Electrophysiological analysis of mitral cells in the isolated turtle olfactory bulb.

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Abstract
An in vitro preparation of the turtle olfactory bulb was developed. Electrophysiological properties of mitral cells in the isolated bulb were analyzed with intracellular recordings. Mitral cells were driven antidromically from the lateral olfactory tract, or activated directly by current injection. Intracellular injections of horseradish peroxidase (HRP) show that turtle mitral cells have long secondary dendrites that extend up to 1800 .mu.m from the cell body and reach around half of the bulbar circumference. There are characteristically, 2 primary dendrites, each supplying separate olfactory glomeruli. Intracellular current pulses showed that whole-neuron resistance ranged from 33-107 M.OMEGA.. The whole-neuron charging transient had a slow time course. The membrane time constant was 24-93 ms by the methods of Rall. The electrotonic length of the mitral cell equivalent cylinder as estimated by Rall''s method was 0.9-1.9. The spikes generated by turtle mitral cells were only partially blocked by tetrodotoxin (TTX) in the bathing medium. The TTX-resistant spikes were enhanced in the presence of tetraethylammonium (TEA), and blocked completely by Co. The implications of the electrical properties for impulse generation in turtle mitral cells are discussed. The mitral cells have dendrodendritic synapses onto granule cells, and the TTX-resistant spikes may therefore play an important role in presynaptic transmitter release at these synapses.