EARLY DEVELOPMENT OF AN IDENTIFIED SEROTONERGIC NEURON IN HELISOMA-TRIVOLVIS EMBRYOS - SEROTONIN EXPRESSION, DE-EXPRESSION, AND UPTAKE

In early-stage embryos of Helisoma trivolvis, a bilateral pair of iden tified neurons (ENC1) express serotonin and project primary descending neurites that ramify in the pedal region of the embryo prior to the f ormation of central ganglia, Pharmacological studies suggest that sero tonin released from ENC1 acts in an autoregulatory pathway to regulate its own neurite branching and in a paracrine or synaptic pathway to r egulate the activity of pedal ciliary cells, In the present study, sev eral key features of early ENC1 development were characterized as a ne cessary foundation for further experimental studies on the mechanisms underlying ENC1 development and its physiological role during embryoge nesis. ENC1 morphology was determined by confocal microscopy of seroto nin-immunostained embryos and by differential-interference contrast (D IC) microscopy of live embryos, The soma was located at an anteriolate ral superficial position and contained several distinguishing features , including a large spherical nucleus with prominent central nucleolus , large granules in the apical cytoplasm, a broad apical dendrite endi ng in a sensory-like structure at the embryonic surface, and a ventral neurite, ENC1 first expressed serotonin immunoreactivity around stage E13, followed immediately by the appearance of an immunoreactive neur ite (stage E14), Both the intensity of immunoreactivity and primary ne urite length were consistently greater in the right ENC1 at early stag es, Serotonin uptake, as indicated by 5,7-dihydroxytryptamine-induced fluorescence, first occurred between stages E18 and E25, At later stag es of embryogenesis (after stage E65), serotonin immunoreactivity disa ppeared, whereas serotonin uptake and normal cell morphology were reta ined, (C) 1998 John Wiley & Sons, Inc.