Variation in electrophysiology and morphology of hippocampal CA3 pyramidal cells

A proportion of pyramidal cells in region CA3 of the mammalian hippocampus generate bursts of action potentials when stimulated with an intracellular injection of depolarizing current. Although a previous study has suggested that burst-type cells are more likely to be located in subregion CA3a than CA3b, it has been unclear if, or how, this burst-type firing was related to cell morphology. In the present study, a sample of pyramidal cells located in subregions CA3a, b and c were recorded intracellularly. Many of these cells were filled with Lucifer yellow, allowing correlation of gross morphology with electrophysiology. Contrary to previous results, it was determined that the proportion of cells which generated bursts did not differ significantly across CA3 subregions. It was found, however, that cells with somata located close to the stratum pyramidale (s.p.)/oriens border ('deep' cells) were more than twice as likely to generate burst-type responses than were cells located closer to stratum radiatum ('shallow' cells). One notable morphological feature of the deep cells was the greater length of the initial portion of their apical dendrite, as measured from soma to primary branching point. This observation is consistent with the hypothesis that burst-type responses are generated or modulated by ion channels on this section of the dendrite.