DENDRODENDRITIC AND AXOAXONIC SYNAPSES IN THE THALAMIC RETICULAR NUCLEUS OF THE ADULT-RAT

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.