NEURITE BRANCH DEVELOPMENT OF AN IDENTIFIED SEROTONERGIC NEURON FROM EMBRYONIC HELISOMA - EVIDENCE FOR AUTOREGULATION BY SEROTONIN

Previous studies have shown that in select neurons, neurite outgrowth can be regulated by the same neurotransmitter that is synthesized and released by those neurons. However, it is not known whether such an au toregulatory mechanism is utilized during the normal course of nervous system development in either invertebrates or vertebrates. In the pre sent study, we tested this hypothesis on the first pair of identified serotonergic neurons to be expressed in embryos of the pulmonate gastr opod, Helisoma trivolvis. Embryonic neurons C1 (ENC1) elaborate a ster eotyped pattern of neurite outgrowth prior to the differentiation of s ubsequent serotonergic neurons. Embryos were treated with either p-chl orophenylalanine (pCPA) or 5-hydroxytryptophan (5-HTP) to lower or rai se embryonic serotonin content, respectively. High-performance liquid chromatography with electrochemical detection was used to measure the effects of these treatments on serotonin content, and serotonin immuno histochemistry was carried out to quantify the extent of neurite outgr owth of ENC1. Embryonic serotonin content was significantly reduced at both 24 and 48 hr after treatment with 0.02% pCPA, whereas dopamine l evels were unchanged. Although the proximal neurite outgrowth of ENC1 appeared unaffected by the pCPA treatment at both of these time points , the distal outgrowth in the target cell region appeared more profuse . This effect on outgrowth was quantified by counting the number of ne urite branch points, which was significantly increased both 24 and 48 hr after pCPA treatment. In contrast, 5-HTP treatment resulted in an i ncrease in embryonic serotonin content and a significant decrease in t he number of ENC1 branch points. Treatment with dopamine had no effect on the pattern of ENC1 neurite outgrowth. Together, these results sup port the hypothesis that a neuron may utilize its own transmitter in a n autoregulatory fashion to regulate neurite formation during embryoni c development. (C) 1995 Academic Press, Inc.