1. The aim of these experiments was to determine how electrical stimulation of identified neurones in culture influences their growth on defined substrates. Single Retzius cells isolated from the central nervous system (CNS) of the leech were plated in culture dishes coated with the plant lectin Concanavalin A or with extracellular matrix extract containing leech laminin to promote neurite outgrowth. Stimuli were applied by a fine tungsten microelectrode placed close to the cell surface. The efficacy of electrical stimulation was checked occasionally by recording intracellularly with a microelectrode. 2. After the period of stimulation had ended, there was a short delay before neurones plated on leech laminin retracted their neurites. Of 112 neurones, only 11 failed to respond to stimulation. Neurite retraction in each cell was non-uniform, some processes retracting while others did not. After having retracted, most neurites subsequently showed clear regrowth. The degree of retraction depended on the duration of the stimulus train: whereas a few minutes was sufficient to produce observable effects, prolonged periods of stimulation caused more extensive retraction. Trains of impulses at 4 s-1 were equally effective when they were delivered in intermittent bursts or continuously. 3. The time in relation to growth at which stimuli were applied was of critical importance. Neurones stimulated during the phase of rapid outgrowth on leech laminin did not retract their neurites, which continued to elongate during and after stimulation. Neurones that had not retracted during the early phase were stimulated again later, when extension and outgrowth of neurites had ceased or slowed. At this stage stimulation was followed by retraction and subsequent regrowth. 4. Retzius cells plated on a substrate of Concanavalin A instead of leech laminin failed to show any retraction after stimulation. 5. To investigate the possible role of Ca2+, cells were grown with raised concentrations of Mg2+ in the bathing fluid. Raised [Mg2+] did not influence the rate or the extent of neurite outgrowth. It reduced, but did not block, the effects of electrical stimulation. Earlier experiments have shown that growth on Concanavalin A occurs without obvious Ca2+ entry following stimulation. Together with the present experiments, the results suggest that Ca2+ entry following impulses in cells grown on laminin is responsible for the massive retraction.
