High-frequency oscillations are not necessary for simple olfactory discriminations in young rats

Individual olfactory bulb mitral/tufted cells respond preferentially to groups of molecularly similar odorants. Bulbar interneurons such as periglomerular and granule cells are thought to influence mitral/tufted odorant receptive fields through mechanisms such as lateral inhibition. The mitralgranule cell circuit is also important in the generation of the odor-evoked fast oscillations seen in the olfactory bulb local field potentials and hypothesized to be an important indicator of odor quality coding. Infant rats, however, lack a majority of these inhibitory interneurons until the second week of life. It is unclear if these developmental differences affect olfactory bulb odor coding or behavioral odor discrimination. The following experiments are aimed at better understanding odor coding and behavioral odor discrimination in the developing olfactory system. Single-unit recordings from mitral/tufted cells and local field-potential recordings from both the olfactory bulb and anterior piriform cortex were performed in freely breathing urethane-anesthetized rats (postnatal day 7 to adult). Age-dependent behavioral odor discrimination to a homologous series of ethyl esters was also examined using a cross-habituation paradigm. Odorants were equated in all experiments for concentration (150 ppm) using a flow dilution olfactometer. In concordance with the reduced interneuron population, local field potentials in neonates lacked detectable odor-evoked gamma-frequency oscillations that were observed in mature animals. However, mitral/tufted cell odorant receptive fields and behavioral odor discrimination did not significantly change, despite known substantial changes in local circuitry and neuronal populations, over the age range examined. The results suggest that high-frequency local field-potential oscillations do not reflect processes critical for simple odor discrimination.