1 1 Symposium Chemosensory Receptors Satellite DEVELOPMENT ...

237 Poster Central Olfaction and Chemical EcologyTWO DISTINCT CLASSES OF EXCITATORYGLUTAMATERGIC INPUTS ONTO OLFACTORY BULBGRANULE CELLSBalu R. 1 , Strowbridge B. 1 1 Neurosciences, Case Western ReserveUniversity, Cleveland, OHGranule cells mediate lateral and self-inhibition of mitral cells andare critical for sculpting mitral cell output patterns. Despite theirimportance in controlling olfactory bulb output, little is known aboutthe fundamental properties of excitatory synaptic transmission ontogranule cells. In addition, the functional properties of excitatory inputsonto proximal granule cell spines remain a mystery. We used minimalstimulation techniques and quantal analysis combined with whole cellpatch-clamp recording in olfactory bulb slices to study excitatorytransmission at single granule cell spines. We found two distinct classesof excitatory inputs onto granule cells. Inputs onto distal spines in theexternal plexiform layer (presumably from mitral cell secondarydendrites) showed strong paired pulse depression due to an increase intransmission failures on the second stimulus. In contrast, EPSCs fromproximal inputs in the granule cell layer (possibly from mitral cell axoncollaterals or centrifugal feedback inputs) showed paired-pulsefacilitation accompanied by a decrease in failure rate on the secondstimulus. These two types of synapses also showed markedly differentresponses to trains of EPSCs that mimic bursts of mitral cell actionpotentials during sniffing. 50 Hz stimulus trains rapidly silencedtransmission at distal synaptic contacts after 2-3 shocks, while the samestimuli produced initial facilitation followed by steady state depressionat proximal synapses. These two classes of synapses are thus expectedto have distinct effects on granule cell output and the time course offeedback inhibition onto mitral cells. Supported by NIH grants F30-DC007274 (to R.B.) and R01-DC04285 (to B.W.S)238 Poster Central Olfaction and Chemical EcologyACTIVATION OF METABOTROPIC GLUTAMATERECEPTORS (MGLUR1) IN THE GLOMERULAR LAYER (GL)AND GRANULE CELL LAYER (GCL) OF THE OLFACTORYBULB ENHANCES SYNAPTIC INHIBITION OF MITRALCELLS (MCS)Dong H. 1 , Hayar A. 2 , Ennis M. 1 1 Anatomy and Neurobiology,University of Tennessee Health Science Center, Memphis, TN;2 Neurobiology and Developmental Sciences, University of Arkansas forMedical Sciences, Little Rock, ARmGluRs are densely expressed on granule cells (GCs) andjuxtaglomerular neurons and may modulate inhibitory dendrodendriticsynapses onto MCs. We investigated the actions of the group I mGluRagonist DHPG on spontaneous IPSCs (sIPSCs) and TTX-insensitiveminiature IPSCs (mIPSCs) recorded in MCs in rodent olfactory bulbslices. Bath-applied DHPG in intact slices increased sIPSC frequency at1 µM, and increased mIPSC frequency at 3 µM. IPSCs were blocked bygabazine (10 µM). The mGluR1 antagonist LY367835 (100 µM)blocked DHPG's enhancement of mIPSC frequency in most MCs. Focalpressure application of DHPG (1 mM) in the GL or GCL increasedsIPSC frequency; however, an increase in mIPSC frequency was onlyobserved when DHPG was puffed in the GL. In slices in which the GLwas excised, bath-applied DHPG at 100 µM did not alter mIPSCfrequency, but it increased sIPSC frequency at 10 uM. Taken together,these results suggest that DHPG-evoked excitation of GCs orperiglomerular neurons increases spike-dependent GABAergicinhibition of MCs. Further, DHPG appears to presynaptically facilitatespike-independent release of GABA from periglomerular cells. Grants:DC06356, DC07123, DC03195.239 Poster Central Olfaction and Chemical EcologyGROUP I METABOTROPIC GLUTAMATE RECEPTORS AREDIFFERENTIALLY EXPRESSED BY TWO POPULATIONS OFOLFACTORY BULB GRANULE CELLSHeinbockel T. 1 , Hamilton K.A. 2 , Matthew E. 3 1 Anatomy, Howard Univ,Washington, DC; 2 Cellular Biology & Anatomy, Louisiana State UnivMedical Center, Shreveport, Shreveport, LA; 3 Anatomy &Neurobiology, Univ of Tennessee Health Science Center, Memphis, TNAt least two classes of main olfactory bulb granule cells (GCs) can bedistinguished based on soma location, either deep in the GC layer(dGCs) or superficially in the mitral cell layer (MCL) interspersed withmitral somata. Little is known about the physiological properties of thedGCs vs. superficial GCs (sGCs). We explored the role of mGluRs inregulating activity GC in slices from wildtype (WT) and mGluRknockout (KO) mice using patch-clamp electrophysiology. In WT mice,bath application of the group I/II mGluR agonist ACPD or the selectivegroup I agonist DHPG, but not Group II or III agonists, depolarized andincreased the firing rate of both populations of GCs. The two GCpopulations responded differentially to DHPG in mGluR1 and mGluR5KO mice. DHPG activated sGCs in slices from mGluR5, but not frommGluR1, KO mice. By contrast, dGCs responded to DHPG in slicesfrom mGluR1, but not from mGluR5, KO mice. Both GC populationslacked an axon and had apical dendrites that extended into the externalplexiform layer (EPL). dGCs had a long apical dendrite that crossed theMCL and then ramified in the superficial EPL. sGCs branched almostimmediately, i.e., close to the cell body and sent dendrites into the deepEPL. Dendritic spines were observed on both dGCs and sGCs. Theseanatomical results agree with previous studies that the morphologicalproperties of GCs vary with laminar depth. The presentpharmacological results suggest that sGCs are more similar to mitralcells than dGCs in terms of mGluR expression, i.e., both mitral andsGCs express mGluR1 but not mGluR5. Support: Whitehall Foundationand PHS grants DC03195 & DC00347.240 Poster Central Olfaction and Chemical EcologyGLUTAMATE AUTORECEPTORS ON DENDRITES OFEXTERNAL TUFTED (ET) CELLSMa J. 1 , Lowe G. 1 1 Monell Chemical Senses Center, Philadelphia, PAIn principal neurons of the main olfactory bulb (MOB), glutamateautoreceptors provide a presynaptic mechanism for modulatingneuronal activity during dendrodendritic neurotransmission. Actionpotential synchronization of mitral cells projecting to one glomerulusrelies on AMPA autoreceptors on dendritic tufts. Here we show thatAMPA autoreceptors are also expressed on tufts of another type ofMOB principal neuron, the ET cell. In whole-cell voltage clamprecordings of ET cells in rat MOB slices, with 1 µM TTX, 50 µMbicuculline, 100 µM APV, depolarizing voltage pulses (100 ms, -60 mVto 0 mV) activated Ca 2+ currents plus a slow tail current that waspotentiated by 100 µM cyclothiazide (charge transfer 35 ± 5 pC, decay τ= 78 ± 37 ms, n = 13) and abolished by NBQX. In 300 µM NAS, thiscurrent was strongly attenuated (charge transfer 43 ± 10% of control),accelerated (τ = 39 ± 15 ms) and restored after drug wash out (77 ±14%, n = 8). This indicates a major contribution from Ca 2+ -permeantAMPA receptors. In 500 µM Cd 2+ , uncaging Ca 2+ in ET cell tuftsloaded with 6 mM DM-nitrophen evoked a biphasic current (durations3.8 ± 1.2 ms, 16 ± 7 ms, n = 4) which we attribute to ET autoreceptors.The ET cells projecting to one glomerulus fire periodic spike burstssynchronized in part by gap junction couplings. We suggest that slowAMPA autoreceptor EPSPs may help maintain burst synchrony,analogous to their role in mitral cell spike synchrony. During bursts,Ca 2+ auto-permeation may regulate or sustain glutamate exocytosisinitially triggered by voltage-gated Ca 2+ channels as backpropagatingaction potentials invade the ET cell tuft. Supported by: NIH DC042808-04 (GL).60