Neurite Outgrowth and Synapse Formation By Lymnaea Neurons: Towards a Characterization of Molluscan Neurotrophic Factors

Abstract

For the last several years we have endeavoured to characterize the endogenous neurotrophic factors of the pulmonate snail, Lymnaea stagnalis. When plated in brain conditioned medium, motoneurons, interneurons and neurosecretory cells of Lymnaea all exhibited neurite outgrowth. We showed earlier that the motoneurons and interneurons respond to a Nerve Growth Factor-like (NGF-like) component of conditioned medium. Neurosecretory cells, however, appear to be under the control of a factor derived from the endocrine dorsal body cells. Most recently, we were surprised to learn that motoneurons and interneurons also respond to another vertebrate neurotrophic factor, i.e., Ciliary Neurotrophic Factor (CNTF). The outgrowth in CNTF differs both in appearance and in functional outcome. Specifically, neurites in CNTF are thin, have small growth cones and elongate more quickly than their counterparts in NGF. Regarding functional consequences, both dopaminergic and peptidergic interneurons fail to form synapses on target neurons when cultured in CNTF, but do so in NGF, as expected from our previous studies in conditioned medium. These data shed new light on the functions of neurotrophic factors and provide the first evidence that outgrowth and synaptogenesis can be differentially regulated.