Ultrastructural characteristics and conduction velocity of olfactory receptor neuron axons in the olfactory marker protein-null mouse

Olfactory receptor neuron (ORN) axon diameters and the conduction velocity of the compound action potential along ORN axons were studied in olfactory marker protein (OMP)-null mice and genotypically matched controls. The comp ound action potential was distinguished from postsynaptic field potentials by its shorter latency, its persistence following application of cobalt or kynurenic acid that blocked postsynaptic responses, and its ability to foll ow paired-pulse stimulation at 300 Hz. Blockade of the postsynaptic field r esponses by kynurenic acid indicates that in the mouse, as in the rat, glut amate is the olfactory nerve transmitter. The mean conduction velocity of O RNs in wild-type control mice was 0.47+/-0.19 (S.E.M.) m/s (n=5), similar t o the conduction velocity reported for other mammals. The mean diameter of ORN axons in control mice was 0.202+/-0.005 and 0.261+/-0.006 mu m in the O MP-null mice. This increase in fiber diameter in the OMP-nulls predicts an increase in impulse conduction velocity. However, the mean conduction veloc ity of OMP-null mice, 0.38+/-0.03 m/s (n=6), was not significantly differen t from control (P>0.1). The conduction velocity predicted by the increase i n fiber diameter in OMP-null mice was within the 95% confidence interval of the measured value. Thus, OMP-null ORNs are normal with respect to the con duction velocity of their axons. The number of axodendritic synapses in the glomeruli of OMP-null mice is higher than in congenic wild-type mice. (C) 2000 Published by Elsevier Science BN. All rights reserved.