Axonal function and ionic regulation in the central nervous system of a phytophagous insect (Carausius morosus).

A limited number of invertebrate species, notably some freshwater molluscans and certain phytophagous insects, are known to possess body fluids whose composition is extremely specialized. In particular the sodium level may be very low in these species, attaining an extreme condition in the haemolymph of a coleopteran such as Timarchia tenebriosa in which the concentration of this cation amounts to only i-6 mM./l. (Duchateau, Florkin & Leclercq, 1953). It has been frequently remarked that such low sodium concentrations are difficult to reconcile with the conventional membrane theory for the propagation of the action potential, which depends upon the presence of a high concentration of this cation in the fluid bathing the axon surface. Recent investigations carried out in this laboratory have been concerned with an attempt to elucidate the ionic basis of axonal conduction in the nerve cord of the stick insect, Carausius morosus, in which the sodium concentration of the haemolymph averages only about 15-0 mM./l., while the magnesium level is as high as 53 mM./l. (cf. Wood, 1957). Preliminary electrophysiological studies involving the use of extracellular electrode techniques indicated that the electrical conduction processes in an abdominal ganglion became sodium-dependent when the peripheral fibrous and cellular nerve sheath was removed (Treherne, 1965 a). Radioisotope studies also demonstrated the existence of an appreciable rapidly-exchanging sodium fraction in the central nervous tissues of this species (Treherne, 19656). This rapidly-exchanging fraction, which was identified as the extracellular sodium, did not appear to result from a simple Donnan equilibrium with the haemolymph, such as was demonstrated in the abdominal nerve cord of the cockroach (Treherne, 1962), but was found to be dependent upon an apparent active uptake of this cation from the haemolymph. More recently it was shown that the entire central nervous system is surrounded by a fat-body sheath in this species (Maddrell & Treherne, 1966). It was subsequently demonstrated that there was an appreciable potential of some 15-20 mV across the neural fat-body sheath, the interior being positive with respect to the bathing medium (Treherne & Maddrell, 1967). This positive potential appeared to result from a chloride diffusion potential across the fat-body sheath, although it did not appear to be directly associated with the maintenance of the relatively high concentration of sodium and chloride ions at the axon surfaces, for it was shown that the axons could function for extended periods in preparations from which the neural