Odor representation and discrimination in mitral/tufted cells of the rat olfactory bulb

Abstract

Extracellular single-unit responses to odorants with various properties were recorded from mitral/tufted cells over large areas of the olfactory bulb of anesthetized rats. Each cell was exposed to one stimulus set consisting of five different odorants each at five concentrations. The resulting concentration-response profiles were compared. All mitral/tufted cells examined responded to two or more odorants, and the largest proportion of the cells were sensitive to all five odorants. Cells unresponsive to all five odorants regardless of concentration were not observed. Mitral/tufted cells sensitive to all three of the odorants that are known to evoke maximal electro-olfactograms in different regions of the olfactory epithelium were distributed widely throughout the olfactory bulb. There were no significant differences in latencies of odor responses either across recording sites or across odorants. A comparison of the concentration-response profiles suggested that all of the mitral/tufted cells were equally capable of responding to any odorant with their own distinctive pattern, but that the cells tended to show an identical pattern rather than variable pattern of response to different odorants. Five mitral/tufted cells isolated within 800 microns of one electrode track showed different concentration-response profiles. Of 18 simultaneously recorded spike pairs with different amplitudes and discharge patterns recorded incidentally through one electrode at different sites, 10 had different and 8 had identical response patterns to odorants. These results suggest that: (1) mitral/tufted cells are sensitive to a broad spectrum of odorants, but respond with their own patterns to odorants; (2) odor discrimination is not uniform in neighboring cells, and a discrimination unit is comprised of a single cell.

Sparse and Selective Odor Coding by Mitral/Tufted Neurons in the Main Olfactory Bulb

“…In other studies, M/Ts responded to subsets of related compounds within a defined structural class, although selectivity across broader chemical categories was not addressed Mori et al, 1992;Katoh et al, 1993). Still other studies found relatively indiscriminate activation by various stimuli within small odorant panels (Motokizawa, 1996;Lehmkuhle et al, 2003Lehmkuhle et al, , 2006. Analogous neurons in the insect antennal lobe appear broadly tuned (Perez-Orive et al, 2002;Mazor and Laurent, 2005), although other insect findings support selectivity (Wang et al, 2003).…”

Sparse Odor Coding in Awake Behaving Mice

“…However, this explanation appears unlikely, because reduction of NMDA transmission in urethane-anesthetized rats by various NMDA receptor antagonists does not lead to noticeable changes in the mitral cell spontaneous firing rate (Wilson et al, 1996). It is also true that anesthetics that have not been reported to block NMDA receptors, such as urethane, yield similar reductions in mitral cell spontaneous rate (Pager, 1983;Motokizawa, 1996;Kay and Laurent, 1999). We argue therefore that modulatory centrifugal inputs that are sensitive to the animal's state of vigilance, such as serotonin-containing fibers from the dorsal raphe nucleus and cholinergic fibers from the basal forebrain (Shipley and Ennis, 1996;Haberly, 2004), could modulate mitral cell spontaneous activity.…”

Chemotopic odorant coding in a mammalian olfactory system

“…Therefore, the failure of these spatially (and temporally) unconstrained approaches to find evidence for tuning (c.f. : Motokizawa, 1996) cannot be taken as evidence that tuning of odorant responses does not occur.…”

“…It has been argued that this condition may have obscured a broader responsiveness of these cells than would have been detected if the animals were differently anesthetized (Motokizawa, 1996) or entirely awake (Bhalla and Bower, 1997;Kay and Laurent, 1999;Rinberg et al, 2006a). Awake animals, such as those we study using 2DG, show complex spontaneous mitral cell activity against which it appears to be easier to detect information about the animal's behavioral state (e.g., hunger, expectation of odor, alertness, or responses to any odor independent of its identity) than to detect information directly relevant to odor quality (Pager, 1974a,b;Bhalla and Bower, 1997;Kay and Laurent, 1999;Rinberg et al, 2006a), at least when the recordings are made in arbitrary locations in response to arbitrarily chosen odorants.…”