The isolated turtle olfactory bulb was used to characterize synaptic interactions between mitral and granule cell dendrites. First, blockade of antidromic invasion of mitral cell dendrites caused a large decrease in the size of the inhibitory postsynaptic potential (IPSP) recorded in mitral cells, indicating that the IPSP results in large part from activity in the dendrites of mitral cells. Second, direct depolarization of mitral cells was followed by an IPSP. In the presence of tetrodotoxin, depolarization of mitral cells evoked calcium spikes (as would be expected for presynaptic membranes) followed by IPSP's. These findings demonstrate that regenerative sodium spikes and axonal pathways are not required for this inhibitory pathway and that the inhibition is indeed a result of local dendritic interaction. Such a pathway provides an obligatory synaptic inhibition of mitral cells following their activation and emphasizes the tight coupling that exists with reciprocal dendrodendritic synapses.
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