Calcium permeable AMPA receptors and autoreceptors in external tufted cells of rat olfactory bulb

Abstract

Glomeruli are functional units of the olfactory bulb responsible for early processing of odor information encoded by single olfactory receptor genes. Glomerular neural circuitry includes numerous external tufted (ET) cells whose rhythmic burst firing may mediate synchronization of bulbar activity with the inhalation cycle. Bursting is entrained by glutamatergic input from olfactory nerve terminals, so specific properties of ionotropic glutamate receptors on ET cells are likely to be important determinants of olfactory processing. Particularly intriguing is recent evidence that α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors of juxta-glomerular neurons may permeate calcium. This could provide a novel pathway for regulating ET cell signaling. We tested the hypothesis that ET cells express functional calcium-permeable AMPA receptors. In rat olfactory bulb slices, excitatory postsynaptic currents (EPSCs) in ET cells were evoked by olfactory nerve shock, and by uncaging glutamate. We found attenuation of AMPA/kainate EPSCs by 1-naphthyl acetyl-spermine (NAS), an open-channel blocker specific for calcium permeable AMPA receptors. Cyclothiazide strongly potentiated EPSCs, indicating a major contribution from AMPA receptors. The current-voltage (I-V) relation of uncaging EPSCs showed weak inward rectification which was lost after > ~ 10 min of whole-cell dialysis, and was absent in NAS. In kainatestimulated slices, Co 2+ ions permeated cells of the glomerular layer. Large AMPA EPSCs were accompanied by fluorescence signals in fluo-4 loaded cells, suggesting calcium permeation. Depolarizing pulses evoked slow tail currents with pharmacology consistent with involvement of calcium permeable AMPA autoreceptors. Tail currents were abolished by Cd 2+ and NBQX, and were sensitive to NAS block. Glutamate autoreceptors were confirmed by uncaging intracellular calcium to evoke a large inward current. Our results provide evidence that calcium permeable AMPA receptors reside on ET cells, and are divided into at least two functionally distinct pools -postsynaptic receptors at olfactory nerve synaptic terminals, and autoreceptors sensitive to glutamate released from dendrodendritic synapses. Keywordsglutamate; glomerulus; dendrite; uncaging; fluorescence; rectification Olfaction begins with the binding of odorants to olfactory receptors which transduce sensory input into action potentials in axons of the olfactory nerve. Axons of cells expressing the same receptor gene converge to stereotypic glomeruli in the superficial layer of the olfactory bulb, forming a topographic neural map of receptor activation (Mombaerts et al., 1996). The olfactory nerve terminals make direct excitatory synapses on dendrites of bulb output neurons, Address correspondence to: Dr. Graeme Lowe, Monell Chemical Senses Center, 3500 Market St, Philadelphia, PA 19104-3308, phone: 215-573-5110, fax: 215-898-2084, email: loweg@monell.org. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for ...

External Tufted Cells Drive the Output of Olfactory Bulb Glomeruli

“…5B). (Ma and Lowe, 2007), APs also induced an AMPA receptor-mediated autoexcitation that caused a voltage rebound that followed the spike (Fig. 5C).…”

“…Patch pipette (ϳ3-6 M⍀) contained the following (in mM): 135 K-gluconate, 2 MgCl 2 , 0.025 CaCl 2 , 1 EGTA, 4 Na-ATP, 0.5 Na-GTP, and 10 HEPES, pH 7.3 (ϳ280 mOsm/L). For studies of synaptic transmission, we added 10 mM glutamate in the internal solution of the presynaptic ET cell to avoid transmitter depletion (Ma and Lowe, 2007) and the resulting rundown of the postsynaptic M-cell response. For all voltage-clamp recordings, we replaced K-gluconate with 120 mM Cs-MeSO 4 , 20 mM tetraethylammonium-Cl, and 5 mM 4-aminopyridine.…”

“…Glomerulus-specific excitatory connections exist between M cells (Schoppa and Westbrook, 2002;Urban and Sakmann, 2002;Christie et al, 2005;Pimentel and Margrie, 2008) and, as we show, between ET and M cells. Action potentials in these cells induce dendritic release of glutamate and autoexcitation (Aroniadou-Anderjaska et al, 1999;Isaacson, 1999;Friedman and Strowbridge, 2000;Salin et al, 2001;Westbrook, 2001, 2002;Ma and Lowe, 2007). Asynchronous afferent activity leading to action potentials in ET or M cells could spread through the coupled network and thus increase excitability in the period that leads up to population bursts in M and ET cells.…”

“…Although the mechanism that drives the rhythmic activity of the glomerular network is at present unclear, external tufted (ET) cells, a population of juxtaglomerular cells, have recently received much attention (Hayar et al, 2004a(Hayar et al, ,b, 2005Antal et al, 2006;Zhou et al, 2006;Ma and Lowe, 2007;Liu and Shipley, 2008). These glutamatergic neurons fire bursts of action potentials (APs), receive monosynaptic inputs from olfactory neurons, and establish excitatory synaptic connections on periglomerular and short axon cells, two other types of juxtaglomerular neurons (Hayar et al, 2004a(Hayar et al, ,b, 2005.…”

Monosynaptic and Polysynaptic Feed-Forward Inputs to Mitral Cells from Olfactory Sensory Neurons

“…For ET cell recording and studies of synaptic transmission in paired recordings, we used an internal solution containing (in mM) 135 K-gluconate, 2 MgCl 2 , 0.025 CaCl 2 , 1 EGTA, 4 Na-ATP, 0.5 Na-GTP, 10 HEPES, and 10 glutamate, pH 7.3) (Osm/L ϳ280). Glutamate was necessary to avoid transmitter depletion and the resulting run-down of the postsynaptic cell response (Ma and Lowe, 2007;De Saint Jan et al, 2009). Alexa-Fluor 594 (4 M) (Invitrogen) was also added to the internal solution to confirm the glomerular projection of the recorded cell.…”