Accessibility of the insect nervous system to a neurotoxic polypeptide

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Abstract
The binding and accessibility of an insect selective neurotoxic polypeptide derived from scorpion venom (AaIT, Mr = 8 kDa) to the cockroach nervous system was studied by the aid of binding assays and light microscopy autoradiography resulting in the following information: The toxin possesses high paralytic potency to the cockroach (ED50= 4 ng per 100 mg of body weight) and high binding capacity (13.8 ± 1.8 pmol per mg protein) to its nervous system. When incubated with dissected and anatomically desheathed central nervous tissue the radioiodinated toxin ([125I]AaIT) revealed an obvious high affinity binding specificity. In contrast, the skeletal muscles of the cockroach were completely devoid of specific binding sites of the [125I]AaIT. The latter observation provides additional support for the sodium channel selectivity of the AaIT toxin. Autoradiography of [125I]AaIT injected into the body cavity in doses inducing a fast paralysis revealed that the central and most parts of the peripheral nervous system are impermeable to the toxin. In the above injection assays the toxin was shown to bind to the terminal branches of motor nerves at their close proximity to the skeletal muscles. Computerized image analysis of the autoradiographical data followed by electron microscopy of the neuromuscular junctions suggest an accessibility of the terminal axonal membranes to the toxin. The above data, supported by previous electrophysiological studies, led to the conclusion that the peripheral terminal outbranchings of the motor nerves serve as the main sites of accessibility to paralytic polypeptide neurotoxins.