecticide and its potential for biotechnological application.