Sprouting by isolated Helisoma neurons: enhancement by glutamate.

We tested glutamate for its ability to modulate neurite outgrowth from isolated neurons of the adult snail, Helisoma trivolvis. Although glutamate did not induce neurite outgrowth from neurons maintained in defined medium, nevertheless it showed a dose-dependent ability to enhance the activity of conditioned medium. We concluded that glutamate can enhance the release and/or activity of CNS derived sprouting factor(s) present in conditioned medium. The general conclusion to be drawn from this study is that the ability of a neurotrophic factor(s) to promote neurite outgrowth can be regulated by a neurotransmitter. This mechanism may be important in the regulation of trophic factors in the adult nervous system.

Neurite outgrowth in molluscan organ and cell cultures: the role of conditioning factor(s).

Isolated neurons from adult central ganglia of the snail Helisoma were cultured in vitro in modified Liebowitz L-15 medium. Such neurons displayed electrical excitability comparable to that in acutely dissected ganglia. Isolated neurons remained spherical in defined medium throughout culture durations up to 2 weeks. This static morphology was contrasted by the significant neuritic outgrowth which occurred from neurons maintained in medium with co-cultured intact Helisoma brains or in brain conditioned medium. A morphological sequence of growth cone formation and neurite extension occurred only in the presence of a conditioning factor(s) with a mode of action which included tight binding of the conditioning factor to the substratum. Under these conditions, the two primary neuronal phenotypes, electrical excitability and complex neuronal architecture, could be affected independently in adult molluscan neurons cultured in vitro.