The Journal of Neuroscience, May 15, 2000, 20(10):3909-3914
Dendritic Depolarization Efficiently Attenuates
Low-Threshold Calcium Spikes in Thalamic Relay Cells
X. J.
Zhan,
C. L.
Cox, and
S. Murray
Sherman
Department of Neurobiology, State University of New York, Stony
Brook, New York 11794-5230
Thalamic relay cells respond in two distinct modes, burst and
tonic, that depend on a voltage-dependent, low-threshold,
transient Ca2+ current
(IT), and these modes relay different
forms of information to cortex. IT
activation evokes a low-threshold spike (LTS), producing a burst of
action potentials. Modulatory inputs from cortex and brainstem are
known to activate metabotropic receptors on relay cell dendrites at
which the T channels underlying IT may be
concentrated. We thus investigated the influence of activating these
receptors on the LTS, using current-clamp intracellular recording in an in vitro slice preparation of the cat's lateral
geniculate nucleus. We found a strong correlation between LTS amplitude
and the number of action potentials evoked in the burst. We then found
that activation of either metabotropic glutamate or muscarinic
receptors produced a hyperpolarizing shift in the sigmoid relationship
between LTS amplitude and the initial holding potentialwithout
affecting the maximum LTS amplitude or slope of the relationship. This
hyperpolarizing shift in the voltage dependency of LTS amplitude is
best explained by space-clamp limitations and significantly more
depolarization of T channels near the dendritic location of activated
receptors than at the soma. Thus, nonretinal modulatory inputs may have a stronger influence on IT and number of
action potentials generated in a burst than previously imagined from
somatic recording, because the EPSP amplitudes generated by these
inputs at the dendritic location of most T channels are greater than
after their electrotonic decay recorded at the soma.
Key words:
thalamus; T channel; IT; burst firing; relay cell; lateral geniculate nucleus; metabotropic
glutamate receptor; muscarinic receptor
Copyright © 2000 Society for Neuroscience 0270-6474/00/20103909-06$05.00/0
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