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First published online February 20, 2004
Journal of Experimental Biology 207, 1113-1125 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00866

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Mechanisms and significance of reduced activity and responsiveness in resting frog tadpoles

Thomas D. Lambert^*, Jenny Howard, Andy Plant, Steve Soffe and Alan Roberts

University of Bristol, School of Biological Sciences, Bristol, UK

^* Author for correspondence at present address: University of Hohenheim, Institute of Physiology, Garbenstrasse 30, 70593, Stuttgart, Germany (e-mail: lambert{at}uni-hohenheim.de)

Accepted 5 January 2004

Hatchling Xenopus laevis tadpoles spend most of their time attached to objects or the water surface by mucus secreted by a gland on the head. While attached, swimming activity and responsiveness to swim-initiating stimuli are reduced over long periods of time. We have investigated the mechanisms and significance of this apparent long-term inhibition. In behavioural experiments we show, firstly, that innervation of the cement gland and GABA_A-mediated inhibition are necessary for attachment to reduce responsiveness, and secondly, that denervation of the cement gland increases tadpole activity and increases their predation by damselfly nymphs (Zygoptera). To investigate the neuronal pathway from the cement gland to GABA_A inhibition, we have devised an immobilized, inverted tadpole preparation where a weight attached to the mucus simulates the force as it hangs. Simulated attachment reduces responsiveness and spontaneous fictive swimming activity. We have recorded the activity and responses of trigeminal neurons innervating the cement gland. They are spontaneously active and simulating attachment results in a sustained increase in this activity. We propose that hanging from a mucus strand increases firing in cement gland afferents. This leads to tonic GABA inhibition that reduces tadpole activity and responses, and leads to fewer attacks by predators.

Key words: GABA, Xenopus, tadpole, trigeminal, immobility, cement gland, tonic inhibition

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