Time-course of declining electrical activity in guinea-pig olfactory cortex after olfactory bulb removal

Studies on unmyelinated axons and varicosities in the olfactory cortex

The main afferent input to the olfactory cortex from the olfactory bulbs is via the lateral olfactory tract (LOT). The axons within the lateral olfactory tract are myelinated. On leaving the LOT, they lose their myelination as they fan out over the layer immediately beneath the pial surface to make en passant synaptic connections with dendrites from neurones within the olfactory cortex. Using the guinea-pig, a semiquantitative electron micrographical study was made of the density and dimensions of these unmyelinated axons and the varicosities they create. The unmyelinated axons were very fine (0.17 +/- 0.004 micron in diameter) and punctuated at 2 microns intervals by varicosities containing a single type of vesicle. The electrophysiological consequences of this close varicosity spacing is that axonal and varicosity membranes behave electrically as single units.

Presynaptic Na/Ca action potentials in unmyelinated axons of olfactory cortex

(1) Pial surface slices of guinea-pig olfactory cortex were cut to have a thickness of 150 micron. Action potentials were recorded from the sectioned ends of the unmyelinated afferent axons originating from the lateral olfactory tract (LOT). These potentials were prolonged by the K-channel blocker 3,4-diaminopyridine (0.1 mmol/l) and further lengthened by tetraethylammonium (10 mmol/l). The action potential was also greatly prolonged by partly replacing the K+ in the bathing solution by Cs+. (2) These prolonged action potentials were shortened by Cd2+; Gd3+ (gadolinium); Ni2+; Mn2+; Co2+, in order of potency. The residual early component of the action potential was tetrodotoxin (TTX) sensitive. In contrast, the LOT action potential was little affected by Ca-channel blockade. (3) Organic Ca-channel blockers either had no effect (0.05 mmol/l nifedipine), or depressed the early and later phases of the prolonged action potential equally (0.05-0.5 mmol/l verapamil or 0.05-0.2 mmol/l diltiazem). (4) A propagated action potential was also obtained in solution containing TTX and low Na+. This potential was supported by Ca2+, Sr2+ or Ba2+ and completely suppressed by Cd2+. (5) The later parts of the action potential, after K-channel blockade, had a pharmacological sensitivity towards Ca-channel blockers matching that of synaptic transmission. This suggests the falling phase of the action potential is caused by charge carrier (mainly Ca2+) passing through Ca-channels that have similar properties to, or are the same as those which open prior to transmitter release.

Levels and synthesis of glutamate and aspartate in the olfactory cortex following bulbectomy

Guinea pigs were unilaterally bulbectomised and the contents of aspartate, glutamate and GABA measured in slices of olfactory cortex taken from the lesioned and intact hemispheres. Two days after the operation there was a fall in the aspartate and glutamate levels, which persisted for over 120 days, whereas gamma-aminobutyric acid (GABA) showed a transient fall followed by a small rise. The fall in glutamate and aspartate was much greater in small, thin slices containing a high density of nerve terminals. The synthesis of 13C aminoacids from [13C]glucose during electrical stimulation was greater in the slices taken from the normal side than in those from the operated side. The GABA synthesis, however, was four times greater on the lesioned side. This time-course for the fall in acidic amino acids correlates with the fall in electrical responses, and this lends weight to the idea that aspartate and/or glutamate mediate synaptic transmission in the area.