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Golgi-impregnated and gold-toned preparations of cat hippocampus were studied under the light and electron microscope in order to characterize synapses in contact with the axon initial segments of pyramidal neurons. The initial segment of the axon emits spine-like appendages and it was sometimes seen to be in apparent contact with thin varicose fibers that climbed along it. The latter type of varicose axon terminal segments occurred at the base of pyramidal neurons in all regions of the hippocampus and in the subiculum: they are 20 to 40 w long, contain 3 to 15 varicose swellings, and are interconnected by thin horizontal or arcadic fibers. It was established by electron microscopy of gold-toned material that light microscopically identified varicosities of these terminal segments were in symmetrical synaptic contact exclusively with axon initial segments; four of the initial segments in the CA1 region could be traced back to pyramidal neurons. Thus the pyramidal cell of the cat hippocampus receives input along its axon initial segment from boutons that probably originate from axoaxonic cells of the type found previously in the cerebral cortex and monkey hippocampus. Antiserum to glutamate decarboxylase was used to study the distribution and nature of cells and processes that bound the antiserum. Several different types of immunoreactive cell bodies were found in all layers in regions CA1 to CA3, suggesting that many kinds of GABAergic interneurons are present in the hippocampus. Fibers and varicosities that were immunoreactive for glutamate decarboxylase were also found in all layers. The most conspicuous patterns formed by immunoreactive varicosities were in the pyramidal cell layer, where they occurred as pericellular nets around all pyramidal neurons. Immunoreactive processes or varicosities also followed both apical and basal dendrites, but a very high density of immunoreactive varicosities was seen around fine (diameter 2 to 4 pm) tube-like structures that could sometimes be seen to lie at the base of pyramidal neurons. Electron microscopic analysis of immunoreactive structures that had first been identified in the light microscope established that the varicosities outlining such tube-like structures were boutons in symmetrical synaptic contact with the axon initial segments of pyramidal neurons; the other immunoreactive varicosities were boutons in symmetrical contact with cell bodies and apical and basal dendrites of pyramidal neurons. Detailed study of 19 axon initial segments showed that immunoreactive boutons occur most commonly along more distal parts of the initial segment, where they almost completely surround the axon and form symmetrical synapses not only along the main axon, but also with its spines. Since the glutamate decarboxylase-immunoreactive boutons comprised 92% of all symmetrical synaptic boutons along the initial segments, it was concluded that the axoaxonic cell uses y-aminobutyrate as its transmitter. It is suggested that the pyramidal neuron receives input from more than one type of GABAergic ’ We are grateful to Mrs. K. Boczko and Miss S. Thomas for excellent ’ To whom correspondence should be addressed. technical assistance. This work was supported by the Wellcome Trust, 3 Visiting Scientist from the Department of Neuroanatomy, Institute the Royal Society, the E. P. A. Cephalosporin Trust, the International of Higher Nervous Activity, Osaka University Medical School, 4-3-57 Cultural Institute (Budapest), the Japanese Society for the Promotion Nakanoshima, Kitaku, Osaka, Japan. Scholar of Herbert von Karajan of Sciences, the Hungarian Academy of Sciences, and Grants NS 13224 Neuroscience Research Trust. and EY 03909 from the National Institutes of Health.

the journal of neuroscience/the journal of neuroscience

Glutamate decarboxylase immunoreactivity in the hippocampus of the cat: distribution of immunoreactive synaptic terminals with special reference to the axon initial segment of pyramidal neurons