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Fig. 2. Visually driven regulation of intrinsic neuronal excitability in
Xenopus tectal neurons improves stimulus detection in vivo.
(A) Tectal neurons from tadpoles exposed to a visual stimulation protocol show
increased excitability in response to injected depolarizing current. Left:
input-output relationships in response to square pulse depolarizations of
varying amplitudes. Neurons from visually stimulated animals (open triangles;
N=25) show significantly more spiking activity in response to
injected current than do controls (filled circles; N=20). Right:
representative recordings from these neurons showing the differences in
response to current injection. (B) Increases in the excitability of tectal
neurons are the result of changes in voltage-dependent Na+
currents. Na+ currents (left) but not K+ currents
(right) show a significant increase in peak amplitude in visually stimulated
animals. An increase in these Na+ currents, which are depolarizing
and excitatory, would account for the increase in excitability seen in
visually stimulated tadpoles. Open symbols represent recordings from visually
stimulated neurons (N=46), and closed symbols are controls
(N=25). K+ currents: trans, transient; sust, sustained;
Ca2+ currents: I-plateau. (C) Increased excitability of
tectal neurons improves visual stimulus detection in semi-intact tadpoles.
Recordings of visually evoked field potentials were made from the tectum in
control and stimulated tadpoles. The recordings shown represent five trials
from a control cell, and five trials recorded from a cell in a tadpole
previously subjected to the visual stimulation protocol. The neuron from the
visually stimulated animal fires more action potentials than the control in
response to a subsequently presented light stimulus (open arrow), indicating
that the sensitivity to visual stimuli is increased in tadpoles with previous
visual stimulation experience. Figure adapted from Aizenman et al.
(Aizenman et al., 2003) and
reproduced with permission.
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