Motor Patterns for Two Distinct Rhythmic Behaviours Evoked by Excitatory
Amino Acid Agonists in the Xenopus Embryo Spinal Cord.
SOFFE, S. R.
School of Biological Sciences, University of Bristol, Woodland Road,
Bristol, BS8 1UG, United Kingdom.
APStracts 2:0333N, 1995.
SUMMARY AND CONCLUSIONS
1. Mechanisms underlying the selective expression of different motor patterns
in vertebrates are poorly understood. Immobilised, spinal Xenopus embryos are
used here to examine the motor patterns evoked by various concentrations of
excitatory amino acids. 2. Relatively low concentrations of NMDA (40-60 #M),
kainate (7-8 #M), AMPA (5 #M) evoked motor root discharge characteristic of
swimming. Brief applications of higher concentrations of kainate (20-40 #M),
AMPA (25-30 #M), quisqualate (5 #M) and glutamate (1-4 mM) evoked sequences of
a quite different motor pattern: struggling. This is characterised by a longer
cycle period, increased burst duration and a reversed longitudinal pattern of
motor root discharge. The struggling pattern was never evoked by higher
concentrations of NMDA (300-500 #M), but was evoked by 30 #M AMPA or 5 #M
quisqualate in the presence of 50 #M D-AP5. 3. Intracellular recordings from
presumed spinal motoneurons showed quite different patterns of activity during
agonist evoked swimming and struggling. The patterns were like those described
previously during sensory evoked behaviour. 4. Caudal applications of
excitatory amino acids that produced struggling discharge did so only at
caudal motor roots, while caudal applications of NMDA evoked swimming activity
throughout the spinal cord. 5. During excitatory amino acid applications,
sensory Rohon-Beard neurons depolarised up to 7 mV, during evoked struggling,
but did not fire. The results show that expression of the struggling pattern,
like swimming, is not critically dependent on sensory discharge. The results
are also consistent with the idea that expression of the two very different
motor patterns for swimming or struggling in this simple vertebrate
preparation can be controlled by the level of excitation within the spinal
motor circuitry, and need not involve the activity of a specific external
neuromodulator.
Received 8 June 1995; accepted in final form 7 November 1995.
APS Manuscript Number J369-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95