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Developmental attenuation of the pre-ecdysis motor pattern in the tobacco hornworm, Manduca sexta

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Summary

At the culmination of each molt, the larval tobacco hornworm exhibits a pre-ecdysis behavior prior to shedding its old cuticle at ecdysis. Both pre-ecdysis and ecdysis behaviors are triggered by the peptide, eclosion hormone (EH). Pre-ecdysis behavior consists of rhythmic abdominal compressions that loosen the old larval cuticle. This behavior is robust at larval molts, but at the larval-pupal molt the only comparable behavior consists of rhythmic dorso-ventral flexions of the anterior body. These flexions appear to be an attenuated version of the larval pre-ecdysis behavior because (1) they show the same EH dependence, and (2) the motor patterns recorded from EH treated, deafferented larval and pupal preparations are similar except that the pupal pattern is much weaker. Both patterns are characterized by rhythmic, synaptically-driven bursts of action potentials in motoneurons MN-2 and MN-3, which occur synchronously in all segments. However, the synaptic drive to the motoneurons and their resultant levels of activity are reduced during the pupal pre-ecdysis motor pattern, especially in posterior abdominal segments. Although the dendritic arbors of both motoneurons regress somewhat during the larval-pupal transformation, this does not appear to be the primary source of diminished synaptic drive because regression is greatest in the segments in which synaptic inputs remain the strongest. The developmental weakening of the pre-ecdysis motor pattern thus may be due to changes at the interneuronal level.

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Abbreviations

A2, A3...:

abdominal segments 2, 3, etc.

ALE :

anterior lateral external muscle

day L3 :

third day of the 5th larval instar

day P0 :

the day of pupal ecdysis

DN a :

anterior branch of the dorsal nerve

EH :

eclosion hormone

HPLC :

high performance liquid chromatography

TP :

tergopleural muscle

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Author notes

  1. Carol I. Miles

    Present address: Section of Neurobiology and Behavior, Seeley G. Mudd Hall, Cornell University, 14853-2072, Ithaca, NY, USA

Authors and Affiliations

  1. Department of Entomological Sciences, Wellman Hall, University of California, 94720, Berkeley, CA, USA

    Carol I. Miles & Janis C. Weeks

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  1. Carol I. Miles
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  2. Janis C. Weeks
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Miles, C.I., Weeks, J.C. Developmental attenuation of the pre-ecdysis motor pattern in the tobacco hornworm, Manduca sexta . J Comp Physiol A 168, 179–190 (1991). https://doi.org/10.1007/BF00218410

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