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.