The penetration of acetylcholine into the central nervous tissues of an insect (Periplaneta americana L.).

The precise physiological role of acetylcholine in synaptic transmission in the insect central nervous system has been a subject of some speculation and controversy. One of the main difficulties involved in postulating the existence of a conventional cholinergic system in the insect central nervous system has been the relative insensitivity of intact ganglia to applied acetylcholine solutions (Roeder, 1948; Twarog & Roeder, 1956; Yamasaki & Narahashi, i960; Treherne, 1962ft). This apparent difficulty was at least partially resolved by the results of experiments which were interpreted as demonstrating that acetylcholine was effectively prevented from penetrating into the central nervous tissues by the presence of a peripheral diffusion barrier which was identified with the fibrous and cellular nerve sheath (Twarog & Roeder, 1956; O'Brien, 1957; O'Brien & Fisher, 1958). More recently it has been demonstrated that the exchanges of a variety of inorganic ions and molecules take place relatively rapidly between the haemolymph and the tissues of the central nervous system in Periplaneta (Treherne, 1961a, b, 1962a) and Carausius (Treherne, 1965). The present investigation was carried out in an attempt to throw some light on this paradoxical situation by direct measurement of the rate of penetration of acetylcholine into the central nervous tissues of Periplaneta americana.