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 ±â€¯14%) in the distance traveled (p < .05), followed by an increase (90 ±â€¯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 ±â€¯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.

Central and peripheral neurotoxicity induced by the Jack Bean Urease (JBU) in Nauphoeta cinerea cockroaches.

BACKGROUND: Ureases of Canavalia ensiformis are natural insecticides with a still elusive entomotoxic mode of action. We have investigated the mechanisms involved in the neurotoxicity induced by Jack Bean Urease (JBU) in Nauphoeta cinerea (Olivier). METHODS: To carry out this study we have employed biochemical and neurophysiological analysis of different cockroach organ systems. RESULTS AND CONCLUSIONS: The injection of the insects with JBU (0.75-6µg/g animal), although not lethal within 24h, caused significant inhibition of the brain acetylcholinesterase activity (60±5%, p<0.05, n=6). JBU (1.5µg/200µL), acetylcholine (0.3µg/200µL) or neostigmine (0.22µg/200µL), induced a positive cardiac chronotropism (∼25%) in the cockroaches (p<0.05, n=9). JBU (6µg/g) increased the insects' grooming activity (137±7%), similarly to octopamine (15µg/g) (p<0.05, n=30, respectively). Pretreating the insects with phentolamine (0.1µg/g) prevented the JBU- or octopamine-induced increase of grooming activity. JBU (6µg/g) caused 65±9% neuromuscular blockade in the cockroaches, an effect prevented by bicuculline (5µg/g) (p<0.05, n=6). JBU (6µg/g) decreased the frequency whilst increasing the amplitude of the spontaneous neural compound action potentials (1425±52.60min-1, controls 1.102±0.032mV, p<0.05, n=6, respectively). Altogether the results indicate that JBU induces behavioral alterations in Nauphoeta cinerea cockroaches probably by interfering with the cholinergic neurotransmission. The neuromuscular blocking activity of JBU suggests an interplay between acetylcholine and GABA signaling. GENERAL SIGNIFICANCE: The search for novel natural molecules with insecticide potential has become a necessity more than an alternative. Understanding the mode of action of candidate molecules is a crucial step towards the development of new bioinsecticides. The present study focused on the neurotoxicity of Canavalia ensiformis urease, a natural insecticide, in cockroaches and revealed interferences on the cholinergic, octopaminergic and GABA-ergic pathways as part of its entomotoxic mode of action.