Contact between identified leech neurones in culture prevents retraction of neurites following electrical activity.

Neurites of individual leech neurones in culture retract in response to electrical stimulation or K+ depolarisation. The aim of these experiments was to assess whether retraction of neurites is prevented by the establishment of cell-cell contacts. Retzius and anterior pagoda (AP) cells were isolated from leech ganglia and plated either as single cells or in pairs. A profusion of well-developed neurites was observed after 5-9 days in culture on laminin-rich leech extracellular matrix extract. Brief trains of action potentials in single cells caused neurites to retract reversibly by approximately 26% of their average length, confirming and extending and earlier results. In contrast, retraction after stimulation was significantly decreased in cultures of paired cells that had formed synapses or contacts between their processes. Even free neurites that had not made contact failed to retract; many neurites continued to elongate during and after electrical stimulation. These results indicate that the retraction of neurites induced by electrical stimulation is prevented by the establishment of intercellular contacts. By stabilising the neurites of neurones contacting potential target cells, the search for additional targets and the strengthening of connections that are being made can continue in the presence of electrical activity.