Response properties of rat olfactory bulb neurones

Abstract

SUMMARY1. Response properties of rat olfactory bulb neurones were studied by recording single-unit activity evoked by stimuli delivered by an air-dilution olfactometer. Neural responses were quantified as peristimulus time histograms by averaging the responses evoked by eight presentations of the same stimulus. Animals were stimulated by means of controlled artificial sniffs produced by a vacuum source connected to the choana. 4 2. Evoked activity was divided into on-and after-responses. Three types of on-responses were distinguished on the basis of temporal patterns of activity evoked by odorous stimuli. Neurones exhibited the same type of response when driven by different odorants. For a given neurone, the size of an evoked response depended on the odorant used as a stimulus substance. The stimulating effectiveness of particular odorants varied between different neurones.3. After-responses were divided into two classes: phasic (05-3 sec after stimulus offset) and tonic (3 sec-2 min after stimulus offset). After-responses were associated with a subset of neurones making each type of on-response and with units producing no clear on-response. It is suggested that the occurrence ofan after-response in a given neurone is not dependent on the nature of the evoked on-response.4. Intensity-related changes ofon-responses were characterized by three measures: number of action potentials evoked during the stimulus event, latency of actionpotential bursts, and rate of activity during the action-potential bursts. The number of evoked action potentials was not monotonically related to stimulus concentration. In contrast, there was a clear relationship between the pattern of evoked activity and stimulus concentration.5. It is proposed that the timing of excitation and inhibition is dependent on the concentration of the stimulus and that the number of evoked action potentials depends on the odorant used as a stimulus substance and the concentration at which it is delivered.

Physiological evidence for two classes of mitral cells in the rat olfactory bulb

“…Antidromic activation of axons in the lateral olfactory tract (LOT) can identify neurons that project from the MOB, but in many previous studies data from mitral and tufted cells were combined, and the cells were referred to as ‘mitral/tufted’ or ‘M/T’ cells (e.g., Buonviso et al, 1992; Imamura et al, 1992; Yokoi et al, 1995). In other prior studies (e.g., Mair, 1982; Chaput & Holley, 1980), cells were identified by depth in the MOB, but without antidromic activation. Such cells could be mitral cells, granule cells, or deep tufted cells.…”

“…Possibly the temporal pattern (bursting) or anatomical differences such as the extent of lateral dendrites or specific interactions with inhibitory interneurons in the glomerular or external plexiform layers are more important in coding odor information. Measurements of mitral cell responses to odor stimulation has revealed that some cells are excited while others are inhibited (e.g., Wellis et al, 1989; Nagayama et al, 2004), that the response can depend on odor concentration (e.g., Mair, 1982; Harrison & Scott, 1986, but also Chalansonnet & Chaput, 1998), and that responses in some cells are modulated by respiration (Macrides & Chorover, 1972; Chaput & Holley, 1980; Chaput et al, 1992; Buonviso et al, 2006). This range of response characteristics and their underlying neural circuits could form a more coherent pattern when more is known about the physiological subclasses of mitral cells.…”

“…However, only a few studies have measured and reported the mean rate for the spontaneous activity; many more have examined a stimulus-evoked response and the spontaneous activity was estimated from a short pre-stimulus period. These previous estimates of spontaneous activity show a high degree of variability ranging from 0 to 42 spikes/sec (Green et al, 1962; Phillips et al, 1963; Mair, 1982; Shepherd, 1963; Yamamoto et al, 1963; Getchell & Shepherd, 1975; Chaput & Holley, 1980; Mori & Takagi, 1978; Harrison & Scott, 1986; Meredith, 1986; Chaput & Lankheet, 1987; Imamura et al, 1992; Yu et al, 1993; Ogawa, 1998; Nagayama et al, 2004; Rinberg et al, 2006). These studies are subject to a variety of methodological problems, including: uncertainty of the identity of the cell as a mitral cell; effects of anesthesia; bias in selecting/isolating cells; and short lengths of recording.…”

Chemotopic odorant coding in a mammalian olfactory system

“…The possibility of the spatial code being transformed into a temporal code for olfactory information also has been raised many times, based on observations that the temporal pattern of activity of mitral cells differs for different odorants, potentially allowing such differences to carry the coded odorant information (Kauer, 1974;Mair, 1982;Meredith, 1986;Harrison and Scott, 1986). In addition to temporal patterns of spiking in individual mitral cells, widespread odorant-evoked oscillatory patterns of a variety of periodicities also have been demonstrated in various olfactory systems (Adrian, 1950b;Freeman and Skarda, 1985;Freeman and Viana Di Prisco, 1986;Laurent and Naraghi, 1994;Gelperin, 1999;Kay, 2003).…”

Respiratory cycle as time basis: An improved method for averaging olfactory neural events

“…Respiratory modulation of electro-encephalographic (EEG) and local field potential (LFP) signals has been extensively described in OB and OC (Boudreau & Freeman, 1963;Freeman, 1975;Freeman & Schneider, 1982;Freeman, 1983;Buonviso et al, 2003, Fontanini et al, 2003. Such a respiratory modulation has also been described in cell unitary activities in the OB mitral/tufted (Walsh, 1956;Macrides & Chorover, 1972;Chaput & Holley, 1980;Onoda & Mori, 1980;Mair, 1982;Pager, 1985;Chaput, 1986) and was recently described in the piriform cortex (PC) (Wilson, 1998;Litaudon et al, 2003). Rather than a variation in overall firing frequency, odor-evoked cell responses have often been reported as a temporal reorganization of their discharges into inhalation-related bursts of spikes.…”