Blockade of mGluRs reduces GC-mediated feedback inhibition of mitral cells. (A) Depolarizing current pulses (100 ms, 300 pA, indicated by bar beneath trace) injected into a mitral cell in the presence of TTX and TEA evoked a Ca²⁺ spike followed by a GC-mediated feedback IPSPs (at arrow). Middle trace shows that the mGluR antagonist LY341495 reduced the IPSP, and subsequent addition of the GABAa receptor antagonist gabazine abolished any remaining feedback IPSP as well as spontaneous IPSPs (lower trace). (B) Group data from similar experiments. *P < 0.05. Adapted from Ref. 35.

DHPG failed to increase mIPSC frequency in mitral cells after excision of the glomerular layer (GL, Sub-GL slices). Examples of biocytin-filled mitral cells recorded in intact (A1) and Sub-GL (A2) slices; arrows mark the apical dendrites. In panel A2 the GL and ONL layer (ONL) have been excised and the mitral cell apical dendrite is truncated within the EPL. (B1) Voltage clamp recordings in normal ACSF show that the frequency of sIPSCs in mitral cells is higher in intact slices (upper trace) than in Sub-GL slices (lower trace). (B2) Bar graph of group data showing lower basal IPSC frequency in Sub-GL slices and that DHPG readily increases the frequency of sIPSCs recorded in normal ACSF in intact and Sub-GL slices. (C) DHPG readily increased sIPSCs in a mitral cell recorded in a Sub-GL slice. (D) DHPG application did not affect the frequency of mIPSCs (recorded in the presence of TTX) in a mitral cell recorded in a Sub-GL slice. Adapted from Ref. 37.

Relative contribution of GCs and PG cells to DHPG evoked-IPSCs in mitral cells. (A) Experimental approach. Two pipettes were positioned to focally puff DHPG (20 nL, 1 mM in ACSF) into the GL and GCL, respectively, while recording IPSCs in a mitral cell. DHPG puffs into the GL (B1) or GCL (B2) increased the frequency of sIPSCs (left column) recorded in normal ACSF; same cell in panels B1 and B2. (B3, B4) Recordings from the same mitral cell in panels B1 and B2 after perfusion with CNQX plus APV plus TTX. DHPG puffs into the GL (B3), but not the GCL (B4), increased the frequency of mIPSCs. (C) Group data showing the effects of DHPG and KCl puffs into the GL or GCL on the frequency of sIPSCs and mIPSCs. *P < 0.05, compared with control, paired t tests. KCl puffs increase IPSCs in all cases. Adapted from Ref. 37.

DHPG increases mIPSCs in ET cells. Upper trace: voltage clamp recordings (HP = −60 mV) from an ET cell in the presence of CNQX (10 μM), APV (50 μM), and TTX (1 μM). Because the chloride equilibrium potential in these conditions was near 0 mV, mIPSCs were recorded as inward currents (downward deflections). The four lower traces correspond to recordings from the first trace displayed at faster time scale. DHPG (30 μM) application produces a slow inward current associated with a robust increase in the frequency of mIPSCs. Subsequent application of Cd²⁺ (100 μM) decreases basal and DHPG-evoked mIPSCs. Inset at left shows diagram of recording from an ET cell in the GL. Adapted from Ref. 37.

Group I and group I/II mGluR agonists directly and indirectly excite deep GCs (dGCs). Upper two traces: bath application of the group I/II mGluR agonist ACPD (50 μM, at lower bars) depolarized and increased spiking in this rat dGC (top trace). Application of blockers of fast synaptic transmission (synaptic blockers; CNQX, 10 μM; APV, 50 μM; gabazine, 5 μM) to the same cell reduced the baseline spontaneous synaptic activity (bottom trace); the depolarizing response to ACPD under these conditions was smaller than that without synaptic blockers and did not reach spike threshold. Lower two traces: the group I mGluR agonist DHPG (50 μM) depolarized and increased the firing rate of this typical mouse dGC in normal ACSF (top trace). In the same cell, blockers of fast synaptic transmission reduced baseline spontaneous synaptic activity and DHPG evoked a more moderate depolarization (bottom trace). Adapted from Ref. 35.

Activation of group I mGluRs increases the frequency of spontaneous GABAergic input to mitral cells. Voltage clamp recordings (HP = 0 mV) of mitral cells show that the group I mGluR agonist, DHPG, dose-dependently increased the frequency of GABAergic sIPSCs (A) and mIPSCs (C); gabazine eliminated all IPSCs, as shown in lower traces. sIPSCs were recorded in normal ACSF (Control) and mIPSCs were recorded in the presence of CNQX (10 μM), APV (50 μM), and TTX (1 μM). (B and D) Group data showing the dose–response relationship for DHPG effects on sIPSC (n = 6) and mIPSC (n = 7) frequency and amplitude (inset bar graphs) recorded in a 1-min epoch. *P < 0.05 compared with control, ANOVA followed by Neuman–Keuls post-hoc comparisons. DHPG did not affect the amplitude of sIPSCs or mIPSCs. Adapted from Ref. 37.

Blockade of mGluRs attenuates M/T cell–evoked excitation of GCs. (A) Upper trace: single shocks (marked by arrow in lower trace) applied to the GL to directly activate M/T cells and evoke excitatory postsynaptic potentials (EPSPs) and spikes in a mouse GC. Middle trace: application of LY341495 (100 μM) markedly reduced evoked excitation of the same GC in a reversible manner (lower trace). (B) Bar graphs of group data (n = 10) show the percentage of GL–evoked spikes before (control) and during application of LY341495. *P < 0.05. (C) LY341495 did not significantly alter the resting membrane potential in the group of 10 cells tested. (D) Regression analysis revealed that the reduction of evoked spikes in individual GCs varied with the degree of hyperpolarization. Data are fitted with a regression line (r = −0.73; P = 0.01). (E) LY341495 did not alter the mean amplitude of the EPSP (n = 10 cells). Adapted from Ref. 35.

mGluR inactivation reduces the amplitude, duration, and spatial extent of M/T cell–evoked excitation of GCs. (A) Optical recording setup used for voltage-sensitive dye imaging. Upper panel shows a videographic image of a 400-μm-thick forebrain-MOB slice. Hexagonal frame demarcates the border of the 464-element photodiode array. The position of the LOT stimulation electrode is indicated. Lower panel shows a higher-magnification videographic image of the area within the hexagon in upper panel. (B) Upper row shows images of optical activity elicited by one shock to the LOT. Each panel shows the amplitude of optical responses, recorded by each of 464 photodiodes. Signal amplitudes, expressed as a percentage above mean baseline values, are color coded. Intervals beneath the images are relative to the onset of LOT stimulation. Lower row: application of LY341495 reduced the LOT-evoked optical signals. CFF, centrifugal fibers; gl, glomerular layer; epl, external plexiform layer; mcl, mitral cell layer; gcl, granule cell layer; LOT, lateral olfactory tract. Adapted from Ref. 35. (In color in Annals online.)

Deep and superficial GCs differentially express mGluR1 and mGluR5. Upper four panels: The group I mGluR agonist DHPG (50 μM) depolarized and increased spike firing in superficial GCs (sGCs) and deep GCs (dGCs) in olfactory bulb slices from wild-type mice. Photomicrograph montage of a biocytin-filled sGC (A) and dGC (B); note soma location in the mitral cell layer (MCL) for the sGC. GCL, granule cell layer; GL, glomerular layer; EPL, external plexiform layer. (C) Bath application of DHPG depolarized and increased the discharge of an sGC (upper trace). Lower trace: DHPG applied in the presence of blockers of fast synaptic transmission (CNQX, APV, gabazine) resulted in smaller membrane depolarization in the same sGC. (D) DHPG also depolarized and increased the firing of dGCs (upper trace). Lower trace: DHPG applied in the presence of blockers of fast synaptic transmission resulted in smaller depolarization of the same dGC. Middle four panels: (A) I–V curves for sGCs in slices from mGluR5^−/− (a) and mGluR1^−/− mice (b), before (control, open circles) and during bath application of DHPG (50 μM, solid circles). Plots represent group data (mean ± SEM) from four sGCs (a) and five sGCs (b). DHPG evoked a current in mGluR5^−/− (a) but not mGluR1^−/− (b) mice. (B) Similar experiments for dGCs in slices from mGluR5^−/− (a) and mGluR1^−/− (b) mice. No DHPG-induced current was recorded from dGCs in slices from mGluR5^−/− mice, whereas a robust current was observed in slices from mGluR1^−/− mice. The inward current of dGCs in slices from mGluR1^−/− mice diminished near the estimated K+ equilibrium potential (−96 mV). DHPG evoked a significant inward current (P < 0.05 compared with control) at −90 mV and more positive voltages. All experiments were performed with blockers of fast synaptic transmission and TTX in the bath. Lower illustration: MOB circuit, depicting mGluR1 expression by mitral cells and sGCs and mGluR5 expression by dGCs. Superficial dendritic extensions of deep and superficial GCs interact with different portions of M/T cell lateral dendrites. Adapted from Ref. 36.