D. Pinault et al., DENDRODENDRITIC AND AXOAXONIC SYNAPSES IN THE THALAMIC RETICULAR NUCLEUS OF THE ADULT-RAT, The Journal of neuroscience, 17(9), 1997, pp. 3215-3233
Currently, it is believed that cell-cell communications occur in the t
halamic reticular nucleus (RT) during thalamocortical operations, but
the anatomical substrate underlying these intrinsic interactions has n
ot been characterized fully in the rat yet. To further our knowledge o
n this issue, we stained juxtacellularly rat RT neurons with biocytin
or Neurobiotin and examined their intrinsic axon collaterals and ''axo
n-like processes'' at both light and electron microscopic levels. Of 1
11 tracer-filled RT cells for which the axon could be followed from it
s origin up to the thalamus, 12 displayed short-range, poorly ramifyin
g varicose local axon collaterals, which remained undistinguishable fr
om parent distal dendrites, raising the question as to whether their v
aricosities were presynaptic terminals. Correlated light and electron
microscopic observations of the proximal part of these intrinsic varic
ose 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 w
ere nonimmunoreactive for GABA. Similarly, the so-called ''axon-like p
rocesses'' 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 so
me 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 exis
tence of several short symmetric dendrodendritic synapses and numerous
puncta adhaerentia between component dendrites. In conclusion, dendro
dendritic junctions might be a prominent anatomical substrate underlyi
ng interneuronal communications in the RT of the adult rat. Furthermor
e, 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 essenti
al to verify whether excitatory axoaxonic synapses with the axon hillo
ck are a general feature in the RT.