Non-pyramidal neurons in the guinea pig hippocampus. A combined Golgi-electron microscope study.

Morphological characteristics of non-pyramidal neurons in the guinea pig hippocampus (regions CA1 and CA3) were analyzed by a correlated light and electron microscopic approach. Following Golgi impregnation, the cells were first studied under the light microscope and classified according to the location of their cell bodies and the distribution of their dendrites in the different hippocampal layers. Next, the Golgi impregnated non-pyramidal neurons were gold-toned and deimpregnated, allowing an electron microscopic analysis of the identified structures. With regard to cell body location and dendritic pattern, non-pyramidal cells are a rather heterogeneous group of neurons. Their perikarya were found in all hippocampal layers and their dendrites had a less regular orientation when compared to pyramidal neurons and granule cells. Two basic types, i.e., "vertical" and "horizontal" non-pyramidal neurons are described. Many cells were of an intermediate type with dendrites extending in all directions. Non-pyramidal cell dendrites were mostly devoid of spines but exhibited numerous varicosities. Non-pyramidal cell axons could sometimes be seen extending towards the pyramidal cell layer. A surprising uniformity was observed when the impregnated, identified non-pyramidal neurons were studied in the electron microscope. Their perikarya exhibited a well-developed endoplasmic reticulum and indented nuclei. Both the cell bodies and the varicose dendrites were densely covered with synaptic boutons which mainly formed asymmetric synaptic contacts. Only occasionally were symmetric synaptic contacts observed. Non-pyramidal cell dendrites extending into the stratum lucidum of CA3 were found to be contacted by the giant boutons of mossy fiber axons. In addition to synaptic contacts, the dendrites of gold-toned non-pyramidal neurons formed gap junctions with neighboring dendrites. The results are discussed in relation to recent immunocytochemical studies which have shown non-pyramidal neurons in the hippocampus to contain gamma-aminobutyric acid and/or various neuropeptides.