Innervation of Entorhinal Principal Cells by Neurons of the Nucleus Reuniens Thalami. Anterograde PHA‐L Tracing Combined with Retrograde Fluorescent Tracing and Intracellular Injection with Lucifer Yellow in the Rat

The innervation of dendrites of identified entorhinal principal cells by fibres originating in the nucleus reuniens thalami was studied in the rat. The lectin Phaseolus vulgaris‐leucoagglutinin (PHA‐L, anterograde tracer) was injected into the nucleus reuniens and the fluorescent dye Fast Blue (retrograde tracer) into the hippocampus. After survival, perfusion‐fixation and the preparation of brain slices, entorhinal neurons retrogradely labelled with Fast Blue were intracellularly injected with the dye Lucifer yellow to introduce a specific marker into their dendrites. The transported PHA‐L and the injected Lucifer yellow were visualized through dual peroxidase immunohistochemistry. Varicosities on PHA‐L labelled reuniens fibres abut ascending and descending Lucifer yellow‐filled secondary dendrites of multipolar and pyramidal principal entorhinal neurons that possess either spiny or sparsely spiny dendrites, but they do not appose the perikarya of these cells. In the electron microscope, PHA‐L labelled boutons in the entorhinal cortex were observed forming asymmetrical synaptic contacts with dendritic spines (50%) or shafts (50%). The results indicate that direct thalamic input occurs on dendrites of neurons in the entorhinal cortex which project to the hippocampus.