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Volume 17, Number 9,
Issue of May 1, 1997
pp. 3215-3233
Copyright ©1997 Society for Neuroscience
Dendrodendritic and Axoaxonic Synapses in the Thalamic Reticular
Nucleus of the Adult Rat
Received July 29, 1996; revised Jan. 21, 1997; accepted Jan. 28, 1997.
Didier Pinault1,
Yoland Smith1, 2, and
Martin Deschênes1
1 Centre de Recherche en Neurobiologie, Hôpital
de l'Enfant-Jésus, Département de Physiologie,
Faculté de Médecine, Université Laval, Québec,
Canada, G1J 1Z4, and 2 Division of Neuroscience, Yerkes
Regional Primate Center and Department of Neurology, Emory University,
Atlanta, Georgia 30322
Currently, it is believed that cell-cell communications occur in
the thalamic reticular nucleus (RT) during thalamocortical operations,
but the anatomical substrate underlying these intrinsic interactions
has not been characterized fully in the rat yet. To further our
knowledge on this issue, we stained juxtacellularly rat RT neurons with
biocytin or Neurobiotin and examined their intrinsic axon collaterals
and "axon-like processes" at both light and electron microscopic
levels. Of 111 tracer-filled RT cells for which the axon could be
followed from its origin up to the thalamus, 12 displayed short-range,
poorly ramifying varicose local axon collaterals, which remained
undistinguishable from parent distal dendrites, raising the question as
to whether their varicosities were presynaptic terminals. Correlated
light and electron microscopic observations of the proximal part of
these intrinsic varicose axonal segments revealed that their
varicosities and intervaricose segments were, in fact, postsynaptic
structures contacted by a large number of boutons that, for the most,
formed asymmetric synapses and were nonimmunoreactive for GABA.
Similarly, the so-called "axon-like processes" stemming from the
soma or dendrites also were identified as postsynaptic structures. Two unexpected observations were made in the course of this analysis. First, the hillock and initial segment of some RT axons were found to
receive asymmetric synaptic inputs from GABA-negative terminals. Second, examination of serial ultrathin sections of dendritic bundles
cut in their longitudinal plane revealed the existence of several short
symmetric dendrodendritic synapses and numerous puncta adhaerentia
between component dendrites. In conclusion, dendrodendritic junctions
might be a prominent anatomical substrate underlying interneuronal
communications in the RT of the adult rat. Furthermore, excitatory
axoaxonic synapses on the axon hillock, initial segment, and local axon
collaterals might represent a powerful synaptic drive for synchronizing
the firing of RT neurons. Future studies are essential to verify
whether excitatory axoaxonic synapses with the axon hillock are a
general feature in the RT.
Key words:
axon hillock;
axon initial segment;
cell-cell
communication;
correlated light and electron microscopy;
juxtacellular
labeling;
thalamic network
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