Journal of Neuroscience, Vol 14, 999-1010, Copyright © 1994 by Society for Neuroscience
Growth cone "collapse" in vivo: are inhibitory interactions mediated by gap junctions?
LR Wolszon, WQ Gao, MB Passani and ER Macagno
Department of Biological Sciences, Columbia University, New York, New York 10027.
In the leech embryo, oppositely directed axons of homologous anterior
pagoda (AP) neurons overlap with each other extensively within the nerves
that link adjacent ganglia, and inhibit each other's further growth (Gao
and Macagno, 1987b). During this 5-8 d period of inhibition, the axons
begin to grow thin, and eventually they retract completely. However,
deletion of one overlapping AP cell results in the renewed growth of the
remaining AP cell's axon, which then innervates territory vacated 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 chromophores suggested direct
contact between their axons at the light level, and this was confirmed by
serial-section electron microscopic analysis. Morphological features
resembling aspects of gap junctions were observed where the projections
were closely apposed, and subsequent electrophysiological recordings
demonstrated electrical coupling between the mutually inhibited axons.
Confirmation that these projections communicate via gap junctions was
obtained using intracellular injection of 5-HT as a tracer, followed by
anti-5-HT immunohistochemistry. The tracer passed selectively between AP
neurons. We propose that the gap junctions formed between the transient
projections of the developing AP neurons may mediate the exchange of the
signals that permit homologs to recognize each other and to inhibit the
further forward progress of these projections.
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