Greater excitability and firing irregularity of tufted cells underlies distinct afferent‐evoked activity of olfactory bulb mitral and tufted cells

Abstract

Key pointsr The two classes of principal neurons in the mammalian main olfactory bulb, mitral and tufted cells, respond with different firing latencies and rates to afferent-evoked input; how these differences in activity arise is incompletely understood.r Tufted cells receive stronger afferent-evoked excitation than mitral cells, but this difference alone is insufficient to account for the greater afferent-evoked firing in tufted versus mitral cells.r Mitral and tufted cells exhibit significant intrinsic functional differences; compared to mitral cells, tufted cells fire action potentials with shorter durations and faster afterhyperpolarizations and exhibit twofold greater firing rate-current curve gains and peak rates.r Tufted cells exhibit diverse firing modes, including tonic firing and irregular stuttering, and on average fire more irregularly than mitral cells.r Collectively, stronger afferent excitation, greater intrinsic excitability and more irregular firing in tufted cells combine to drive distinct responses of mitral and tufted cells to sensory input.Abstract Mitral and tufted cells, the two classes of principal neurons in the mammalian main olfactory bulb, exhibit morphological differences but remain widely viewed as functionally equivalent. Results from several recent studies, however, suggest that these two cell classes may encode complementary olfactory information in their distinct patterns of afferent-evoked activity. To understand how these differences in activity arise, we have performed the first systematic comparison of synaptic and intrinsic properties between mitral and tufted cells. Consistent with previous studies, we found that tufted cells fire with higher probability and rates and shorter latencies than mitral cells in response to physiological afferent stimulation. This stronger response of tufted cells could be partially attributed to synaptic differences, as tufted cells received stronger afferent-evoked excitation than mitral cells. However, differences in intrinsic excitability also contributed to the differences between mitral and tufted cell activity. Compared to mitral cells, tufted cells exhibited twofold greater excitability and peak instantaneous firing rates. These differences in excitability probably arise from differential expression of voltage-gated potassium currents, as tufted cells exhibited faster action potential repolarization and afterhyperpolarizations than mitral cells. Surprisingly, mitral and tufted cells also showed firing mode differences. While both cell classes exhibited regular firing and irregular stuttering of action potential clusters, tufted cells demonstrated a greater propensity to stutter than mitral cells. Collectively, stronger afferent-evoked excitation, greater intrinsic excitability and more irregular firing in tufted cells can combine to drive distinct responses of mitral and tufted cells to afferent-evoked input.

Intraglomerular Lateral Inhibition Promotes Spike Timing Variability in Principal Neurons of the Olfactory Bulb

“…However, that study relied on a theoretical network model based on experimentally undemonstrated differences in PG cell inputs on mitral and tufted cells. More recently, Burton and Urban (2014) suggested that the different firing phases of mitral and tufted cells may be attributable to a greater intrinsic excitability and a stronger afferent-evoked excitation in tufted cells.…”

“…Experimental protocols were approved by the National Institute of Health and Medical Research and the National Center of Scientific Research guidelines. Horizontal olfactory bulb slices were prepared from 14-to 30-d-old transgenic mice of either sex expressing the EYFP under the control of the Kv3.1 K ϩ channel promoter (Metzger et al, 2002) or the enhanced green fluorescent protein (EGFP) under the control of the calretinin (CR) promoter (Caputi et al, 2009). Note that, in these mice, the fluorescent protein expression may differ from the expression pattern of the Kv3.1 protein (Metzger et al, 2002) or the CR protein (Caputi et al, 2009).…”

“…Horizontal olfactory bulb slices were prepared from 14-to 30-d-old transgenic mice of either sex expressing the EYFP under the control of the Kv3.1 K ϩ channel promoter (Metzger et al, 2002) or the enhanced green fluorescent protein (EGFP) under the control of the calretinin (CR) promoter (Caputi et al, 2009). Note that, in these mice, the fluorescent protein expression may differ from the expression pattern of the Kv3.1 protein (Metzger et al, 2002) or the CR protein (Caputi et al, 2009). Mice were killed by decapitation, and the bulbs were dissected rapidly in ice-cold oxygenated (95% O 2 /5% CO 2 ) solution containing the following (in mM): 83 NaCl, 26.2 NaHCO 3 , 1 NaH 2 PO 4 , 2.5 KCl, 3.3 MgSO 4 0.5 CaCl 2 , 70 sucrose, and 22 D-glucose, pH 7.3 (osmolarity, 300 mOsm/L).…”

“…The dura was removed before stabilizing the bulb with agarose (2% w/v). After surgery, the animal was placed under a custombuilt two-photon laser scanning microscope (Charpak et al, 2001) equipped with a 40ϫ water-immersion objective (numerical aperture, 0.8) and a Mira 900 mode-locked titanium:sapphire laser (Coherent). Anesthesia was switched to isoflurane (1-2%, 3 L/min in moisturized air/O 2 ), and its depth was monitored throughout by respiration rates along with lack of response to toe pinch.…”

“…PG cells expressing CR are three times more numerous than CB-expressing PG cells and constitute the most abundant immunohistochemical subtype (Panzanelli et al, 2007;Parrish-Aungst et al, 2007;Whitman and Greer, 2007). We used transgenic mice expressing EGFP under the control of the CR promoter to identify these PG cells (Caputi et al, 2009). In CR-EGFP mice, only 56% of the cells immunoreactive for CR were also EGFP ϩ (1173 of 2098 cells), but almost all EGFP ϩ PG cells were immunoreactive for CR (1173 of 1185 cells, n ϭ 6 mice; Fig.…”