GROWTH CONE COLLAPSE IN-VIVO - ARE INHIBITORY INTERACTIONS MEDIATED BY GAP-JUNCTIONS

In the leech embryo, oppositely directed axons of homologous anterior pagoda (AP) neurons overlap with each other extensively within the ner ves that link adjacent ganglia, and inhibit each other's further growt h (Gao and Macagno, 1987b). During this 5-8 d period of inhibition, th e axons begin to grow thin, and eventually they retract completely. Ho wever, deletion of one overlapping AP cell results in the renewed grow th of the remaining AP cell's axon, which then innervates territory va cated by the killed cell. Thus, each neuron can detect the presence of the other, and adjust its branching pattern accordingly. To begin to explore how these neurons detect and inhibit each other, we tested for direct communication between them. Dye fills with fluorescent chromop hores suggested direct contact between their axons at the light level, and this was confirmed by serial-section electron microscopic analysi s. Morphological features resembling aspects of gap junctions were obs erved where the projections were closely apposed, and subsequent elect rophysiological recordings demonstrated electrical coupling between th e mutually inhibited axons. Confirmation that these projections commun icate via gap junctions was obtained using intracellular injection of 5-HT as a tracer, followed by anti-5-HT immunohistochemistry. The trac er passed selectively between AP neurons. We propose that the gap junc tions formed between the transient projections of the developing AP ne urons may mediate the exchange of the signals that permit homologs to recognize each other and to inhibit the further forward progress of th ese projections.