Selective effects of 3-methylindole on the olfactory system: An anatomical and behavioral study

The effect of 3-Methylindole (3-MI) on axonal transport of horseradish peroxidase (HRP) from olfactory epithelium to the olfactory bulb and on odor detection was examined. In Experiment I, rats given 0, 150, 350, or 400 mg/kg of 3-MI were sacrificed 7, 12, 22, or 92 days later, and their olfactory bulbs were examined. 3-MI had selective effects on axonal transport that were both dose- and time-dependent. At the largest dose and shortest survival time, only a few bulbar glomeruli, generally in the mid lateral and ventromedial sectors of the bulb, had reaction product. For each dose there was a clear pattern of recovery in axonal transport as a function of time. The most severe deficits in axonal transport at lower doses and or longer survival times were in the dorsal and dorsomedial sectors of the bulb. In Experiment II, four trained rats treated with 400 mg/kg dose initially performed at chance but were able to relearn a simple odor detection task (0.15% isoamyl acetate vs. air). Controls and 3-MI treated rats trained on a visual task, however, had no deficits in retention. Experimental animals had, on average, less than 6% of glomeruli with detectable levels of reaction product. In Experiment III, three experimental animals tested on different concentrations of each of 4 different odors performed more poorly than controls but each was able to detect one or more concentrations of each odor and there was no evidence for a specific anosmia. These rats had very light to dense axonal transport to approximately 12% of their olfactoly bulb glomeruli. Further, bulbar regions identified with 2-DG in prior studies with these odors had little or no transport. Four other animals which had axonal transport in approximately 6% of their olfactory bulb glomeruli performed at chance on each of these odor detection tasks. These results demonstrate that 3-MI has a highly selective effect on connections of the bulb with receptor neurons and that rats with minimal input to restricted parts of the bulb are able to detect a variety of odors.