The synaptic connections of basket cell axons in the developing rat hippocampal formation

L Seress,C E Ribak

doi:10.1007/bf02423498

Abstract

Recent studies have indicated that hippocampal basket cells in both the dentate gyrus and Ammon's horn develop their somal and dendritic features during the first two postnatal weeks in rats. Their axon terminals form exclusively symmetric synapses that are found as early as 5 postnatal days in both regions. The present study used Golgi-electron microscopic material from 10 and 16 day old rats to demonstrate that the axon terminals of basket cells form synapses not only with somata, dendrites, and dendritic spines as reported for adult material but also with axon initial segments. However, the terminals forming synapses with axon initial segments and dendritic spines represent only a minor portion of the total number of basket cell terminals. Quantitative results indicate that 36-62% of the total number of these terminals form axosomatic synapses and 32-50% form axodendritic synapses depending on the analyzed cell. These data indicate that hippocampal basket cells have an axonal distribution similar to that found for cortical basket cells.

Highlights

The light microscopic features of the basket cells have been described for both the neocortex and hippocampal formation by the pioneering investigators who used the Golgi impregnation method (Lorente de N6 1934; Ramdn y Cajal 1911)
Immunocytochemical studies have shown that basket cells of the hippocampal formation contain glutamate decarboxylase (GAD), the synthesizing enzyme for the inhibitory neurotransmitter GABA (Ribak et al 1978; Seress and Ribak 1983)
Identified basket cells with an extensive axonal plexus were selected from the CA 1 and CA 3 areas of Ammon's horn and from the granule cell and inner molecular layers of the dentate gyrus

Summary

Introduction

The light microscopic features of the basket cells have been described for both the neocortex and hippocampal formation by the pioneering investigators who used the Golgi impregnation method (Lorente de N6 1934; Ramdn y Cajal 1911). Subsequent studies have confirmed this synaptic relationship in adult and developing hippocampal formations (Kosaka et al 1984; Kunkel et al 1986; Liibbers and Frotscher 1987; Seress et al 1989; Somogyi et al 1983) This finding raises the question whether basket cells of the hippocampal formation form synapses with all parts of the target neurons or only with the cell bodies, while other specialized local circuit neurons form synapses with these other postsynaptic sites. Some overlap in termination can be expected because large basket cells of the visual cortex were reported to form synapses with all parts of the target pyramidal cells with a preference for the cell bodies and dendrites (Somogyi et al 1983) They showed that the same axon branch can contact the cell body of one neuron and a dendrite of another. Such results contradict the concept of selectivity for the postsynaptic site of basket cell axons

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