THE HIPPOCAMPAL CA3 NETWORK - AN IN-VIVO INTRACELLULAR LABELING STUDY

The intrahippocampal distribution of axon collaterals of individual CA 3 pyramidal cells was investigated in the rat. Pyramidal cells in the CA3 region of the hippocampus were physiologically characterized and f illed with biocytin in anesthetized animals. Their axonal trees were r econstructed with the aid of a drawing tube. Single CA3 pyramidal cell s arborized most extensively in the CA1 region, covering approximately two-thirds of the longitudinal axis of the hippocampus. The total len gth of axon collaterals in the CA3 region was less than in CA1 and the axon branches tended to cluster in narrow bands (200-800 mu m), usual ly several hundred microns anterior or posterior to the cell body. The majority of the recurrent collaterals of a given neuron remained in t he same subfield (CA3a, b, or c) as the parent cell. CA3a neurons inne rvated predominantly the basal dendrites, whereas neurons located prox imal to the hilus (CA3c) terminated predominantly on the apical dendri tes of both CA1 and CA3 cells. Two cells, with horizontal dendrites an d numerous thorny excrescences at the CA3c-hilus transitional zone, we re also labeled and projected to both CA3 and CA1 regions. All CA3 neu rons projected some collaterals to the hilar region. Proximal (CA3c) n eurons had numerous collaterals in the hilus proper. One CA3c pyramida l cell in the dorsal hippocampus sent an axon collateral to the inner third of the molecular layer. CA3c pyramidal cells in the ventral hipp ocampus had extensive projections to the inner third of the dentate mo lecular layer, as well as numerous collaterals in the hilus, CA3, and CA1 areas, and several axon collaterals penetrated the subiculum. The total projected axon length of a single neuron ranged from 150 to 300 mm. On the basis of the projected axon length and bouton density (mean interbouton distance: 4.7 mu m), we estimate that a single CA3 pyrami dal cell can make synapses with 30,000-60,000 neurons in the ipsilater al hippocampus. The concentrated distribution of the axon collaterals (''patches'') indicates that subpopulations of neurons may receive dis proportionately denser innervation, whereas innervation in the rest of the target zones is rather sparse. These observations offer new insig hts into the physiological organization of the CA3 pyramidal cell netw ork. (C) 1994 Wiley-Liss, Inc.