Non-pyramidal hippocampal projection neurons: a light and electron microscopic study.

Following injections of horseradish peroxidase conjugated with wheat germ agglutinin into the medial nucleus accumbens of the rat, a large number of projecting pyramidal neurons in the hippocampus were retrogradely labelled. In addition to this major projection, a few retrogradely labelled cells were tentatively identified at the light microscopic level as non-pyramidal neurons. These presumptive non-pyramidal neurons were found in all hippocampal layers, although they were mainly outside the stratum pyramidale, in the stratum oriens. Ultrastructurally, in serial sections, the non-pyramidal nature of 20 of these neurons was confirmed by their characteristic features such as deeply indented nuclei, occasional intranuclear inclusions, and symmetric and asymmetric synaptic contacts with their somata. Possible transmitters used by these neurons are discussed.