FREQUENCY AND DENDRITIC DISTRIBUTION OF AUTAPSES ESTABLISHED BY LAYER-5 PYRAMIDAL NEURONS IN THE DEVELOPING RAT NEOCORTEX - COMPARISON WITHSYNAPTIC INNERVATION OF ADJACENT NEURONS OF THE SAME CLASS

Synaptic contacts formed by the axon of a neuron on its own dendrites are known as autapses, Autaptic contacts occur frequently in cultured neurons and have been considered to be aberrant structures, We examine d the regular occurrence, dendritic distribution, and fine structure o f autapses established on layer 5 pyramidal neurons in the developing rat neocortex. Whole-cell recordings were made from single neurons and synaptically coupled pairs of pyramidal cells, which were filled with biocytin, morphologically reconstructed, and quantitatively analyzed, Autapses were found in most neurons (in 80% of all cells analyzed; n = 41). On average, 2.3 +/- 0.9 autapses per neuron were found, located primarily on basal dendrites (64%; 50-70 mu m from the soma), to a le sser extent on apical oblique dendrites (31%; 130-200 mu m from the so ma), and rarely on the main apical dendrite (5%; 480-540 mu m from the soma), About three times more synaptic than autaptic contacts (ratio 2.4:1) were formed by a single adjacent synaptically coupled neuron of the same type. The dendritic locations of these synapses were remarka bly similar to those of autapses. Electron microscopic examination of serial ultrathin sections confirmed the formation of autapses and syna pses, respectively, and showed that both types of contacts were locate d either on dendritic spines Or shafts, The similarities between autap ses and synapses suggest that autaptic and synaptic circuits are gover ned by some common principles of synapse formation.