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
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
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Fig. 1. Diagrams of experimental setups. (A) Cross section of the dish used in
behavioural experiments. Tadpoles could be unattached on the bottom of the
dish, or attached by their cement gland mucus to a piece of wire, which was
fixed to the side of the dish. (B) Scale diagram of the Xenopus
tadpole to show the location of strokes to the head and tail used in
behavioural and electrophysiological experiments (shaded areas indicate region
and arrows indicate direction). Scale bar, 1 mm. (C) Diagram of the head of
the tadpole showing the mandibular nerve of the trigeminal ganglion
innervating the cement gland. The site of the lesion is shown by a dotted
line. (Modified from Roberts and Blight,
1975 .) (D) Simulating cement gland attachment. The immobilised
tadpole was pinned to a Sylgard block that was rotated so that the tadpole
pointed downwards. A weight attached to the cement gland mucus was able to
hang freely. An extracellular suction electrode on an intermyotomal cleft
monitored fictive swimming activity from a motor nerve. Arrows show the
direction of rotation of the block.
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Fig. 4. Multiple-unit activity in trigeminal sensory neurons innervating the cement
gland is increased by attaching a weight to the mucus strand. (A) Diagram of
the head end of the inverted tadpole preparation. The weight attached to the
cement gland mucus simulates attachment. Multi-unit activity is recorded by a
suction electrode on the trigeminal ganglion. (B) Examples of a 30 s duration
multi-unit recording showing spontaneous spike activity in the unattached
state and increased activity when the weight is attached. (C) Combined results
from 7 animals showing a sustained increase in trigeminal activity over a 60
min period of weight attachment and a return to control levels after the
weight is removed. Activity is measured as the number of spikes in each 30 s
period and is normalised relative to that at 25 min for each recording. Values
are means + S.D.
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Fig. 2. Cement gland attachment reduces responses to touch and light dimming.
Percentage of tadpoles responding to stimulation when unattached (black bars),
and at increasing times after attachment (1, 10 and 30 min, white bars). (A)
Responses to a tail stroke are significantly reduced in all attached groups
compared to unattached (P<0.01 for all,  2=43, 27
and 44, d.f.=1,  corrected for multiple tests using Dunn–Sidak
method). (B) Responses to dimming are also reduced during attachment. With no
responses after 1 and 10 min, attached responses are grouped and there is a
significant reduction from unattached (P<0.001,
2=36.3, d.f.=1).
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Fig. 3. Simulated cement gland attachment reduces responses to dimming and
spontaneous swimming of immobilised tadpoles. A suction electrode on the motor
nerve or ventral root (vr) records fictive swimming. (A) Examples of fictive
responses to a dim (LED). When unattached, rhythmic bursts in the vr indicate
that swimming is initiated, but with the weight attached there is no response.
(B) Percentage of dims initiating fictive swimming in the unattached (black
bar) and weight attached (white bar) states. Values are means +
S.D. (N=16). Weight attachment significantly
reduced the % of responses to dimming (**P<0.001). (C) The
frequency of spontaneous fictive swims measured over 10 min in the unattached
(black bar) and weight attached (white bar) states (means +
S.D., N=12). Weight attachment significantly
reduces the number of spontaneous swims (*P=0.009).
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Fig. 5. Spontaneous activity of single trigeminal units and responses to the
attachment of a weight to the cement gland mucus. (A-C) recordings from 3
units (lower traces) with the instantaneous frequency plot (logarithmic scale)
of each unit shown above the recording. Spontaneous activity is followed by an
initial transient response to attachment of the weight before firing levels
out to a rate greater than spontaneous activity.
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Fig. 6. Extracellular recording from a single trigeminal ganglion unit in the
unattached and attached state. The instantaneous frequency plot and interspike
interval (ISI) histogram accompanying each trace were constructed from a
single unit discriminated on the basis of spike shape. (A) Spontaneous
activity in the unattached state. (B) In the weight-attached state, firing
rate is increased. For each recording the accompanying ISI histogram is
unimodal and skewed. The coefficient of variation
(CV=S.D./mean ISI, measure of regularity) values show
firing to be irregular. Firing rate over the 5 min period (total number of
spikes/total time period) is also shown.
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Fig. 7. Cement gland mechanosensory neuron unattached activity is not influenced by
dimming (LED trace moves down), or fictive swimming. (A,B) Examples from two
tadpoles. Spontaneous activity was recorded by an electrode on the trigeminal
ganglion (trig; vertical scale bar, 0.3 mV) and a ventral root electrode
monitored fictive swimming activity (vr; vertical scale bar, 0.1 mV).
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Fig. 8. Blocking impulses generated in receptor endings with anaesthetic abolishes
the spontaneous activity of cement gland mechanosensory neurons. (A) Diagram
of the preparation. An electrode on the trigeminal ganglion recorded
multi-unit activity. A microperfusion nozzle was positioned so the
microperfusate flowed over the distal end of the mandibular nerve and the
cement gland. The flow of the bath perfusion ensured the microperfusate was
washed away and did not contact the trigeminal ganglion. Microperfusion of
anaesthetic (0.1% MS-222) onto the distal mandibular nerve as it innervates
the cement gland produced the following effects. (B) MS-222 abolished the
trigeminal response (trig) to pulling the cement gland mucus. Tungsten wire
mounted in a glass capillary is attached by a lever to a loudspeaker cone, the
input to which is shown (pull). The tungsten wire pulls on the cement gland
mucus when the line (pull) moves down. Horizontal scale bar, 2.5 s (C) MS-222
abolished spontaneous activity. Horizontal scale bar, 60 s. (D) MS-222 did not
abolish the antidromic spikes produced by a 300 µs stimulus pulse to the
hindbrain (at arrows). Subthreshold stimulation (thin line) is shown together
with a response (thick line). After washing off MS-222 by switching
microperfusion to saline, spontaneous activity returned. Horizontal scale bar,
14 ms.
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© The Company of Biologists Ltd 2004
