Synaptic excitatory and inhibitory interactions at distal dendritic sites on mitral cells in the isolated turtle olfactory bulb.

The olfactory nerves terminate exclusively on the distal dendritic tufts of mitral cells in the olfactory bulb, which makes this a favorable model for analysis of synaptic responses in distal dendrites. Intracellular recordings of responses to olfactory nerve volleys have been obtained in the isolated turtle olfactory bulb. Single mitral cells usually responded with excitatory postsynaptic potentials (EPSPs) to volleys in two different bundles, indicating convergence from separate receptor neuron populations. Paired volleys revealed long-lasting inhibition of a test EPSP by a conditioning volley. This could be shown to be independent of the inhibition of mitral cells by granule cell interneurons in the deeper layers. The results suggest that excitatory and inhibitory synaptic interactions in the glomerular layer are important in the processing of olfactory inputs. The results also provide an exception to the classical doctrine that synaptic inhibition is preferentially sited near the cell body and axon hillock in order to control impulse generation there. Our findings of inhibitory actions on distal dendrites of mitral cells may provide a model for similar actions on distal dendrites of other central neurons.