INTERNEURONS IN THE STRATUM LUCIDUM OF THE RAT HIPPOCAMPUS - AN ANATOMICAL AND ELECTROPHYSIOLOGICAL CHARACTERIZATION

The anatomical and electrophysiological properties of neurons in the s tratum lucidum of the CA3 subfield of the hippocampus were examined by using patch-pipette recordings combined with biocytin staining. This method facilitated the analysis of the morphological features and pass ive and active properties of a recently described class of spiny neuro ns in the stratum lucidum, as well as aspiny neurons in this region. S ome, but not all, synaptic inputs of both types of neurons were found to arise from the messy fiber system. The axons of spiny neurons in th e stratum lucidum were heavily collateralized, terminating primarily i n the stratum lucidum and stratum radiatum of CA3, and to a lesser ext ent in the stratum pyramidale and stratum oriens. Only a few axonal pr ojections were found that extended beyond the CA3 region into CA1 and the hilus. Aspiny neurons fell into two classes: those projecting axon s to the stratum lucidum and stratum radiatum of CA3 and those with ax on terminations mainly in the stratum pyramidale and stratum oriens. T he electrophysiological properties of spiny and aspiny neurons in the stratum lucidum were similar, but on average, the aspiny neurons had s ignificantly higher maximal firing rates and narrower action potential half-widths. The results demonstrate that a diverse population of neu rons exists in the region of messy fiber termination in area CA3. Thes e neurons may be involved in local-circuit feedback, or feed-forward s ystems controlling the flow of information through the hippocampus. (C ) 1997 Wiley-Liss, Inc.