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The Journal of Neurophysiology Vol. 81 No. 5 May 1999, pp. 1999-2016
Copyright ©1999 by the American Physiological Society
Department of Anatomy and The Neuroscience Training Program, University of Wisconsin, Madison, Wisconsin 53706-1532
Bartlett, Edward L. and
Philip H. Smith.
Anatomic, Intrinsic, and Synaptic Properties of Dorsal and
Ventral Division Neurons in Rat Medial Geniculate Body. J. Neurophysiol. 81: 1999-2016, 1999.
Anatomic, intrinsic, and synaptic properties of dorsal and ventral
division neurons in rat medial geniculate body. Presently little is known about what basic synaptic and cellular mechanisms are
employed by thalamocortical neurons in the two main divisions of the
auditory thalamus to elicit their distinct responses to sound. Using
intracellular recording and labeling methods, we characterized anatomic
features, membrane properties, and synaptic inputs of thalamocortical
neurons in the dorsal (MGD) and ventral (MGV) divisions in brain slices
of rat medial geniculate body. Quantitative analysis of dendritic
morphology demonstrated that tufted neurons in both divisions had
shorter dendrites, smaller dendritic tree areas, more profuse
branching, and a greater dendritic polarization compared with stellate
neurons, which were only found in MGD. Tufted neuron dendritic
polarization was not as strong or consistent as earlier Golgi studies
suggested. MGV and MGD cells had similar intrinsic properties except
for an increased prevalence of a depolarizing sag potential in MGV
neurons. The sag was the only intrinsic property correlated with cell
morphology, seen only in tufted neurons in either division. Many MGV
and MGD neurons received excitatory and inhibitory inferior colliculus (IC) inputs (designated IN/EX or EX/IN depending on
excitation/inhibition sequence). However, a significant number only
received excitatory inputs (EX/O) and a few only inhibitory (IN/O).
Both MGV and MGD cells displayed similar proportions of response
combinations, but suprathreshold EX/O responses only were observed in
tufted neurons. Excitatory and inhibitory postsynaptic potentials
(EPSPs and IPSPs) had multiple distinguishable amplitude levels
implying convergence. Excitatory inputs activated
-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and
N-methyl-D-aspartate (NMDA) receptors the
relative contributions of which were variable. For IN/EX cells with
suprathreshold inputs, first-spike timing was independent of membrane
potential unlike that of EX/O cells. Stimulation of corticothalamic
(CT) and thalamic reticular nucleus (TRN) axons evoked a
GABAA IPSP, EPSP, GABAB IPSP
sequence in most neurons with both morphologies in both divisions. TRN
IPSPs and CT EPSPs were graded in amplitude, again suggesting
convergence. CT inputs activated AMPA and NMDA receptors. The NMDA
component of both IC and CT inputs had an unusual voltage dependence
with a detectable DL-2-amino-5-phosphonovaleric acid-sensitive component even below 
70 mV. First-spike latencies of
CT evoked action potentials were sensitive to membrane potential regardless of whether the TRN IPSP was present. Overall, our in vitro
data indicate that reported regional differences in the in vivo
responses of MGV and MGD cells to auditory stimuli are not well
correlated with major differences in intrinsic membrane features or
synaptic responses between cell types.
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