Ultrastructure of stratum lacunosum-moleculare interneurons of hippocampal CA1 region.

Intracellular recordings were obtained from nonpyramidal neurons (interneurons) in stratum lacunosum-moleculare (L-M) of the CA1 region of guinea pig hippocampal slices. These interneurons had response characteristics that distinguish them from pyramidal cells and other interneuron types: the L-M neurons had relatively broad action potentials with large spike afterhyperpolarizations, high input resistance and little spike-firing adaptation, and low spontaneous activity. Lucifer Yellow (LY) and horseradish peroxidase (HRP) were injected intracellularly into physiologically identified L-M interneurons, and the cells were characterized morphologically using light and electron microscopy. L-M somata were fusiform-shaped (15 x 25 micron), had multiple processes, and were located at the border between stratum (str.) lacunosum-moleculare and str. radiatum. L-M dendrites coursed through str. lacunosum-moleculare and projected into str. radiatum. L-M axons made axodendritic synaptic contacts primarily in str. lacunosum-moleculare and str. radiatum, but also in str. moleculare of the dentate gyrus. These axodendritic synaptic contacts were made onto spiny dendritic processes (presumably pyramidal cell and granule cell dendrites) and onto aspinous dendrites (presumably interneuron dendrites), and appeared to be of the symmetric type (type 2), characteristic of inhibitory synapses. In separate groups of animals, selective lesions were made of afferents to the CA1 and dentate regions of hippocampus, and subsequent degeneration of contacts and L-M interneuron somata and dendrites was examined at the ultrastructural level. Fibers originating from contralateral and ipsilateral CA3 region, and from ipsilateral entorhinal cortex, were found to make synaptic contact onto presumed L-M interneurons. Degenerating terminals appeared to be of the asymmetric type (type 1), characteristic of excitatory synapses. These morphological data are consistent with electrophysiological results showing that L-M interneurons can mediate feedforward inhibition of CA1 pyramidal cells.