High-Frequency Oscillations Are Not Necessary for Simple Olfactory Discriminations in Young Rats

Abstract

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 mitral-granule 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 crosshabituation 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 ␥-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.

Disynaptic Amplification of Metabotropic Glutamate Receptor 1 Responses in the Olfactory Bulb

“…Similar patterns of oscillatory activity are observed in slices stimulated electrically (Lagier et al, 2004), suggesting similar circuits are recruited in vivo and in vitro. Moreover, they are absent in young animals in vivo (Fletcher et al 2005), similar to long-lasting depolarization in vitro (Fig. 1d), suggesting they might be driven in older animals by slow EPSPs in mitral cells.…”

“…In contrast behavioral studies indicate that olfactory discrimination is fast (ϳ200 ms) (Uchida and Mainen, 2003;Abraham et al, 2004), although seemingly at the expense of accuracy for closely similar odors (Uchida and Mainen, 2003;Abraham et al, 2004;Khan and Sobel, 2004). Accordingly, odor identification seems intact in young rats in which odors do not evoke slow patterns of activity (Fletcher et al, 2005). These studies raise the question of the benefit of slow neural processes.…”

Ontogeny of cortical synaptic depression underlying olfactory sensory gating in the rat

“…First, although behavioral odor habituation is expressed to a similar extent and time course throughout development [6,7], we wanted to confirm that the . In all procedures described below, a concentric bipolar stainless steel stimulating electrode was placed on the lateral olfactory tract (LOT) and field potentials were recorded from layer I of the aPCX using tungsten microelectrodes (A-M Systems).…”

“…First, although behavioral odor habituation is expressed to a similar extent and time course throughout development [6,7], we wanted to confirm that the cortical synaptic depression believed to underlie odor habituation was also expressed throughout the postnatal period. Second, we wanted to confirm the role of group III mGluRs in this synaptic depression throughout development.…”

“…Second, we wanted to confirm the role of group III mGluRs in this synaptic depression throughout development. Although the olfactory system undergoes major postnatal developmental changes in cell number and circuitry [4], the mitral/tufted cell projection via the lateral olfactory tract (LOT) to the aPCX is present and functional near birth [7,15,16], and rats are reliant on olfactory information for survival from birth [17].…”