DIFFERENCES BETWEEN SOMATIC AND DENDRITIC INHIBITION IN THE HIPPOCAMPUS

Hippocampal synaptic inhibition is mediated by distinct groups of inhi bitory cells. Some contact pyramidal cells perisomatically, while othe rs terminate exclusively on their dendrites. We examined perisomatic a nd dendritic inhibition by recording from CA3 inhibitory and pyramidal cells and injecting biocytin to visualize both cells in light and ele ctron microscopy. Single perisomatic inhibitory cells made 2-6 termina ls clustered around the soma and proximal pyramidal cell processes. De ndritic cells established 5-17 terminals, usually on different dendrit es of a pyramidal cell. Perisomatic terminals were larger than those f acing dendritic membrane. Perisomatic inhibitory cells initiated the m ajority of simultaneous IPSPs seen in nearby pyramidal cells. Single I PSPs initiated by perisomatic inhibitory cells could suppress repetiti ve discharge of sodium-dependent action potentials. Activation of inhi bitory fibers terminating on dendrites could suppress calcium-dependen t spikes. Thus, distinct inhibitory cells may differentially control d endritic electrogenesis and axonal output of hippocampal pyramidal cel ls.