Journal of Neuroscience, Vol 8, 765-775, Copyright © 1988 by Society for Neuroscience
Hormonal regulation of the shape of identified motoneurons in the moth Manduca sexta
JW Truman and SE Reiss
Department of Zoology, University of Washington, Seattle 98195.
Motoneurons MN-1 and MN-3 in the moth Manduca sexta go through 3 distinct
phases during postembryonic life. During larval life their neuritic fields
show allometric growth in concert with larval growth. Larval neurites are
then lost during the larval-pupal transition, which is followed by the
outgrowth of adult-specific neurites during adult development. In MN-1, the
adult-specific arbor typically results from the outgrowth of neurites
ipsilateral to the cell body. However, in a small percentage of cells,
ipsilateral branches are not present and contralateral branches extend
across the midline to fill the vacant space. This altered form of MN-1 is
thought to result from the early failure of ipsilateral neurite extension.
The steroid hormones, the ecdysteroids, are responsible for the outgrowth
of adult neurites. The onset of the latter is correlated with the
ecdysteroid increase that promotes adult differentiation and does not occur
under conditions such as diapause, in which the normal steroid rise is
absent. Artificial replacement of ecdysteroids, however, induces the
adult-specific growth. This action of ecdysteroids to cause a change in
neuronal form requires the absence of juvenile hormone (JH). Application of
JH mimics prior to the onset of the program of adult outgrowth blocks this
outgrowth. MN-1 and MN-3 show different times of JH sensitivity, which
appear to be correlated with different times of neurite outgrowth. It is
concluded that the role of JH is to maintain the status quo of central
neurons and prevent changes in form in response to ecdysteroids. In the
absence of JH, the ecdysteroids can then exert morphogenetic changes, but
the nature of these actions, neurite outgrowth or regression, is likely a
function of the developmental history of the cell.
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