The metabolism of acetylcholine in the intact central nervous system of an insect (periplaneta americana l.).

The insect central nervous system is known to be remarkably insensitive to applied acetylcholine. This effect was first demonstrated in the terminal abdominal ganglion of the cockroach when it was shown that synaptic transmission was unaffected by concentrations of up to io~ M acetylcholine in the bathing solution (Roeder, 1948). This level of acetylcholine is more than two orders higher than the concentration which evokes the discharge of impulses in vertebrate sympathetic ganglia (cf. Bronk, 1939). The insensitivity of this insect ganglion to applied acetylcholine was first accounted for by the hypothesis that this substance was prevented from penetrating the tissues of the central nervous system by the presence of a peripheral diffusion barrier. This barrier was identified with the fibrous and cellular nerve sheath (Twarog & Roeder, 1956; O'Brien, 1957; O'Brien & Fisher, 1958). Subsequent work has shown, however, that synaptic transmission continued following the injection of massive amounts of acetylcholine beneath the nerve sheath in ganglia of Locusta (Harlow, 1958) and Periplaneta (Treherne, 19626). In addition, it has more recently been demonstrated that C-labelled acetylcholine penetrates rapidly into the tissues of the intact abdominal nerve cord of the cockroach (Treherne & Smith, 1965). In the latter experiments the rate of the rapid initial influx of acetylcholine was found to be equivalent to those rates previously measured for the exchanges of inorganic ions etween the haemolymph and the extracellular fluid of the central nervous tissues (Treherne, 1961, 1962 a, 1965). Furthermore, recent electron microscopic investigations of the insect neuropile have shown that the limited intercellular spaces in the regions of synaptic activity (Smith & Treherne, 1965) are confluent with the extensive peripheral extracellular system (Smith & Treherne, 1963). The demonstration of the relatively rapid penetration of acetylcholine into the extracellular system leaves two possible explanations which may account for the ability of insect nerves to function in high concentrations of acetylcholine in the bathing medium. The first possibility is that synaptic transmission is not effected by the conventional cholinergic system in the cockroach central nervous system. The second possibility which remains is that the cholinergic system operates in the ganglia of this insect, the extremely high cholinesterase activity of the insect central nervous tissues (cf. Chadwick, 1963; Colhoun, 1963) being sufficient to reduce drastically the