A COMPARISON OF RAT HIPPOCAMPAL MOSSY CELLS AND CA3C PYRAMIDAL CELLS

1. There is a long-standing debate about whether the large spiny cells in the hilar region of the hippocampus should be classified as pyrami dal cells of Ammon's horn or as a distinct cell type of the dentate gy rus. The rationale for grouping these hilar neurons (termed ''mossy ce lls'') with pyramidal cells of Ammon's horn is shared characteristics. In the present study we have compared the morphological and physiolog ical characteristics of mossy cells and nearby CA3c pyramidal cells wi th the use of a rat hippocampal slice preparation. 2. Biocytin-labeled neurons were examined on the basis of soma area, location, shape, num ber of primary dendrites, extent of dendritic spines, dendritic locati on, and axon trajectories. Mossy cells had larger soma areas than CA3c pyramidal cells, and they had more large complex spines (thorny excre scences) on their proximal dendrites and somata than CA3c pyramidal ce lls. Mossy cell dendritic trees and axon collaterals ramified in diffe rent regions of the hippocampus than dendrites and axons of CA3c pyram idal cells. 3. Intrinsic physiological properties, and spontaneous and evoked synaptic properties, were measured and compared. Mossy cells h ad significantly higher input resistances, smaller amplitude burst aft er hyperpolarizations, smaller amplitude action potentials, less spike -frequency adaptation, and more anomalous rectification than CA3c pyra midal cells. 4. Mossy cells had spontaneous excitatory postsynaptic po tentials (EPSPs) that were significantly higher in frequency and large r in amplitude than CA3c pyramidal cells. A larger porportion of mossy cells than CA3c pyramidal cells responded to perforant path stimulati on with depolarizing postsynaptic potentials without any apparent hype rpolarization. Conversely, a smaller proportion of mossy cells than CA 3c pyramidal cells responded to perforant path stimulation with inhibi tory postsynaptic potentials (IPSPs), and spontaneous IPSPs were more difficult to detect in mossy cells. 5. The intrinsic physiological pro perties of mossy cells endow these cells with potent excitatory mechan isms but relatively fewer inhibitory control processes than CA3c pyram idal cells. Recordings of spontaneous and evoked PSPs suggest that mos sy cells receive more excitatory input and less inhibitory input than CA3c pyramidal cells. These intrinsic and synaptic properties of mossy cells may explain this cell type's exceptional vulnerability to excit otoxic damage by intense afferent stimulation. 6. In summary, mossy ce lls were significantly different from CA3c pyramidal cells in many of their morphological, intrinsic physiological, and synaptic properties. These results support the view that mossy cells are not simply a cont inuation of the CA3 pyramidal cells; rather, mossy cells are best cate gorized as a distinct cell type of the dentate gyrus.