Sprouting and connectivity of embryonic leech heart excitor (HE) motor neurons in the absence of their peripheral target |
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Authors: | Dr John Jellies Diane M Kopp |
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Institution: | (1) Department of Biological Sciences, Western Michigan University, 49008 Kalamazoo, MI, USA;(2) Neurobiology Research Center, University of Alabama at Birmingham, 35294 Birmingham, AL, USA;(3) Present address: Department of Zoology, University of Texas, 78712 Austin, TX |
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Abstract: | The rhythmic pumping of the hearts in the medicinal leech,Hirudo medicinalis, is neurogenic and mediated by a defined circuit involving identified interneurons in a central pattern generator (CPG) and
segmentally iterated motor neurons that drive the heart muscle. During early embryogenesis, presumptive heart excitor (HE)
motor neurons extend many axon branches into the body wall; they later innervate the heart while retracting the supernumerary
peripheral axons, and only much later in development receive synaptic input from the central pattern generator (Jellies, Kopp
and Bledsoe (1992)J. Exp. Biol., 170, 71–92.)- In this study, HE motor neurons were deprived of an early interaction with the heart by surgical ablation
of a circumscribed portion of body wall including the heart primordium. Anatomical and electrophysiological data were obtained
using intracellular techniques to examine the hypothesis that peripheral interactions with the developing heart provide instructive
cues for the final differentiation of these neurons. Target-deprived HE motor neurons continued to extend multiple axons in
ventral, lateral and dorsal body wall throughout late embryonic and into postembryonic stages and they extended anomalous
axons within the CNS. This resembles the early embryonic growth of HE motor neurons before heart tube differentiation. Furthermore,
HE motor neurons deprived of heart contact exhibited tonic activity similar to the situation during early development before
they are contacted by the CPG interneurons. In contrast, sham-operated and contralateral HE motor neurons oscillated normally.
These results suggest that heart tube contact is specifically required for at least some aspects of HE development and provide
a framework in which to identify cell-cell interactions that are involved in matching neurons and targets to generate behaviorally
relevant neural circuits. |
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Keywords: | Hirudo medicirudis synaptogenesis muscle growth cone peripheral nerves central pattern generator |
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