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First published online June 27, 2008
Journal of Experimental Biology 211, 2346-2357 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.017384
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A muscarinic cholinergic mechanism underlies activation of the central pattern generator for locust flight

Edgar Buhl, Klaus Schildberger and Paul A. Stevenson*

University of Leipzig, Institute of Biology II, Talstr. 33, 04103 Leipzig, Germany


Figure 1
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  		Fig. 1. Electromyograms of flight motor activity induced by (A) wind stimulation (wind, ~6 m s–1) compared to motor activity evoked by bath applied cholinergic agonists (perf, B–F) in deafferented locust preparations. The top traces of each panel show continual sequences as recorded from the right hindwing depressor muscle (Dh-r) and the lower traces show details of the pattern as recorded from the right hindwing elevator (Eh-r) and depressor (Dh-r) and the depressor left fore- (Df-l) and hindwing homologous (Dh-l) muscles. (B) The muscarinic agonist pilocarpine (5 mmol l–1) initiates flight motor activity interrupted by pauses. (C) Acetylcholine (100 mmol l–1) induces continuous rhythmic motor activity with occasional interspersed sequences that resemble flight. (D) Eserine (1 mmol l–1) induces a short flight sequence. (E) The cholinergic agonist carbachol (5 mmol l–1) induces flight motor activity at exceptionally high frequency. (F) Nicotine (1 mmol l–1) induces a short burst of uncoordinated motor activity only. Scale bar, 10 s upper traces, 100 ms lower traces.

 

Figure 2
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  		Fig. 2. Comparison of key features of the flight motor pattern in deafferented locust preparations released by various treatments: (from left to right) wind stimulation, pilocarpine, carbachol, octopamine, dopamine, tyramine and finally by wind and pilocarpine after amine depletion. Values are means + s.d., from 100 cycles, 20 from each of five animals for each condition. (A) Rhythm frequency. Note the elevated frequency of the cholinergic-induced patterns. (B) Depressor elevator (DE) and elevator depressor (ED) latencies. The DE latency is longer than the ED latency. (C) Phase. The phase of the elevator in the depressor cycle is greater for flight released by the amines. (D) Hind–forewing (HF) latency. The forewing depressor muscles lag several milliseconds behind the homologous hindwing muscles in all cases. (E) Left–right wing (LR) latency. The homologous depressor muscles of the two body sides are activated in near synchrony for all treatments. Asterisks in A, C and D indicate significant differences from the wind-induced flight motor pattern (unpaired two-tailed t-test); asterisks in B indicate significant differences between the DE and ED latencies (paired two-tailed t-test). *P<0.05; **P<0.01; ***P<0.001.

 

Figure 3
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  		Fig. 3. Reversible muscarinic blockade of flight initiation. (Ai,Aii,Aiii) Responses to wind before, during and after (wash) perfusion with atropine (10 mmol l–1) in the same deafferented preparation. (B) Atropine (10 mmol l–1) and (C) scopolamine (10 mmol l–1) both block flight initiation by pilocarpine (5 mmol l–1). Top traces show condensed, and lower traces expanded excerpts of electromyograms as in Fig. 1. Scale bar: 10 s upper traces, 100 ms lower traces.

 

Figure 4
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  		Fig. 4. Flight initiated by stimulating the tritocerebral commissure giant interneurone (TCG) and its reversible blockade by atropine. (A) Pictogram of the TCG interneurone [after Bacon and Tyrer (Bacon and Tyrer, 1978Go)] showing the site of electrical stimulation (tcc, posterior tritocerebral commissure) and recording (con, connective; Sog, Pro, Meso, Meta, suboesophageal and the three thoracic ganglia, respectively). (Bi) TCG stimulation (stim; 20, 0.1 ms, 5 V pulses, 200 Hz) evokes bouts of flight muscle activity as shown by electromyograms of right (Dh-r) and left (Dh-l) hindwing depressor muscles. (Ci) Expanded record of the sequence marked in Ai (arrow). Note the cross talk of an elevator muscle (e) revealing rhythmic alternation with depressor muscle activity (d). Asterisks mark stimulus artefacts. (Bii,Cii) Same recordings and animal showing that flight initiation by TCG stimulation is fully blocked during atropine (10 mmol l–1) superfusion and completely restored after washing with saline (Biii,Ciii). Scale bars, A, 1 mm; B, 10 s; C, 100 ms.

 

Figure 5
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  		Fig. 5. Amine-induced flight motor activity. Top traces: initial response to bath application; lower traces: expanded excerpts of electromyograms, as in Fig. 1. (A) Octopamine (500 mmol l–1), (B) dopamine (200 mmol l–1) and (C) tyramine (10 mmol l–1) all induce fictive flight, but not (D) serotonin (10 mmol l–1) or (E) the precursor amino acid tyrosine (10 mmol l–1). (F) Atropine (10 mmol l–1) blocks flight initiation by octopamine. Scale bar, 10 s upper traces, 100 ms lower traces.

 

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  		Fig. 6. Flight initiation and amine depletion. (A,B) Photomicrographs of sagittal sections of a thoracic ganglion (anterior left, dorsal top) processed for octopamine immunocytochemistry. (A) In DMSO-treated locusts the well-known octopaminergic DUM neurones, for example, are strongly labelled (arrow, Nomarski optics) and immunoreactive varicosities are visible in the dorsal neuropil (insert, normal light microscopy). (B) Corresponding sections from a reserpine pre-treated animal verifies effective depletion of octopamine from the nervous system (arrow: unlabelled DUM neurone; insert: same neuropil region as in A). (C) Wind- and (D) pilocarpine (5 mmol l–1, bath applied)-induced flight motor activity in deafferented locust preparations pre-treated with 500 µg reserpine (top traces: initial response; lower traces: expanded excerpts, as in Fig. 1). Scale bars, A,B 100 µm; C,D 10 s upper traces, 100 ms lower traces.

 

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