DISTRIBUTION AND ROLE OF ASPARTATE IN THE NERVOUS-SYSTEM OF THE CHAETOGNATH SAGITTA

Cholinergic control of locomotory muscles in chaetognaths is monitored by diffuse transmitter release through layers of collagen fibers that form the connective stratum of the hydroskeleton. Despite the lack of morphologically defined synaptic junctions, the control of locomotor activity in chaetognaths is highly specific and allows complex behavio ral patterns. This complexity suggests the existence of neuromediators acting to modulate the effects of the main motor neurotransmitter, ac etylcholine, on muscular contraction. Immunocytochemical investigation s performed in Sagitta friderici by using antibodies directed against L-aspartate revealed the presence of the amino acid within abundant fi ber networks regularly distributed in the head, trunk, and tail and wi thin discrete groups of cell bodies. In addition to known components o f the sensory and motor nervous systems, L-aspartate immunoreactivity revealed previously undescribed intraepidermal networks of axonal prof iles. With the exception of two giant anterior fibers radiating from t he ventral ganglion, L-aspartate immunoreactive processes were usually thin and varicose, occasionally making an anastomosis. As indicated b y electron microscopy, L-aspartate-immunoreactive varicosities apposed to the connective stratum were filled with synaptic-like vesicles but displayed no synaptic differentiation. Physiologic investigations sug gested a potent inhibitory effect of L-aspartate on acetylcholine-indu ced muscle contraction. The wide distribution pattern of immunoreactiv e profiles suggests an important role of L-aspartate in motor and sens ory functions in chaetognaths. Although classified among excitatory am ino acids in vertebrates, aspartate may function as an inhibitory modu lator of acetylcholine-induced muscle contraction in these enterocoelo us gastroneuralians. (C) 1997 Wiley-Liss, Inc.